The effect of core gap on different facesheet thicknesses of Nomex honeycomb sandwich structures subjected to in-plane compression

Core gap defects commonly occur during manufacturing of large sandwich structures made of many core pieces, owing to the misalignment of the adjacent core, which may reduce the mechanical performance of sandwich structures. Therefore, it is important to characterize the core gap effect to investigate the structural integrity of such structures. Furthermore, the occurrence of brittle failure in carbon/epoxy/Nomex honeycomb structures can be mitigated by varying the facesheet thickness, thereby inducing a progressive crushing response. In this study, the influence of the core gap on different facesheet thicknesses of Nomex honeycomb sandwich structures subjected to in-plane compression was investigated. The in-plane compression samples were characterized without and with 20 mm core gaps in the gauge length of the sample. The nominal thickness, t_f, of the carbon/epoxy facesheets was varied from 0.25 mm (Type 1) to 2 mm (Type 8) by increasing the number of plies from one to eight. Changes in the failure modes were observed as the facesheet thickness was gradually increased. The maximum compressive load was predicted using a two-parameter Weibull reliability model through the analysis of variance (ANOVA) tool, and the deviation between the predicted and experimental values was less than 5 %. The maximum load sustained by the Nomex honeycomb sandwich structure increased from 1.81 to 28.6 kN across the range of facesheet thicknesses. In contrast, the strength of the core-less structure increased from 0.29 to 27.9 kN within this thickness range. Removing the core had a much greater effect on those samples based on thin facesheets. However, the drop in load and elastic stiffness in similar samples based on thicker facesheets (from t_f =1.25 mm to 2 mm) was less than 5 % while the specific energy absorption was less than 36 %.