How Does Autofrettage Work?
Autofrettage works by yielding a portion of the wall thickness of the material of a high pressure carrying element to produce a beneficial compressive stress on the product’s ID surface when the applied pressure is removed. Essentially, autofrettage works as if the pressure carrying element has a band clamp on its outer circumference, minimizing or eliminating the pressure vessel ID from experiencing the negative effects of pressure pulsations. Generally, cracks will tend to initiate from the largest stress riser or the largest defect on the ID surface. The only time this is not the case is when large stress risers with residual tensile stresses are positioned on the outer perimeter of the part. The following graphic illustrates the process in two steps. In the first step, a very high pressure is applied to the part. The inner most bore will generally experience a small plastic deformation; while the lesser stressed OD will expand elastically. When the pressure is released in step two, the elastic stresses in the OD of the part overcomes the lesser material in the bore and instills a compressive hoop stress which is largest at the inner surface. It is even possible to plastically deform the entire wall of the pressure carrying part and still impart large beneficial compressive stresses at the bore. There is very little measurable dimensional difference between a part that has undergone autofrettage and a part that has not.
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