外文翻译1

1 1. Introduction

Direct restorative composites experience considerable mechanical challenge

during function, especially those indicated for posterior restorations. Thus, in order to

withstand the mechanical stress generated by the biting forces, these composites

contain a high percentage of inorganic reinforcing filler.

A huge variation in the size, shape and constitution of filler particles can be

observed in the different commercial resin composites, even for those of the same

category or from the same manufacturer. Improvements in filler technology for

composites increased the variety of options available and even classifications of such

materials have been suggested based on the morphology of the filler particles.

Studies have shown the influence of the size and shape of the filler particles on

the mechanical properties of dental composites. These particle characteristics

determine what Braem et al. called “maximum particle packing fraction”, which is the

ratio of true particle volume to the apparent volume occupied by the particles in the

composite. According to the authors, important mechanical properties, such as

Young's modulus, depend upon this ratio. Also, the presence of small spherical

particles has been related to a high percentage of filler in the commercial composites,

improving the mechanical properties.

Nanotechnology has become a reality in different areas of engineering with the

development, through physical and chemical methods, of materials and functional

structures containing particles within a size interval of 0.1–100 nm. It is also one of

the most noticeable advances in composite filler technology, involving the

incorporation of silica fillers of nanometer size. Nanofillers are found in microfill and

some hybrid composites that can be considered predecessors of the newer

nanoparticulate composites. A study evaluating the mechanical properties of

experimental composites with or without nanofillers was carried out by Musanje and

Ferracane, who observed a positive effect of the presence of nanofiller particles,

expressed by an improvement in flexural strength, surface hardness (H) and fracture

toughness (Kc). 2 Nanoparticulate composites bring the perspective of creating another category of

universal resin composite that joins the optical properties and the polishability

required for anterior restorations with the mechanical properties demanded for

posterior restorations. However, relatively little information about these new materials

is available in the dental literature.

Strength

(σ) is an important property for a restorative material. It is dependent

upon the material's microstructure, composition, testing method, environment and

failure mechanisms. Strength values are valuable when representing information

about the flaw population with potential to cause the failure of

a restoration or

prosthesis, and thus, must be interpreted within a context that involves the analysis of

failure and structural reliability, rather than an isolated result. The presence of

structural defects with potential to become critical defects, such as microcracks,

grains or internal voids depend upon the volume of the material structure.

Measurement of the strength of composites is often performed through flexural

tests. The test indicated by the International Standards Organization to evaluate the

strength of polymer-based restorative materials is the 3-point bending test (3PBT).

This test employs bar-shaped specimens that bend under compressive loading equally

distant from the lower supports, promoting tensile stresses in the lower surfaces that

are more likely related to the fracture initiation (Fig. 1A). The test configuration tends

to confine the area submitted to the stress between the supporting rollers and the

loading rollers. The so-called 4-point bending test (4PBT) uses the same bar-shaped

specimens, but a different configuration for load appliance based on two load

cylinders over the upper surface of the specimens (Fig. 1B) that tend to expose a

higher flaw containing area of the material to the stress when compared to the 3PBT.

Therefore, it is expected that higher strengths are measured with the three-point

bending test. 3

Fig. 1

3-point (A) and 4-point (B) bending tests.

Statistical parameters are commonly applied to data from mechanical strength

tests to determine the level of structural reliability of the materials. The Weibull

modulus (m), or shape parameter, describes the variation in the distribution of

strength values from different materials and also establishes a direct relationship with