Fatigue crack propagation behaviour in thick steel weldments

The purpose of this paper is to study the fatigue crack growth (FCG) behaviour of the steel and weldments of a railway bridge.

Tests were carried out on compact tension (CT) specimens using the thickness (B=32 mm) of a structural detail. The test matrix included three R values and three material conditions: base material (BM), heat affected zone (HAZ) and weld metal (WM). An evaluation of opening load behavior was carried out. The full field measurement of the residual stress perpendicular to the crack plane was performed using the contour technique. A simplified finite element analysis supported the interpretation of the results. Scanning electron microscopy (SEM) observation of the fracture surface of BM and HAZ specimens was carried out.

Extensive crack closure effects were found in the welded specimens. Important through‐the‐thickness variation of residual stress was found using the contour technique. The residual stress fields of HAZ and WM specimens led to slowing down the FCG rate in the initial stages of crack propagation and to uncommon fracture surfaces. When the opening load effect was taken into consideration it was found that the da/dN vs ΔK of the different types of specimens are approximately identical. The ratio (striation spacing)/(da/dN) decreases up to approximately unity as a/W increases. In the specimens analyzed, FCG rates below approximately 2E‐7 to 3E‐7 m/cycle are associated with approximately constant striation spacing values, which could be considered a conservative upper bound of the real crack growth rate.

Fatigue crack growth behavior of thick welded steel CT specimens was analyzed on the basis of tests including full field residual stress measurements, crack closure behavior and striation spacing, allowing for the simultaneous consideration of all those aspects. It is shown that the striations spacing provides no more than a conservative upper bond of the real crack propagation rate.