Stress range
Stress range is the difference between the highest and lowest stress in a cyclic load. It directly determines how fast a fatigue crack initiates in the weld. A large stress range drastically shortens the service life.
Physical-Chemical Mechanism of Action
Under cyclic loading, the stress constantly alternates between an upper and a lower value. The difference between these two values is called the stress range. It describes the intensity of the load fluctuation. In the material, local stress peaks occur at notches or discontinuities. These peaks drive the growth of a microcrack once the stress range exceeds a threshold value.
Metallurgical Relevance & Risks in Welding
Welds almost always have notches: undercut, root gaps, or slag inclusions. At these points, the external stress range is amplified. Even small stress ranges can initiate a crack after many load cycles. High stress ranges quickly lead to fatigue fracture, often without visible warning. Components under tensile stress and cyclic loading are especially at risk.
Workshop Practice & Action Guidelines
• Keep the stress range as low as possible through design. Avoid sharp cross-section changes and sudden stiffness transitions.
• Grind the weld transitions free of notches. Remove undercut and spatter.
• Use post-weld treatments like shot peening or hammering to introduce compressive residual stresses. They reduce the effective stress range.
• Check the actual loading. Halving the stress range can increase the life by a factor of 5 to 10 (depending on the notch case).
• Use non-destructive testing to detect cracks early. Once a crack is a few millimeters long, only repair welding helps.