Low velocity impact behavior of composite sandwich panels

Low velocity impact behavior of composite sandwich panelsPatrick M.Schubel *,Jyi-Jiin Luo,Isaac M.DanielRobert R.McCormick School of Engineering and Applied Science,Center for Intelligent Processing of Composites,Northwestern University,2137Tech Drive,Evanston,IL 60208,USAReceived 24June 2004;revised 23October 2004;accepted 22November 2004This paper is submitted in honor of Professor Jack R.Vinson of the University of Delaware.This is a tribute recognizing Prof.Vinson’s outstanding and enduring contributions to the field of composite materials in general and more specifically to structural applications of composites and composite sandwichstructures.In addition,one of the authors (Daniel)feels privileged and thankful to Professor Vinson for his personal friendship.AbstractComposite sandwich structures are susceptible to low velocity impact damage and thorough characterization of the loading and damage process during impact is important.The objective of this work is to study experimentally the low velocity impact behavior of sandwich panels consisting of woven carbon/epoxy facesheets and a PVC foam core.Instrumented panels were impacted with a drop mass setup and the load,strain,and deflection histories were recorded.Damage was characterized and quantified after the test.Results were compared with those of an equivalent static loading and showed that low velocity impact was generally quasi-static in nature except for localized damage.A straightforward peak impact load estimation method gave good agreement with experimental results.A contact force–indentation relationship was also investigated for the static loading case.Experimental results were compared with analytical and finite element model analysis to determine their effectiveness in predicting the indentation behavior of the sandwich panel.q 2005Elsevier Ltd.All rights reserved.Keywords:A.Carbon fiber composite;B.Impact behavior;D.Mechanical Testing;Sandwich panels1.IntroductionThe use of composite sandwich structures is expanding in the aerospace and marine industries,as well as in other areas where a lightweight material with high in-plane and flexural stiffness is needed [1].The concept behind these structures is the separation of relatively stiff,strong and thin facesheets by a lightweight and thicker flexible core.The proper design and application of sandwich construction depends on a thorough characterization and understanding of not only the sandwich constituent materials (facesheets,core,and adhesive),but also of the structure as a whole under quasi-static and dynamic loadings.Sandwich structures are known to be susceptible to impact damage by foreign objects [2,3].This type of damage and more specifically,the response of composite sandwich panels under low velocityimpact,is the focus of this study.Although the induced damage may not be readily apparent,its effects on the strength and reliability of the structure can be quite detrimental.Several common failure modes have been identified,including core indentation/cracking,facesheet buckling,delamination within the facesheet,and debonding between the facesheet and core [4–6].Because of its complex nature,the investigation of low velocity impact on composite sandwich structures remains an active research topic and has received much posite sandwich beams have been studied to charac-terize failure processes and damage [7–11].This two-dimensional approach somewhat simplifies analysis and gives a cross-sectional view of the damage.However,a sandwich panel can provide a more complete deformation and damage response.Most of the previous impact research conducted on panels has focused on the analysis of impact dynamics,the characterization of impact damage,and the determination of the post-impact mechanical properties of the composite structure [2].A wide range of materials,geometries,and facesheet layups have been used [12–17].Composites:Part A 36(2005)1389–1396/locate/compositesa1359-835X/$-see front matter q 2005Elsevier Ltd.All rights reserved.doi:10.1016/positesa.2004.11.014*Corresponding author.Tel.:C 184********.E-mail address:p-schubel@ (P.M.Schubel).In this study,simply supported sandwich panels consisting of woven-carbon/epoxy facesheets and PVC foam core were loaded under central point impact in a drop weight apparatus.Identical sandwich panels were also tested under central point quasi-static loading for com-parison.The low velocity impact response of the current sandwich panel was determined by means of a detailed load–strain analysis and damage characterization.Because of the relatively dense core and woven carbon facesheets, the contact forces,along with the impact energy requiredto produce damage,are quite high compared to thefindings in the referenced works above.Although much research already exists on the study of low velocity impact of composite structures,new configurations are continually being developed and need to be characterized.The contact force–indentation relationship for sandwich panels is an area that requires additional investigation as well. Analytical models and afinite element analysis were compared with experimental results on indentation for the static loading case.2.Experimental2.1.Constituent materials and fabricationThe facesheets of the sandwich panel were woven-carbon fabric/epoxy laminates(AGP370-5H/3501-6S).This AS4-based carbon fabric was afive-harness satin weave with the same tow count