An investigation of residual and thermomechanical stress and strain in the turbine housing of a turbocharger

This thesis contains the results of a novel combination of experimental measurements and simulation techniques to characterise the stress/strain behaviour throughout a typical lifecycle of a turbine housing. During operation the turbine housing is subjected to thermomechanical fatigue (TMF). The accurate validation of residual stress (both before and after operation) and stresses induced due to applied loading can be challenging and internal residual stresses in areas prone to fatigue crack initiation are difficult to measure accurately.

Experimental measurements of residual stress were obtained using two techniques. Neutron diffraction, a non-destructive technique based on the measurement of the shift in atomic lattice spacings induced by stress, was used to measure internally within the turbine housing both before and after the application of thermomechanical load cycles. The contour method, a destructive technique, based on the measurement of deformed surface contours after stress-relief induced by cutting was used to obtain measurements on a turbine housing before thermal cycling. High temperature strain gauges were used to measure strain due to applied thermomechanical load.

Simulations of the residual stresses present in the turbine housing at the end of the production process which included an annealing heat treatment showed very low <±10MPa levels of stress within the bulk of the turbine housing. Contour measurements showed a similar tendency toward low levels of residual stress. In contrast the neutron diffraction results revealed higher levels, tensile in nature and up to 150MPa in magnitude. Evidence of strain hardening was observed in the neutron diffraction measurement after thermal cycling with comparable behavior present in simulation models. The TMF simulation was validated against strain gauge measurements, observed trends were very similar. The magnitude of simulated and measured applied strain showed some variation and was within 10%-50%. However, the accuracy of the model was reasonable enough to predict the fatigue crack locations when assessed against a housing subjected to an extended durability test.