Stress Analysis and Fatigue Life Determination of Engine Valves

Fatigue Life of Girders with Trapezoidal Corrugated Webs Richard Sause,M.ASCE1;Hassan H.Abbas,A.M.ASCE2;Robert G.Driver,M.ASCE3;Kengo Anami4;andJohn W.Fisher,Hon.M.ASCE5Abstract:Fatigue design criteria are necessary to design steel corrugated web girders for highway bridges.The paper presents research on the fatigue life of steel bridge I-girders with trapezoidal web corrugations.Eight large-scale test girders were fabricated from HPS 485W steel and fatigue-tested in four-point bending.The web-to-flangefillet welds were made using semiautomatic gas metal arc welding ͑GMAW͒or robotic GMAW.Fatigue cracks initiated in the tensionflange at the web-to-flangefillet weld toe along the inclined web folds and adjacent bend regions,and propagated in theflange.The results demonstrate that steel corrugated web I-girders exhibit a fatigue life that is longer than that of conventional steel I-girders with transverse stiffeners.For design of corrugated web I-girders,the Category BЈdesign curve of the AASHTO LRFD specifications is recommended forfinite life fatigue design calculations,and a value of96.5MPa ͑14.0ksi͒is recommended for the constant amplitude fatigue limit.DOI:10.1061/͑ASCE͒0733-9445͑2006͒132:7͑1070͒CE Database subject headings:Fatigue;Girders;Webs;Bridges,highway;Corrugating.IntroductionGirders with steel corrugated webs have been used in highway bridges in Europe and Japan͑Abbas2003͒.Different corrugation geometries are possible,but trapezoidal corrugations have been widely used in practice.Recent research at Lehigh University, conducted in cooperation with others͑Sause et al.2003a,b;Sause 2003͒has investigated the design and fabrication of steel corru-gated web highway bridge girders.This paper presents research on the fatigue life of steel corrugated web bridge I-girders with trapezoidal corrugations.The research objectives were:͑1͒to develop fatigue life data for steel corrugated web I-girders with trapezoidal corrugations, and͑2͒to propose fatigue design criteria for steel corrugated web I-girders for bridges.Fatigue tests on large-scale girder specimens with full-scale trapezoidal webs were conducted.This paper summarizes and analyzes the test results.Fatigue design criteria are recommended.BackgroundFatigue Design SpecificationsThe AASHTO LRFD bridge fatigue design provisions͑AASHTO 2004͒are based on work by Fisher et al.͑1970,1974͒,which showed that stress range and detail type control the fatigue life of welded details,and that for a given detail the fatigue life,in terms of number of cycles to failure,N,is related to the stress range,S r, as follows:log N=log A−B log S r͑1͒where log A=log N axis intercept and B=slope.Application of this log-log transformation to fatigue test data resulted in a normal distribution of the data,and a lower bound͑S r–N͒design curve was obtained from the mean curve by shifting1.96s down the log N͒axis,where s is the standard deviation,as follows:log A lower bound=log A mean−1.96s͑2͒This procedure defined the original fatigue resistance of detail Categories A,B,C,D,and E of the AASHTO fatigue design provisions.Keating and Fisher͑1986͒refined these fatigue detail categories using additional test results,proposed a single slope,−3͑i.e.,B=3͒,for all the detail categories,defined detail Categories BЈand EЈand proposed constant amplitude fatigue limits͑CAFLs͒for all the detail categories.The factor of1.96 used in Eq.͑2͒is also from Keating and Fisher͑1986͒.These refinements are the basis for the AASHTO fatigue design provisions͑AASHTO2004͒,summarized in ing the standard deviation for each category͑Table1͒,as reported by Keating and Fisher͑1986͒,the mean curve for each category has been calculated from the design curve using Eq.͑2͒,and is plotted in Fig.1.1Joseph T.Stuart Professor of Structural Engineering and ATLSS Center Director,Dept.of Civil and Environmental Engineering, Lehigh Univ.,Bethlehem,PA18015͑corresponding author͒.E-mail:rs0c@2Visiting Research Scientist,ATLSS Center,Dept.of Civil and Environmental Engineering,Lehigh Univ.,Bethlehem,PA18015; formerly,Graduate Research Assistant.3Associate Professor,Dept.of Civil and Environmental Engineering, Univ.of Alberta,Edmonton AB,Canada T6G2W2.4Assistant Professor,Dept.of Infrastructural Systems Engineering, Kochi Univ.of Technology,Tosayamada-cho,Kochi782-8502Japan.5Professor Emeritus and ATLSS Center Director Emeritus,Dept.of Civil and Environmental Engineering,Lehigh Univ.,Bethlehem,PA 18015.Note.Associate Editor:Scott A.Civjan.Discussion open until December1,2006.Separate discussions must be submitted for individual papers.To extend the closing date by one month,a written request mustbefiled with the ASCE Managing Editor.The manuscript for this paper was submitted for review and possible publication on February10,2005; approved on July15,2005.This paper is part of the Journal of Struc-tural Engineering,V ol.132,No.7,July1,2006.©ASCE,ISSN0733-9445/2006/7-1070–1078/$25.00.1070/JOURNAL OF STRUCTURAL ENGINEERING©ASCE/JULY2006Previous Research on Corrugated Web Girder Fatigue LifeHarrison ͑1965͒tested two I-girders ͓Fig.2͑a ͔͒with sinusoidal corrugations,Beams 1and 2,in four-point bending.The web-to-flange fillet welds were 6.35mm ͑0.25in.͒in size.The corruga-tion depth was 76.2mm ͑3in.͒and the corrugation wavelength was 610mm ͑24in.͒and 419mm ͑16.5in.͒for Beams 1and 2,respectively.The stress range in the bottom flange,based on mea-sured strains,was 185MPa ͑26.9ksi ͒and 156MPa ͑22.6ksi ͒for Beams 1and 2,respectively.Beam 1failed from a fatigue crack propagating from a flame cut edge of the flange after 1.1million cycles.Web-to-flange weld cracks were not reported and 1.1mil-lion cycles can be regarded as a lower bound for these cracks.Harrison ͑1965͒reported a number of web-to-flange weld cracks in Beam 2,which failed after 2.35million cycles.Korashy and Varga ͑1979͒tested 11girders ͓Fig.2͑b ͔͒,stiff-ened using discrete web corrugations in four-point bending.The corrugations had a wavelength of 257mm ͑10.1in.͒and a depth of 78.4mm ͑3.09in.͒.The web-to-flange fillet weld size was 4mm ͑0.16in.͒.The calculated stress range on the top of the bottom flange varied from 132MPa ͑19.2ksi ͒to 216MPa ͑31.4ksi ͒.Cracks initiated at the web-to-flange weld toe within the corrugated regions and propagated into the flange leading to fracture.Ibrahim ͑2001͒reported results from six girders ͓Fig.2͑c ͔͒with trapezoidal corrugations tested in four-point bending.The corrugations were 75mm ͑2.95in.͒deep with a wavelength of 434mm ͑17.1in.͒.The bend radius between the inclined fold and the longitudinal fold was 27mm ͑1.1in.͒and the corrugation angle was 36.9°.The web-to-flange fillet welds were 5mm ͑0.20in.͒in size.The calculated stress range on the top of the bottom flange varied from 64.7MPa ͑9.4ksi ͒to 131MPa ͑18.9ksi ͒.All six girders failed from fatigue cracks that initiated at the web-to-flange weld toe along an inclined fold and propa-gated in the bottom flange within the constant moment region.The point of crack initiation was generally at the end of an inclined fold where the bend region begins.The previous experimental results are plotted in Fig.1with the mean S r –N curves of the AASHTO fatigue detail categories.The figure suggests that the fatigue life of corrugated web girders lies between the fatigue life of Categories B and C.Fatigue Life Tests Test SpecimensSmall-scale specimens generally have longer fatigue life than large-scale specimens ͑Fisher et al.1970,1974;Keating and Fisher 1986͒.Therefore,large-scale fatigue test specimens,representative of bridge girders,were used for the present study ͓Fig.2͑d ͔͒.Fig.3shows the nominal geometry of the fatigue test girders,which were made of A709HPS 485W steel ͑ASTM 2001͒.The test girders were designed for four-point loading with a constant moment test region between the load points.Table 1.AASHTO Fatigue Categories CategoryConstant A ͑MPa 3͒log A ͑lower bound ͒slog A ͑mean ͒CAFL ͑MPa ͒A 8.20ϫ101212.91380.22113.3470165B 3.93ϫ101212.59440.14712.8825110B Ј2.00ϫ101212.30100.147a 12.589282.7C 1.44ϫ101212.15840.06312.281869C Ј1.44ϫ101212.15840.063b 12.281882.7D 7.21ϫ101111.85790.10812.069648.3E 3.61ϫ101111.55750.10111.755531E Ј1.28ϫ101111.1072——17.9aNot provided in the original work by Keating and Fisher ͑1986͒;s assumed equal to that of Category B.bNot provided in the original work by Keating and Fisher ͑1986͒;s assumed equal to that of CategoryC.Fig.1.Mean S r –N curves for AASHTO fatigue detail categories with test data from previousresearchFig.2.Corrugated web girder cross sections used in previous and current experimental investigations ͑drawn to same scale ͒JOURNAL OF STRUCTURAL ENGINEERING ©ASCE /JULY 2006/1071The corrugated web of the test girders had a full-scale trap-ezoidal geometry ͓Fig.3͑c ͔͒,which was based primarily on shear strength considerations ͑Sause et al.2003b ͒.The corrugation angle,␣,and the bend radius,r ,however,were selected with fatigue life in mind.Research by Anami et al.͑2005͒suggests that the fatigue life increases as ␣decreases.However,to satisfy shear strength requirements,Lindner and Huang ͑1995͒suggest that ␣should not be less than 30°.␣for the test girders was 36.9°.The bend radius,r ,of 120mm ͑4.7in.͒gives an r /t w ratio of 20.This large bend radius maintains fracture toughness in the bend region of the web by minimizing the plastic strain during the web forming process.This plastic strain,estimated to be 2.5%͑Abbas 2003͒,should produce only a small reduction in fracture tough-ness ͑Kaufmann et al.2001͒.The large bend radius also reduces stress concentration at the web-to-flange fillet weld toe in the bend region under fatigue loading ͑Ibrahim 2001;Anami et al.2005͒.A web-to-flange weld size of 8mm ͑5/16in.͒was used even though 6mm ͑1/4in.