Numerical Analysis and Experimental Study for Fatigue Life Behavior of HSLA and UHSS Welded Joints

Abstract

Welded steel joints are widely used for the structural components in the on/off-highway industry. Due to increasing regulations around CO2 emissions and competitive demand for improved fuel economy, there is a significant focus on the lightweighting of structures in the machinery components deriving the use of higher strength steel grades. It’s given that the lightweighting objectives should be achieved without compromising on functional performance, life-long durability, and safety among many others. HSLA (high-strength low alloy steel) and UHSS (ultra high-strength steel) grades are of specific interest considering their favorable mechanical properties along with excellent weldability. During this study, 345-MPa HSLA and 690-MPa UHSS steel grades were selected. Understanding the fatigue behavior of high-strength steel welded joints is of specific interest because fatigue properties for some of the steel grades can degrade from the thermal effects of welding. Keeping this objective in mind, an extensive and systematic experimental fatigue test study was performed on the double fillet T-joints welded from HSLA and UHSS steel grades. The experimental investigation included detailed metallurgical analysis such as microstructure and hardness variations across three distinct zones of welded joints, residual stress measurements using X-ray diffraction, and weld toe geometry measurements using dental molds. Further, detailed numerical finite element and fatigue analysis were carried out to predict the total fatigue life (crack initiation plus propagation) of the welded joints, based on the total fatigue life approach. The total fatigue life approach is an advanced fatigue analysis method which allows estimating the total life of the structure without the need to divide into arbitrary crack initiation and propagation phases, making this an attractive method for engineering practice. Finally, the outcome from the numerical analysis has been compared with the experimental fatigue test results for the welded joints made from HSLA and UHSS steel grades, showing good correlations.