EFFECTIVE ELASTIC MODULUS OF BONE-LIKE HIERARCHICAL MATERIALS

A shear-lag model is used to study the mechanical properties of bone-like hierarchical materials. The relationship between the overall effective modulus and the number of hierarchy level is obtained. The result is compared with that based on the tension-shear chain model and finite element simulation, respectively. It is shown that all three models can be used to describe the mechanical behavior of the hierarchical material when the number of hierarchy levels is small. By increasing the number of hierarchy level, the shear-lag result is consistent with the finite element result. However the tension-shear chain model leads to an opposite trend. The transition point position depends on the fraction of hard phase, aspect ratio and modulus ratio of hard phase to soft phase. Further discussion is performed on the flaw tolerance size and strength of hierarchical materials based on the shear-lag analysis

SIMULATIONS OF MECHANICAL BEHAVIOR OF POLYCRYSTALLINE COPPER WITH NANO-TWINS

Mechanical behavior and microstructure evolution of polycrystalline copper with nano-twins were investigated in the present work by finite element simulations. The fracture of grain boundaries are described by a cohesive interface constitutive model based on the strain gradient plasticity theory. A systematic study of the strength and ductility for different grain sizes and twin lamellae distributions is performed. The results show that the material strength and ductility strongly depend on the grain size and the distribution of twin lamellae microstructures in the polycrystalline copper

A Method to Determine Material Length Scale Parameters in Elastic Strain Gradient Theory

With specimen size decrease for advanced structural materials, the measured mechanics behaviors display the strong size effects. In order to characterize the size effects, several higher-order theories have been presented in the past several decades, such as the strain gradient theories and the micro-polar theories, etc. However, in each higher-order theory, there are several length scale parameters included, which are usually taken as the material parameters and are determined by using the corresponding theoretical predictions to fit experimental results. Since such kind of experimental approaches needs high techniques, it is very difficult to be performed; therefore, the obtained experimental results are very few until now; in addition, the physical meanings of the parameters still need to be investigated. In the present research, an equivalent linkage method is used to simply determine the elastic length parameters appeared in the elastic strain gradient theory for a series of typical metal materials. We use both the elastic strain gradient theory and the higher-order Cauchy-Born rule to model the materials mechanics behaviors by means of a spherical expanding model and then make a linkage for both kinds of results according to the equivalence of strain energy densities. The values of the materials length parameters are obtained for a series of typical metal systems, such as the face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP) metals

Trans-scale characterization of interface fracture in peel test for metal film/ceramic substrate systems

In order to describe the interfacial fracture behaviors of the metal thin film with nano- or microscale thickness peeled on the ceramic substrate, a trans-scale mechanics model has been adopted. In the trans-scale mechanics model, both the strain gradient effect and surface/interface effect are considered. In addition, two fracture process models are used in present study, which are the cohesive zone model and the virtual internal bond model. Using the trans-scale mechanics theory and the interface models, the size effect of the interfacial separation strength between the metal thin films and the ceramic substrates is analyzed systematically by using the peel test. The results show that the fracture process zone size could be taken as the indicator of the trans-scale interface fracture characterization. The interface effect should be considered when the fracture process zone size is at the nanoscale, and the obtained interfacial separation strength is much higher than the conventional separation strength. The material length scale parameters of the metal films are determined by comparing the interfacial energy release rate predicted by the scale theories with the experimental results, which shows that the material length scale parameter could be regarded as the size of active plastic zone in the small scale yielding case during the peeling process

Effect of plasticity and adhesion on the stick-slip transition at nanoscale friction

When an external force is suddenly applied to the material interface, how much the critical force is required for the onset of slip? To answer this, we used molecular simulations to study the slip initiation during a nano-scratch. The shear load is applied by a very soft spring to achieve the constant force boundary. Using this model, we studied how much plasticity and adhesion affect the critical shear load. Results show that the slip initiation depends on contact depth because the dominated factor for stick-slip changes from interfacial adhesion in shallow contact to dislocation plasticity in deep contact. Also, the initiation stress of slip depends on temperature; the critical shear stress follows a temperature dependence of similar to TlnT when dislocation plasticity prevails

Head-on impact of metal microparticles: Aggregation or separation?

During the head-on particle collision, the adhesion plays a more important role as theparticle size decreases to micro size; the increasing surface effect makes the particle prefer to aggregate. While on the other hand, as the impact velocity increases, particles prefer to separate because of the larger elastic repulsive interaction. Another factor, which cannot be ignored during the impact of metal microparticles, is the dislocation plasticity which shows the rate and size effect. In this work, taking nano-plasticity behavior into account, our molecular simu-lations revealed two critical impact velocities for the transition of particle collision from separation to aggre-gation, and these two velocities are quantified by the analytical models proposed in this study. The low critical velocity for particle aggregation is dominated by adhesion, while in contrast, the high critical velocity for ag-gregation is dominated by dislocation plasticity, where the dislocation density in the particle after the collision is proportional to the impact velocity. With these findings, an analytical model was proposed to determine the critical particle size, below which no separation will be found whatever the impact velocity is. And this critical size is proportional to the ratio of surface energy to stacking fault energy