Study On Fatigue Testing And Life Prediction Method Of Spot Welded Joints Of Steel

Spot welding is an essential technology for joining parts in automobile industry. Thousands of spot welded joints exist in automobile body in white. The strength of these spot welded joints will affect the performance of whole structure when the vehicle is running. Therefore, it is extremely meaningful and economically valuable to conduct durability analysis for spot welded joints.This thesis focuses on the spot welded joints of 4 steels that are commonly used in automotive structure, and has performed systematically experiments and theoretical research on two of the most widely used specimen types- tensile shear and coach peel specimen. The effect of base material, weld nugget diameter, sheet thickness and failure mode on the fatigue life of spot welded joints has been discussed. It is found that for the spot welded joints of steel used in this thesis, the influence of load ratio(R=0.1 and R=0.3) on fatigue life is negligible. And different steel grades also have similar fatigue performance. Fatigue life is mainly depended on specimen types, sheet thicknesses, and weld nugget diameters. Different specimen types have shown various failure modes in tensile and fatigue tests.Two common finite element analysis models of spot welded joints have been introduced and established in this thesis. By using very coarse meshes, the nodal forces and moments in the critical area have been obtained and used to represent the force status of the nugget under fatigue loading. The results of these two models have been compared. And Rupp's structural stress has been chosen as the fatigue damage parameter and has been calculated based on the finite element analysis results. Then stress-life(S-N) curves have been constructed. It is shown that the structural stress based on the nine initial empirical factors cannot reduce the difference in fatigue lives induced by different geometries and different specimen types.Based on the fatigue testing data of spot welded joints published by others, this thesis, utilizing a classic nonlinear generalized reduced gradient algorithm, has proposed an optimization procedure for the determination of empirical factors included in Rupp's structural stress calculation. It has been proved that, by choosing suitable empirical factors, Rupp's structural stress is able to correlate the fatigue lives of joints for different geometries and different specimen types. And it has similar results when using those two finite element models. The S-N curves which are used to predict fatigue life have been obtained through this structural stress method, and most of the data points are located in the “x5” scale band. Is has been concluded that Rupp's structural stress is capable to be used on fatigue failure analysis on spot welded joints as a fatigue damage parameter. The effectiveness of the optimization procedure on the accurate calculation of Rupp's structural stress is also validated.