Toughness intrinsic mechanisms

Moreover, it is evident that the intrinsic mechanical strength and fracture toughness of such nanoporous structures is low [55, 65]. Insofar, gross dealloying represents a serious form of corrosive deterioration. In addition to this, there is little doubt with respect to an association between dealloying and the stress corrosion cracking of certain alloys. 

The treated phenomena and investigations summarized above are at the centre of the topic of this special two-volume edition of the Advances in Polymer Science on Intrinsic molecular mobility and toughness of polymers . However, little has been said about the mutual relations between molecular configuration, polymer morphology, nature, intensity and cooperativity of sub-Tg relaxation mechanisms, and mode of solicitation on the one hand and their effect on disentanglement, craze formation, yield behaviour and ultimate strength on the other. These subjects will be critically evaluated in the subsequent presentations of this Volume. 

In the case of thermosets toughened with thermoplastics particles (Sec. 13.4), this mechanism may be of a considerable importance because of the intrinsic toughness and/or ductility of these particles. 

Interestingly, the ductile-brittle transition observed for the MIM system provided an opportunity to assess the material fracture toughness, which was not possible using classical fracture mechanics tests due to the intrinsic brittleness of the MIM system. The measurement of the critical crack length, Lc, in the contact plane at the onset of brittle propagation allows estimation of a fracture toughness K C = a x+JnLc in the order of 0.85 MPa m1/2, i.e. much less than that of a poly(methylmethacrylate) homopolymer (1.20 MPa m1/2). 

The difference in volume fraction of MC carbides and the nature of the residual phases are reflected in the mechanical and fracture test data. Yield strengths at all temperatures were nearly identical for both alloys in the standard STD A condition. This indicates that the intrinsic alloy strength, which is derived from precipitations of y and y" phases, was not influenced by carbides and other phases. Ductility, fracture toughness, and the FCGR were apparently strongly influenced by carbides and particularly by the Laves phases, which tended to precipitate in the grain boundaries. 

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. 

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