Crack micromechanisms

Fig. 12. Micromechanics of ductile reinforcement particles yielding within process 2one and particles bridging in crack wake.

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

Other researchers have substantially advanced the state of the art of fracture mechanics applied to composite materials. Tetelman [6-15] and Corten [6-16] discuss fracture mechanics from the point of view of micromechanics. Sih and Chen [6-17] treat the mixed-mode fracture problem for noncollinear crack propagation. Waddoups, Eisenmann, and Kaminski [6-18] and Konish, Swedlow, and Cruse [6-19] extend the concepts of fracture mechanics to laminates. Impact resistance of unidirectional composites is discussed by Chamis, Hanson, and Serafini [6-20]. They use strain energy and fracture strength concepts along with micromechanics to assess impact resistance in longitudinal, transverse, and shear modes. 

Doll, W. and Konczol, L. Micromechanics of Fracture under Static and Fatigue Loading Optical Interferometry of Crack Tip Craze Zones. Vol. 91/92, pp. 137 — 214. 

Hard layer and soft layer combined together can reduce the intrinsic stress of the whole coating [17,18,22-27]. Samples 4, 5, and 6 have higher critical load than that of monolayer A and B. For Samples 5 and 6, no obvious crack occurs during the scratch test. Sample 5 has the highest hardness and reduced elastic modulus among the multilayer samples, and the interfaces in Sample 5 also contribute to scratch resistance. So it has the best micromechanical properties here. 

Anstice P. D. and Beaumont P.W.R. (1981). Hygrothermal aging effects on the micromechanisms of crack extension in glass fiber and carbon fiber composites. In Proc. ICF 5 (Francois D. et al.. eds.). Pergamon Press, Oxford, Vol. I, pp. 473-483. 

Harris B. (1980). Micromechanisms of crack extension in composites. Metal Sci. 14, 351-362. 

Thermosetting epoxy polymers are widely employed in structural engineering applications and thus a knowledge of the mechanics and mechanisms of the fracture of such materials is of vital importance. The present Chapter discusses the fracture of epoxy polymers, concentrating on the use of a continuum fracture mechanics approach for elucidating the micromechanisms of crack growth and identifying pertinent failure criteria. 

Hagan J. T., 1979, Micromechanics of crack nucleation during indentations, J. Mater. Sci., 14, 2975-2980. 

It is important to note that within the plastic zone there is an energy dissipation originating from the deformation micromechanisms (SDZs, CSCs, or CDCs) occurring. The amount of energy thus dissipated represents almost the whole energy involved in crack propagation. 

In the case of BPA-PC, the thin film investigation of deformation micromechanisms (Sect. 4.2) shows that CDCs occur around 60 °C. So, it is unlikely that the craze at the crack tip occurring at - 20 °C, or above, could be a CDC. The observed MW dependence of failure originates from the above described mechanism with CSCs. 

Abstract When subjected to a mechanical loading, the solid phase of a saturated porous medium undergoes a dissolution due to strain-stress concentration effects along the fluid-solid interface. Through a micromechanical analysis, the mechanical affinity is shown to be the driving force of the local dissolution. For cracked porous media, the elastic free energy is a dominant component of this driving force. This allows to predict dissolution-induced creep in such materials. 

Let us consider the case where the pore space is a network of saturated cracks. In order to implement the classical micromechanical estimates of the strain concentration tensor A introduced in (14), the cracks are modelled as flat oblate spheroids. For simplicity, a uniform crack radius a is considered. N denotes the crack density. For an isotropic distribution of crack orientations, the macroscopic behavior derived from (15) is isotropic as well (Deude et al., 2002) ... 

Deude, V., Dormieux, L., Kondo, D. and Maghous, S. (2002) Micromechanical approach to non linear poroelasticity application to cracked rocks, Journal of Engineering Mechanics 128(8), 848-855... 

The earliest works of trying to model different length scales of damage in composites were probably those of Halpin [235, 236] and Hahn and Tsai [237]. In these models, they tried to deal with polymer cracking, fiber breakage, and interface debonding between the fiber and polymer matrix, and delamination between ply layers. Each of these different failure modes was represented by a length scale failure criterion formulated within a continuum. As such, this was an early form of a hierarchical multiscale method. Later, Halpin and Kardos [238] described the relations of the Halpin-Tsai equations with that of self-consistent methods and the micromechanics of Hill [29],... 

M.F. Horstemeyer et al Using a micromechanical finite element parametric study to motivate a phenomenological macroscale model for void/crack nucleation in aluminum with a hard second phase. Mech. Matls. 35, 675-687 (2003)... 

F. Costanzo, D.H. Allen Micromechanics and homogenization of inelastic composite materials with growing cracks. J. Mech. Phys. Solids 44, 333-370 (1996)... 

Y. Xue et al Micromechanisms of multistage fatigue crack growth in a high-strength aluminum alloy. Acta Mater. 55, 1975-1984 (2007)... 

Typical experimental data on the crack growth rate vs stress intensity factor in LMIE conditions are shown in Figure 7.91. Large cracks are at the top portion (>2mm) and small cracks (<2 mm) are located at the bottom of the figure and reflect the kinetics of growth of cracks. The micromechanism of crack growth is different in the upper and lower regions. 

The two micromechanism of LMIE are (i) the dissolution-condensation model (DCM) and (ii) the adsorption-induced localized slip model. In the DCM mechanism the crack is filled with melt and grows by dissolution of atoms from the crack tip where the chemical potential is increased due to applied stress.1,2 In the second model,... 

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