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It is shown that autofrettage at low temperatures is superior to autofrettage at room temperature in enhancing the fatigue resistance of thick-walled tubes against pulsating internal pressure. The physical reason is based on the well-known temperature dependence of the mechanical behaviour of metals and alloys which generally exhibit an enhancement of both the yield stress and strain hardening behaviour at lower temperatures. As a consequence, significantly larger compressive residual hoop stresses can be introduced during pressurization at low temperatures than at room temperature. Experimental data obtained on thick-walled tubes of the metastable austenitic stainless steel AISI 304 L which were subjected to pulsating internal pressure at room temperature after autofrettage at temperatures between -110°C and room temperature are presented. These data demonstrate convincingly the advantages offered by low-temperature autofrettage in enhancing both the fatigue life in the finite-life region and the fatigue endurance limit in comparison with autofrettage at room temperature. In conclusion, some specific materials requirements for optimum low-temperature autofrettage performance are discussed. NOMENCLATUREAF = autofrettage c = degree of autofrettage d = inner diameter of tube d, = diameter of circular plastic zone in tube D = outer diameter of tube p = internal autofrettage pressure plmX = autofrettage pressure for 100% autofrettage (c = 100%) T, RT = temperature and room temperature, respectively AuraF = sum of two distinct stress increments at temperature TAF A U~, ' .~ = stress increment: difference between yield stresses uO,' at temperatures T and RT T,, = (low) temperature of autofrettage = stress increment: difference between flow and yield stresses at a given temperature T E = plastic strain ut.p =tangential (hoop) stress under an internal pressure p u ,~.~ = von Mises equivalent stress under an internal pressure p u:~, uFs = axial and radial residual stress after unloading respectively urs = compressive residual tangential (hoop) stress after unloading 4; = von Mises equivalent residual stress after unloading u~,~ = 0.1% proof stress (yield stress) uuss = flow stress at fracture: ultimate tensile strength C T~, , ,~ =yield stress, 0.1% proof stress at a temperature T oE,* = flow stress after strain hardening at a temperature T u~.~, u~.~, u~~.~ = flow stress after I%, 3% and 18% plastic strain, respectively 595 596 H. MUGHRABI et al.

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Pressure pulsations in hydraulic systems, generated by reciprocating pumps, can cause serious problems with regard to plant safety and reliability. In particular, fatigue problems arise in high-pressure piping systems. The available knowledge is not sufficient for an accurate computation of pressure peaks in the piping of reciprocating pumps. A number of calculation models are available which, however, neglect both fluid compressibility and friction. This contribution presents a calculation method which allows a precise modelling of various pump installations. Comparison of calculated and experimental data shows a good agreement and provides a validation of the computational model.

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