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In true corrosion fatigue, the fatigue-crack-growth rate is enhanced by corrosion; this effect is seen in all three regions of the fatigue-crack growth-rate diagram. The diagram on the left is a schematic of crack-growth rate under true corrosion fatigue; the curve shifts to a lower stress-intensity-factor range in the corrosive environment.
The crack growth rate behaviour with respect to the alternating stress intensity can be explained in different regimes (see, figure 1) as follows Regime A: At low growth rates, variations in microstructure, mean stress (or load ratio), and environment have significant effects on the crack propagation rates. It is observed at low load ratios ...
As such, the volume of the oxides can be interpreted as a wedge inserted into the crack, reducing the effect stress intensity range. Experiments have shown that oxide-induced crack closure occurs at both room and elevated temperature, and the oxide build-up is more noticeable at low R-ratios and low (near-threshold) crack growth rates. [11]
Material failure theory is an interdisciplinary field of materials science and solid mechanics which attempts to predict the conditions under which solid materials fail under the action of external loads.
The stress intensity factor at the crack tip of a compact tension specimen is [4] = [() / / + / / + /] where is the applied load, is the thickness of the specimen, is the crack length, and is the effective width of the specimen being the distance between the centreline of the holes and the backface of the coupon.
Underloads: Small numbers of underloads increase the rate of growth and may counteract the effect of overloads. Overloads: Initially overloads (> 1.5 the maximum load in a sequence) lead to a small increase in the rate of growth followed by a long reduction in the rate of growth.
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The rate of growth can be predicted with a crack growth equation such as the Paris-Erdogan equation. Defects such as inclusions and grain boundaries may locally slow down the rate of growth. Variable amplitude loads produce striations of different widths and the study of these striation patterns has been used to understand fatigue.