Modelling of low-energy/low-velocity impact on Nomex honeycomb sandwich structures with metallic skins

Abstract

Abstract.In the aircraft industry, manufacturers have to decide quickly whether an impacted sandwich needs repairing or not. Certain computation tools exist at present but they are very time consuming and they also fail to perfectly model the physical phenomena involved in an impact. In a previous publication, the authors demonstrated the possibility of representing the NomexTM honeycomb core by a grid of nonlinear springs and have pointed out both the structural behaviour of the honeycomb and the influence of core-skin boundary conditions. This discrete approach accurately predicts the static indentation on honeycomb core alone and the indentation on sandwich structure with metal skins supported on rigid flat support. In this study, the domain of validity of this approach is investigated. It is found that the approach is not valid for sharp projectiles on thin skins. In any case, the spring elements used to model the honeycomb cannot take into account the transverse shear that occurs in the core during the bending of a sandwich. To overcome this strong limitation, a multi-level approach is proposed in the present article. In this approach, the sandwich structure is modelled by Mindlin plate elements and the computed static contact law is implemented in a non linear spring located between the impactor and the structure. Thus, it is possible to predict the dynamic structural A c c e p t e d m a n u s c r i p t response in the case of low-velocity/low-energy impact on metal-skinned sandwich structures.A good correlation with dynamic experimental tests is achieved. Keywords.Sandwich structures, low velocity/low energy impact, finite element nonlinear analysis, indentation. 1-IntroductionSandwich structures exhibit static properties such as high stiffness-to-weight ratio and high buckling loads which are of great importance in the aeronautics field. Nevertheless, the current applications on commercial airplanes remain mainly limited to secondary structures like control surface or floor panels. In the field of helicopters where stress levels are lower, full sandwich structures are already in flight. In fact, one of the main limitations is linked to a lack of knowledge on the effects induced by impact damages [1]. However, in service, such structures are exposed and are often impacted during taxiing manoeuvres or take-off. They can also be damaged by tool drops during maintenance operations [2] and [3]. In such circumstances, the aircraft manufacturer has to inform the users very quickly of whether the impacted structure needs a repair or not. The study presented in this article is a step toward the final objective, which is to provide the aerospace industry with an inverse method. In this method, a 3D picture of the damage is first made by the airline company. Then, based on the shape of the impact, the impactor's shape is found and both the impact damage and the residual strength would be computed. All the approaches presented have a common feature which is to consider the honeycomb as a homogeneous material. Recent...

Experimental analysis and modeling of the buckling of a loaded honeycomb sandwich composite

“…In this region the crushing and fracture of the cores start ( Figure 5). Depending on the core densification at the end of Region 3, an increase in the compressive stress is observed, as was reported in [16][17][18][19][20][21][22][23][24] .…”

“…They have determined that the critical buckling load of Nomex is higher than that of aluminum comb composite panels for all cell sizes. Castanié et al 17 showed that the compression load is essentially taken by the vertical edges of the hexagonal cell. From the axial compression collapse tests on the aluminum honeycomb sandwich panel specimen, various potential influential parameters, i.e., the core height, core-cell thickness and panel aspect ratio, it was observed that the core height would be a crucial parameter affecting the sandwich panel's ultimate compressive strength.…”

Experimental and numerical analysis of the shear nonlinear behaviour of Nomex honeycomb core: Application to insert sizing

“…In the related literature, the main topic is clearly the compressive response, a phenomenon that has been studied for a long time (see McFarland since 1965 [12] or Wierzbicki [13]). Many authors have studied the compressive behavior -mainly to analyse the energy absorbing capabilities of honeycomb ( [12][13][14][15][16][17] for example) or, more precisely, the crush behavior of the core after a low-velocity/low-energy impact on a sandwich [17][18][19][20][21][22][23][24]. Today, the tendency is to use very refined finite element analysis with explicit code to model the complex failure mode following crushing of the Nomex Honeycomb core.…”

“…However, by understanding the structural behavior of the hexagonal cell and by making an analogy with the postbuckling of stiffened structures [22], it is possible to propose simplified discrete models that are very accurate [22][23][24].…”

A Digital Image Correlation Method For Tracking Planar Motions Of Rigid Spheres: Application To Medium Velocity Impacts

“…An accelerometer is sometimes placed on the impactor, but its use is restricted to low velocity impacts performed with drop weight devices [7] and the projectiles cannot be easily instrumented with an accelerometer in the context of medium velocity impacts obtained with gas-guns.…”