Failure micromechanisms

Gencur SJ, Rimnac CM, Kurtz SM. Failure micromechanisms during uniaxial tensile fracture of conventional and highly crosslinked ultra-high molecular weight polyethylenes used in total joint replacements. Biomaterials 2003 October 24(22) 3947-54. 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

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. 

For a consideration of filler-network breakdown at increasing strain, the failure properties of filler-filler bonds and filler clusters have to be evaluated in dependence of cluster size. This allows for a micromechanical description of tender but fragile filler clusters in the stress field of a strained mbber matrix. A schematic view of the mechanical equivalence between a CCA-filler cluster and a series of soft and hard springs is presented in Figure 22.9. The two springs with force constants... 

CNTs have extremely high stiffness and strength, and are regarded as perfect reinforcing fibers for developing a new class of nanocomposites. The use of atomistic or molecular dynamics (MD) simulations is inevitable for the analysis of such nanomaterials in order to study the local load transfers, interface properties, or failure modes at the nanoscale. Meanwhile, continuum models based on micromechan-ics have been shown in several recent studies to be useful in the global analysis for characterizing such nanomaterials at the micro- or macro-scale. 

Wells J.K. and Beaumont P.W.R. (1982). Construction and use of toughness maps in a fracture analysis of the micromechanisms of composite failure. In Composite Materials Testing and Design. ASTM STP 787 (I.M. Daniel ed.), ASTM, Philadelphia, PA, pp, 147-162. 

Bowles, D.E. and Griffin, O.H. (1991b). Micromechanics analysis of space simulated thermal stresses in composites, part II Multidirectional laminates and failure predictions. J. Reinforced Plast. Composites 10, 522-539. 

Fatigue resistance increases with the [PU] up to 50Z, while energy absorption determined from dynamic properties and pendulum impact tests varies directly with the [PU], The micromechanism of failure Involves the generation of discontinuous growth bands associated with shear yielding rather than crazing. 

With respect to micromechanisms of failure, at low values of Ak, discontinuous growth bands whose spacings correspond to many cycles of loading were observed (26,32). Figure 10 shows the effect of composition on r, the spacing of the bands, the yield stress o (estimated from the Dugdale relationship, r ifK /80 ), and tlJe... 

The commercial composite materials being marketed today are optimized in order to make the interfacial properties acceptable in the sense that they will not fail at such low levels as to detract from the overall composite behavior. Considering a unidirectional specimen, where the fibers are all aligned parallel to each other, commercial systems can be described by a rule of mixtures661 relationship (Fig. 10). Properties of the matrix and fiber can be linearly combined based on the volume fraction of each constituent. For example, the longitudinal tensile modulus is the sum of the proportion of each component. The interface in these systems is considered ideal in that it efficiently transmits forces between fiber and matrix without failure. Using this model as a basis for micromechanical analysis and discussion, the magnitude of the forces present at the interface can be predicted. 

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. 

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. 

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],... 

H. D. Epinosa and R. J. Clifton, Place Impact Experiments for Investigating Inelastic Deformation and Damage of Advanced Materials, in Experiments in Micromechanics of Failure Resistant Materials, AMD Vol. 130, ed. K.-S. Kim, ASME, New York, 1991, pp. 37-56. 

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... 

Conceptually, the problems associated with the optimization of specific mechanical properties by variations of structure and morphology are the same in rubber-filled systems, ass-bead filled systems and semicrystalline polymers. When the fracture properties are singled out, our understanding of the relationships between macroscopic failure and local failure is hampered by the limited knowledge of stress transfer in statistically nonhomogeneous structures. The increased use of composites theory and micromechanics to address these problems would appear to be appropriate. 

An interesting aspect in the understanding of the micromechanics of the interface is the fibril failure mechanism in the crazing regime. As pointed out in the previous section, two mechanisms are possible chain scission or chain dis-... 

Mastrangelo, C.H. Adhesion-related failure mechanisms in micromechanical devices. Tribology Lett. 1997, 3 (3), 223-238. 

FCP resistance for the SINs increases with PU content up to 50% and is better in the prepolymer material than in the one-shot material, since the former always has larger values of percent energy absorption. With respect to micromechanisms of failure, the generation of discontinuous growth bands associated with shear yielding is involved in the SINs from the one-shot procedure. On the other hand, the fracture surfaces of the SINs from the prepolymer procedure show extensive stresswhitening phenomenon which is associated with the cavitation around PU domains and localized shear deformation. 

Fracture Surface Morphology. While a complete study of fracture surface morphology and the micromechanisms of failure is still in progress, preliminary examination revealed major differences between the modified and neat PVC s. These are now being interpreted in order to elucidate the micromechanism of failure. 

Micromechanical failure criterion for FGM architecture studied via disk-bend testing of ZrOj/Ni composites... 

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