Javier LLorca

A numerical approximation to the elastic properties of sphere-reinforced composites

Abstract Three-dimensional cubic unit cells containing 30 non-overlapping identical spheres randomly distributed were generated using a new, modified random sequential adsortion algorithm suitable for particle volume fractions of up to 50%. The elastic constants of the ensemble of spheres embedded in a continuous and isotropic elastic matrix were computed through the finite element analysis of the three-dimensional periodic unit cells, whose size was chosen as a compromise between the minimum size required to obtain accurate results in the statistical sense and the maximum one imposed by the computational cost. Three types of materials were studied: rigid spheres and spherical voids in an elastic matrix and a typical composite made up of glass spheres in an epoxy resin. The moduli obtained for different unit cells showed very little scatter, and the average values obtained from the analysis of four unit cells could be considered very close to the “exact” solution to the problem, in agreement with the results of Drugan and Willis (J. Mech. Phys. Solids 44 (1996) 497) referring to the size of the representative volume element for elastic composites. They were used to assess the accuracy of three classical analytical models: the Mori–Tanaka mean-field analysis, the generalized self-consistent method, and Torquatos third-order approximation.

A numerical investigation of the effect of particle clustering on the mechanical properties of composites

The effect of the reinforcement spatial distribution on the mechanical behavior was investigated in model metalmatrix composites. Homogeneous microstructures were made up of a random dispersion of spheres. The inhomogeneous ones were idealized as an isotropic random dispersion of spherical regions—which represent the clusters—with the spherical reinforcements concentrated around the cluster center. The uniaxial tensile stress-strain curve was obtained by finite element analysis of three-dimensional multiparticle cubic unit cells, which stood as representative volume elements of each material, with periodic boundary conditions. The numerical simulations showed that the influence of reinforcement clustering on the macroscopic composite behavior was weak, but the average maximum principal stress in the spheres—and its standard deviation—were appreciably higher in the inhomogeneous materials than in the homogeneous ones (up to 12 and 60%, respectively). The fraction of broken spheres as a function of the applied strain were computed from experimental values of the Weibull parameters for the strength of the spheres, and the local stress computed in the simulations. It was found that the presence of clustering greatly increased (by a factor between 3 and

Mechanical properties of single-brin silkworm silk

Mechanical tests were performed on single brins of Bombyx mori silkworm silk, to obtain values of elastic modulus (E), yield strength, tensile breaking strength, and shear modulus (G). Specimen cross-sectional areas, needed to convert tensile loads into stresses, were derived from diameter measurements performed by scanning electron microscopy. Results are compared with existing literature values for partially degummed silkworm baves. The tensile modulus (16 ± 1 GPa) and yield strength (230 ± 10 MPa) of B. mori brin are significantly higher than the literature values reported for bave. The difference is attributed principally to the presence of sericin in bave, contributing to sample cross-section but adding little to the fibers ability to resist tensile deformation. The two brins in bave are found to contribute equally and independently to the tensile load-bearing ability of the material. Measurements performed with a torsional pendulum can be combined with tensile load-extension data to obtain a value of E/ that is not sensitive to sample cross-sectional dimensions or, therefore, to the presence of sericin. The value of E measured for brin can be used together with this result to obtain G = 3.0 ± 0.8 GPa and E/G = 5.3 ± 0.3 for brin. The latter value indicates a mechanical, and therefore microstructural, anisotropy comparable to that of nylon.

Multiscale modeling of composite materials: a roadmap towards virtual testing.

A bottom-up, multiscale modeling approach is presented to carry out high-fidelity virtual mechanical tests of composite materials and structures. The strategy begins with the in situ measurement of the matrix and interface mechanical properties at the nanometer-micrometer range to build up a ladder of the numerical simulations, which take into account the relevant deformation and failure mechanisms at different length scales relevant to individual plies, laminates and components. The main features of each simulation step and the information transferred between length scales are described in detail as well as the current limitations and the areas for further development. Finally, the roadmap for the extension of the current strategy to include functional properties and processing into the simulation scheme is delineated.

Microstructural factors controlling the strength and ductility of particle-reinforced metal-matrix composites

A micromechanical model is developed to simulate the mechanical response in tension of particle-reinforced metal-matrix composites. The microstructure of the composite is represented as a three-dimensional array of hexagonal prisms with one reinforcement at the centre of each prism. The shape, volume fraction and state (either intact or broken) of the reinforcement is independent for each cell, so the interaction among all these factors could be studied. The tensile response of the composite is determined from the behaviour of the intact and damaged cells, the fraction of damaged cells being calculated on the assumption that the reinforcement strength follows the Weibull statistics. The model is used to determine the microstructural factors which provide optimum behaviour from the point of view of the tensile strength and ductility. The analyses included the effect of the matrix and reinforcement properties, the reinforcement volume fraction, the interaction between reinforcements of different shape and the heterogeneous distribution of the reinforcements within the composite.