͒was sufficient for strength requirements.A larger weld size leads to larger internal weld defects,increased residual stresses,and larger stress concentration at the weld toe,all of which lead to a reduced fatigue life ͑Fisher et al.1970,1974͒.The weld was undermatched with a yield stress of 345MPa ͑50ksi ͒.The fabricator of the test girders avoided weld-ing stop-starts within the bend region ͑Sause 2003͒.Most welding stop-starts were located on a longitudinal fold and occasionally on an inclined fold.Eight fatigue test specimens were fabricated.Six girders ͑G1A–G6A ͒were welded using semiautomatic gas metal arc welding ͑GMAW ͒.Two girders ͑G1B and G4B ͒were later refab-ricated from Girders G1A and G4A and rewelded using robotic GMAW.After the tests of Girders G1A and G4A were completed,the damaged bottom flange and approximately 50mm ͑2in.͒ofweb above the flange were removed using plasma cutting.Manual grinding was used to treat the cut edge and a new HPS 485W bottom flange was robotically welded to the web.The superiority of these robotic welds,especially the weld toe geometry,compared to the semiautomatic welds,was apparent by visual examination.The size of the test girder flanges was based on the capacity of the loading equipment and the required stress ranges.The rela-tively small flanges were susceptible to flange transverse bending due to shear as well as secondary effects such as flange distortion and nonuniform web contribution to flexure ͑Abbas 2003͒.The flange stresses at some locations within the shear region of the fatigue test girders were shown analytically and experimentally to be substantially higher than the stresses within the constant moment region due to flange transverse bending ͑Abbas 2003͒.Ultrasonic impact treatment ͑UIT ͒was applied to the web-to-flange weld toe on the bottom flange at these locations,shown in Fig.3͑b ͒,to inhibit fatigue cracking ͑Takamori and Fisher 2000͒,and the UIT prevented cracking in these locations.UIT was not applied within the constant moment test region between the load points.Test Setup,Instrumentation,and ProcedureA photograph of the test setup is shown in Fig.4.The girders were tested in four-point bending with two load points,spaced 3m ͑9.8ft ͒apart and symmetric about midspan.Loads were ap-plied at 261cycles/min ͑4.35Hz ͒using two synchronized jacks that were braced in the transverse direction.Load cells measured the applied loads.Bearings provided simply supported conditions.The test matrix is given in Table 2in the order the girders were tested.The nominal stress range varied from 103MPa ͑15.0ksi ͒to 138MPa ͑20.0ksi ͒.The stress ratio,R ,varied between 0.10and 0.13.The nominal stress is defined as the longitudinalstressFig.3.Fatigue test specimens ͑Girders G1A–G6A͒Fig.4.Fatigue test setup1072/JOURNAL OF STRUCTURAL ENGINEERING ©ASCE /JULY 2006on the top surface of the bottomflange from overall bending moment only,calculated according to simple beam theory. The measured cross-sectional dimensions were utilized in this calculation,and the web contribution toflexure was neglected ͑Abbas2003͒.Two strain gauge schemes were used to determine stresses in the bottomflange.Thefirst scheme͓Fig.3͑b͔͒,used for Girder G2A,placed strain gauges near the bend region.The strain gauges were placed along theflange centerline and at a distance of 13mm͑0.5in.͒from theflange tips.Test results showed that most fatigue cracks initiated along the inclined fold,and there-fore,the second scheme͓Fig.3͑b͔͒,used for Girders G1A and G3A–G6A,placed pairs of gauges at a transverse section through the middle of the inclined fold,51mm͑2in.͒from theflange centerline.Strain rosettes were used at selected locations.Fatigue tests were run continuously when possible.The girders were in-spected twice per day͑i.e.,approximately every188,000cycles͒, when cracks were not expected,and continuously when cracks were initiating and girders were near their fatigue life.The tests were terminated after a fatigue crack reached theflange tip.Test ResultsLoad and strain data were acquired twice per day during the fatigue tests.The measured load range was used to control the tests,and stresses were calculated from measured strains using elastic theory.The measurements showed that the longitudinal stresses on the top of the bottomflange at some locations within the test region were larger than the nominal stresses.This behav-ior is a result offlange plate distortion as described by Abbas ͑2003͒.Strain rosette measurements showed that the direction of the major principal stress varied between2and11°from the longitudinal direction,but the difference between the longitudinal and principal stress range amplitude was small.Girders G2A,G1A,and G4A were tested at a nominal stress range of138MPa͑20.0ksi͒.Each girder failed from a fatigue crack that propagated in the bottomflange from the web-to-flange fillet weld toe within the constant moment test region.The scatter in N is rather small͑as shown in Table2͒.Fatigue cracks in these girders were detected after approximately1million cycles.Cracks as small as1–2mm͑0.04–0.08in.͒were initially observed at the intersection of the web-to-flange weld with theflange.Nearly simultaneous initiation of multiple cracks on different inclined folds and adjacent bend regions was observed. Fatigue cracks were not detected along the longitudinal folds. Fig.5shows a typical multiple crack pattern along an inclined fold.The point of fatigue crack initiation was usually associated with a nonuniform weld toe geometry introduced by the semi-automatic GMAW process,referred to herein as a weld ripple.The fatigue cracks propagated in theflange at an angle of approximately10°from perpendicular to the longitudinal axis, which is consistent with the principal stress direction,determined from strain rosettes.The observed crack growth rate varied between cracks.Eventually one crack,near the end of an inclined fold͑i.e.,near the bend region͒,became the main crack,propa-gated through theflange thickness and then to theflange tip. The main fatigue crack of G4A is shown in Fig.6.A posttest survey of cracks is summarized in Table3.The longitudinal web folds are numbered1–14in Fig.3͑b͒,and the inclined web folds are identified using the adjacent longitudinal folds.For example, Fold1-2is the inclined fold between longitudinal Fold1and longitudinal Fold2.Table3shows that for G2A,G1A,and G4A, fatigue cracks were observed along both the north͑N͒and south ͑S͒sides of nearly every inclined fold within the test region.TheTable2.Summary of Test ProgramGirder aNominalstress range b͑MPa͒Stressratio R͑S min/S max͒Numberof cyclesto failure,NFailurelocationG2A1380.101,418,100cG1A1380.101,448,000cG4A1380.101,303,500cG5A1030.13Ͼ7,316,500dG6A1030.132,563,400cG3A1030.13Ͼ7,645,100dG4B1380.101,980,000cG1B1100.123,500,000ea A and B denote semiautomatic GMAW and robotic GMAW welding, respectively.b At the top surface of the tensionflange in the longitudinal direction based on simple beam theory.c Flange fromfillet weld toe on inclined fold.d No failure.e Flange fromfillet weld start-stop on longitudinalfold.Fig. 5.Example of multiple fatigue cracks atfillet weld toe ͑Girder G2A͒Fig.6.Main fatigue crack of Girder G4AJOURNAL OF STRUCTURAL ENGINEERING©ASCE/JULY2006/1073abundance of cracks indicates that the girders attained their fatigue life and reduces concerns about stress variability within the test region.Girders G5A,G6A,and G3A were tested at a nominal stress range of103MPa͑15.0ksi͒.Girder G5A developed two closely spaced fatigue cracks,approximately6.4mm͑0.25in.͒in length,in the bottomflange at the web-to-flange weld toe on the north side of Fold9-10after approximately1,992,200cycles.Strain measurements showed that the longitudinal stress range in front of the crack location was approximately120MPa͑17.4ksi͒, which is16%greater than the nominal stress range and substan-tially higher than stress ranges determined from strain gauges at other locations within the test region.Also,the web was clearly misaligned in the region of Fold9-10.Consequently the cracks on the north side of Fold9-10were repaired by grinding to eliminate the crack tip,followed by air hammer peening to intro-duce compressive residual stresses,and the test was resumed. G5A survived7,316,500cycles without failure before the test was discontinued.Posttest inspection did notfind any additional cracks in the bottomflange.Girder G6A developed a small fatigue crack in the bottom flange at the web-to-flange weld toe on the north side of Fold6-7 after1,895,100cycles.The crack initiated from what appeared to be a weld stop-start.The longitudinal stress range in theflange in front of the crack location was109MPa͑15.9ksi͒,slightly higher than the nominal stress range.The test was resumed and five more cracks were detected in theflange along the north side of three different inclined folds,which suggested the girder was near its fatigue life.The cracks were not repaired and the test was resumed until failure occurred at2,563,400cycles.The crack that caused failure propagated from a weld stop-start in the web bend region.Inspection showed that the web-to-flangefillet weld geometry was clearly less uniform on the north side than on the south side͑Abbas2003͒.The results for Girder G3A are similar to those of G5A.An approximately2mm͑0.08in.