Stress ratio
The stress ratio R is the quotient of minimum stress σ_min and maximum stress σ_max of a cyclic load. It characterizes the mean stress condition: R = -1 means pure alternating load, R = 0 means pure pulsating tensile load. In welded components, residual stresses increase the local R and lower the fatigue strength.
Physical-Chemical Mechanism of Action
Under cyclic loading, the stress fluctuates periodically between an upper value σ_max and a lower value σ_min. The stress ratio R = σ_min / σ_max (with sign) describes how much this fluctuation deviates from the mean stress. If R is negative, the stress changes direction (tension-compression cycling). If R is zero or positive, the stress remains in the tensile or compressive range. The larger R becomes, the higher the mean stress for the same stress amplitude. This drives the growth of fatigue cracks because the crack tips are less relieved.
Metallurgical Relevance & Risks in Welding
After cooling, weld seams contain high tensile residual stresses that can be close to the yield strength. These residual stresses superimpose on the service load. This shifts the local stress ratio in the weld significantly upward. Even if a component is externally tested with R = -1, the critical weld region often cycles with R ≈ 0.3 to 0.5. This considerably reduces the fatigue strength. If the true local R is underestimated, premature cracking occurs at weld transitions, especially in high-strength steels or thick plates.
Workshop Practice & Action Guidelines
Always determine the stress ratio from the actual load sequence on the component, not from idealized assumptions. For welded structures without residual stress relief, use at least R = 0.5 in the fatigue life calculation, even if the external load is alternating. Reduce local tensile stress peaks by grinding the weld transitions, shot peening, or TIG dressing. This lowers the mean stress and the effective R. For critical parts, measure residual stresses using X-ray or hole-drilling methods to determine the real R.