Non-destructive evaluation of cracking in tubular T-joints using vibration procedures

Cheng, Shumin (1998) Non-destructive evaluation of cracking in tubular T-joints using vibration procedures. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis presents an experimental and analytical study of the initiation and growth of cracks in tubular T-joint members of the type used in some off-shore platforms. Cracks were developed experimentally under fatigue loading conditions. -- The early stages of formation and growth of the cracks have been studied by fracture surface analysis, using scanning electron microscopy. Their growth has also been followed by modal testing. It has been shown that a number of significant changes occur in the static and dynamic response characteristics of the T-joints, as cracks initiate and grow. In particular, it has been shown that strain gauge modal testing provides a means of detecting cracks at a much earlier stage of formation than has been possible using alternative techniques. -- The modal analysis methods developed in this thesis rely on the use of strain/acceleration frequency response functions to detect the presence of growing cracks. Three parameters were found to be significant in detecting the presence of cracks, and these include: a) response in the quasi-static region; b) response in the anti-resonant region; and c) non-linearity in the frequency response functions. The factors contributing to the observed changes are discussed. -- In addition, measured (static) strains and stresses were also found to provide good indications of the presence of growing fatigue cracks. Abrupt changes in strain gauge outputs were observed consistently as small thumb-nail cracks, present in the early stages of crack growth, coalesced to form single cracks of much longer length. -- Parametric equations were developed to relate the fatigue life of the tubular specimen to the strains/frequencies measured at the various strain gauge locations. In addition equations were also developed to relate the crack size to the strains/frequencies measured at various locations. These equations could be utilized to predict the remaining life of the joint as well as the probable crack size at the critical location near the gauge. -- Finite element analysis was used to predict the modal response of crack-free and cracked structures to dynamic excitation. Excellent agreement was observed between experiment and theory in all cases. -- A lumped mass model has been developed to simulate the behavior of the fatigue cracks during dynamic testing. It has been shown that the structure in the vicinity of the cracks exhibits non-linear stiffness and compliance as the fatigue cracks open and close.

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