Of course, once that's been put into service, the mill no longer has a spare, so there's usually considerable interest in knowing why the first one failed and whether its fate portends the fate of the replacement unit. In this particular project, we were asked to conduct a failure analysis on the cracked roll to determine the cause of failure. Subsequent laboratory analysis showed the cracking to be the result of fatigue. As you've probably guessed by now based on the title of this story, the answer is residual stresses. Although the stresses induced by service loading were low, the roll had residual stresses approaching the yield strength of the material.
Residual stresses are stresses present in a component without the presence of service loading.
Measurement of Residual Stress Over the years, there have been a number of methods devised to measure residual stresses. Many of these methods are destructive in nature, involving steps such as cutting and sectioning the part, machining off successive surface layers, or trepanning. Another method, which is entirely nondestructive, is by X-ray diffraction strain measurement, but this technique is limited in its applicability due to the bulk and complexity of the equipment and by its ability to measure only very shallow surface layers. Additional nondestructive techniques have been developed in recent years that utilize ultrasonics, but these have not yet achieved wide acceptance in industry. When the hole is drilled, it relieves the residual strains in the immediate vicinity of the hole, which are measured and then related to the relieved principal stresses through a series of equations.