Dominant Fracture Mechanisms

Figure 2.10 shows the fracture surface of syntactic foam containing low and high volume fractions of microspheres, respectively. It is clearly shown that step structures prevail for the microstructures containing low volume fractions of microspheres. Step structures are absent in the microstructures of syntactic foam containing 50 vol% hollow microspheres. Instead, the dominant fracture mechanism that... 

To summarize, for autohesion (polymer-polymer welding), there will be rapid (Rouse) interdiffiision, which occurs to distances comparable to the radius of gyration of the entanglement molecular weight. Me, say 3 nm (O Table 2.4). The interface is very weak and could be described in terms of the nail solution, with chain pullout being the dominant fracture mechanism. 

In the present section, attention will focus on the size of fragments created in a violent fragmentation event. The objective will be to explore some theoretical ideas which appear important to the dynamic fragmentation process. The two underlying phenomena that have dominated theoretical efforts in this area of dynamic fracture mechanics are the presence of an inherent flaw structure, and energy balance in the fracture process. 

At strain rates below e, energy-limited spall is expected to proceed through a brittle (fracture dominated) spall mechanism while for strain rates greater than 8, a ductile (flow-dominated) mechanism is expected. 

Fig. 6.12. Toughness maps depicting contours of predicted fracture toughness (solid lines in kJ/m ) for (a) glass-epoxy composites as a function of fiber strength, Uf, and frictional shear stress, tf and (b) Kevlar-cpoxy composites as a function of at and clastic modulus of fiber, Ef. The dashed line and arrows in (a) indicate a change in dominant failure mechanisms from post-debonding friction, Rif, to interfacial debonding, Sj, and the effect of moisture on the changes of Of and Tf, respectively. Bundle debond length...

The modern silk sample not subjected to heat or light stress displayed fracture Types 2, 5, 7, and 11. The dominant fracture type exhibited by these fibers was Type 7 50 of the modern silk fractures were of this type. This finding indicates that surface flaws largely govern the fracture mechanism of modern silk. A smaller number of fracture Types 2, 5, and 11 were observed. This observation indicates that at least some of these fibers also exhibit ductility, a moderate loss of interfibrillar cohesion, and large internal voids, although their influence in fiber fracture is considerably less than that of surface flaws. 

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