In English

Critical plane approach to low cycle thermal fatigue of welds in exhaust manifolds

Jakob Alm
Göteborg : Chalmers tekniska högskola, 2018. 34 s.
[Examensarbete på avancerad nivå]

Components in internal combustion engines are often subjected to temperature cycling that lead to low-cycle fatigue due to thermal expansion and contraction. Furthermore, these thermal loads may be multiaxial and non-proportional in nature which means that traditional fatigue evaluation methods are not sucient. Welds constitute another complication since they are often more sensitive to fatigue damage than the base material. This is in large part due to the notch e ect associated with the boundaries of the weld in addition to the presence of tensile residual stresses near the weld. At Volvo Cars, there is an interest in developing a new methodology for numerically evaluating the low cycle fatigue life of welds during thermal cycling. Methods that can account for non-proportional loading are of particular interest. One of the most prevalent methods for non-proportional multiaxial fatigue evaluation is the critical plane approach. Evaluating weld fatigue also requires treatment of the weld geometry to resolve the stress-strain gradients in the vicinity of the weld. This thesis explores the possibility of combining the critical plane approach with common weld modelling techniques to accurately model low cycle thermal fatigue. The Smith-Watson-Topper model using the maximum normal stress to the critical plane was chosen as the critical plane fatigue model and compared to a traditional strain-based fatigue evaluation methodology. Welds were modelled with shell elements, solid elements and with the e ective notch method. Weld residual stresses were accounted for by considering them as a tensile mean stress combined with a mean stress correction. To assess and compare the methods, a welded exhaust manifold subjected to low cycle thermal fatigue was evaluated by using Finite Element Analysis (FEA). It was shown that both the critical plane approach and traditional strain-based fatigue evaluation o ered a conservative fatigue life estimate compared to available experimental values. It was also shown that modelling the weld with shell elements resulted in fatigue life estimates within the margin of error for the experimental values. Using the e ective notch method gave the lowest fatigue life estimate but the fatigue failure location was predicted to be the weld toe which corresponded with known failure locations from experimental testing. Mean stress e ects were shown to have a negligible impact at the considered fatigue lives. It is important to note that further numerical and experimental validation is needed before deploying the methodology in an industrial setting.

Nyckelord: Weld fatigue; Critical plane approach; Exhaust manifold; Low cycle fatigue; Thermal fatigue;

Publikationen registrerades 2018-07-04.

CPL ID: 255473

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