Autofrettage pressure

The autofrettage treatment (Fig. 1.4-9, B) is certainly one of the oldest, but still very useful methods to create beneficial residual stresses in thick-walled components (e.g., pipes). The autofrettage pressure must be adjusted to a level so that the material in the thick wall is plastically strained within a certain percentage (e.g., 50 %), the rest staying only elastically strained. 

After the removal of the autofrettage pressure the part shows the compressive residual stresses required to reduce the operational stresses effectively (see Chapter 1 Introduction Fig. 1.4.10). Successful autofrettage treatment needs tough and sufficiently strong steels which should offer a certain potential for strain-hardening without unacceptable embrittlement. 

Autofrettage pressure is the pressure that causes initial overstrain of the bore to occur. It is that pressure that results in plastic deformation, beginning at the inside surface of the cylinder. This is initial autofrettage pressure or minimum autofrettage pressure . The maximum autofrettage pressure is the pressure resulting in plastic yielding across the entire wall thickness. 

Pi = autofrettage pressure, pounds per square inch P2 = unload pressure, pounds p square inch Ononnaily zero)... 

Upon unloading of the autofrettage pressure a change in unit radial strain, Aer, occurs. 

Under autofrettage pressure the radial, tangential, and lijngitudinal stresses for re 9 fe. are as follows. 

Figure 14.14 shows the residual hoop (tangential) stress resulting from unloading the autofrettage pressure. Tl ... 

Solution, The maximum autofrettaging pressure of 60 ksi is less than twice the lower bound pressure P. Accordingly, Ae stress distribution resulting ftom a pressure drop of 60 ksi is in the elastic range, as shown in Fig. 15.4. From Eq. 15.1,... 

Basically, this process is accomplished by partially overstraining the irmer shape of the bore by a hydraulic or mechanical overloading. The apphed autofrettage pressure determines the amount of material at the bore that is yielded and thus determines the amount of residual compressive stresses when the pressure is removed. These residual stresses reduce the hoop stresses when the normal operation pressure is applied and improve the fatigue hfetime of the component tremendously. 

Farrahi, G.H., Hosseinian, E., and Assempour, A. (2008) On the material modelling of the autofrettaged pressure vessel steels. ASME Pressure Vessel and Piping Division Conference, July 27-31, 2008, Chicago, IL, Paper No. PVP 2008-61482. 

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