DYNAMIC-RESPONSE OF TUBULAR T-JOINTS UNDER THE INFLUENCE OF PROPAGATING CRACKS

This paper presents an analytical study on the vibration response of t ubular T-joints for detecting the existence of cracks along their inte rsections. The ABAQUS finite element program was utilized for carrying out the analysis. Frequency response functions were obtained for a jo int with and without cracks. The joint was modeled with 8-node degener ate shell elements having 5 degrees of freedom per node. Line spring e lements were used to model the crack. The exact crack configuration (s emielliptical shape, Fig. 5(b)), as observed from numerous experimenta l fatigue crack investigations at;the critical location, has been achi eved through a mapping function, that allows a crack in a planar eleme nt to be mapped on to the tube surface. The natural frequency changes with respect to crack depth show little changes, being 4.82 percent fo r a 83-percent crack depth for the first mode. On the other hand signi ficant changes have been observed for bending moment and curvature as a function of crack depth. For an 83-percent chord thickness crack, a 97-percent change in bending moment at points around the crack vicinit y, and 34.15 to 78 percent change in bending moments, for those locati ons far away from the crack location, have been observed. Natural freq uency change should be combined with other modal parameters such as '' bending moment (or bending strain)'' and ''curvature'' changes for cra ck detection. The presence of the crack can be detected at locations f ar away from the crack location using such sensors as strain gages.