Low velocity impact behaviour of glass fabric/epoxy honeycomb core sandwich composites

In this paper, the low velocity impact characteristics of honeycomb sandwich panels with varying cell sizes such as the maximum load absorbed, absorbed energy and failure modes were investigated. Low-velocity impact tests were carried out at different energy levels using an instrumented drop-weight apparatus as per ASTM D3029-95. Honeycomb core sandwich composite panels were made with glass fabric/epoxy honeycomb core with different cell sizes i.e., 8, 12, 16 and 20 mm using the vacuum bag molding technique. Tests were conducted at impact energies that ranged ranging from 7 to 50 J. The energy absorption mechanism and the damage process are analysed with parameters derived from the load-time and energy-time curves. It is observed that at low energy levels, the core buckles while it gets crushed at higher levels. This buckling of the core is due to damage to the epoxy resin that allows the glass fiber to bend freely, and core crushing is due to the breakage of the glass fiber reinforcement within the core. It is inferred that for a given core height, as the cell size increases, the peak load at which cracking occurs in the face sheet decreases drastically. This is attributed to the fact that a core with a smaller cell size has fewer load paths to distribute the absorbed energy. It is evident from the post impact damage analysis that three types of failure modes occur viz., crack initiation, cracking of the face sheet along with debonding, and the crushing of cells. The common damage pattern observed on the front facing has a square shape which is the same as the shape of the indentor.