How do you evaluate the fatigue performance of welded joints in metals?

1 Factors affecting fatigue

The fatigue performance of welded joints in metals depends on several factors, such as the geometry and size of the joint, the welding process and parameters, the material properties and microstructure, the loading conditions and environment, and the presence and size of defects. These factors can affect the stress concentration, crack initiation, crack propagation, and fracture resistance of the joint. For example, a sharp notch or a large defect can increase the stress concentration and reduce the fatigue life of the joint. Similarly, a high heat input or a rapid cooling rate can cause residual stresses and microstructural changes that can reduce the ductility and toughness of the joint. Therefore, you need to consider these factors when designing, fabricating, inspecting, and testing welded joints for fatigue.

2 Fatigue failure modes and mechanisms

The fatigue failure of welded joints in metals can occur in different modes and mechanisms, depending on the stress state, the crack location, and the fracture mode. Common fatigue failure modes include base metal failure, toe failure, root failure, and throat failure. These mechanisms can then be grouped into two types: high-cycle fatigue, with crack initiation and propagation at low stress levels and high number of cycles; or low-cycle fatigue, with crack initiation and propagation at high stress levels and low number of cycles. High-cycle fatigue is characterized by slow and stable crack growth rate with a ductile or mixed fracture mode; while low-cycle fatigue is characterized by fast and unstable crack growth rate with a brittle or mixed fracture mode.

3 Fatigue testing and analysis

Fatigue testing and analysis of welded joints in metals are conducted to determine the fatigue strength, life, and damage of the joint under different loading conditions and environments. The fatigue testing methods can be divided into two categories: Constant amplitude testing, where the loading amplitude is kept constant throughout the test and results are usually presented as S-N curves; and variable amplitude testing, where the loading amplitude varies according to a specified spectrum or a random sequence, with results usually presented as damage accumulation or cumulative damage models. Similarly, the fatigue analysis methods can be divided into two categories: nominal stress approach, which is simple and conservative but does not account for local stress concentration or weld quality effects; and notch stress or strain approach, which is more accurate and realistic but requires more detailed information and calculations. Standards and codes such as ASTM, ISO, AWS, BS provide recommendations for the test specimens, loading conditions, failure criteria, data analysis, and design curves for different types of welded joints in metals.

4 Fatigue design and improvement

The fatigue design and improvement of welded joints in metals are aimed at increasing their fatigue strength and life. To achieve this, stress concentration must be reduced, weld quality must be enhanced, material properties must be improved, and defects must be minimized. Common techniques for doing this include geometric optimization to improve load-bearing capacity by using smooth transitions, fillet welds, large radii, and small angles; welding optimization to reduce residual stresses, distortion, and microstructural changes through low heat input, preheating, post-weld heat treatment, controlled cooling, and filler metals; post-weld improvement to modify the surface condition, residual stress state, or microstructure of the joint through grinding, polishing, peening, shot blasting, hammering, or laser shock peening; and inspection and repair to detect and remove defects through non-destructive testing such as ultrasonic radiographic magnetic particle or dye penetrant testing followed by welding grinding or patching.

5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?