Fatigue damage and life assessment of welded joints based on energy methods

Faertom Pakandam, Ryerson University

Date of Award

2009

Degree Type

Thesis

Degree Name

Master of Applied Science (MASc)

Department

Mechanical Engineering

First Advisor

Ahmad Varvani-Farahani

Abstract

The present study intends to evaluate fatigue damage of different welded joints under loading conditions and their response on fatigue lifetime. The main variables influencing the fatigue life of a welded joint are: applied stress amplitude, material properties, geometrical stress concentration effects, and size and location of welding defects. In order to carry out the study, calculations have been performed using the parameters in three energy-based models. Calculations have been carried out separately for each model from the original experimental data obtained from available literature related to each welded joint. The data variables used as a basis for the calculations of the energy-based models for different welded joints, dimensional and geometrical information on the welded joints, and stress versus endurance cycle tables obtained from the tests performed on the welded joints. All the mentioned variables are parameters influencing the fatigue life of a welded joint. Fatigue damage assessments were performed and discussed based on earlier developed energy damage approaches consisting of: (i) the hysteresis loop based parameter of Masing type material, (ii) the notch stress-intensity based parameter and (iii) the critical plane/energy based parameter.

In evaluating fatigue damage of welded joints, these approaches were discussed based on the comparison of energy-lifetime diagrams obtained from each energy model and how readily coefficients/constants are determined and employed in the parameters. In addition, a finite element analysis was performed on selected welded joints to obtain local peak stress values and their location. Numerically obtained stress concentration factor and fatigue notch factor values were also compared with their analytical values. To assess fatigue damage of welded joints based on various energy models, different sets of experimentally obtained fatigue data performed by different laboratories under uniaxial loading conditions available in literature were chosen. The welded joints used in this study were butt joint, cruciform joint, butt-ground joint, and butt-strap fillet joint. The welded joint base metals included low carbon structural steel, aluminum alloys, and carbon steel. The energy models were compared for their energy-fatigue life curve slopes and their ability to converge the related nominal stress-life scatter. The energy values calculated based on their models included the effect of variables of cyclic stresses. Important results were concluded for welded joints from the study including: the relation between fatigue notch factor and fatigue strength, the stress-life diagram slope and fatigue resistance, the ability of the energy models to reflect the fatigue notch factor, and merits and disadvantages of each energy model.