͒long fatigue crack was detected in theflange at the weld toe along Fold8-9after1,992,200cycles. The longitudinal stress range in theflange in front of the crack location was110MPa͑16.0ksi͒.The crack was repaired by grinding and peening,the test was resumed,and G3A survived 7,645,100cycles without failure before the test was discontinued.Posttest inspection did notfind any additional cracks in the bot-tomflange͑Abbas2003͒.Girder G4B was tested at a nominal stress range of138MPa ͑20.0ksi͒and failed after1,980,000cycles from a bottomflange fatigue crack propagating from the web-to-flange weld toe along an inclined fold.The robotic GMAW increased the fatigue life by 42%compared to girders welded using semiautomatic GMAW. Increased uniformity in the weld toe geometry is thought to be responsible for the longer fatigue life.Girder G1B was tested at a nominal stress range of110MPa ͑16.0ksi͒,which is the CAFL for Category B͑see Table1͒. Multiple cracks were found at2,800,000cycles along Fold7-8 where the stress range was approximately125MPa͑18.2ksi͒, 14%greater than the nominal stress range.These cracks were repaired by grinding and peening,and the test was resumed. Girder G1B failed after3,500,000cycles from a bottomflange crack propagating from a weld stop-start at the middle of a longitudinal web fold.Crack Propagation and Fracture SurfacesExamination of the fracture surfaces of the main fatigue cracks of the girders welded using semiautomatic GMAW verified that the cracks initiated at the web-to-flange weld toe͑Abbas2003͒. The examination showed the influence of the weld toe geometry on crack propagation,and suggested three stages of crack life ͑Fig.7͒:͑1͒crack initiation at the weld toe;͑2͒crack propagation into theflange along the weld toe;and͑3͒crack propagation into theflange away from the weld toe.Fatigue crack initiation͑Stage1͒generally occurred at the web-to-flangefillet weld toe where the surfaces of two weld ripples intersect with theflange surface.The angle between the flange longitudinal axis and the tangent to the intersection of theTable3.Posttest Survey of Fatigue Cracks in Girders G2A,G1A,G4A, and G6AInclined foldG2A G1A G4A G6A N S N S N S N S1-2————————2-3————————3-4————————4-5163—111—5-6—54112——6-734411032—7-8156745——8-96421441—9-10114472——10-114—24912—11-12————————12-13————————13-14————————Total1625251836186—Fig.7.Observed crack patterns for semiautomatic GMAW processwelds1074/JOURNAL OF STRUCTURAL ENGINEERING©ASCE/JULY2006weld surface with the flange surface at the weld toe,␣int ,tends to be largest at this location.For Girders G2A and G4A,␣int at the crack initiation point was measured to be approximately 70°as shown in Fig.7.Crack propagation along the weld toe ͑Stage 2͒was usually several millimeters in length ͓5mm ͑0.20in.͒for G2A and 7mm ͑0.28in.͒for G4A ͔as shown in Fig.7.A clear pattern of crack propagation was not observed for Stage 2.In the final stage of propagation ͑Stage 3͒,the fatigue crack turns into the flange and propagates perpendicular to the principal stress direction.For Girders G2A and G4A,␣int at the beginning of Stage 3was measured to be approximately 50°as shown in Fig.7.The fracture surface of the main crack of Girder G4A is shown in Fig.8.Crack propagation “beach”marks are visible on the fracture surface and are semielliptical in shape,suggesting that the crack propagation pattern can be idealized as shown in Fig.8͑b ͒.The ratio of the minor axis of the ellipse to the major axis of the ellipse was found to be rather constant and approxi-mately equal to 0.75.Analysis of Test ResultsAnalysis Based on Nominal StressesBased on the nominal stresses,the test data from the current study are plotted in Fig.9with the mean S r –N curves for the AASHTO fatigue detail categories.The data from Ibrahim ͑2001͒,which are particularly relevant because the angle of corrugation is the same,are plotted for comparison.The test data generally fall in a band between the mean for Category B and the mean for Category C,and the mean for Category B Јfits the data reasonably well.The robotically welded girders exhibited an increased fatigue life,but their data points are also below the mean for Category B.The test data are plotted in Fig.10with the design S r –N curves for the AASHTO fatigue detail categories.The data here include all existing fatigue test results relevant to corrugated web bridge girders.Although most data from previous studies are above the Category B design curve,the data from the current study are often below Category B.The fatigue life of the Category B Јdesign curve provides a reasonable lower bound to the test results.Remaining Fatigue Life AnalysisThe test results show that multiple fatigue cracks initiated at the web-to-flange weld toe along the inclined web folds within the constant moment test region.The tests were terminated when the most critical crack propagated through the flange thickness and to the flange tip.Here,test data for other less critical fatigue cracks are used to estimate additional fatigue life data.Where possible,the remaining fatigue life ͑the remaining number of cycles to failure,N r ͒of the largest partially propagated crack along each inclined web fold was analytically estimated and added to the past life ͑number of cycles the crack experienced during the physical test,N p ͒to determine the total life for each of these cracks.The past life,N p ,equals the number of cycles at the end of the physical test for Girders G2A,G1A,G4A,and G6A cracks,or the number of cycles at the time of repair for Girders G5A and G3A cracks.The fracture surface studies suggested that,during the final propagation stage ͑Stage 3͒,the fatigue cracks could be assumed semielliptical in shape,with the ratio of the minor semiaxis of the ellipse,a ,to the major semiaxis of the ellipse,c ,equal to0.75.Fig.8.Fracture surface from Girder G4A and crack propagationassumptionsFig.9.Fatigue test data for corrugated web girders ͑with trapezoidal corrugations ͒with mean S r –N curves for AASHTO fatigue detailcategoriesFig.10.Fatigue test data for corrugated web girders with design S r –N curves for AASHTO fatigue detail categoriesJOURNAL OF STRUCTURAL ENGINEERING ©ASCE /JULY 2006/1075Fig.8͑b͒shows assumptions used in estimating N r.The dimen-sion a i is the initial crack depth,estimated as0.75c i where c i is the observed initial crack length.a f is thefinal crack depth, which equals theflange thickness,t f,because,after the crack propagates through theflange plate thickness,the remaining number of cycles needed to propagate the crack to theflange tip is negligible.The remaining life,N r,is estimated by integrating the crack propagation per cycle,da/dN=C͑⌬K͒n͑Paris1962͒,as follows:N r=1C͵a i a f da⌬K n͑3͒where aϭcrack size;and⌬Kϭstress intensity factorrange.The constant C is equal to9.69ϫ10−4mm5.5/kN2͑3.6ϫ10−10in.5.5/kip2͒and the constant n is equal to3,as proposed by Barsom and Rolfe͑1999͒for ferrite–pearlitesteels.These values were selected because they provide goodcorrelation between the test and remaining life analysis results.⌬K was calculated as follows͑Albrecht and Yamada1977͒:⌬K=F͑a͒S rͱ͑4͒where F͑a͒=F E F S F W F G=crack size dependent correction factor;F E=crack shape correction factor;F S=front free surface correction factor;F W=finite plate width correction factor;and F G=stress gradient correction factor.The longitudinal stress range,S r,in front of the crack was calculated from strain measurements.For an elliptical crack in an infinite solid under uniform tension,F E is͑Irwin1962͒F E=ͭ͵0␲/2ͫ1−ͩ1−a2c2ͪsin2␸ͬ1/2d␸ͮ−1͑5͒where␸=integration parameter.For a/c equal to0.75,F E is1.38. For a semielliptic edge crack,various researchers͑e.g.,Tada et al. 2000͒suggest the following for F S:F S=1+0.12ͩ1−a cͪ͑6͒which suggests that for long shallow cracks͑i.e.,aӶc͒;F S approaches the theoretical value of 1.12for an edge crack in a semi-infinite plate;and for a semicircular crack,F S=1.For a/c equal to0.75,F S equals1.03.Based on analysis of a plate withfinite width and a central through-thickness crack of size2a subjected to a uniform tensile stress͑Paris and Sih1964͒,it can be deduced that F W for the current case isF W=ͱ2t f␲a tanͩ␲a2t fͪ͑7͒where t f=flange thickness.F G accounts for local stress concen-tration.As a fatigue crack grows into a plate away from the initiation point at an attachment,it grows away from the stress concentration at the attachment.The largest crack along the inclined folds had partially propagated into theflange plate,so for estimating the remaining life of these cracks,it is reasonable to neglect the stress concentration and F G was taken as1.The remaining fatigue life analysis considered only theflange regions along inclined web folds where cracks were detected and flange strain measurements were available.Table4shows that longitudinal stress ranges determined from strain measurements in front of the crack location on the top of theflange are alwaysTable4.Remaining Fatigue Life AnalysisGirder/foldMeasuredstress range͑MPa͒Ratioof measuredto nominal stress rangec i͑mm͒a i͑mm͒Remaininglife,N rTotallife,N tG2A/7-8146 1.0611.18.375,8001,493,800 G1A/4-5150 1.09 4.8 3.6262,2001,710,200 G1A/5-6155 1.12——01,448,000 G1A/6-7146 1.067.9 5.9144,7001,592,700 G1A/7-8148 1.079.57.1101,3001,549,300 G1A/8-9145 1.059.57.1108,9001,556,900 G1A/9-10142 1.0310.37.798,0001,546,000 G1A/10-11140 1.0111.18.386,9001,534,900 G4A/4-5147 1.07 3.2 2.4421,0001,724,500 G4A/5-6146 1.068.7 6.5125,2001,428,700 G4A/6-7153 1.11——01,303,500 G4A/7-8149 1.08 6.4 4.8188,4001,491,900 G4A/8-9152 1.1019.814.970001,310,500 G4A/9-10167 1.2110.37.760,0001,363,500 G4A/10-11156 1.1312.79.544,8001,348,300 G5A/9-10120 1.16 6.4 4.8361,2002,353,400 G6A/4-5111 1.07 2.4 1.81,264,5003,827,900 G6A/6-7109 1.067.9 5.9346,9002,910,300 G6A/8-9120 1.16 4.0 3.0619,2003,182,600 G6A/10-11126 1.21——02,563,400 G3A/8-9110 1.0612.79.5128,1003,564,200 Note:Cracks with data in bold were not used in regression analysis.1076/JOURNAL OF STRUCTURAL ENGINEERING©ASCE/JULY2006。