Fatigue of particle-and whisker-reinforced metal-matrix composites

Abstract The reinforcement of metallic alloys with ceramic particles or whiskers has generated a new family of composite materials. They have matured during the last 20 years, and are currently used in structural components subjected to cyclic loads. This was partially possible thanks to a large research effort aimed at characterizing their behavior in fatigue. The results of this activity constitute a fairly coherent body which relates the micromechanisms of cyclic deformation to the overall fatigue performance. They are presented in this review, which is divided in seven sections. After the introduction, the microstructural changes induced by the dispersion of the ceramic reinforcements are described. This is followed by two sections devoted to an analysis of the micromechanisms of cyclic deformation from the microstructural and mechanical viewpoints. The next two sections are focused on the origins of crack nucleation and the kinetics of crack propagation upon cyclic loads. The overall fatigue performance of these composites is examined in the last part, which emphasizes their advantages and limitations as compared to the unreinforced counterparts. The effects of the processing, thermo-mechanical treatments, microstructural features, environmental factors and loading conditions are included in each section to provide a comprehensive picture of the fatigue performance of these composites.

Particulate fracture during deformation

The mechanisms of deformation and failure in a 2618 Al alloy reinforced with 15 vol pct SiC particilates were studied and compared with those of the unreinforced alloy, processed by spray forming as well. Tensile and fracture toughness tests were carried out on naturally aged and peak-aged specimens. The broken specimens were sliced through the middle, and the geometric features of fractured and intact particulates were measured. The experimental observations led to the conclusion that failure took place by the progressive fracture of the particulates until a critical volume fraction was reached. An influence of the particulate size and aspect ratio on the probability of fracture was found, the large and elongated particulates being more prone to fail, and the fracture stress in the particulates seemed to obey the Weibull statistics. The dif- ferences in ductility found between the naturally aged and peak-aged composites were explained in terms of the number of broken particulates as a function of the applied strain. Numerical simulations of the deformation process indicated that the stresses acting on the particulates are higher in the peak-aged material, precipitating the specimen failure. Moreover, the compressive residual stresses induced on the SiC during water quenching delayed the onset of particulate breakage in the naturally aged material.

Mechanical properties of directionally solidified Al2O3–ZrO2(Y2O3) eutectics

Abstract The relationship between microstructure and mechanical properties was studied in Al2O3–ZrO2 eutectic rods. The material, produced by directional solidification using the laser-heated float zone method, was formed mainly of colonies consisting of a fine interpenetrating or ordered network of ZrO2 and α-Al2O3 surrounded by a thick boundary region that contained pores and other defects. The flexure strength of the eutectic rods was excellent (>1.1 GPa) owing to the small critical defect size and the high toughness (7.8 MPa m ). No microstructural changes were observed after about 1 h of exposure at 1700 K, and the eutectic oxide maintained a very high strength up to this temperature. The nature of the critical defects that led to fracture, the toughening micromechanisms, and the differences between the longitudinal and transverse strength are discussed in the light of the microstructural features of the material.

Mechanical properties of silkworm silk in liquid media

Abstract Tensile tests have been performed on silkworm silk fibres submerged in liquid environments (water, acetone, ethanol and isopropanol). Liquid media were initially chosen in order to weaken non-covalent interactions specifically. However, only immersion in water leads to a decrease in the mechanical properties of silk, indicating the weakening of hydrogen bonds. Immersion in acetone, ethanol and isopropanol leads to an increase in the stiffness of the fibre. In addition, all three organic solvents produce similar force–displacement curves, which can be explained by the desiccating effect that these solvents exert on silk. These results indicate that water disrupts hydrogen bonds initially present in the amorphous phase, while the other solvents eliminate water and contribute to the formation of new hydrogen bonds in the amorphous phase of silk. This interpretation was developed through the shear lag model of the elastic modulus ( E ) of silk, and a good agreement has been found between the model and the experimental values of E .

Fractographic analysis of silkworm and spider silk

An investigation is presented of the fracture surfaces of three different silks produced by two silkworms (Attacus atlas (A. atlas) and Bombyx mori (B. mori)) and one spider (Argiope trifasciata (A. trifasciata)). Tensile tests up to fiber failure were performed at a strain rate of 0.0002 s � 1 , and the fracture surfaces of the broken fibers were analyzed through a scanning electron microscope. The nominal relative humidity during the tests was 60% and the average temperature was 20 C. A. atlas silk was formed of bunches of microfibrils of � 1 lm in diameter embedded in a soft matrix, which were pulled out from the matrix during fracture. B. mori fibers were made up of two brins of irregular shape embedded in a proteinaceous coating. Failure occurred by fracture of the brins, whose fracture surface presented a fine globular structure corresponding to the ends of the nanofibrils of 1–2 lm in length and � 100 nm in diameter, which form the B. mori silk brins according to the analysis of the brins by atomic force microscopy. A. trifasciata fibers were circular and exhibited a defined core-skin structure. The skin fracture surface was featureless while the core showed a globular structure similar to that of B. mori although slightly shallower. The fractographic observations were discussed in the light of current knowledge of the microstructure of each fiber and the corresponding mechanical properties. 2002 Elsevier Science Ltd. All rights reserved.