Nondestructive detection of cracks in tubular T-joints using vibration characteristics

This paper presents the results of an experimental investigation of the non destructive inspection (NDI) of cracks in an intermediate scale tubular T-j oint similar to those used in offshore platforms. To simulate the real situ ation, cracks were generated under fatigue loading. Modal testing and analy sis were then carried out to study the vibration characteristics of the tub ular T-joints. Strain gages were used as the main transducers to acquire da ta because strains are more sensitive to the presence of cracks. Strain fre quency response functions were particularly analyzed. Three salient phenome na were observed: (a) antiresonance shifts, (b) quasi-static phenomenon, an d (c) nonlinearity, all of which are indicative of the presence of cracks, even at a normalized crack size (the ratio of cracking area to load-bearing area) as small as 0.07. Therefore, the method developed in this study rend ers a more promising NDI technique than the frequency monitoring technique. Finite element analyses were also conducted to confirm the experimental re sults and to depict the mode shapes.