Approach to Formulation of Nonderstructive Quality Assessment Criterion of High-Strength Steel Lifting Hooks

摘要

The present investigation attempts to formulate the analytical quality assessment criteria of lifting hooks made of 4140 grade (400 ℃ temper) high-strength steel using linear elastic fracture mechanics (LEFM) procedures. According to the existing standard specifications, the proof load test is mandatory for the acceptance/rejection of these components. Also, there is a provision that the hooks should be checked by nondestructive testing (NDT) techniques and must be free from obvious defects, but there is no stipulation of acceptance limit of defect, if any. It is therefore not realistic and is insufficient, because sometimes tested hooks are found to fail in actual service. In the present paper, acceptance limit that is, the maximum allowable defect sizes for dynamic loads with various stress ratios are quantitatively estimated from the computed defect growth urges. The maximum allowable operating cycles for nondestructively detectable minimum defects are tabulated, from which a logical quality assessment criteria has been formulated for the assessment of the fatigue life of the hooks. This investigation is helpful for the safe operation of the hooks.