Degradation in strength

Figure 3. Degradation in strength as a function of time in various environments. All of the tests were conducted at room temperature.

Figure 4. Degradation in strength as a function of time for Nylon 6 samples in three different concentrations of NO, at 81° C. Final fracture was at room temperature. Concentrations are (a) 0.86% (b) 2.58% (c) 5.16%.

If the flaw is not cracklike, ordinary stress concentration factors may be used in conjunction with the failure criteria of the preceding section to predict failure. If the flaws (inherent or introduced) are cracklike, however, stress concentration factors are useless because linear elasticity theory predicts an infinite concentration factor at the crack tip. For these situations, experiment shows a degradation in strength as schematically represented in Region II of Figure 1. 

Griffith Theory. Griffith provided the first analysis of the degradation in strength as a function of flaw size by considering the problem of a small through-the-thickness line crack in a thin sheet of brittle material as illustrated in Figure 2 (13). 

The degradation in strength with increasing flaw size is less severe for... 

Figure 9.2 The degradation in strength that occurs under stress can be followed by considering the inert strength as a reference state.

At high temperatures (> 600°C) the composites degrade in strength and toughness due to oxidation of C fibers and/or these interface layers and prevention of fiber pullout (brittle fracture mode). Studies are in progress to increase oxidation resistance by use of a CVD-SiC overlayer which seals the surface of the porous composites. 

Griffis, C.A., Nemes, J.A., Stonesfiser, F.R., and Chang, C.I. (1986) Degradation in strength of laminated composites subjected to intense heating and mechanical loading. /. Compos. Mater., 20(3), 216-235. 

The chemical stability of alumina is related to its phase stability, whereas the phase changes of zirconia result in degradation in strength and wear resistance. Release of substances from zirconia and alumina implants to the surrounding tissue is very low and neither local nor systemic effects have been reported. 

Figure 10.10 The graph shows three possible results for an environmental test. Result A is clearly ideal as there has been no degradation in strength. Result B shows an initial fall in strength but no further degradation, and provided the strength remaining still adequately meets the specification, this result may be acceptable. Result C is clearly unacceptable as the strength has continued to fall during the test...

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