Crack initation

Nylon is an attractive engineering polymer widely used in engineering applications. It exhibits high resistance to crack initation and imparts high unnotched impact toughness. Presence of notch leads to crack propagation and finally to embrittlement [58, 59]. 

Figure 8.11 Delamination resistance of symmetric cross-ply carbon fibre—epoxy (IM7/977-2) under quasi-static mode I loading, average initiation (init), average propagation (prop) and maximum prop values (averages from two to five laboratories and standard deviations) for testing from a starter crack thin-film insert and from a mode I precrack.

When subjected to stresses sufficient to initate and propagate cracks, polymers emit sound waves with frequencies ranging from the upper acoustic hmit (ca. 10 kHz) to 10 MHz that may be detected with suitable transducers. A well-isolated acoustic emission (AE) system may used to detect deformation events including the slow to fast brittle crack transition in PMMA, fatigue in SEN samples, and crack propagation under high hydrostatic pressure [53]. 

Herefrom and because of the different elastic properties between the wire and the rollers and the shearing strain aroused by the extrusion process a fretting fatigue exposure of the rollers is the result. This exposure is able to initate viable fretting fatigue cracks which can lead to failure of the rollers. 

Fig. 4.12. Plane of load P versus deflection 6 including the fracture resistance locus for constant F. The value of F can be estimated from fracture initation data for two values of initial crack length 2a. Adapted from Suresh (1998).

The observation that R eations inhibit the initation of SCC (fig. 12) and CFC (fig. 14) on plain surfaces is clearly related to the ability of the inhibitor to prevent corrosion pits from developing and growing into significant stress raisers. With R cations present, some superficial surface attack does develop, but at a slow rate and with a low-risk profile. Thus, a long period of time or a large number of cycles is required before the stress localization, necessary to initiate cracking, develops. Hence, resistance to SCC is increased and CFC failure is delayed. 

Ero- sion Color Change Crack- ing Craz- ing Gloss Init. Aged ... 

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