Fracture Toughness and Crack Path Deviations in Laser Welded Joints

Abstract

The resistance of laser welded joints to cleavage failure has been examined using fatigue cracked fracture toughness specimens and Charpy tests. The apparent toughness of a range of weld microstructures was determined, the lowest being for a crack located in the weld metal. Sharp cracks deviate into the microstructure with the lowest apparent toughness adjacent to the tip and propagate down the toughness gradient. Charpy tests differ in that the crack consistently propagates into the softer material, and extends down the yield strength gradient adjacent to the tip. Charpy tests can thus give misleading information about the fracture behaviour of mismatched weldments, as crack path deviations may mask a potentially dangerously low toughness weld metal. Recommendations are given on supplementing Charpy weld characterisation with the fracture mechanics tests to ensure conservatism in mismatched weldments.