Application of the Finite Element Method to Crack Evaluation of Warships

摘要

The effects of artificial cracks in high stress components of the Halifax class frigate are investigated using two finite element based methodologies currently being considered for use in the Improved Ship Structural Maintenance Management (ISSMM) project. In one method, static stress intensities are determined for each crack configuration using a one percent exceedance wave load in sag. In the other method, the crack growth is predicted for a spectral wave loading corresponding to 100 hours in five meter head seas. A total of fifteen local models in the high stress region of the ship are investigated separately (three crack lengths for each of five locations). No evidence of uncontrolled crack growth is found using either of the two methods for any single crack configuration. The smallest margin of safety (ratio of fracture toughness to predicted stress intensity) is found to he 2.08. Crack growth rates are predicted for a 100 hour voyage in five meter head seas conditions. In this severe sea state the largest average crack growth rate is found to he 8.55 mm/ hour, but at present this result has not been validated. Crack growth predictions can be extended to loadings of different magnitudes using the assumed linearity between wave height and response.