Interphase crack

According to data /3/, the AE sources in the fibrous composites are plastic deformation and cracking of the die material, shift stratification on the fibre-die interphase border, fibre destmction and stretching fibres out of the die. 

Because of their greater thickness, CAA oxides serve to protect the metal surface from corrosion better than thinner oxides but the important factor for bond durability is the stability of the outer oxide structure when water diffuses to the oxide-polymer interphase. Accordingly, it would be expected that the performance of CAA treated adherends would be similar, although no better, than that of PAA, or BSAA. The wedge test data shown in Fig. 20 and other work [29,77,97,98] support this and demonstrate that when these processes are done correctly the wedge test crack will be forced to propagate entirely within the adhesive. Similar arguments are likely with BSAA adherends, also. 

Structural applications of composite materials require not only acceptable static mechanical properites but the ability to withstand the generation and propagation of cracks without premature failre. For example, impact resistance, fracture toughness and fatigue resistance are desireable composite properties. Fiber-matrix structure at the interphase can affect the values attainable for these properties. 

IFSS of the sized fibers vs. the bare. These results indicate that the application of the sizing and the consequent formation of the interphase have resulted in an increase in the level of fiber-matrix adhesion. It could reasonably be expected that under the multiaxial state of stress at the fiber-matrix interphase, stronger fiber-matrix adhesion would reduce the tendency to grow an interfacial crack, thereby placing more energy into driving the matrix crack as observed for the sized fiber. 

Interphase Adhesion. Adhesion between phases must be excellent if a crack or craze traveling in the matrix is to propagate into the dis-... 

Critical flaw size predicted to cause a crack to kink out of the BN interphase, plotted against the ratio of the interfacial fracture resistance to the fracture resistance of Si3N4 (adapted from ref. [30]). 

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... 

The interphase provided by the adhesion promoter may be hard or soft and could affect the mechanical properties. A soft interphase, for example, can significantly improve fatigue and other properties. A soft interphase will reduce stress concentrations. A rigid interphase improves stress transfer of resin to the filler or adherend and improves interfacial shear strength. Adhesion promoters generally increase adhesion between the resin matrix and substrate, thus raising the fracture energy required to initiate a crack. 

Altstadt V (1997) Fatigue crack propagation in homopolymers and blends with high and low interphase strength. In European Conference on Macromolecular Physics -Surfaces and Interfaces in Polymers and Composites, Lausanne, Switzerland... 

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