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The through-the-thickness measurement of residual stress in a thick welded steel compact tension specimen by the contour method

Compact tension specimens are widely used for fatigue crack growth characterization. It is well known that when using this type of specimen for welded samples, residual stresses affect the crack growth behaviour, in particular, reducing the crack growth rate when the initial notch is parallel to the weldment. Several approaches have been used to characterize the residual stress field in welded compact tension specimens, including traditional destructive relaxation-based methods, as well as non-destructive diffraction-based techniques. The problem with the approaches presented so far is that the former do not provide through-the-thickness results, whereas the latter, although capable of providing through-the-thickness measurements, are only available in very few facilities worldwide. The interest in thick welded samples, where substantial through-the-thickness variations of residual stress fields are to be expected, leads to the need for a full-field characterization. In the present work, the residual stress perpendicular to the crack growth path in two welded steel compact tension specimens was measured by the contour method, an emerging destructive method. A full surface map of the residual stresses was obtained. This full field information on the residual stress distribution reveals substantial through-the-thickness variations. High compressive stresses were found in the centre of both specimens. The compact tension specimen with the crack plane in the centre of the weld bead presented slightly higher compressive residual stresses. The strong variation of the stress field, both in the longitudinal as well as in the through-the-thickness directions, emphasizes the need for full-field residual stress measurements for the correct interpretation of fatigue crack growth tests when using this type of specimen.