Safety Assessment And Fatigue Life Calculation Of T-plate Containing Crack

The working level of offshore crane is relatively high and fatigue cracks can be found on it during the routine inspection. However due to its high cost and long cycle of maintenance, the productivity is seriously affected. Therefore it is vital to commence safety evaluation on the cracked structure. Failure assessment diagram (FAD) is usually used to evaluate the safety of welded structures containing cracks. There are three assessment levels in BS7910. As the assessment level increases, accuracy is improved and more data is required. This standard is widely used in safety assessment of pressure vessels and pipelines. To construct level 3C curves of the T-plate with through thickness crack and surface crack, the ABAQUS software and Shao Yongbo’s experimental data are utilized. In order to verify the validation of FAD for cracked T-plate, the constructed level 3C curves and the conventional level 2A curve are compared; the tendencies of both curves agree very well with each other. This paper offers the critical load of cracked T-plate according to level 2A evaluation and calculates the fatigue life from initial crack size to critical crack size under given spectrum. Following is the main work of this paper.(1) Each assessment level and the necessary input data of the British Standard BS7910 are introduced in detail.(2) For the purpose of accurate calculation of J integral value, the crack tip element is chosen, by numerical simulation and partial analytical calculation, through elastic and elastic-plastic analysis.(3) The level 3C curves of cracked T-plate are constructed according to BS7910.(4) The constructed level 3C curves are compared with the conventional level 2A curve and it is found that the T-plate can be evaluated in accordance with FAD and the level 2A curve is more conservative. Then the critical loads of T-plate with different crack sizes are calculated according to level 2A curve.(5) Based on the given stress spectrum, the fatigue life from initial crack size to critical crack size for four different cracked T-plate are calculated according to Miner cumulative damage theory and fracture mechanics theory.