in the warp andfill directions.The matrix is an amine-cured epoxy resin.The facesheets were laminates made of four plies of prepreg,resulting in a cured plate with a thickness of1.37mm and afiber volume ratio of 0.62.Table1lists measured mechanical properties of this woven carbon composite.The core used for the sandwich panel was a closed-cell PVC foam(Divinycell H250obtained from DIAB).This foam is relatively dense compared to other commonly used foams for core materials.The core was25.4mm thick. Table2lists selected mechanical properties of Divinycell H250.The sandwich panel was fabricated by bonding the cured facesheets to the core material with Hysol9430adhesive,a room temperature curing epoxy resin.The facesheets and core were bonded together and cured under vacuum.The final sandwich panel was a square plate of27.9!27.9cm dimensions with an overall thickness of2.82cm and a mass of0.83kg(Fig.1).Three plates were fabricated for this study,one for quasi-static testing and two for impact testing.2.2.Quasi-static testingTo determine the level to which dynamic processes should be considered in low velocity impact testing,a sandwich panel wasfirst tested under a quasi-static loading for subsequent comparison with the impact loading case. The comparison involves correlating load and strain levels between the two types of tests.In both loading situations, the sandwich panel was simply supported on rollers along two parallel edges.The supports were steel bars of25.4mm diameter and werefixed at a distance of25.4cm.An indenter with a spherical surface of12.7cm radius introduced load at the center of the panel.The relatively large radius reduced the effects of large local strains in the contact rge local strains caused by indentation can induce early failure of the sandwich facesheet[19].The panel was instrumented with16strain gages at various locations on the top and bottom facesheets.Selected strain gage locations are shown in Fig.2.The deflection of the sandwich plate at the load point was recorded by the stroke of the Instron servo-hydraulic machine,which alsoTable1In-plane mechanical properties of carbon fabric/epoxy composite used for sandwich facesheets[18]Property Value Density,r(kg/m3)1600Fiber volume ratio,V f(%)62Warp modulus,E1t(GPa)77Fill modulus,E2t(GPa)75Major Poisson’s ratio,n120.07In-plane shear modulus,G12(G Pa) 6.5Out-of-plane shear modulus,G31(GPa) 5.1Out-of-plane shear modulus,G32(GPa) 4.1Warp tensile strength,F1t(MPa)963Ultimate warp,fill tensile strain,3ult1t;2t(%) 1.3Table2Selected mechanical properties of sandwich core material:Divinycell H250 Property Value Density,r(kg/m3)250In-plane modulus,E1(MPa)240Out-of-plane modulus,E3(MPa)403 Transverse shear modulus,G13(MPa)115In-plane compressive strength,F1c(MPa) 4.6 Transverse shear strength,F13(MPa)5Fig.1.Photograph offinished composite sandwich panel.P.M.Schubel et al./Composites:Part A36(2005)1389–1396 1390recorded the load levels during the test.The displacement of the bottom facesheet directly below the load point was recorded with an extensometer setup.The plate was loaded slowly until the first indication of damage initiation and then carefully unloaded.2.3.Impact testingA drop tower apparatus with a free-falling mass was used to impact the sandwich plates.The impactor (tup)surface was spherical with a radius of 12.7cm and the total mass of the dropped carriage was 6.22kg.The support conditions were identical to those of the static testing case.The panels were subjected to impact with increasing heights of mass drop until damage was induced.This corresponds to an impact energy range of 7.8–108J.Impact velocities ranged from 1.6to 5m/s.After the initial contact,the drop mass was held manually to prevent repeated impacts.Impact loads were acquired with a piezoelectric force transducer located between the impactor and the carriage.The position of the mass was recorded during the impact event with a non-contact linear displacement sensor that detects a metal target through inductive technology.By differentiating the displacement–time curve,the velocity of the drop mass was determined just before,during,and just after impact.The plate was instrumented with strain gages on its upper and lower facesheets,at identical locations as in the quasi-static test.The dynamic load,position and strain histories of the impact event were captured with digitizing oscilloscopes.3.Results and discussion 3.1.Quasi-static loadingLoad vs.displacement curves for the static loading test are shown in Fig.3.The displacements of both the top and bottom facesheets at the panel center are given.The difference between the top and bottom facesheet deflections is due to indentation of the facesheet with core crushing.The top deflection increases at a nearly linear rate and a portion is recovered upon unloading.The portion that is not recovered is due to permanent indentation.Facesheet damage was initiated at a load of 17.3kN.The total indentation can be obtained by subtracting the bottom facesheet deflection from the upper deflection,and will be discussed in a following section.Eight strain readings were taken on the upper facesheet and eight on the lower one.As the sandwich panel underwent deformation,these strains could be monitored to find where the highest strains occurred.In general,the readings were classified as two types,near-and far-field.The near-field strains were recorded sufficiently close to the load point at the center of the top facesheet.Far-field