Controlled fracture in tough polymers

The fracture of most semi-crystalline polymers, notably polyethylene, polypropylene and nylon, cannot be described by LEFM based on the theory of Griffith and Irwin because large-scale yielding occurs at the crack tip prior to failure. For these materials and for 

The development of brittle fracture, as outlined in Sections 12.2.1 12.2.3 above, is directly applicable to the failure of many glassy polymers, including PMMA 

The 7-integral approach was initiated by Rice [64] and developed by Begley and Landes [65], It is most instructive to follow the exposition adopted by Landes and Begley [66] and later by Chan and Williams [67], 

Sumpter and Turner [68] have expressed J as the sum of elastic and plastic components Jg and Jp, respectively  

For single-edge-notch blend specimens when 

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Toughness assessment of ductile polymers is still a matter of debate. A sensitive way to characterise the mechanical performance of these materials, and to rank them, is to determine their ductile-brittle transitions. Test speed can thus be varied over several decades of test speed, while keeping the temperature constant, or a wide range of temperature can be scanned in controlled steps at given velocity. In the first case, the higher the speed at which the tough-to-brittle transition occurred, the better the grade in terms of fracture resistance. In the latter case, the lower the temperature at which the brittle-to-ductile transition occurred, the more suited the material for impact applications. 

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