# Stability and Failure of the Edge-Closed Honeycomb Sandwich Panels with Face/Core Debonding
## 1 Introduction  
Honeycomb sandwich composite is one of the most effective material structures applied to improve the bending stiffness and reduce the weight of the structure. In recent years, it has been widely used in aerospace aircraft structures such as the fuselage, the wings, the rudders and so on [1]. For instance, the proportions of honeycomb sandwich used in B-S8 high-altitude bombers and F-111 fighters as high as 85% and 90%, respectively [2]. However, defects of honeycomb sandwich structures, such as cracks, debonding, core crush, and laminate delamination, etc., which generally caused by alternating loads, external impacts, lightning strike and environment during manufacturing and using, greatly affects the mechanical properties. Among them, face/core debonding is the most universal and dangerous defect in honeycomb sandwich structures which undermines the structural integrity seriously [3]. Studies on characterizing face/core debonding of sandwich structures have been conducted. In order to measure the mode I critical fracture toughness, the most common test method adopted is single cantilever beam (SCB) sandwich specimen which has been selected as the candidate of ASTM International test standard [4]. The SCB test method is easy to perform; [PERSON] et al. [5] used this test method to measure mode I critical fracture toughness of various sandwich configurations, and concluded that the fracture toughness increased with the rise of core density, face-sheet thickness, and core cell size. [PERSON] et al. [6] also adopted this method to examine the effect of face and core moduli and thickness on crack rotation in SCB sandwich specimen. For the measurement ofmode II critical fracture toughness, the Cracked Sandwich Beam (CSB) test [7] and End-Notched Flexure (ENF) test [8] are the common test methods. As for the test method under mix-mode condition, the Mixed Mode Bending (MMB) test is used [9; 10], but the possible range of mode mixity phase angle in a MMB test is limited [11]. Therefore, [PERSON] and [PERSON] [11] adopted the Double Cantilever Beam-Uneven Bending Moments (DCB-UBM) test method to overcome the limitation of MMB test method. Compressive strength is seriously affected by face/core debonding of sandwich structures [12]. The comprehensive experimental study on compression failure of sandwich structures with face/core debonding was firstly carried out by [PERSON] and [PERSON] [13]. The results showed that failure of sandwich structures was mainly caused by buckling of debonding face sheet, and followed by rapid propagation of the debonding. Test results given by [PERSON] and [PERSON] [14] showed that panels with square debonding failed at lower loads than those with circular debonding under in-plane compression, and the interface (core/resin) failure increased with the core density increased. [PERSON] et al. [15] studied the edgewise compressive strength of Carbon Fiber Reinforced Plastic (CFRP) sandwich structure with debonding area of different shapes and sizes, which showed that the size of the rectangular debonding defect had a slight effect on edgewise compressive strength, and the larger area of the elliptical debonding caused edgewise compressive strength to decrease by 18%. [PERSON] et al. [16] studied the effects of panel thickness and debonding length on the compressive strength and failure mechanism of composite sandwich panels through numerical simulation and experimental methods. The results showed that specimens of thicker sheets and smaller debonding area had higher buckling loads and failure strength. [PERSON] and [PERSON] [17] demonstrated that the failure load and debonding behavior of asymmetric sandwich structures with aluminum sheets were related to the size and depth of delamination by in-plane compressive tests. In terms of simulation, the buckling behavior of sandwich structures was developed by [PERSON] and [PERSON] [18] based on high-order sandwich theory.