Micromechanical evolution

An important aspect of micromechanical evolution under conditions of shock-wave compression is the influence of shock-wave amplitude and pulse duration on residual strength. These effects are usually determined by shock-recovery experiments, a subject treated elsewhere in this book. Nevertheless, there are aspects of this subject that fit naturally into concepts associated with micromechanical constitutive behavior as discussed in this chapter. A brief discussion of shock-amplitude and pulse-duration hardening is presented here. 

Two examples of path-dependent micromechanical effects are models of Swegle and Grady [13] for thermal trapping in shear bands and Follansbee and Kocks [14] for path-dependent evolution of the mechanical threshold stress in copper. 

The evolution of T, is just an exercise in mesoscale thermodynamics [13]. These expressions, in combination with (7.54), incorporate concepts of heterogeneous deformation into a eonsistent mierostruetural model. Aspects of local material response under extremely rapid heating and cooling rates are still open to question. An important contribution to the micromechanical basis for heterogeneous deformation would certainly be to establish appropriate laws of flow-stress evolution due to rapid thermal cycling that would provide a physical basis for (7.54). 

The latest development are micromechanical sensors. Their development began with the large-scale introduction of silicon micromachined pressure sensors to the automotive industry in the nineties, which entailed a massive price reduction. Then acceleration sensors for airbag firing, yaw rate sensors and more were introduced. Many devices are still being discovered. The next step is product evolution, with introduction times between a few years and over a decade, as shown in Tab. 2.2. Once customers in the industry have accepted a product, investment in large-scale production can go ahead. It helps to find more applications for the product The time scale for the product evolution process varies from about five... 

For an elastic behavior of the solid phase, fl 1 only depends on the loading parameters, that is the macroscopic strain E and the fluid pressure p, and on the evolving morphology of the r.e.v.. We formally account for this evolution of the morphology through a parameter , which micromechanical interpretation will be obtained in particular cases. 

Estimates for the macroscopic drained stiffness tensor Chom(f) as a function of the morphological parameter can be derived from various micromechanical techniques. The micromechanical approach classically refers to the concept of strain concentration tensor, denoted here by A. By definition, in an evolution... 

Based on a local dissolution law, the micromechanical approach is able to discuss the effects of the local heterogeneity of the mechanical affinity on the dissolution process and to predict the evolution of the pore space morphology. Whenever it is possible to describe the latter by a scalar parameter , (22) yields its evolution (t) which captures the chemomechanical coupling in so far as it controls the evolution of the poroelastic coefficients in (13). Nevertheless, the implementation of this modelling requires to be able to determine the microscopic strain state along the fluid-solid interface by appropriate micromechanical techniques. 

The micromechanical experiments in a SEM, characterize the damage evolution on preselected areas of film on substrate systems. 

Unfortunately, the initiation and evolution of crazes do not concern only the majority of thermoplastic glassy polymers, which exhibit brittle behavior. Crazes usually also constitute the dominant micromechanism for failure when many polymers generally considered tough are subjected... 

Kim and Michler have observed the relationship between morphology and strain micromechanisms in cases of both rigid and elastomeric filler growth of voids, by cavitation or debonding [7,31]. Oshyman has reported a transition, at a certain fraction of filler, correlated to the evolution from macroscopic homogeneous strain to micromechanisms such as crazes. It is in fact a transition between independent mode and correlated mode of strain micromechanims [32]. 

Figure 6.2 Typical evolution at various voltages of SACE glass gravity-feed drilling using a cylindrical tool (cathode) of 0.4 mm diameter with a force of 0.8 N acting on it. The electrolyte (30 wt% NaOH) level above the workpiece is about 1 mm. Reprinted from [131] with the permission of the Journal of Micromechanics and Microengineering.

Figure 6.10 Evolution of SACE glass gravity-feed drilling in the machining voltage-drilling depth plane. Reprinted from [84] with the permission of the Journal of Micromechanics and Microengineering.

We consider an elastic solid weakened by a set of microcracks. The elastic free energy, used as thermodynamic potential, can be estimated by using micromechanics approaches (Krajcinovic 1989, Pensee and Kondo 2001). In this work, we assume an isotropic distribution of microcracks. We limit the present study to the case of fully open microcracks. However we account for an energy coupling between damage evolution and plastic flow. Therefore, the thermodynamic potential for dry material is obtained ... 

Lee, B. J., Parks, D. M., and Ahzi, S. (1993b) Micromechanical modeling of large plastic deformation and texture evolution in semi-crystalline polymers, J. Mech. Phys. Solids, 42, 1651-1687. 

Govindjee and Simo [52] developed a similar model on the basis of the micromechanical structures of the carbon black particles and rubber matrices the relaxation processes in the material were described through stress-like internal variables. These variables are governed by dissipative evolution equations, and interpreted as the nonequilibrium stresses due to the interaction between the polymer chains. Holzapfel [128] proposed a model in which the internal energy is... 

Lee et al. [218,219] defined a dedicated polymer micromechanical model in which the material is represented by an aggregate of layered two-phase composite inclusions (Figure 1.16). Large plastic deformation and texture evolution... 

The studies that have been conducted using this method thus far have provided insight into the similarities and differences in stress-strain behavior, morphological evolution, and fracture micromechanisms between uniaxial and triaxial loading conditions for different UHMWPE formulations. Because we have shown that UHMWPE formulation significantly affects the response to a notch, we believe that UHMWPE formulation must be considered... 

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