Fracture testing of the heat affected zone from welded steel pipes using an in situ stage

The structural assessment of welded joints requires that the weld metal and the heat-affected zone (HAZ) have adequate fracture toughness values. The measurement of these quantities is still controversial due to the inhomogeneous properties of the welds, which affects standardized methodologies. This work presents local measurements of the fracture toughness of the heat-affected zones from welded steel pipes. Fracture tests were performed inside the scanning electron microscope (SEM) using small specimens, allowing the direct measurement of the crack tip opening displacement (CTOD) on the surface. CTODs of approximately half a millimeter were attained for crack growth of about one-tenth of a millimeter before the crack crossed the fusion line. Three methods for computing the CTOD were compared, and the differences among the results were discussed. Furthermore, the surface strain field, calculated by measuring the distortion of a regular grid, showed evidence of an influence of HAZ inhomogeneity.

► CTOD and surface strain fields measured from cracks in the heat affected zone of welds using in situ SEM fracture tests.
► CTOD direct measurements compared to traditional formulations in the literature based on homogeneous-material models.
► The technique can measure CTOD without being affected by the inhomogeneity, which affects the other measures.
► The strain fields at the crack tip presented significant differences with those from the HRR model.