Impact Damage Characterization and Damage Tolerance of Composite Sandwich Airframe Structures.

摘要

The use of composite sandwich construction is rapidly increasing in current and future airframe designs. Typically, these sandwich constructions use thin gage composite facesheets (0.020 inch to 0.045 inch) which are co-cured to honeycomb and foam cores. Due to the nature of these structures, damage tolerance is more complex than conventional laminated structures. Besides typical damage concerns such as through penetration and delamination, additional modes including core crushing and facesheet debonding must also be addressed. This complicates the certification process by introducing undefined Allowable Damage Limits (ADL) and Critical Damage Thresholds (CDT) as related to the ultimate and limit load carrying capability of the structure. In this report, the preliminary results of the damage resistance and tolerance experiments on sandwich panels are presented. The testing capabilities developed at the Wichita State University to support this program are presented in detail. The effect of impactor size on the impact resistance and residual strength properties was investigated. The effectiveness of traditional nondestructive inspection (NDI) methods in detecting and quantifying the damage distribution in the sandwich panels was studied and the salient results were presented. The damage metrics used for quantifying the damage distributions are planar damage area and residual indentation depth. The use of residual indentation in conjunction with a typical visual inspection protocol for preliminary damage detection was appraised. The characteristic damage states due to different impactor sizes were identified using destructive inspection and further correlated with the NDI damage metrics. The effects of different damage states were quantified by conducting uniaxial edgewise-compressive tests on the impacted specimens. The failure mechanisms governing the sandwich panels with different damage modes were identified and reported.