Weak interfaces

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When relating interface structure to strength, the literature is replete with analyses, which are based on the nail solution [1,58], as shown in Fig. 10. This model is excellent when applied to very weak interfaces (Gic 1 J/m ) where most of the fracture events in the interface occur on a well-defined 2D plane. However, the nail solution is not applicable to strong interfaces (Gic 100-1000 J/m ), where the fracture events occur in a 3D deformation zone, at the crack tip. In Fig. 10, two beams are bonded by E nails per unit area of penetration length L. The fracture energy G c, to pull the beams apart at velocity V is determined by... 

As shown in Fig. 10, the nail solution applies to relatively weak interfaces, G c 1 J/m, where all the fracture events occur on one plane and the matrix holding the molecular nails does not deform but offers frictional resistance. In the next section, we explore how the fracture energy can increase by several orders of magnitude as we proceed from nails to nets. 

Fig. 6.13. The Cook-Gordon (1964) mechanisms tensile debonding occurs at the weak interface ahead of crack tip as a result of lateral stress concentration and crack tip is effectively blunted.

The experimental results indicated that the recycle did generally Increase hardness and decrease strength as expected. Effects on modulus ranged from negligible to negative, suggesting that the recycle either inhibited cure or simply introduced weak Interfaces. 

This drawback can be at least partially eliminated by blending PLLA with other polymers (26-29). In addition, ABS has been used for blending (30). The blends were prepared laboratory mill equipped with a twin-screw. It turned out that uncompatibilized blends of PLLA and ABS have a morphology with big phase size and a weak interface. The blends exhibit poor mechanical properties with low elongation at break and decreased impact strength. 

Polymers of different structures do not form homogeneous blends. Mixture of polymers that differ in structure and polarity are compatible with one another only to a very limited extent. They separate in the melt or at the latest after cooling in the solid state, as a result of weak interfaces between the components and local stress concentrations. Therefore, they cannot be... 

The reduced hardness and improved machinability are attributed primarily to the crack deflection process. It can be seen in Fig. 13.8 that the composite showed obvious particle pullout and significant crack deflection along interphase boundaries due to the weak interface bonding. The crack deflection mechanism (absorbing fracture energy and blunting crack tip) could lead to an increase in machinability. As described above, the thermal expansion... 

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