Numerical determination and experimental verification of the optimum autofrettage pressure for a complex aluminium high‐pressure valve to foster crack closure

Abstract

The determination of the optimum autofrettage pressure enables a clear improvement of the fatigue life for an internally highly pressurized component. The autofrettage process induces residual compressive stress after the release of a single static overload pressure, leading to plastic deformation at the inner wall whereas the outer part is only elastically stressed. This autofrettage pressure is clearly above the subsequent pulsating operating pressure range. Due to the complex geometry of the aluminium valve body, a detailed elastic–plastic finite element analysis is used to determine the critical area and the optimum autofrettage pressure. Based on an experimental stress–strain curve, three important load steps are simulated in a non‐linear way. The FKM guideline is used to assess fatigue life and crack initiation with detailed subsequent experimental verification. Even if small cracks occur, residual compressive stresses prohibit crack growth (nonpropagating crack), which can be analytically verified by fracture mechanical considerations (crack closure effect).

Analysis of residual stress relaxation of aluminum alloys EN AW 6061/‐82 T6 under cyclic loading

“…Autofrettage induces compressive stresses at the inner surface of internally high-pressurized components to extend fatigue life and is adaptable for different geometries of ductile metal components, for example, pressure valves, 6,7 pumps, 27 pressure vessels, and injection systems. 11,21 If cyclic relaxation of residual compressive stresses occurs, mean stress σ z,m,af.…”

“…The mechanical properties of EN AW 6061 T6 (AlMg1SiCu) were analyzed before the fatigue experiments with uniaxial tensile tests of five cylindrical specimens. A detailed description of these tests is given in the work of Repplinger et al 7 Furthermore, three flat, uniaxial test specimens were made of EN AW 6082 T6 (AlSi1MgMn). Their geometry, as well as the uniaxial test procedure, follow the standards ASTM E8/E8M‐16a 38 and BS EN ISO 6892‐1 39 .…”

“…Furthermore, autofrettaged components with bore intersections showed a strong improvement of fatigue life and a limitation of the fatigue crack growth. [1][2][3][4][5][6][7] But relaxation, if occurring, could reduce residual stress and would lead to higher mean stress (F-G) and hence to lower fatigue life.…”

“…Autofrettage is a single static well-controlled overloading pressure to induce residual compressive stresses at the critical inner surface for internally pressurized tubes in service. This compressive residual stress highly increases fatigue life even at bore intersections for steel [1][2][3][4][5] and aluminum 6,7 materials. In the case of more complex geometries like high-pressure valves, the optimum autofrettage pressure is more difficult to find, but still very beneficial as compressive residual stresses help even after crack initiation as closing mechanism, 8 if stress relaxation can be excluded.…”

“…A previous work by Repplinger et al 7 deals with an autofrettaged, complex aluminum valve structure of EN AW 6061 T6. They compared the numerically determined crack limit, based on the crack closure effect, to real fatigue cracks at several numbers of pressure cycles.…”