Low cycle fatigue analysis of marine structures

• Main Author: Ng, Wen Hao.
• Other Authors: Ong Lin Seng
• Format: Final Year Project
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Materials::Material testing and characterization
• Online Access: http://hdl.handle.net/10356/53980
• Institution: Nanyang Technological University

Fatigue fracture has affected the aircraft industry largely due to the high cyclic loadings of the airplane. However, in recent years, marine structures are found to fail at loading cycles lower than the design life. This brings to the attention of researchers about low cycle fatigue which occur at cycles lesser than 104 cycles. Fatigue damage accumulated in the lower cycle life can be recorded and assessed, thereby predicting the failure of the structure. The damage is due to the stresses at notches of higher stress concentrations. At these notches, yielding occurs even though the macrostructure may still be in the elastic regions. Therefore, approaches used to obtain the pseudo stresses at the notches are done via the strain values. The first approach using the empirical Neuber’s relationships provide a fast and indicative representation of the pseudo stresses experienced due to the nominal loadings. The other approach is using non-linear finite element analysis. Both of which utilizes ANSYS Parametric Design Language to obtain the required values. The model joint used in this analysis is a cruciform joint which is common in most marine structures due to the use of longitudinal and transverse backings to provide higher sectional modulus. In this report, the approaches will be adopted, compared and discussed. It shows that the empirical Neuber’s approach results in a set of pseudo stress values lower than the non-linear approach and hence a practical fatigue experiment might be needed to justify this. Also, it is derived that that there is minimal effect of modelling the notch radius on the pseudo stress values. However, the effect of variation of Young’s Modulus due to undermatching or overmatching of welds can have considerable effect on the pseudo stresses.