应激、心理承受力与身心健康272?InternationalChineseApplicationPsychologyJournal呈04?【.?.虑激,心理承受力舆身心健康昧建文王滔摘要通遏理蒲分析得出以下结擒:鹰激是影孥身心健康的重要原因,但是只有曹臆激耩成封佃礁的心理承受力的威旮峙才影搴其身心健康.②心理承受力是人们抵抗麈力.保持健康的重要心理资源.根操心理承受力水平差异,可以把人们分成三佃群髓;脆弱人群,敏感人群和耐受人群.③心理承受力的蚤樟有赖于愿激因素的激登,但遗高的愿激水平刖畲耗盎心理承受力的能量.醐懿弱愿激;心理承受力;身心健康Stress,MentalResllienceandPsychosomaticHealth.ChenJianwen.School ofEducation,HuazhongUniversityofScienceandTech~wlogy,Wukan430074,P.R.China AbstractThroughtheoreticanalysis,thecoclusionsareasfollows:(1)Stressis animportantfactortOinfluencepsychoso-matichealth,whichproduceresultsonlyifstressthreatensourmentalresilienc e.(2)MentalresilienceisanimportantmentalresourceforUStOresiststressandkeephealth.AccordingtOthedifferenceof ourmentalresil{ence,threegroupsarepro-duced:brittlegroup,sensitivegroupandresilientgroup.(3)Thefunctionofme ntalresiliencedependsonevokingofstress,however,excessivestresswouldexhausttheresourceofourmentalresilience. KeywordsStress:Mentalresilience;Psychosomatichealth1弓f言愿激是佃{澧感受到外在匿力而引起身心异常反臆的遗程.适佃遏程包括三佃连靖性暗段一:首先,佃髓遭遇到刺激情境,即所谓的”愿激源”;然后,佃髓恕斌到适獯愿激源封自己身心安全耩成威督;最后随之座生一些心身反愿,比如焦虑紧强,恕知扭曲,言蓓不蜴,生理异常等.在适里,如果倔髓的愿激反愿在愿激情境消失后鹰随之恢稷的裤焉心身反愿;如果愿激遏强或持绩愿激啼同遏畏,使心身反愿持绩存在,但是并不伴以器贯性改蹩的裤属心身障礓;如果愿激反愿尊致器赏性蹩化的,刖裤属心身疾病Lz].颓然,臆激封佃健身心健康有很大影謇.现代心理神经免疫学研究表明l-3,愿激鲤由雨佃相互作用的系统封身心健康的座生影謇:生理免疫系统心理免疫系统.生理免疫系统是佃髓抵抗匿力影謇的生理资源.塞利热焉[“,有槭礁储存着有限的生理道愿能量以愿封匿力.啻臆激源采酸晴,佃礁利用自身的通臆能量抗拒臆激源,但如果愿激源持缋存在,那么佃礁的通愿能量新新耗盎,有械健就畲追入衰竭暗段,各檀心身疾病就朝始表现出技街,一旦蚤现有毒有害资讯,要采取遗滤,遮罩等瓣法,及畸将其遗滤,遮罩掉,不给其立足之地,净化绸上碾境.3.3制定和完善舆互骈绸相lI目的法律法规,依法打挚铜上遮法犯罪行属很多国家都十分注意加强垌络管制,美国已趣有法律规定禁止13威以下免童使用ICQ聊天.我们要借錾圆外封互磷绢的管理缝验,抓紧制定和完善舆五磷缡相嗣的法律法规,提法制的角度解决互骈绸的寅面影謇问题,目前我国已有”纲上警察”,随着互骈搁的蚤展普及, 必须增加警力,造一步加强辋络监察,封登现利用垌络退行的建法犯罪活勤,追行依法打挚.3.4封”绸络成瘾稼合症”及早预防”辋络成瘾综合症”是一檀心理疾病,患者整天鏖寝忘食,只知舆雹脂打交道, 性情孤僻,展重影孥身心健康,目前全国各高校都有”纲络成癌综合症”患者,焉此而辍学者占了学校辍晕人数的8O%以上.因此,高校愿及早加以预防.①中凰.垂中科技犟教育科擘研究院(湖北武溪)430074封大晕生追行辋络自我管理教育.第一步,明罐上垌的目的是焉了方便工作,擘留知诫舆调筛生活,不要焉物所役;第二步,上绸遗程中臆保持平静心憩,不宜遗分投入;第三步,控制上辋晴,保持正常的生活,工作,学留规律,小到一日三餐,大到参加教育,寻找工作都要合理安排.4参考文献[1:涂敏霞.平等互助:现代攀校邱生jI羁系的新境界,童代青年研究,2000.2C2]艰冠梓.互磷绸;童代中国青年的”曼刃剑”,中国青年研究. 2001,6[3焉倩.装旭.青少年纲上交往特站及所存在瑚题的分析.童代青年研究,2000.3[4:中国青少年研究中.中圜青少年蛋展基金合.新莘子一童代大莘生研究赧告.文出版社,2002,12国除中釜慈用-理擎雄萎2.004年第l巷第4期柬.同揉.心理免疫系统是佃艘抵抗鉴力影譬的心理资源,拉雅器斯属:j,愿激的意羲是建立在佃谴封愿教源的韶知评估基莲上的,因此,封庭激的稽桠或消衽的身心反愿部依耪于人们的知能力,佃髓的韶知是身心星力惑的重要原因,也是我们免受鉴力之苦的屏障随之,建立在匿力恕知基琏上的愿封襄略就是重要的心理资源了近举,相吉多研究焉,佃人封鉴力的愿封策略是重要的心理资源但是,在现青生活中,我套现,吉鉴力采盛_琦,有些人桩有的垦强,自信,柒靓等心理素黄,于是.采取馈枉的愿对策略,提而戟臃匿力;而有些人性格软弱,性情悲视,缺乏自信,于是面封鉴力畸他j要么束手熹策,要么消枉逃避.由此我们焉,佃人愿封策略只是心理资源的外颓的心理行焉表现,而真正的心理资源愿亥是佃人内在心理素剪.心理素贸的内涵非常竖富,而真正抵抗外在墨力的心理素贾是心理承受力.它才是徊人遭遇的外在座力舆佃人身心健康之罔的中介调筛雯量,可以起到外在屡力封佃人身心健康中攀的缓申作用’..根檬上述分析,佃人的身心健康状况,不馑受到愿激水平的影警而且受到佃人心理承受力的稽枉调茚.换句话说,fli!1人的身心健康状况舆愿激水平成反比,而奥徊人的心理承受力成正比用公式表示:H=R/S.H表示身心健康状况,可以j,-q各撞身心症状指襟采衡量:S表示愿激水平,在平均意羲上,它可以用外在匿力刺激的强度和持久性采舒量:R表示佃人心理承受力,是一獯内隐的心理资源.属了造一步弄清楚三佃雯量之蠲贵贯性醐系,我们不妨丙丙鲒合,造行封蒲.2虑激水平舆身心健康首先,我仍通遗限定心理承受力的燮化,就可以辊测到愿激水平舆身心健康的函数嗣系.即假设R是常数.身心健康(H)舆愿激水平tS)醐系就可以用以下公式表示:H=R/s.适佃奎式具有丙方面的现育意羲:其一,就佃腔而言,在持定的心理承受力水平下,徊人的臆激水平舆其身心健康水平是成反比『列醐系.也就是说,佃人承受的愿激水平越低,佃人身心受到的冲挚就越小,那么佃人的身健康状况就越好:相反徊人承受的愿激水平越高.佃人身心受到的:中警就越大,那幺徊人的身心健康状况愿越差.追惹味着,音佃人的心理承受力作属一徨人格素臂是相封德定的峙候, 佃人遭遇的穗激水平的爱化是造成其身心健康水平波勤的重要原因.其二,就群髓而言,忽略佃毽之筒的心理承受力水平的差异.那么.人们的身心健康水平也舆其所感受的愿激水平成反比例醐系.在不同的峙刻或不同的情景下,有些人承受很高的愿激水平,他们的身心受到的中肇也小一些, 那么他们的身心健康状况就差一些;而有些人随的愿激水平低一些,池们的身心受到的;中挚也小一些,邵么他们的身心健康状况也就稍微好一些适意味着,封群髓采说,人们遭遇的愿激水平的差异是尊致其身心健康状况差异的重要原因,逢一步分析,捉佃髓方面薛,磨激水平舆身心踺康并非丽翠的线性函敷醐系.如果以心理承受力作属胡筛燮量,恋激水平舆身心健豪的朝系大致趣屡三佃陪段:工身一健康状慈.起初,并不是任何强度的外在巫力都官引起学生的牙心反愿.通弱的匿力刺激并不官辊测到明颞的身心症状反癃,因此心理承受力的抵抗作用也不明颞,身心庭于艮好的逸康状悲.三,身心愿激状慈吉量力刺激适到一定的强度?悃人的身心反愿跫化官明颓表现出来.身心症状指揉的雯化也被明颈地祝测到,学生的心理承受力也官被激蚤出采充蚩缓冲和抵搐的作用三身心衰退.随着匿力强度的不新增大,到芝一定的程度,佃髓徙稽枉愿封蚌向消枉退缩或放弃.迄徊许候,身心症状指棵也逐新接近某佃枉限,再增加瘪激的强度也不舍51起太大的身心症状指檩的燮化,此畸,徊澧心理已接近崩溃的遣缘L如下国).一L,涟康水干慨\另心做献急t\,殚承\,鼻心裒j:恢态力临饩\\擞l床单圈l愿急水平舆身心健康水平的黼系3心理系受力舆身心健康我们再采看心理承受力舆身心健康之简的醐系.假设愿激水平是穗定的,即S是一侗常数,那么心理承受力(R) 奥--I’心健康(H)之罔的嗣系就可以用以下公式表示:H:R/S.逼佃公式也有丙方面的现贵意羲:其一,徒群{澧方面采说,肯A.{rl在遭遇同揉的愿激情境或愿激事件啼,由于人们心理承受力水平的差异,他们封愿激的恕知和感受不同, 于是,他们对愿激的身心反愿也合有差异.有的人心理承受力水平高,他们抵抗蜃力的能力也强,于是他们的身心健康状况就要好一些;有的人心理承受力水平低,他们抵抗麈力的能力也弱,于是他们的身心健康状况就要差一些.因此, 心理承受力差异是A.4r{身心健康状况差异的重要原因.其二,绽佃髓方面说,佃髓也许在不同的畸剥遭遇相同的愿激事件,但是由于佃谴的心理承受力的脊挥程度不同,那么倡髓的所表现的身心反庭程度也不同.如果佃{澧能够自主蚤择能勤性,充分激餮和利用自身的心理资源,稽桓愿封愿激情境,那么倡髓就能表现良好的身心健康状慈:相反,如果侣髓表现出较差的身心健康状怨.在违里,佃髓心理承受力的激蚤水平是影窨佃髓身心健康水平波勐的重要原因.追一步分析,徒群髓方面译,心理承受力水平舆身心健康也并非旃罩线性明系.人们遭遇到某獯程度的廪激水平, 是否畲立即表现出明颓的身心症状反虑.徒而影窨身心健康状慈,要看佃人心理承受力舆愿激水平的匹配情况.根谍心理承受力舆愿激强度的匹配明系,可以把人们分属三往-27{?InternationalChineseApplicationPsychologyjournal!:::人群:其一脆弱人群.相封于特定的愿激水平,适些人心理承受能力桓低,根本龚法抵抗适獯愿激水平的;中攀.于是,一旦受到特定水平的愿教,就焉上座生明颞的身心症状反恿.他们封外在刺激是易感的.其二是敏感人群.适些人的心理承受能力水平舆特定刺激强度的愿激庭于匹配状慈.也就是说,相对于特定的愿激水平,追些人的?22理承受能力正庭于中等水平,能够调勤起来抵抗外在刺激的;中挚.如果他们能够有效蚤挥自己的心理承受力,则不合尊致身一22症状,如果不能蚤挥,刖畲尊致身一22症状.他们对外在刺激是相古警觉的.其三是耐受人群.造些人的心理承受力水平很高,特定水平的愿激根本不官封他们的心理承受力形成挑戟或槽成威膏,因此他们也不合由于造些刺激的来畴而表现出身心的症状,他们封外在刺激是弹性的(如下圆).高署,继.囔水.低脆人群最感人群//////辟7R受圈2心理承受力舆身心健康水平的嗣系4膊激水平舆心理承受力最后.我们来看愿激水平舆心理承受力的嗣系.假设身心健康庭于相封穗定的水平,即身心健康水平(H)是一佃常数.那么心理承受力(R)舆愿激水平(S)的嗣系可以用以下公式来表示:R—HS,造佃公式现膏意羲在于:在身心健康水平保待穗定的情况下.佃人的承受的愿激水平舆佃人一22理承受力的卺挥水平有非常密切的醐系.如果佃髓的遭遇的愿激水平低,佃人可能心理承受力激蚤水平低;而如果佃髓遭遇的愿激水平高,那么,佃髓就自觉激蚤心理承受力,充分利用自身心理资源.横棰愿封愿激情境,键而也能堆持较穗定的身心健康水平.由此可见,外在愿激因素是内在心理承受力蚤挥作用的重要刺激因素.遣一步分析,愿激水平舆心理承受力之阳的嗣系短屉三倔登展燮化的喈段:第一是心力潜伏睹段,曹佃谴感受到,恿激水平遏低畸,并不需要佃髓激蚤自己的意志努力,主勤利用自身的资源,因此佃人心理承受力的抵抗作用并不明颓.第二是横桓抵抗喈段,啻愿激水平提高的一定的程度, 佃髓感觉到封自身的挑戟或威膏峙,佃髓就合横枉行勃起来,充分激蚤和利用自身的心理资源,由此,心理承受力的抵抗作用明颞地表现出来,啻然也是承受力接受考验的峙候.第三是心力衰竭暗段,啻愿激强度造一步加大,超遏一定的限度,那么佃髓觉得任恐自己怎檬努力,都熹法戳臃麈力,或者持绩的高强度匿力使佃畦有限的?22理资源消耗殆盏,那么佃髓就合放弃努力,此峙,心理承受力的水平反而下降甚至消失(如下圆).在适里,我们可以看出,?22理承受力舆愿激水平并非旃罩的线性函数嗣系,起初,心理承受力的水平合随着愿激水平的提高而新新提高,但是,啻愿激水平提高的一定的程度,佃人心理承受力水平反而舍下降,适说明佃人的心理承受力有一佃畴界值,即心理承受力登挥的最佳水平.在畴界值附近,佃人心理承受力起到横枉的抵抗作用;而运在畴界值之下,心理承受力畿乎不合受到考验;这在畴界值之上,佃人心理承受力遭受打挚和破壤.jJ承爱力激发水J低心力tjc阶段秘Ii2j氍抗段心板陂:心承受编俏/\—————————————I般激水甲圈3虑激水平舆心理承受力水平的嗣系憩鲒上述分析,我们可以得到以下结榆:第一,愿激是影謇身心健康状况的重要原因.佃人遭遇的愿激水平变化是造成其身心健康水平波勤的重要原因, 群髓遭遇的愿激水平差异是葶致其身心健康状况差异的重要原因.但是并非所有的愿激都合引起佃畦的身心症状反愿,只有啻愿激水平封佃畦的心理承受力耩成挑皲或威膏峙,才舍影謇的佃人的身心健康.第二,心理承受力是人们抵抗外在麈力,保持身心健康的重要心理资源.心理承受力水平及其蚤挥程度不同,人们的身心健康的水平也不同.根橡心理承受力的水平可以把人们分焉三佃群髓:脆弱人群,敏感人群和耐受人群.第三,愿激因素的刺激封佃畦的心理承受力具有激登作用.遏小的外在刺激不合引起明颞的身心反愿.只有臆激水平逵到一定程度,佃髓的心理承受力能量才合激登出来,蚤挥抵抗和缓冲调筛作用.但是,遏高的愿激水平刖合耗蛊心理承受力能量.5参考文献[1:朱教先.健康心理擎.教育科擎出版社,2002.313～3l4[2]黄希庭,人格心理擎.浙江教育出版社,2002,619～620ra.~BrannonL.FeistJ;李新锵,林宜美,睬美君,睐碧玉谭.?,72理出版社,l999,l24～l38[4]SelyeH.Thestressoflife,NewY ork:McGraw—Hil1.1956[5]LazarusRS.FolkmanS,Sress.appraisal,andcoping,NewY ork{ Springe,l993[6涑建文,王滔脯于大擎生理承受力的畿佃基本题.现代教育科攀(高教研究).2004,76(3):62～66。

奋斗的正反对比论证英文作文回答例子1:Title: The Duality of Struggle: A Discourse on the Positive and Negative Aspects of EndeavorIntroduction:In the journey of life, the theme of struggle emerges prominently. It is a multifaceted phenomenon that can lead to both positive and negative outcomes. This essay explores the dual nature of struggle, presenting arguments for both its advantages and disadvantages.Positive Aspects of Struggle:1. Personal Growth: Struggle serves as a catalyst for personal development. Through facing challenges, individuals cultivate resilience, perseverance, and adaptability. These qualities contribute to character building and foster a sense of self-improvement.2. Achievement: Struggle often precedes achievement. Theobstacles encountered along the way provide opportunities for individuals to demonstrate their capabilities and accomplish their goals. Success attained through struggle is often more fulfilling and rewarding.3. Innovation: Necessity is the mother of invention. Struggle prompts individuals to think creatively and seek innovative solutions to overcome obstacles. In challenging situations, ingenuity flourishes, leading to advancements in various fields.4. Empathy: Experiencing struggle fosters empathy towards others facing similar challenges. It builds solidarity and a sense of community as individuals come together to support each other through difficult times. Shared struggle strengthens social bonds and fosters compassion.Negative Aspects of Struggle:1. Stress and Anxiety: Struggle can induce stress and anxiety, negatively impacting mental and emotionalwell-being. Constant pressure to overcome obstacles may leadto burnout, exhaustion, and even mental health disorders such as depression and anxiety.2. Failure and Disappointment: Not all struggles end in success. Failure is an inevitable part of the journey, and repeated setbacks can lead to feelings of disappointment, self-doubt, and demotivation. The fear of failure may deter individuals from taking risks and pursuing their aspirations.3. Social Inequality: Struggle is not experienced equally by all. Socioeconomic disparities create unequal access to resources and opportunities, amplifying the challenges faced by marginalized communities. Systemic barriers perpetuate cycles of poverty and hinder upward mobility.4. Conflict and Division: Struggle can breed conflict and division within societies. Competition for limited resources may fuel tensions and lead to social unrest. Ideological differences and power struggles exacerbate societal divisions, hindering collective progress.Conclusion:The duality of struggle is undeniable. While it can catalyze personal growth, foster innovation, and build solidarity, it also poses challenges to mental well-being, perpetuates social inequalities, and fuels conflict. Acknowledging both the positive and negative aspects of struggle is essential for navigating its complexities effectively. By harnessing its potential for growth and addressing its pitfalls, individuals and societies can strive towards a more balanced and inclusive future.