strains were those recorded on the bottom facesheet and sufficiently far from the load point.Fig.4shows load vs.strain plots for three locations on the top and bottom facesheets.The tensile reading on the bottom facesheet directly below the load point (gage number 3in Fig.2)is far-field and increases in a general linear manner with the load.This is also the case for the compressive far-field strain on the top facesheet,measured 50.8mm from the center point of the plate and aligned perpendicular to the supports (gage 2in Fig.2),although the strain levels were somewhat lower.However,the tensile near-field strain gage,located 25.4mm from the center point and aligned perpendicular to the supports (gage 1in Fig.2)is highly non-linear and reaches strain levels above those in the far-field range.These readings offer clues about the deformation profile of the panel as it is loaded.A plate with the current support and loadingFig. 2.Diagram showing panel dimensions and selected strain gage locations.Displacement (mm)L o a d (k N )Fig.3.Load vs.displacement curves for upper and lower facesheets of sandwich panel under quasi-static loading.P.M.Schubel et al./Composites:Part A 36(2005)1389–13961391conditions will exhibit a global in-plane compressive strain on its top surface.Near the load point,local effects become apparent and tensile strains are produced due to indentation.On the bottom facesheet,global bending effects dominate and tensile strains only are present.After the static test,damage in the plate was evaluated.Permanent indentation was visibly apparent and measured 1.3mm in depth at the panel center.The upper facesheet was ultrasonically scanned to determine the extent of damage in the composite and bond with the core.Damage results are presented in a following section.3.2.Impact testingFig.5shows the strain histories for the strain gage location corresponding to gage 1in Fig.2for various impact energies.The peak tensile strain increased with increasing impact energy and,excluding the highest energy impact,the strain pulse duration was consistent.For the higher energy drops,some permanent strain was apparent after the impact,which would indicate onset of damage.Also of importance in the 108J impact,the peak strain of 1.25%is quite close to the material ultimate tensile failure strain of 1.3%[18].The load history of each impact event was acquired and the peak force at each impact energy level was obtained (Fig.6).For low impact energies,the load pulse is sinusoidal and at higher energies the sinusoidal nature of the pulse is preserved with some fluctuations possibly indicating facesheet damage.The pulse duration was relatively constant for all events.Because of the assumed sinusoidal pulse,a relatively straightforward load history estimation method can be used to predict the maximum load for a given impact energy.The load pulse can be represented as a half-sine wave f ðt ÞZ P 0sin 2p Tt(1)where P 0is the maximum load.T is twice the pulse duration (period)and stays constant.The maximum load was obtained by computing the difference of moment of momentum by integrating f (t )from 0to T /4as P 0Z2p mv T(2)where m is the mass of the impactor and v is the velocity of the impactor just before impact which can be found by direct measurement or energy balance.The incoming velocity can also be expressed in terms of impact energy to give the maximum load as a function of impact energy P 0Z2p ffiffiffiffiffiffiffiffiffi2mE p (3)where E is the impact energy for a specific impact event.The pulse duration can be determined by direct examination of the load history,or by making the assumption that the force–deflection response is linear under low velocity impact.L o a d (k N )Strain (%)Fig.4.Load–strain response at near-and far-field locations of sandwich panel in quasi-statictest.S t r a i n (%)Time (ms)Fig.5.Impact strain histories at near-field gage location 1.F o r c e (k N )Time (ms)Fig.6.Impact load histories showing peak impact load and pulse duration.P.M.Schubel et al./Composites:Part A 36(2005)1389–13961392Then,T can be estimated byT Z 2pffiffiffiffimkr (4)where the panel stiffness k (2.87MN/m)can be found from the initial slope of the static load vs.deflection curve of the upper facesheet (Fig.3).In the current study,the aim was to predict the load response using theoretical methods to determine both the impactor velocity and the pulse duration.However,the measured T from Fig.6(8.8ms)is close to the theoretical value of 9.2ms.The predicted maximum loads shown in Fig.7as a function of impact energy are in good agreement with the experimental results.This method is advantageous because,by running one quasi-static loading test,the impact response of the sandwich panel can be predicted without conducting a single impact parison of quasi-static and impact testsIn comparing the quasi-static and impact loading response of composite sandwich panels,the load–strain characteristics at corresponding locations were used.For the impact case,the peak load and strains for each impact event were used in the analysis.If the loads are the same at identical facesheet locations and strain levels,then,the low velocity impact behavior of the sandwich plate is very similar to the quasi-static case.However,any observed differences can be explained.Fig.8shows a comparison,at gage location 1on the top facesheet,of the static load–strain response and the peak impact load vs.strain behavior.At equivalent strains,the impact load