Characterization of Damage Edge Distance Effects on the Residual Strength of Metallic Honeycomb Sandwich Panels Subjected to Edge-wise Compression

Abstract

Honeycomb sandwich panels are a type of structural composite that is widely used in aerospace applications because of their exceptional stiffness and strength-to-weight properties. Throughout their service life, many sandwich panels will experience damage in the form of dents on the facesheet caused by low-velocity impacts with foreign objects. This damage, depending on the size and location, can cause a significant reduction in the residual strength of the sandwich panel and affect the way the loads are carried by the structure. Although all known aircraft structural repair manuals have a minimum required distance between a dent and the edge of the sandwich panel, the technical data packages supporting these limitations are subject to intellectual property restrictions. No academic literature has been found discussing and quantifying the effects of damage edge distance on sandwich panels.

The current work, taking advantage of existing industry standards where possible, presents a new experimental approach to characterizing the damage edge distance effects on dented sandwich panel specimens subjected to compression after impact tests. The effects are characterized as changes in residual strength, failure mode, and facesheet strain behaviour. Data are collected using a combination of displacement, load cell, and three-dimensional digital image correlation instruments.

The test methodology developed, including test panel fabrication, testing, and data collection using three-dimensional digital image correlation, successfully characterized damage edge distance effects. For smooth circular dents 20 % of panel thickness in depth and 43 mm in diameter, the damage edge distance was found to affect residual strength, failure mode, and facesheet strain at damage edge distances up to 60 mm. The residual strength was found to have an inverse correlation with the damage edge distance. Edge distance effects were observed in the form of asymmetric dent growth and delayed macrobuckling of the sandwich panel. Finally, the facesheet strain became asymmetric in the presence of edge effects. Hypotheses are proposed as to why these effects occurred and how loads transfer from the sandwich panel into the adjacent structure when edge effects are present.