Applicability of fatigue design recommendations for thin plate welded joints by assessment of fatigue crack initiation and propagation life

Abstract

Fatigue design guidelines for welded joints are mainly derived from tests on joints with a thickness of approximately 10 mm. As thickness increases, fatigue strength typically declines, likely due to greater stress concentration or changes in stress distribution. Consequently, these guidelines adjust fatigue strength downward for thicker joints. However, for thinner plates, such as those frequently used in railway cars, test data is limited, making it uncertain whether the guidelines are applicable. This study performed a 4-point bending fatigue test on a 3 mm thick fillet-welded joint to evaluate fatigue strength. Contrary to expectations, reducing the thickness from 9 to 3 mm resulted in a decrease in fatigue strength. Nevertheless, the test results indicated a higher fatigue strength than the recommended value, supporting the applicability of the guidelines to joints with a 3 mm thickness. The study also examined the factors contributing to reduced fatigue strength in thin plates, focusing on the relationship between the number of cycles to crack initiation, local stress range, and crack growth analysis using Paris' law. It was observed that as the thickness decreases, the stress concentration at the weld toe reduces, and the number of cycles to crack initiation increases. However, the decrease in ligament length leads to a shorter crack propagation life.