is consistently higher.As the strain levels increase,the static and impact responses diverge further.The non-linear upturn at the end of the static load–strain response is not seen in the impact data.This divergence could suggest that the deformation profile of the top facesheet in the static test is more severe,meaning that the strains around the contact area are higher.Additionalnear-field strain readings on the upper facesheet confirm this effect as well.At a distance sufficiently far from the loading area,the strains are not influenced by the localized deformation profile caused by the indenter/impactor contact area.Fig.9shows the comparison of the load–strain response of the sandwich panel at gage position 2.The behavior is quite consistent.Far-field strain readings taken from the bottom facesheet also correlate well for static and impact loadings.The far-field strains are outside the region affected by the localized indentation and the load–strain curves increase relatively linearly,unlike the near-field curves which are non-linear at higher loads.Thus,static and low velocity impact loadings produce similar deformation behavior in510152025300255075100125Impact Energy (J)P e a k I m p a c t F o r c e (k N )Experimental Load HistoryEstimation MethodCalculated T = 9.2 msFig.7.Predicted and experimental peak impact load vs.impact energy.Load estimations calculated with theoretical T (9.2ms).5101520253000.51 1.5Strain (%)L o a d (k N )IMPACTQUASI-STATICparison of load–strain response of near-field gage location 1.5101520253000.050.10.150.20.250.3Compressive Strain (%)L o a d (k N )IMPACT QUASI-STATICparison of load–strain response of far-field gage location 2.P.M.Schubel et al./Composites:Part A 36(2005)1389–13961393the plate except for the local effects produced by the indenter/impactor.3.4.Post test damage characterizationThe static and impact-loaded panels were inspected and ultrasonically scanned after the tests for damage.Both plates showed delamination damage around the contact area of the indenter/impactor.Fig.10shows ultrasonic C-scan images for the two loading conditions.The dark areas at the panel center indicate facesheet damage,which B-scans confirmed to be delaminations within the upper facesheet. The lighter areas on the C-scan of the statically loaded panel signify adhesive non-uniformity in the facesheet-core bond but do not affect its response.Upon visual inspection,a pronounced indentation could be seen at the center of the impacted panel which measured0.9mm in depth.All damage in both loading cases was localized and the delaminated areas were roughly the same in size.Although the peak load level in the impact tests was higher than that in the quasi-static test,no increase in the level of damage was apparent.A comparable level of delamination damage was initiated in the static panel at a lower load.Therefore,the static test in general is more conservative when considering localized strain and damage levels,which has been concluded in other sandwich impact studies[20].Along with the load history estimation method,a straightforward damage prediction model can be developed. Running a quasi-static test on a composite sandwich panel yields the panel stiffness as well as a damage initiation load. For a given impact energy,the peak impact force can be estimated.By comparing this peak impact force with the damage initiation load from the quasi-static test,a quick damage prediction can be made.If the predicted peak impact force exceeds that of the static damage initiation, damage is possible in the panel for the given impact energy.3.5.Indentation behavior of sandwich panelsDescribing the behavior of a composite sandwich panel also involves understanding its indentation behavior as it is loaded.Indentation includes local facesheet deformation and core crushing,often interacting in a complex way. Analysis of indentation is more straightforward in a sandwich beam configuration and theoretical contact laws have been successfully established with good agreement with experimental results[19,21].However,quasi-static indentation on a sandwich panel with a rigid sphere presents additional analytical difficulties.Sburlati[22]addressed the problem with a mathematical model based on elastic plate theory with some success,but the model did not extend past the initial linear load–indentation relationship,after which core yielding effects are present.The same is true for the work by Anderson and Madenci[23],where a three-dimensional analytical model based on laminate theory develops complete stress and displacementfields and computes the contact area.These models do not capture the core crushing damage processes and non-linear indentation behavior.Olsson and McManus[24]developed a method to incorporate indentation damage effects into the sandwich panel behavior.In this model,a point-load is assumed and must be corrected during calculation.A‘membrane solution’is described where past the linear region of the contact load–indentation curve,large deflections in the facesheet cause it to be dominated by membrane stresses. The region affected by core crushing is modeled as a membrane,while the rest of the sandwich panel is modeled as a plate on an elastic foundation.This theory results in a load–indentation curve with an initial linear region,a softening upon core yielding followed by a stiffening as the membrane stress becomes dominant.The model seems to describe approximately the indentation