A Probabilistic Study of Composite Impact Damage Design Strain Allowables

An investigation was made to assess the use of a probabilistic method to remove conservatism in the design of composite panels for impact damage. The baseline deterministic method used the barely visible impact damage (BVID) criterion. The probabilistic method accounts for variability of both the impact energy and applied loads with an allowable probability of failure of 10 -7 per flight as specified by the Joint Service Specification Guide (JSSG). A wing panel and a fuselage panel on an existing transport aircraft finite element model were selected for use in the analysis. Estimated impact energy variability was represented using normal distributions for multiple locations on the aircraft and multiple impact threat sources. Variability of the gust design load case was estimated using a discrete gust load analysis and Rice’s exceedance formula. The variability of the allowable panel loads was calculated using Lockheed Martin’s Composite Durability And Damage Tolerance (CDADT) program iteratively to find the zero margin of safety loads for a wide range of impact energies. Probability of failure per flight was calculated by integrating the product of the applied load cumulative probability and the allowable load probability density over the relevant range of loads. The probabilistic analysis method based on the JSSG criterion was found to offer a potential for modest (approximately 10%) weight savings for thick composite panels but no potential for weight savings for thin panels. It was concluded that the probabilistic analysis offers a potential for weight savings for thick panels, which have a high BVID impact energy threshold with a low probability of occurrence.