回答例子2:Title: A Comparative Analysis of the Pros and Cons of StruggleIntroduction:Struggle, an intrinsic facet of the human experience, is a phenomenon that has both positive and negative implications. In this discourse, we embark on a journey to explore the contrasting aspects of struggle, delving into its merits and demerits. By examining both perspectives, we aim to gain a comprehensive understanding of the role struggle plays in shaping individual lives and societal dynamics.Positive Aspects of Struggle:1. Personal Growth and Development:Struggle often serves as a catalyst for personal growth and development. When individuals encounter obstacles, they are compelled to tap into their inner reservoirs of resilience, determination, and resourcefulness. Through overcoming challenges, individuals acquire invaluable life skills, such as problem-solving, adaptability, and perseverance. Each triumph over adversity contributes to the enhancement of one's character and fortitude, fostering a sense of self-efficacy and confidence.2. Achievement and Success:Struggle is frequently intertwined with achievement and success. The pursuit of goals amidst adversity imbues accomplishments with a profound sense of fulfillment and satisfaction. Individuals who endure hardships on their journey to success often cherish their achievements more dearly, recognizing the arduous path they traversed. Moreover, overcoming obstacles instills a profound sense of resilience, equipping individuals with the tenacity to surmount futurechallenges.3. Social Change and Progress:Struggle has historically been a driving force behind social change and progress. Movements advocating for civil rights, gender equality, and environmental sustainability have emerged from the collective struggles of marginalized communities and passionate activists. These struggles have catalyzed legislative reforms, cultural shifts, and paradigmatic changes, thereby fostering a more equitable and just society. Through collective action and perseverance, individuals can effectuate transformative societal change, transcending barriers and prejudices.Negative Aspects of Struggle:1. Psychological Stress and Burnout:Prolonged exposure to adversity can precipitate psychological stress and burnout. The incessant pressure to overcome obstacles may exacerbate anxiety, depression, and feelings of inadequacy. Moreover, individuals grappling with chronic adversity may experience fatigue and disillusionment,compromising their mental well-being and sense of purpose. The relentless pursuit of success amidst adversity may exact a toll on one's psychological resilience and vitality, necessitating measures to safeguard mental health.2. Inequity and Structural Barriers:Struggle is often exacerbated by inequity and structural barriers entrenched within societal systems. Individuals hailing from marginalized communities may face systemic discrimination, socioeconomic disparities, and institutionalized oppression, amplifying the obstacles they confront. While struggle can engender resilience and empowerment, the uneven distribution of opportunities and resources perpetuates cycles of disadvantage and exclusion. Addressing systemic inequities necessitates concerted efforts to dismantle discriminatory practices and foster inclusive environments that empower all individuals to thrive.3. Diminished Quality of Life:Chronic struggle can impede individuals' ability to lead fulfilling and balanced lives. The relentless pursuit of success may engender a hyper-competitive ethos, eclipsingopportunities for leisure, relaxation, and interpersonal connections. Moreover, individuals ensnared in cycles of adversity may grapple with financial instability, inadequate access to healthcare, and precarious living conditions, diminishing their overall quality of life. Striking a harmonious balance between ambition and well-being is imperative to mitigate the detrimental effects of chronic struggle and cultivate holistic flourishing.Conclusion:In summation, the discourse on struggle unveils a nuanced tapestry of both positive and negative ramifications. While struggle serves as a crucible for personal growth, achievement, and societal progress, it also poses formidable challenges pertaining to psychological well-being, equity, and quality of life. Recognizing the multifaceted nature of struggle engenders empathy, resilience, and a commitment to fostering environments that empower individuals to navigate adversity with grace and tenacity. Ultimately, it is through collective endeavors and compassionate solidarity that we cansurmount obstacles, transcend limitations, and forge a more equitable and compassionate world.。

Striving for success is a journey that requires immense dedication,perseverance,and a strong spirit.It is the essence of human endeavor,pushing us to achieve our goals and realize our dreams.Here is an essay that encapsulates the spirit of hard work and determination.In the vast tapestry of life,the thread of hard work weaves through the fabric of success. It is the cornerstone of achievement,the bedrock upon which the edifice of accomplishment is built.The spirit of striving,of pushing oneself to the limits,is what separates the ordinary from the extraordinary.From the early morning hours,when the sun is just a whisper on the horizon,to the late night when the moon reigns supreme,the spirit of hard work is everpresent.It is the farmer who rises before dawn to tend to his fields,ensuring a bountiful harvest.It is the student who stays up late into the night,poring over textbooks and notes,in pursuit of academic excellence.Hard work is not merely about the physical labor or the mental exertion.It is a state of mind,a relentless pursuit of excellence that refuses to be deterred by obstacles or setbacks.It is the athlete who trains tirelessly,pushing through pain and fatigue,in the quest for victory.It is the entrepreneur who toils day and night,risking everything for the chance to turn a dream into reality.The spirit of striving is not limited to the individual.It is a collective force that propels societies forward,driving innovation and progress.It is the team of scientists working tirelessly to find a cure for a disease,the group of engineers designing a more efficient power source,the community coming together to build a school or a hospital. However,the path of hard work is not an easy one.It is fraught with challenges and setbacks,with moments of doubt and despair.Yet,it is in these moments that the true spirit of striving shines the brightest.It is the ability to pick oneself up,to dust off the disappointments,and to forge ahead with renewed determination.The rewards of hard work are not always immediate or tangible.Sometimes,they come in the form of personal growth,the development of character and resilience.Sometimes, they are the intangible assets of knowledge,experience,and wisdom.But ultimately,the true reward of hard work is the satisfaction of knowing that one has given their all,that they have pushed their limits and achieved their best.In conclusion,the spirit of hard work is the driving force behind human progress.It is the flame that fuels our ambition,the wind that propels us forward.It is the essence of our humanity,the testament to our capacity for greatness.Let us embrace the spirit of striving, for it is in the pursuit of excellence that we find our true potential,and it is through hard work that we achieve our dreams.。

pro-EMFATICE ngineering M aterial FATI gue C alculatorTheory BookV2.5<Preliminary English version 2010.03.16>Copyright © 2009 Pro-Lambda SolutionsTable of ContentsA1. Stress-Life Criteria For Uniaxial Fatigue (5)A1.1Manson-Coffin Stress Life (5)A1.2Morrow Stress Life (5)A1.3ASME Stress Life (5)A1.4Basquin Stress Life (6)A1.5BWI Weld Stress Life (7)A1.6User-defined stress life (8)A2. The ”Mean Stress” Effect For Stress-Life Criteria (9)A3. Strain Life Criteria For Uniaxial Fatigue (10)A3.1 Manson-Coffin Strain Life (10)A3.2 Morrow Strain Life (10)A3.3 Smith-Watson-Topper (S-W-T) (10)A3.4 Maximum Shear Strain (11)A3.5 User-defined Strain Life (11)A4. Safety Factor Evaluation For Uniaxial Fatigue (12)A5. Factors Influencing Fatigue Life (13)A6. Fuchs method (14)A7. Neuber's Rule (15)A8. Cyclic Stress-Strain Behavior (15)A8.1 Ramberg-Osgood Stress-Strain Relation (16)A9. Multiaxial Fatigue (18)A9.1 Bi-axiality Ratio Method (18)A9.2 Critical Plane Method (21)A9.2.1 Brown-Miller Equation (21)A9.2.2 Maximum Shear Strain Equation (21)A9.2.3 Dang-van analysis (21)A10. Cumulative Damage (22)References (23)‘f σNomenclatureE the elastic modulus (Young's Modulus)K' the strain hardening coefficientn' the strain hardening exponentb the fatigue strength exponent (Basquin's exponent)the fatigue strength coefficientc the fatigue ductility exponent (the Coffin-Manson exponent)'f ε the fatigue ductility coefficientu σ the ultimate tensile strengthe σ the fatigue Strength Limity σ the yield strengthRA the percent reduction in areaμ Poisson’s ratioH cyclic strength coefficientn strain hardening coefficientσmax the maximum normal stress value for a cycleσmin the minimum normal stress value for a cycle2Δσ the normal stress amplitude value for a cycle σm the mean normal stress value for a cycleεmax the maximum normal strain value for a cycleεmin the minimum normal strain value for a cycle2εΔ the normal strain amplitude value for a cycle 2γΔ the shear strain amplitude value for a cycle Nf the number of Cycles to fatigueln() nature logarithmlog() the logarithm to base 10A1. Stress-Life Criteria For Uniaxial FatigueThe following life criteria are provided for uniaxial stress cycles.A1.1 Manson-Coffin Stress LifeThe stress level is generally very low and the number of cycles to failure is generally very high. The Manson-Coffin stress life criterion is typically used for high cycle fatigue, such as material fatigue due to vibrations of the structure. The Manson-Coffin stress life equation of the S-N curve can be written asb 'f )Nf 2(•)σ(=2σΔ In this life equation, only the stress amplitude is taken into account.A1.2 Morrow Stress Life The Morrow stress life equation of the S-N curve is shown in the followingb m σσσ）（）（Nf 2•=2Δ'f - The Morrow stress life criterion is also used for high cycle fatigue. Besides the stress amplitude, the mean stress effect is included in this life equation,A1.3 ASME Stress LifeThe ASME life criterion is based on the American Society of Mechanical Engineers(ASME) Boiler and Pressure Vessel code. This code uses the percent reduction in area as its principal variable.This criterion is designed for predicting low cycle fatigue in pressure vessels. It assumes that plastic action is the major cause of failure, which may occur in a few thousand cycles.Therefore, only data sources that contain stresses/strains above the yield produce meaningful results. If the data source contains nothing but low stress or strain levels, this criterion predicts infinite life.The ASME equation is solved for the number of cycles to failure. This equation is shown below.RA •σ•01.0+)RA -100100ln(•]Nf4E [=2σΔu A1.4 Basquin Stress LifeThe Basquin stress life is an approximate method for fatigue life evaluation. The Basquin stress life equation is shown