Toughness extrinsic mechanisms

Finally, our interest will be limited here exclusively to the phenomenon of crazing in heterogeneous polymers. Thus, apart from the considerations of improving toughness by manipulation of the processes that govern the craze flow stress and, thus, rendering the extrinsic flaws inoperable that result in craze fracture, we will not consider the mechanics of fracture of crazable polymers. A brief survey of this subject related to the crazing process can be found elsewhere... 

In the Irwin approach, as with the Griffith approach, strength is found to depend on a combination of a material property (intrinsic) and a flaw size (extrinsic). In the linear elastic fracture mechanics approach, however, the material property is T or R and it has a component that depends on the microstructure of the material. Thus, if the mechanisms that increase T for a material can be identified, an approach is available to increase the reliability of brittle materials. It is this philosophy that has been a major driving force in the recent production of ceramics with higher strengths and toughnesses than had previously been considered possible. 

The extrinsic properties of materials are particularly interesting for materials technologists because these can be adjusted with the preparation. Examples of structural properties that depend on the morphology are toughness, yield strength, and wear resistance. The most important extrinsic properties by far of functional materials are mechanical, electrical, optical, and chemical. There are also many combination properties in composites such as thermoelectrical, thermochromic, electromechanical, electrochemical, optoelectric, and optochemical, and these are described in Chapter 9. 

In the above relations, Tq is intrinsic toughness, is the extrinsic toughness mechanism (crack-tip shielding), Rq is the fracture resistance energy and R is the crack resistance energy contribution. The critical condition for crack extension is then given by ... 

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