processesobserved Fig.10.Ultrasonic C-scan images of sandwich panels showing post test delamination damage:(a)statically loaded panel;(b)impact loaded panel(108J).P.M.Schubel et al./Composites:Part A36(2005)1389–13961394in the current study.However,when applied to the current sandwich panel configuration and static loading situation,the stiffening effect was present but the model under-estimated the indentation and the correlation between the membrane solution and experimental results was not close.A finite element model was developed to simulate the load–deflection behavior of the top and bottom facesheets in the static loading case.With this information,a model of the indentation of the sandwich panel was obtained.The model was constructed with ABAQUS,with the facesheets modeled as orthotropic laminae and the core as an isotropic elastic material until yielding.After core yielding,the core was modeled as a plastically yielding material (crushable foam).The indenter was rigid and contact was assumed frictionless.The uniform mesh was three-dimensional,with simply supported boundary conditions along two edges,identical to the experimental setup.Fig.11shows the experimental contact force–indentation curve along with the FE analysis results.The FE model matches quite well the experimental results and captures the stiffening behavior.For comparison,the membrane solution is also plotted which shows the poor agreement with experimental data.Additionally,a plate on an elastic foundation solution for small deflections is shown [25].The linear plate solution correlates well with the behavior at small indentations prior to core yielding.4.ConclusionsBesides the localized effects caused by load contact characteristics,the quasi-static and low velocity impact behavior of composite sandwich panels composed of woven carbon fabric/epoxy facesheets and a PVC foam core investigated in the current study are quite similar.In this respect,the low velocity impact response of plates can be characterized as quasi-static in nature.This conclusion isbased on the comparison of a quasi-static test and multiple impact tests on sandwich panels and an analysis of the load–strain response,as well as a thorough damage evaluation of panels under both types of loading.Therefore,a quasi-static test,which is easier to perform and analyze,can be used to predict related impact response.Localized effects deal mainly with the contact characteristics between the indenter/impactor and the upper facesheet.A static test produces a more pronounced deformation profile and damage processes are initiated earlier.The current results suggest that the quasi-static test is in general more severe in terms of the deformation and strain levels induced in the loaded facesheet.Additionally,a load history estimation method,based on the sinusoidal shape of the impact load pulse,was employed to predict the peak impact force as a function of impact energy.Assuming a linear load–deflection relationship,the pulse period can be calculated from a quasi-static test.The predicted peak loads agreed quite well with the experimen-tal results,meaning low velocity impact behavior can be predicted without running a single impact test.A straight-forward damage prediction method was also put forth which could be used to predict impact damage in the composite sandwich panel by comparing the peak impact load to the static damage initiation load.The contact force–indentation relationship for sandwich panels was also investigated for the static loading case.Some analytical models were studied for their applica-bility to the current sandwich setup.Two [22,23]were found to be limited in their scope because they did not model the panel behavior beyond core yielding.Another model [24]did account for core yielding and facesheet stiffening but underestimated the indentation response for the current configuration.A need exists for an analytical indentation model that can account for the complex interactions that govern the behavior of the structure from small contact area loading,leading to core yielding and ultimately including the stiffening effect observed in sandwich plates.The initial linear indentation behavior until core yielding matched well that of a simple plate on an elastic foundation model.A finite element model was implemented to capture the full response of the panel indentation and was able to match the stiffening behavior seen in the experiments quite well.AcknowledgementsThis research was sponsored by the Office of Naval Research (ONR).We are grateful to Dr Y.D.S.Rajapakse of ONR for his encouragement and cooperation.We also acknowledge the efforts of Dr J.W.Yoo for developing and implementing the ABAQUS finite element model for this paper.123Indentation (mm)C o n t a c t F o r c e (k N )Fig.11.Static loading indentation response comparing experiment,analytical models and FE analysis.P.M.Schubel et al./Composites:Part A 36(2005)1389–13961395References[1]Vinson JR.Sandwich structures.Appl Mech Rev2001;54(3):201–14.[2]Abrate S.Localized impact on sandwich structures with laminatedfacings.Appl Mech Rev1997;50(2):69–82.[3]Abrate S.Impact on composite structures.Cambridge UK:CambridgePress;1998.[4]Hazizan MA,Cantwell WJ.The low velocity impact response offoam-based sandwich posites Part B2002;33: 193–204.[5]Nemes JA,Simmonds KE.Low-velocity impact response of foam-core sandwich composites.J Compos Mater1992;26(4):500–19. 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