below.6b 10•)6S 2σΔ(=Nf Whereb =logS6-logS33 In which,S3 is the stress amplitude at 103 cycles.S6 is the stress amplitude at 106 cycles.Through many years of experience, particularly with steels, empirical relationships between fatigue and tensile properties have been developed. Although these relationships are not soundly based in science, they remain to be useful tools for engineers in assessing fatigue performance. When the S-N curves for steels of varying strengths are plotted as the ratio of endurance limit, i.e., the stress amplitude at 106 cycles, S6, to ultimate tensile strength, σu, all the curves tend to all fall into a single curve, which implies thatS6 ≈ 0.5 σu for σu < 1400 MPaandS6 ≈700 MPa for σu > 1400 MPaIn addition, the stress amplitude at 103 cycles, S3, can be approximated by 0.9 σu, and utilizing these approximations, a generalized S-N curve can be generated for wrought steels.A1.5 BWI Weld Stress LifeThe BWI Weld stress life is based on the British Welding Institute's formulation (BS7608), which uses the weld class as the basis for fatigue life estimate, instead of using material properties.This method predicts fatigue failure in welded joints. The criterion takes into account local stress concentrations which may be due to the weld itself.The weld equation is solved for cycles to failure. This equation is shown below.logd-=)log(Nfa•Δ)σ•log(mstdev+wherea is a constant and m is the inverse slope of the log Sr versus log N curve . Parameters a and m are dependent on the weld class.d is number of standard deviations below the mean value,stdev is the standard deviation of log(Nf).Δis the stress range.σThe fatigue life estimate is thus based on the weld class selected by the user.pro-EMFATIC provides damage life evaluations ranging from Class B through Class W.The user must also enter d (the number of standard deviations below the mean value). This allows the user to make the life estimate more conservative. The "mean" value refers to the average Welding Institute equation for a specific weld class. The equation for each weld class is a curve fit of empirical data. Therefore, some of the data lies above this mean curve and some lies below. The user can make the resulting curve more conservative by reducing it by a number of standard deviations. Typically one or two deviations below the mean aresufficient. Typically, two standard deviations below the mean are used in design situations. This suggests that 98% of the parts will achieve the predicted life.A1.6 User-defined stress lifeTo be added.A2. The ”Mean Stress” Effect For Stress-Life CriteriaMost basic fatigue data is collected in the laboratory by means of testing procedureswhich employ fully reversed loading, i.e. zero mean stress load cycle. However, most realistic service situations involve non-zero mean stresses. Therefore, it is very important to know the influence the mean stress has on the fatigue process, so that the fully reversed laboratory data can be employed in the assessment of real situations.Four correction methods are offered to update the stress amplitude used for stress-based life equations. In the following equations,2σΔ is the effective stress amplitude that includes the mean-stress effect. a. Goodman method:）（m u u σ-σσ•2σΔ=2σΔ b. Soderberg method:）（my y σ-σσ•2σΔ=2σΔ c. Gerber method:）（2m 2u 2u σ-σσ•2σΔ=2σΔ d. Morrow method:）（m'f 'f σ-σσ•2σΔ=2σΔA3. Strain Life Criteria For Uniaxial FatigueThe strain life criterion is typically used for low cycle fatigue; for example, material fatigue due to the thermal cycles of the structure. The stress level may be higher and the number of cycles to failure may be lower.The following algorithms are provided for predicting uniaxial strain life.A3.1 Manson-Coffin Strain Life The Manson-Coffin strain life equation can be written asc 'f b 'f )Nf 2(•ε+Nf 2•Eσ=2εΔ）（）（ In this life equation, only the strain amplitude is taken into account.A3.2 Morrow Strain Life The Morrow strain life equation is shown below.c 'f b m 'f )Nf 2(•ε+Nf)2(•)Eσσ(=2εΔ- The Morrow strain life criterion is also used for low cycle fatigue. Besides the strain amplitude, the mean stress effect is also included in this life equation.A3.3 Smith-Watson-Topper (S-W-T)The Smith-Watson-Topper (S-W-T) is typically used for low-cycle fatigue. The S-W-T life equation of the S-N curve is shown below.c +b 'f 'f b 22'f max )Nf 2(•ε•σ+)Nf 2(•E σ=2εΔ•σ A3.4 Maximum Shear StrainThe maximum shear strain life approach is based on the assumption that the notchshearing strain amplitude will correlate life with the shear strain amplitude in uniaxial test specimens. It uses the shear strain amplitudes on the maximum shear plane for the life equation as following:c 'f b 'f max )Nf 2(•ε•5.1+)Nf 2(•Eσ•3.1=2γΔ where2γΔmax is the shear strain amplitude value in the maximum shear plane for a cycle. A3.5 User-defined Strain LifeTo be added.A4. Safety Factor Evaluation For Uniaxial FatigueOne can use fatigue safety factor to evaluate fatigue strength and predict whether any part of the structure will ever fail in the design life due to cyclic loading.The Goodman fatigue safety factor equation can be written as Safety factor u e u e m σ•σ=σ•+σ•σ2The Gerber fatigue safety factor equation is shown below. Safety factor 2u e 22u e m σ•σ=Δσσ•+σ•σ2Where safety factor is fatigue safety factor.A5. Factors Influencing Fatigue LifeA standardized, fully reversed, strain controlled, fatigue test is used to determine the base-line strain-life relationship for a polished specimen of approximately 6 mm in diameter.For steels in particular, several empirical relationships have been developed which can account for the various factors from a laboratory specimen to a component. The factors are shown below:md the component size factorms the component surface factormt the component load factormo other factorfk/ft the stress concentration factor or strain notch factorThe usual way to account for these effects is through the calculation and application of specific modifying factors.A6. Fuchs methodpro-EMFATIC uses the Fuchs method to include these factors for stress life criteria.Using the Fuchs method, the notched stress life curve may then be approximated by a line drawn between the point representing the notched stress amplitude at 106 reversals (A) and the un-notched stress amplitude at 103 reversals (B).A7. Neuber's RuleNeuber's Rule is used to calculate local stress and strain relationship for strain life criteria. pro -EMFATIC applies Neuber's rule to determine local cyclic stresses and strains from a given notch factor kt. The equation of Neuber's rule is defined as:2e Δ•2s Δ•Kt =2σΔ•2εΔ2 where2s Δ is nominal stress amplitude. 2e Δ is nominal strain amplitude. 2σΔ is local stress amplitude. 2εΔ is local strain amplitude. With the cyclic stress-strain relation, the local stress and strain amplitudes in the equation can be calculated by Newton-Raphson iterations.A8. Cyclic Stress-Strain BehaviorThe cyclic stress-strain curve is used to define the material behavior of structuressubjected to cyclic loading.pro -EMFATIC uses cyclic stress-strain curves to convert the amplitudes of stress cycles to and from the amplitudes of the strain cycles for durability evaluations.A8.1 Ramberg-Osgood Stress-Strain RelationThe Ramberg-Osgood equation is the most commonly used cyclic stress-strain relation for fatigue analysis. The total strain is the sum of the elastic and plastic strains, which can be shown as:n /1)H2σΔ(+E 2σΔ=2εΔ）（Using the Ramberg-Osgood relation, the plastic strain can be very significant even in the monotonic linear region which is assumed for the linear FE analysis.It is recommended that the strain history be used directly for durability evaluation if one of the following strain-based criteria is selected since taking stress history for strain-based evaluations may cause significant errors.Manson-Coffin strain lifeMorrow strain lifeUser-defined strain lifeA9. Multiaxial FatigueMany real engineering design situations, such as rotating shafts, connecting links,automotive and aircraft components and many others, require multiaxial fatigue equations. The multi-axial effects complicate the analysis required to predict fatigue behavior.Biaxial fatigue algorithms are commonly used to take into account the multiaxial loading conditions. Two common approaches are used for biaxial fatigue evaluations.（1）. The biaxiality ratio method—it calculates fatigue results quickly but becomes too conservative when the loading between the two axes is not proportional.（2）. The critical plane method---it takes a lot of time in searching the critical plane but is more accurate than the other method.A9.1 Biaxiality Ratio MethodProportional Loading Assumption is assumed, so that it can count the stress or strain cycles in one direction and decide the corresponding ones in other direction. This assumption implies:a. The principal axes remain unchanged during the loading.b. The biaxiality ratio, r b , between two principal stress (or strain) range is aconstant.yx Δσrba =Δσ ym xm σrbm =σoryx Δεrba =Δε ym xm εrbm =εA9.1.1 Bi-axiality Correction For Stress-Based Life EquationsThe Manson-Coffin stress life, Morrow life, Basquin life, BWI Welds, ASME life criteria, Goodman safety factor and Gerber safety factor can be updated by using the effective stress amplitude (or range) and the effective mean stress of the biaxial cycle in the life equation, in which:~= 22~m m1σ=σrbm （1+）Where rba is the biaxial proportional ratio of stress amplitude ； .rbm is the biaxial proportional ratio of mean stress ；2σΔ~ is the updated normal stress amplitude. ~m σ is the updated normal mean stress.A9.1.2 Biaxiality Correction For Strain-based Life Equations'f b 'c f σΔε )= (1-μ•rbm) (2Nf )+ε•(1-0.5rb m) (2Nf )2E Manson-Coffin strain lifeBy including the stress/strain effect from the secondary loading direction, theManson-Coffin life equation can be derived as:'f m b'cfσ-1-rbmσΔε)=(1-μ•rbm) (2Nf)+ε•(1-0.5rbm)•(2Nf)2E（）Morrow strain lifeBy including the stress/strain effect from the secondary loading direction, the Morrow life equation can be derived as:'f b'cm f'bfσΔσ+σ•(1-μ•rbm) (2Nf)+ε•(1-0.5μ) (2Nf)Δε2•()=2σ•(2Nf)S-W-T lifeThe S-W-T life equation is shown below including the second loading direction effects:A9.2 Critical Plane MethodA9.2.1 Brown-Miller EquationFor most conventional metals, the Brown-Miller algorithm is the preferred algorithm to take into account the non-proportional biaxial loading. The following equation shows Brown-Miller life algorithm.A9.2.2 Maximum Shear Strain EquationMaximum Shear Strain Equation is also a critical plane multiaxial fatigue algorithm; the maximum shear strain life equation is shown below.A9.2.3 Dang-van analysisTBD.A10. Cumulative DamageOnce the number of cycles to failure is determined using the selected life criterion (S-N curve), the damage of the cycle can be calculated by:ii Nf 1=D whereDi is the damage caused by the i-th cycle.Nfi is the number of cycles to failure for the i-th identified cycle.The total damage of a “duty cycle” or an “event” of time history can be calculated by summing up the damage caused by each cycle according to the Palmgren-Miner rule.