Autofrettage of high-pressure components made of ultra-high-strength-steel

This work is primarily concerned with the fatigue life of high-pressure bearing components with intersecting holes, typically used in Diesel engine fuel injection systems. The investigation focuses on specimens with orthogonally intersecting holes that have undergone the process of Autofrettage (single mechanical overload), which is typically used to extend the fatigue life of components loaded by cyclic internal pressure. The Autofrettage process induces advantageous, life-time prolonging residual compressive stresses in the highly stressed areas of the components. The resulting residual stress distribution thus influences the fatigue failure and especially the crack propagation behaviour of the components. In previous works, fracture mechanics based approaches were used to describe the crack propagation behaviour for autofrettaged specimens made of the quenched and tempered steel 42CrMo4. Results showed that crack arrest has to be taken into account when calculating fatigue lives of autofrettaged specimens as the endurance limit is otherwise underestimated. As efforts are made to increase the injection pressures of fuel injection systems, in this work, the benefit of using ultra high strength steel for the application described is investigated. In order to achieve reliable results, material testing with samples made of the ultra-high-strength steel W360 was performed. The resulting test data were used to describe the initial loading and cyclic loading behaviour of the material with a suitable material model. Finite element analysis was then performed to simulate the Autofrettage process and subsequent cyclic loading. Based on the simulation results, possible crack initiation was determined. For predicted crack initiation, the simulated residual stress distribution was used to investigate the crack propagation behaviour with fracture mechanics based approaches of different complexity in order to identify possible crack arrest or crack propagation. Calculated results were compared to experimental test data from component-like specimens. The comparison to the test results showed an overestimation of the predicted fatigue lives. The modelled material behaviour and consequently the residual stress distribution from the simulation models was identified as the decisive factor for the deviation. Still, the comparison showed that the fracture mechanics based approaches are capable of describing the crack arrest and propagation behaviour reliably. Further investigation regarding the modelling of the material behaviour with focus on the Autofrettage process is still required.