Fracture mechanism analysis and life-prolonging investigation of butt weld for ladle crane

Abstract

The butt weld between the T-shaped steel and the main web of ladle crane is a crucial weld to bear the lifting load. Since it is mainly considered to be subjected to compressive stress, this weld is generally not regarded as the key area of fatigue assessment. An obvious crack was found in the butt weld of an in-service ladle crane during a routine inspection. In this paper, the cracking mechanism and life-prolonging scheme were studied by means of field tests and numerical analysis. Firstly, the stress history signal according to the cracked area were acquired with several working cycles. It was concluded that the unconventionally imagined tensile and compressive stress are generated during the running process of the crane, which further led to several high-level stress cycles in a typical working cycle. A Finite Element Model (FEM) of the bridge containing a fine solid mesh of the studied weld was established by using sub-model technology. Combined with the rain flow counting results of the measured stresses and the Equivalent Structural Stress (ESS) solving technique based on the FEM, the ESS spectrum of the cracked area was proposed. The fatigue life of the cracked local area is analyzed based on the stress spectrum and the failure mechanism of the butt weld is revealed. A life-prolonging scheme for the cracked local structure was proposed and the fatigue life of the butt-weld area after reinforcement is predicted by using the ESS spectrum, which verifies the effectiveness of reinforcement. Up to now, the ladle crane has been operating normally for nearly 4 years, no new cracks have appeared in the originally cracked are, providing the effectiveness of the life-prolonging scheme.