Tip Parameter Approximation and Fatigue Growth of Crack Towards Inclined Weld Interface Between Strength Mismatched Steels

Abstract

A Mode I crack growing at an angle other than 90° with respect to the interface of weld between strength mismatched steels in a bimetallic composite is investigated. The steels and the weld have nearly identical elastic and thermal expansion properties. As the crack in parent steel reaches near the interface of the weld, the plastic strain or yield zone at its tip spreads to the weld, which causes load transfer effect, due to strength mismatch across the weld interface, over the crack tip. Yield zone is modeled as Dugdale’s cohesive zone subjected to constant closing cohesive traction. Geometrical formulations are employed to approximate change in volume of parts of cohesive zone on either side of the interface resulting upon inclining the interface from the vertical. A computational model for crack tip in front of the perpendicular weld interface in linear elastic regime is modified to account for the effect of inclination of weld interface from the vertical. Crack tip stress intensity parameter is approximated from the model, cyclic value of which is used to assess the rate of fatigue crack growth towards the weld interface. The model is validated by experiments. Bimetallic compact tension specimens, prepared by electron beam welding of weak ASTM 4340 alloy and strong MDN 250 maraging steels at interface angles of 10° and 15° with the vertical, are subjected to high cycle fatigue. Growth rate of crack is found to dip substantially when it reaches near the interface of ultra strong weld. The results are compared with those of plain and bimetallic specimens with perpendicular weld interfaces. Inclination of weld interface from the vertical by nearly all the angles changes, albeit marginally, the values of crack tip parameter visà-vis those in the case of perpendicular weld interface. Theoretical and experimental results are in good agreement with each other.