Fatigue cracks prevention

In square-forged kellys, the decarburized zone has been removed from the corners of the fillet between the drive section and the upset to prevent fatigue cracks. Hexagonal kellys have machined surfaces and are generally free of decarburized zone in the drive section. 

Ralox TMQ is a highly effective agent that prevents heat-induced aging and also provides minor protection against ozone and fatigue crack failure. Its major field of application is with black elastomers based on SBR, NR, IR, BR and NBR, as well as latices. A highly effective agent, Ralox TMQ is also added to polyethylene that has been cross-linked with peroxide. 

Tomita N, Kitakura T, Onmori N, Ikada Y, Aoyama E. Prevention of fatigue cracks in ul-trahigh molecular weight polyethylene joint components by the addition of vitamin E. J Biomed Mater Res 1999 48 474-8. 

The aim of the protective measures is to prevent corrosion fatigue cracks firom initiating rather than stopping them from propagation once they have been initiated. 

As shown in O Fig. 42.9a, and not the apparently more economical but weaker joint shown in O Fig. 42.9b. Some years ago, the author tried to stiffen an aluminum alloy honeycomb core by filling with a polyurethane foam. This was then scraped down to the level of the honeycomb and the bond made. Unfortunately, the foam prevented the formation of the fillet and the joint failed at a low load. With bonded honeycomb structure, the major defects consist of a lack of attachment between the core and the skin. This may be due to several causes such as locally crushed honeycomb O Fig. 42.10a), skin defects O Fig. 42.10b), or lack of adhesive (O Fig. 42.10c). In themselves, none of these defects may reduce the short-term joint strength. However, as for the lap joint, they may show poor preparation and may provide sites for fatigue crack propagation. 

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