Mechanism crack propagation

The critical energy is about 4,000 erg/cm for PMMA in methylated spirits at room temperature, but the value is lower in benzene and higher in petroleum ether. Thus Tc in this case is much higher than the true surface energy but still much lower than that for mechanical crack propagation. 

The most common interpretation of the mechanism of cracking is based on a periodic electrochemical-mechanical process. This suggests that cracking is an alternating sequence of relatively slow anodic dissolution in the crack base and sudden mechanical crack propagation. In some alloys, intermittent cracking has actually been found, but in many other cases, no evidence of stepwise cracking has been produced. 

Application The authors applied this methodology to the investigation of Si. In particular, they studied brittle fracture mechanisms (crack propagation, determination of the stress-strain curve, and so on), vacancy diffusion, and gliding of a pair of partial dislocations at finite temperature. In all of these applications, the authors used a tight-binding (TB) scheme to perform the quantum calculations, and the Stillinger-Weber potential (SW) for the classical ones. 

Fis. 5 Thermal-cycled samples after vibration. Note mechanical crack propagation at two locations and minimal damage at center ball. 

Other researchers have substantially advanced the state of the art of fracture mechanics applied to composite materials. Tetelman [6-15] and Corten [6-16] discuss fracture mechanics from the point of view of micromechanics. Sih and Chen [6-17] treat the mixed-mode fracture problem for noncollinear crack propagation. Waddoups, Eisenmann, and Kaminski [6-18] and Konish, Swedlow, and Cruse [6-19] extend the concepts of fracture mechanics to laminates. Impact resistance of unidirectional composites is discussed by Chamis, Hanson, and Serafini [6-20]. They use strain energy and fracture strength concepts along with micromechanics to assess impact resistance in longitudinal, transverse, and shear modes. 

The stress corrosion resistance of maraging steel has been evaluated both by the use of smooth specimens loaded to some fraction of the yield strength and taking the time to failure as an indication of resistance, and by the fracture mechanics approach which involves the use of specimens with a pre-existing crack. Using the latter approach it is possible to obtain crack propagation rates at known stress intensity factors (K) and to determine critical stress intensity factors (A iscc) below which a crack will not propagate (see Section 8.9). 

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