Читать книгу Root Cause Failure Analysis - Trinath Sahoo - Страница 103

So far we’ve talked about the gross overloads that can result in immediate, almost instantaneous, catastrophic failures. Another type of failure occurs by means of progressive brittle cracking under repeated alternating or cyclic stresses of an intensity considerably below the normal strength. Although the fracture is of brittle type, it may take some time to propagate, depending on both intensity and frequency of the stress cycles. The number of cycles required to cause fatigue failure at a particular peak stress is quite large but it decreases as stress increases. A very important distinction is that fatigue cracks take time to grow across a part. In a fatigue failure, an incident of a problem can exceed the material’s fatigue strength and initiate a crack that will not result in a catastrophic failure for millions of cycles. There were cases where fatigue failures in 1200 rpm motor shafts that took less than 12 hours from installation to final fracture, about 830 000 cycles. On the other hand, there are cases the crack growth in slowly rotating process equipment shafts has taken many months and more than 10 000 000 cycles to fail.

Fatigue fracture results from the simultaneous action of repeated or fluctuating cyclic stress, tensile stress, and plastic strain. No fatigue crack starts or grows in the absence of any of these three active components. Cyclic stress initiates the crack and tensile stress produces the crack growth. There are many variables in service that influence the fatigue behavior or characteristic pattern. These include the magnitude and frequency of application of the fluctuating stress, the presence of a mean stress, temperature, environment, part size and shape, state of stress and residual stresses, surface finish, surface damages, and microstructure. The occurrence of fatigue may be considered as a three‐stage process.

1 Initiation of the surface or sub‐surface fatigue crack under a fluctuating load.

2 Crack propagation under tensile stress until the section thickness critically bears the imposed load.

3 Sudden fracture under overload

In the presence of a severe stress concentration, multiple crack origins are formed which eventually unite to form a single crack front. When the stress concentration on the surface is absent, cracks propagate more readily near the center of a section than at the surface. While there is a stress concentrating ‘notch, W‐shaped crack fronts are‐observed. For a given material, zone of the final fracture increases with increasing applied load. On rotating or bending the final fracture’, region is often rotated or offset toward the origin in the direction opposite to the direction of rotation. Also, with increasing nominal stress, the final fracture moves toward the center.

Figure 5.9 Schematic representation of the fatigue crack (three stage) phenomenon.