Title: Fracture Based Fatigue Assessment of Crack
Client:
Software used: Ansys Mechanical
Duration: 4 Months
Keywords: crack, residual life, fracture, fatigue, BS 7910, weld, residual stress, fatigue assessment diagram, wind, Palmgren-Miner rule, automated assessment
During the inspection of the structure, several cracks are observed near weld locations. It is required to check the structural integrity of structure and residual life of structure based on fracture-based fatigue calculation. Complexity in this project involves assessment of each crack which will be subjected to 10000 combinations of variable loads during its service life. It is also required to account for the effect of wind load, variable with respect to direction, during assessment. We have performed fracture-based fatigue assessment of cracks based on guidelines of BS 7910. The assessment is performed at different levels of conservatism, considering variable fatigue loads for each crack. Most of the cracks are initiated near the weld region, so the effect of residual stress is also accounted for in the assessment. It evaluates allowable crack dimensions based on Failure Assessment Diagram (FAD) from initial crack size which is used in estimation of residual life of the structure. The fatigue damage for individual loading is calculated using load cycles and allowable cycles as per BS 7910 guidelines. Palmgren-Miner rule is used to calculate cumulative fatigue damage and residual service life. A program is developed for assessment procedure which considers all combinations of loading and is executed for all cracks. This automated assessment procedure has reduced the project execution time by ~75%. Our assessment reports are self-explanatory and gives best understanding to the customer for decision making.
Benchmark: Fracture based fatigue calculations are performed. Detail understanding of fracture mechanics is applied to the problem and Palmgren-Miner rule is used to calculate cumulative fatigue damage and residual life. The calculations have given an insight to the customer regarding the estimated life of the structure.