∑i D =DamageAnd the total number of “duty cycles” or “events” to failure can be calculated by the following equation:Damage 1=N where N is the number of events to failure.References1. Dowling, Norman E. Mechanical Behavior of Materials, Prentice-Hall, 1993, p. 670.2. Tipton, S.M., and Fash, J.W., "Multiaxial Fatigue Life Predictions for the SAE SpecimenUsing Strain-based Approach," in Multiaxial Fatigue: Analysis & Experiments, SAE AE-14, 1989, pp. 67-80.3. ASME Boiler and Pressure Vessel Code, Section III, Division I, Subsection NA, ASME,1974.4. Gurney, T. R., "Fatigue Design Rules for Welded Steel Joints," Welding InstituteResearch Bulletin Vol. 17, No. 5 (May 1976): 115-124.5. Brown, M.W., and Miller, K.J. "High Temperature Low-cycle Biaxial Fatigue of TwoSteels," Fatigue of Engineering Materials and Structure, Volume 1, 1979. pp. 217-225.。

英语作文交通事故八年级Traffic accidents are a serious issue that affects individuals, families, and communities around the world. They can have devastating consequences, leading to injury, loss of life, and significant emotional and financial burdens. As someone in the eighth grade, it is important to understand the gravity of this problem and the steps that can be taken to prevent such incidents.One of the primary causes of traffic accidents is distracted driving. This can include activities such as using a mobile phone, adjusting the radio, or engaging in conversations with passengers. When a driver's attention is diverted from the road, even for a brief moment, it can lead to catastrophic consequences. Distracted driving is particularly prevalent among young drivers, as they may be more tempted to engage in these behaviors.Another significant factor in traffic accidents is speeding. Drivers who exceed the posted speed limit or drive too fast for the current road conditions put themselves and others at risk. Higher speeds reduce the amount of time a driver has to react to unexpected situations,and the force of impact in a collision is significantly greater at higher speeds.Impaired driving, whether due to alcohol, drugs, or fatigue, is also a major contributor to traffic accidents. When a driver's judgment, reaction time, and decision-making abilities are impaired, the likelihood of a crash increases exponentially. This is a particularly dangerous issue, as it not only puts the driver at risk but also endangers the lives of innocent bystanders.In addition to these human factors, there are also environmental and infrastructural elements that can contribute to traffic accidents. Poor road conditions, such as potholes, lack of signage, or inadequate lighting, can create hazardous situations for drivers. Weather conditions, such as heavy rain, snow, or icy roads, can also increase the risk of accidents.The consequences of traffic accidents can be devastating, both in the immediate aftermath and in the long-term. Victims may suffer from physical injuries, ranging from minor cuts and bruises to life-altering disabilities. The emotional trauma of being involved in an accident can also be profound, leading to anxiety, depression, and post-traumatic stress disorder.Beyond the personal impact, traffic accidents also have significantsocietal and economic consequences. The cost of medical care, rehabilitation, and lost productivity can place a heavy burden on healthcare systems and the economy as a whole. In many cases, the financial impact of a traffic accident can be felt for years, affecting not only the victims but their families and communities as well.To address the issue of traffic accidents, a multifaceted approach is necessary. This includes education and awareness campaigns to promote safe driving practices, the implementation of stricter laws and enforcement measures, and the improvement of road infrastructure and vehicle safety features.One important aspect of this approach is the role of education. By teaching young people, such as those in the eighth grade, about the risks and consequences of unsafe driving behaviors, we can help to instill a culture of responsible and cautious behavior on the roads. This can include lessons on the dangers of distracted driving, the importance of obeying traffic laws, and the critical need to remain sober and alert while behind the wheel.In addition to education, the implementation of stricter laws and enforcement measures can also play a crucial role in reducing traffic accidents. This can include harsher penalties for offenses such as speeding, drunk driving, and texting while driving, as well as the use of technology like speed cameras and breathalyzers to detect anddeter these behaviors.Furthermore, the improvement of road infrastructure and vehicle safety features can also contribute to a reduction in traffic accidents. This can include the construction of safer roads with better signage, lighting, and traffic flow management, as well as the development of advanced safety technologies in vehicles, such as collision avoidance systems and autonomous driving capabilities.By addressing the various factors that contribute to traffic accidents, we can work towards creating a safer and more secure environment for all road users. This is a complex issue that requires a collaborative effort from individuals, communities, and governments, but the potential benefits in terms of saved lives and reduced economic burden make it a vital endeavor.As a young person in the eighth grade, it is important to understand the gravity of this issue and to take an active role in promoting safe driving practices. This can involve advocating for better education and awareness campaigns, supporting the implementation of stricter laws and enforcement measures, and encouraging the development of safer road infrastructure and vehicle technologies.By taking these steps and fostering a culture of responsible and cautious behavior on the roads, we can work towards reducing theincidence of traffic accidents and creating a safer and more secure world for all. It is a challenging task, but one that is essential for the well-being of individuals, families, and communities around the globe.。

voiunteer优秀作文英文Volunteering is a noble act that has the power to transform both the lives of those who give and those who receive. It is a testament to the human spirit, a demonstration of our capacity to care for one another and make a positive impact on the world around us. Whether it is feeding the hungry, caring for the elderly, or protecting the environment, volunteering offers individuals the opportunity to contribute their time, skills, and compassion to make a tangible difference in the lives of others.One of the most rewarding aspects of volunteering is the sense of personal fulfillment it can bring. When we engage in acts of service, we tap into a deeper well of purpose and meaning within ourselves. We are no longer mere spectators in the world, but active participants in the betterment of our communities and the alleviation of human suffering. This sense of purpose can have a profound impact on our mental and emotional well-being, as we feel a deeper connection to the world around us and a greater sense of belonging.Moreover, volunteering can be a powerful tool for personal growthand development. By stepping outside of our comfort zones and engaging with diverse populations and new challenges, we are forced to confront our own biases, assumptions, and limitations. We learn to adapt, problem-solve, and think creatively, all while developing a greater appreciation for the experiences and perspectives of others. This growth can translate into tangible benefits in our personal and professional lives, as we become more empathetic, resilient, and adaptable individuals.The impact of volunteering, however, extends far beyond the individual. When we volunteer, we become part of a larger movement of people who are committed to making the world a better place. Whether it is through organized non-profit organizations or grassroots community initiatives, volunteers play a crucial role in addressing some of the most pressing social, environmental, and humanitarian issues of our time. From feeding the hungry and sheltering the homeless to protecting endangered species and restoring natural habitats, the collective efforts of volunteers can have a profound and lasting impact on the well-being of our communities and the health of our planet.One particularly powerful example of the transformative power of volunteering can be seen in the work of organizations that provide support and resources to marginalized communities. In many parts of the world, there are individuals and families who face significantbarriers to accessing basic necessities, such as food, clean