Repeated and multiple hail impacts on honeycomb sandwich panels: An experimental and numerical study

Abstract

Sandwich panels are often utilized in structures to enhance stability, reduce weight, and simplify repairs. Airplanes equipped with honeycomb core sandwich panels are susceptible to hail impact damage. This type of damage can cause barely visible impact damage (BVID), which affects the core but not the face sheets. The main objective of this study is to examine the failure mechanisms of the Nomex honeycomb sandwich panel when subjected to single, repeated (i.e. at the same location), and multiple (i.e. at different locations) hail impacts. The experimental study was conducted to investigate the hail’s behavior upon impact on rigid structures and, subsequently, to assess the failure modes of sandwich structures under repeated hail impacts. The objective was to develop a suitable numerical model for studying the response of sandwich structures under multiple and random hail impacts. The sandwich panel, which consisted of four layers of woven glass epoxy as the face sheet and a Nomex honeycomb layer with a thickness of 16 mm as a core, was tested for its resilience to repeated hail impacts with 5, 10, 15, and 20 impacts. Four failure mechanisms were observed in the sandwich plate, including delamination growth at the top face sheet and three types of failure in the core. The core failures, initiated with wrinkling of the core walls, progressed to wall rupturing at higher impacts and, after 15 impacts, separated the face sheet from the core. Numerical models were then deployed to investigate the effects of hail size in a single impact and the effect of the random distribution of multiple hail impacts. The severity of damage caused by multiple impacts from lower-energy hail (30 mm in size at 21 m/s) was in a similar level as damage from a single impact by higher-energy hail (>40 mm in size at 30 m/s). The damage was almost initiated after eight impacts under the site, which experienced two close impacts.