Impact Failure Mechanism And Absorption Energy Distribution Of Aircraft Honeycomb Sandwich Structure

For the superior mechanical properties, such as light weight, high stiffness to weight and strength to weight ratios, fatigue and corrosion resistance, better dimensional stability, during the sandwich structures service life, impact damage is the primary reason to degenerate the structure loading carring capacity, such as the bird strike, severe hail storm, dropped tools, collision with the ground equipment and etc. Defects and damages of aircraft sandwich structure directly influence the security. Impact damage mode closely relate to the impact energy, sandwich structure deformation and the material physical damage size appear complicated relationship with the stress, the strain rate and the variable of internal energy. Under impact loading, material existence a variety of dynamic failure modes and cause the micro-structure irreversible changes.Based the homogenization theory can deduce the equivalent density and elastic modulus by the honeycomb core representative unit cell. Under the compressive loading, the core single-layer begin to instability till the double-layer wall achieve its maximal load-carrying capability, core wall start folding with certain wavelengths from the upper side to the lower side. The purpose of quasi-static out-of-plane compressive test is to studying the honeycomb core failure mechanism with different materials and different cellular size, results show honeycomb core failure feature depends on cellular matrix material properties and cellular size.Honeycomb sandwich beam skins can be regarded as I-beam flange cause honeycomb core in-plane tensile stiffness is very small, all the bending moment, bending normal stress, in-plane compression stress and torque moment carried by the skins of sandwich beam. Honeycomb core between the skins is similar to the I-beam web carrying only shear stress but increasing the structure bend stiffness substantially. Three-point bending experiment obtains the sandwich beam relationship between the load and the deflection of the beam mid-point, the theory results can fit the experiment results well. Projectile impact and drop impact test show the sandwich beam with aluminium skins dynamic response and the capacity of absorbed kinetic energy. Honeycomb sandwich beam with different thickness skins can cause different failure mode, for the complexity failure mechanism, it is difficult to understand the structural strength degradation form the external damage. The dynamic response and failure comparison analysis results show the sandwich beam with thicker skins in the test the structural integrity degenerate seriously.The failure modes of sandwich structure with composite skins mainly include matrix cracking, fiber breakage, delamination and honeycomb core crushing damage. Quasi-static indentation experiment is designed to study composite sandwich panel failure mechanism and find its failure modes dependent on skins thickness and boundary conditions, the bottom rigid support condition is the main factors affect the failure. In the drop impact experiment, impact dent depth variable can reflect the increasing of impact energy, the curve fitting the experiment results can predict the impact dent depth by the impact energy.Honeycomb core material is well energy absorption material. Under the out-of-plane compressive loading, wide yield platform in the stress-strain curve illustrate honeycomb core can absorb more energy by cellular wall regularly folding deformation during the plastic crushing yield process. The dynamic impact response of sandwich beam with aluminium alloy skins and Nomex honeycomb core has been studied using a aerodynamic gun projectile impact test under different velocity interval. An analytic model based on the partition energy method and experimental data is proposed to deduce the energy distribution of the the absorption energy from impact projectile initial kinetic energy Eabs via elastic bending deformation Ebend and irreversible damage consumed energy Edam. The results show that ratio Ebend/Eabsof the sandwich beam with thicker skins are lower than the beams with thinner skins. The essence of the difference is varing certain degrees undermine the structural integrity under the impact loading in the sandwich beams with different thickness skins. The similar results also present the irregular of the strain-time history curve account for the destroy of the structural integrity due to blocking the impact loading wave propagating in the structure.