Evaluation of test methods and face-sheet thickness effects in damage tolerance assessment of composite sandwich plates

Composite sandwich materials provide high bending performance-to-weight ratios. However, these materials are vulnerable to impact damages which can drastically reduce their load-bearing capability. Presently there is a lack of standardised test methods for impact assessment. This study compares three different test methods for impact assessment; single skin compression after impact (CAI-SS), sandwich compression after impact (CAI-SW) and four-point bending-after-impact (BAI). The CAI-SS test method shows high compressive strength and strain at failure and the tesr is relatively easy to evaluate. For finite size plates with significant impact damage, the CAI-SS test method is recommended for post impact strength assessment. For large sandwich panels with relatively small impact damages the CAI-SW test method could be more relevant since it includes effects of panel asymmetry generated from the impact damage. The BAI test method may be recommended as an alternative to CAI but quite long specimens are required in order to assure compressive failure in the tested face-sheet, making the test both demanding and expensive. On the other hand, lower load levels are required to break the specimens and there is less need for precise machining during specimen manufacturing. A finite element model including progressive damage evolution was used to estimate the post impact strength. The simulations showed generally good agreement with the experiments.

Automated summary

Composite sandwich materials offer high bending performance-to-weight ratios, but are susceptible to impact damages which can significantly reduce their load-bearing capability. This study compared three different test methods for impact assessment and found that CAI-SS test method is recommended for post impact strength assessment of finite size plates with significant impact damage. CAI-SW test method may be more relevant for large sandwich panels with relatively small impact damages due to inclusion of panel asymmetry effects, while BAI test method could be an alternative to CAI but requires long specimens and is demanding and expensive. A finite element model with progressive damage evolution was used to estimate post impact strength, showing good agreement with experiments.