Nondestructive Detection of Cracks in Tubular T-Joints Using Vibration Characteristics

Abstract

This paper presents the results of an experimental investigation of the nondestructive inspection (NDI) of cracks in an intermediate scale tubular T-joint similar to those used in offshore platforms. To simulate the real situation, cracks were generated under fatigue loading. Modal testing and analysis were then carried out to study the vibration characteristics of the tubular T-joints. Strain gages were used as the main transducers to acquire data because strains are more sensitive to the presence of cracks. Strain frequency response functions were particularly analyzed. Three salient phenomena were observed: (a) antiresonance shifts, (b) quasi-static phenomenon, and (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 renders a more promising NDI technique than the frequency monitoring technique. Finite element analyses were also conducted to confirm the experimental results and to depict the mode shapes.