Failure mechanism, strain, time

The failure phenomena of viscoelastic materials consist of two factors. One is the time dependence of the hulk stress-strain relation of the material. For example, higher strain rate increases the stress level of the material and consequently the stress or the strain at break will he different from those of the case of the lower rate. The second factor is the time dependence of the failure mechanism itself, namely, the time dependence of the failure initiation mechanism. 

A disadvantage of conducting experiments in the microscope is that the experiments can consume a lot of time and are thus expensive. For mechanical tests, it is rare to observe a statistically significant number of specimens also, neither stress-strain measurements nor microscopy is optimal. Nevertheless, the insight gained from direct observation of deformation and failure mechanisms outweigh these disadvantages in some cases. 

Promising results have been obtained, however, using the free volume concept to account for the shift of relaxation times with strain and thermal history and to predict changes of failure stress and failure mechanism [123,124]. 

Analysis methods for hydrogen absorbed in the deposit have been described (65), and instmments are commercially available to detect hydrogen in metals. Several working tests have been devised that put plated specimens under strain and measure the time to failure. A method for cadmium-plated work has been described (66) as has a mechanical test method for evaluating treatments on AlSl 4340 Steel (67). Additional information on testing for hydrogen embrittlement is also available (68). 

Fibrous or particulate filters are not important anymore because membrane filters are relatively compac t and perform veiy well. For filtration by straining, there is an intermediate air velocity at which filtration efficiency is a minimum because different collec tion mechanisms predominate at different ranges of velocity. At low velocities, diffusional and elec trostatic forces on the particle are important, and increased velocity shortens the time for them to operate. At high velocities, inertial forces that increase with air velocity come into play below a certain air velocity, their effect on collection is zero. Surges or brief power failures could change velocity and collection efficiency. 

As was cited in the case of immersion testing, most SCC test work is accomplished using mechanical, nonelecdrochemical methods. It has been estimated that 90 percent of all SCC testing is handled by one of the following methods (1) constant strain, (2) constant load, or (3) precracked specimens. Prestressed samples, such as are shown in Fig. 28-18, have been used for laboratory and field SCC testing. The variable observed is time to failure or visible cracldng. Unfortunately, such tests do not provide acceleration of failure. 

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