Abstract
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.
Highlights
Sandwich structures are widely employed as lightweight solutions in e.g. aerospace, automotive, marine and civil engineering applications
The results show that the relative loss of residual strength for barely visible impact damage (BVID)-impacted specimens is about 28% for symmetric and 25% for asymmetric specimens
The compression after impact (CAI)-SS test method generally shows high compressive strength and strain at failure and is relatively easy to evaluate. It appears to be most suitable for post-impact damage tolerance assessment of the sandwich panels at hand
Summary
Sandwich structures are widely employed as lightweight solutions in e.g. aerospace, automotive, marine and civil engineering applications. At present most sandwich CAI testing follows the same technique as that for laminated composites, i.e. using edge-wise compression of the complete sandwich cross-section Using this method, there is a risk for scatter in data due to uneven load introduction. A detailed evaluation of methods for damage tolerance testing was conducted using three different test methods: edge-wise compression of the complete sandwich cross-section, edge-wise compression of the impacted facesheet alone and 4PB after impact. Both symmetric and asymmetric sandwich configurations were studied, and comparisons made between the results from the different test methods. A progressive damage finite element (FE) model including damage after impact was used to simulate the compression response after impact
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