water, and healthcare. Through the dedicated efforts of volunteers, these communities can receive the support they need to not only survive, but to thrive. Whether it is distributing food and supplies, offering medical care, or providing educational resources, volunteers play a vital role in empowering these individuals and families to break the cycle of poverty and build a brighter future for themselves and their loved ones.Similarly, the environmental sector has benefited immensely from the contributions of volunteers. From planting trees and restoring wetlands to cleaning up beaches and monitoring wildlife populations, volunteers have been instrumental in preserving and protecting the natural world. Their efforts not only have a tangible impact on the health of our ecosystems, but they also inspire others to take action and become stewards of the environment. By fostering a sense of collective responsibility and empowering individuals to make a difference, volunteers play a crucial role in addressing some of the most pressing environmental challenges of our time.Of course, the impact of volunteering is not limited to specific sectors or causes. Volunteers can be found in a wide range of settings, from schools and hospitals to animal shelters and disaster relief efforts. Regardless of the context, their contributions are invaluable and their impact is felt far and wide. Whether it isproviding emotional support to patients, tutoring children in need, or responding to natural disasters, volunteers are the backbone of many of the essential services and initiatives that make our communities more resilient, equitable, and compassionate.Despite the numerous benefits of volunteering, it is important to acknowledge that it is not without its challenges. Volunteers often face logistical and practical hurdles, such as finding the time to volunteer, navigating bureaucratic systems, and dealing with limited resources. Additionally, the emotional and psychological toll of working with vulnerable populations or in high-stress environments can be significant, and volunteers must be equipped with the necessary support and coping mechanisms to avoid burnout and compassion fatigue.However, these challenges should not deter individuals from engaging in volunteer work. Rather, they should serve as a call to action, inspiring us to advocate for greater resources, support, and recognition for volunteers, and to work collectively to address the systemic barriers that hinder their ability to make a difference. By investing in the well-being and empowerment of volunteers, we can ensure that their invaluable contributions continue to have a lasting impact on our communities and our world.In conclusion, volunteering is a powerful and transformative act thathas the potential to change lives, strengthen communities, and create a more just and equitable world. Whether it is through organized non-profit organizations or grassroots community initiatives, volunteers play a crucial role in addressing some of the most pressing social, environmental, and humanitarian issues of our time. By tapping into our innate capacity for compassion and service, we can not only make a tangible difference in the lives of others, but also cultivate a deeper sense of purpose, meaning, and connection within ourselves. As we continue to navigate the challenges and complexities of the modern world, the power of volunteering remains a beacon of hope and a testament to the enduring strength of the human spirit.。

做一个勇敢的追梦者英语作文1When I look back on my journey of pursuing my dreams, I can't help but feel a rush of emotions. There was a time when I had an intense fear of the stage. However, my burning desire to overcome this fear led me to sign up for a speech competition.The moment before I stepped onto the stage, my heart was pounding like a wild drum. My hands were shaking, and my throat felt dry. But I took a deep breath and reminded myself of my goal - to be brave and express myself.When I began to speak, my voice quivered at first. But as I went on, I focused on sharing my thoughts and feelings, gradually forgetting my nervousness. The audience's attentive expressions and occasional nods gave me more confidence.Another instance was when I decided to learn a new musical instrument - the guitar. At the beginning, it was extremely challenging. My fingers hurt from pressing the strings, and I often got frustrated when I couldn't play a piece smoothly.There were moments when I wanted to give up. But the image of me playing beautiful music on stage kept me going. I practiced day after day, hour after hour. Eventually, I made significant progress and could playsome simple songs.These experiences have taught me that as long as we have the courage to pursue our dreams and the perseverance to overcome difficulties, nothing can stop us.2There is a friend of mine who has always been an inspiration to me in my journey. His dream was to become an outstanding athlete. Every day, he would rise before dawn and head to the training ground, rain or shine. He endured countless hours of intense physical training, pushing his limits constantly. Even when faced with injuries and setbacks, he never gave up. The pain and fatigue did not stop him; instead, they made him more determined.There is also a neighbor who has a burning passion for art. To fulfill her artistic dream, she traveled far and wide, seeking knowledge and inspiration. She faced numerous rejections and difficulties but remained steadfast in her pursuit. She spent long hours studying the works of great masters, constantly improving her skills.The stories of these people have deeply touched me. They have taught me that the road to achieving one's dreams is filled with challenges and obstacles. However, with unwavering courage and perseverance, we can overcome them. Their determination and hard work inspire me to keep moving forward, to brave the difficulties on my own path, and to hold ontomy dreams no matter what. I believe that as long as I have the same courage and dedication as they do, I can also turn my dreams into reality.3When we talk about being a brave dream chaser, there are many inspiring figures who have left their marks on the path of pursuing their dreams. Take Jack Ma for instance. In the early days of his entrepreneurial journey, he faced countless doubts and challenges. But he held onto his belief firmly and never gave up. With his unwavering determination, he established Alibaba, a company that has changed the way people do business and shop online.Another remarkable example is Messi. Despite facing physical disadvantages, he relied on his strong willpower and perseverance. Through countless hours of training and hard work, he overcame all the obstacles and became a football superstar. His story teaches us that no matter how difficult the situation is, as long as we have the courage and determination to pursue our dreams, nothing can stop us.I have been deeply moved by these stories. They have encouraged me to take the first step bravely towards my own dreams. I understand that there will be difficulties and setbacks along the way, but I am ready to face them with a positive attitude and unwavering belief. I believe that as long as I keep moving forward and never stop chasing my dreams, I will eventually achieve my goals and create a brilliant future for myself.When I look back on my life, there is one particular experience that stands out vividly, an adventure that truly tested my determination to pursue my dreams.It was a challenging outdoor exploration activity. The weather was extremely bad. Heavy rain poured down, and strong winds howled, making the already difficult terrain even more treacherous. But I had set a goal for myself – to reach the summit of the mountain.The path was slippery, and every step was a struggle. At times, I felt like giving up. Doubts filled my mind. Was it worth it? But deep inside, my passion for achieving my dream kept me going.I reminded myself why I started this journey. The desire to overcome challenges and prove my mettle pushed me forward. I gritted my teeth, held my hiking stick firmly, and took one step after another.Finally, after hours of perseverance and hard work, I reached the summit. The moment I stood there, looking at the breathtaking view, I knew that all the hardships were worth it.This experience taught me that no matter how difficult the path to our dreams may be, as long as we have the courage and determination to persevere, we can overcome any obstacles and achieve our goals. A brave dream chaser never gives up easily.When I look up at the starry sky at night, my heart is filled with countless dreams and aspirations. I dream of becoming a scientist, dedicating my life to solving global problems that plague humanity. The thought of finding solutions to diseases, creating sustainable energy sources, and exploring the mysteries of the universe drives me forward.I envision myself spending countless hours in the laboratory, conducting experiments, and analyzing data. I am not afraid of failures or setbacks, for they are but stepping stones on the path to success. With unwavering determination and a passion for knowledge, I believe I can make significant contributions to the world.Another dream that burns brightly within me is to become an environmental ambassador. Our planet is facing numerous challenges, and I long to do my part to protect and preserve it. I imagine organizing campaigns to raise awareness about pollution, promoting sustainable living, and working towards the conservation of precious natural resources.I know that the road ahead may be filled with difficulties and obstacles, but I am prepared to face them head-on. I will not be deterred by the naysayers or the enormity of the tasks at hand. I am a brave dream chaser, and I will pursue my dreams with all my heart and soul, never giving up until they become a reality.。