Fatigue microcrack

Cooper, C.V. Fine, M.E. Fatigue microcrack initiation in polycrystalline alpha-iron with polished and oxidized surfaces. Metall. Trans. A 1985, 16, 641-649. 

Another important aspect of the fatigue of all materials is the statistical nature of the failure process and the scatter which this can cause in the results. In a particular sample of plastic there is a random distribution of microcracks, internal flaws and localised residual stresses. These defects may arise due to structural imperfections (for example, molecular weight variations) or as a result of the fabrication method used for the material. There is no doubt that failure... 

Results from our fatigue tests have shown that, in the one-to two-month accelerated fatigue tests, blood is a less severe environment than saline or air. This may be due to adsorption of the blood components into the microcracks, resulting in reduced stress concentrations. More work is anticipated to study the effect of blood on the fatigue life of elastomers. 

Interfacial cavitation, microcracking, diffusion and crack branching due to the viscous flow of the glass phase during creep-fatigue. 

The relationship between fatigue life and carbonyl content can be explained as follows according to the photooxidation mechanism of PE, carbonyl groups result mainly from a Norrish type II reaction, i.e., for each carbonyl formation, there is a scission of a segment of a molecule chain. Such scission creates a defect in the structure which can grow and propagate into a microcrack under application of a load. Under cyclic loading, it is understandable that the number of cycles the sample can sustain will be directly related to the number of defects (such as microcracks, microvoids. ..), as is clearly described by equation (2). ... 

All types of deformation-uniaxial, biaxial and cyclic uniaxial stretchlng-enhance the photooxidation of LDPE films. A close relationship exists between the enhancement process and the deformation mechanism more disrupture of the structure results in a larger enhancement. Damages such as microcracks, micro-voids, microcrazes-resultlng from disrupture or fatigue effects-increase the degradation rate while orientation decreases it. 

Fatigue failure in neat polymers or short fibre reinforced polymers involves a phase in which a defect zone initiates, with the development of crazes or microcracks, followed by crack propagation to final rupture [17], This statement means that an analysis of fatigue behaviour can use the tools already developed for metals. This will be demonstrated in the cases that follow. 

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