Deformation and energy absorption of composite egg-box panels

Static compressive tests of composite egg-box panels, whose stacking sequences and number of plies were controlled, were carried out to investigate their deformation behaviour and energy absorption capacity. Silicon rubber moulds were first moulded from an aluminium egg-box panel template. These moulds were in turn used to fabricate composite specimens. Two fabric prepregs, carbon/epoxy plain weave fabric and glass/epoxy 4-harness satin weave were draped over the rubber mould with various stacking angles. The specimens were cured in an autoclave using vacuum bag degassing moulding and an appropriate cure cycle. The nominal stress–strain relations of the specimens were compared and multiply-interrupted compressive tests were used to identify fracture initiation and development. The energy absorption per unit mass of composite egg-box panels were compared with that of an aluminium egg-box panel. From the test results it was concluded that the compressive behaviour of the composite structure is affected by the local stacking sequence of the fabrics and by shear deformation during initial lay-up and draping. By considering the stress–stain behaviour, energy absorption and material cost, the optimal material and draping condition were proposed for a composite egg-box panel.     

Impact behavior and energy absorption of paper honeycomb sandwich panels

Dynamic cushioning tests were conducted by free drop and shock absorption principle. The effect of paper honeycomb structure factors on the impact behavior was analyzed. Results of many experiments show that the dynamic impact curve of paper honeycomb sandwich panel is concave and upward; the thickness and length of honeycomb cell-wall have a great effect on its cushioning properties; increasing the relative density of paper honeycomb can improve the energy absorption ability of the sandwich panels; the thickness of paper honeycomb core has an up and down fluctuant effect on the cushioning properties; with the increase of the thickness of paper honeycomb core, the effect dies down; flexible corrugated paperboard as liners can improve the compression resistance and cushioning properties of paper honeycombs. The research results can be used to optimize the structure design of paper honeycomb sandwich panel and material selection for packaging design.     

Crushing energy absorption of GFRP sandwich panels and corresponding monolithic laminates

Steady quasi-static compression of GFRP monolithic laminates and sandwich panels made of a randomly oriented continuous filament mat/polyester were undertaken. The effects of facing/laminate thickness, trigger collapse system and aspect ratio on their failure mechanisms, hence their energy absorption capability were examined. A numerical model, using a non-linear finite element explicit code, LS-DYNA, was used for pre-analysis of the effect of aspect ratio. A collapse trigger configuration was also studied numerically. The experimental data showed that high values of energy absorbed per unit mass were a predominant feature of the thickest monolithic laminates and sandwich panels with the thickest facings. The monolithic laminates showed higher specific energy than their sandwich panel counterparts. It seems that this difference was due to instability of the sandwich specimens.     

Investigation of bond strength and energy absorption capabilities in recyclable sandwich panels

The paper investigates the energy absorption characteristics of honey comb sandwich panels manufactured from recyclable materials. In this study, honeycomb cores were manufactured from sawdust-polypropylene composites. The composites were thermoformed between matched-dies into half hexagonal profiles which were ultrasonically bonded at their nodes and facesheets to form recyclable sandwich panels. As the bonding efficiency at the nodes and the core-to-facing interface will be governed by several process parameters, an L₈ partial factorial and full factorial design of experiments (DoE) based on the Taguchi method was used. Single lap shear test was used to determine the shear strength of the bond at the nodes, and climbing drum peel test was used to determine the skin-to-core bonding strength in the sandwich panels. The energy absorption characteristic of the honeycombs was determined experimentally by subjecting them to quasi-static compressive loads. The effect of the thickness-to-length ratio (t/l) of the cell walls on the energy absorption was evaluated using finite element (FE) modelling, and energy absorption maps were constructed for various t/l ratios. Taguchi analysis suggested setting all the parameters at their highest levels to obtain maximum lap shear strength at the nodes and indicated that the hold time has negligible influence, following which a L₈ full factorial DoE based on orthogonal arrays was used to maximise the peel load, neglecting the hold time parameter. The results indicated a minor interaction between the welding amplitude and welding time, indicating the increase in welding amplitude should be complimented with the welding time. However, this interaction was marginal compared to the main factor effects. All the panels at different levels failed at the core-to-face interface but the highest peel load values were exhibited by specimens that were welded with amplitude of 22.5 μm, a welding distance of 50% of the facing thickness from the welding interface and a welding time of 1 s. Though the panels exhibited approximately 30% increase in shear strength values compared to the unreinforced cores, their specific shear strength was similar to those of the unreinforced ones (8 × 10³ N m/kg). However, these were achieved at much lower cell densities or t/l values (0.07). The energy absorption diagrams revealed an increasing trend in energy absorption per unit volume with increasing t/l ratio of the honeycomb.     

Impact and post impact behavior of composite sandwich panels

Assessing the residual mechanical properties of a sandwich structure is an important part of any impact study and determines how the structure can withstand post impact loading. The damage tolerance of a composite sandwich structure composed of woven carbon/epoxy facesheets and a PVC foam core was investigated. Sandwich panels were impacted with a falling mass from increasing heights until damage was induced. Impact damage consisted of delamination and permanent indentation in the impacted facesheets. The Compression After Impact (CAI) strength of sandwich columns sectioned from these panels was then compared with the strength of an undamaged column. Although not visually apparent, the facesheet delamination damage was found to be quite detrimental to the load bearing capacity of the sandwich panel, underscoring the need for reliable damage detection techniques for composite sandwich structures.     

Prediction of impact damage on sandwich composite panels

Sandwich structures are extensively employed in the aerospace and automobile industries. The understanding of their behaviour under impact conditions is extremely important for the design and manufacturing of these engineering structures since impact problems are directly related to structural integrity and safety requirements. This paper investigates the damage behaviour of composite sandwich panels with aramid paper honeycomb (NOMEX) and polyetherimide (PEI) foam cores under transverse impacts at high velocities. A numerical model was developed using the dynamic explicit finite element (FE) structure analysis program PAM-CRASH. For both sandwich structures numerical analysis reproduces physical behaviour observed experimentally in high velocity impact tests.     

Mechanical behaviour of cellular core for structural sandwich panels

This paper deals with the analysis of the mechanical properties of the core materials for sandwich panels. In this work, the core is firstly a honeycomb and secondly tubular structure. This kind of core materials are extensively used, notably in automotive construction (structural components, load floors...). For this study, three approaches are developed: a finite element analysis, an analytical study and experimental tests. Structural members made up of two stiffs, strong skins separated by a lightweight core (foam, honeycomb, tube...) are known as sandwich panels. The separation of the skins by the core increases the inertia of the sandwich panel, the flexure and shear stiffness. This increase is obtained with a little increase in weight, producing an efficient structure to resist bending and buckling loads. A new analytical method to analyse sandwich panels core will be presented. These approaches (theoretical and experimental) are used to determine elastic properties and ultimate stress. A parameter study is carried out to determine elastic properties as a function of geometrical and mechanical characteristics of basic material. Both theoretical and experimental results are discussed and a good correlation between them is obtained.     

Compression and impact testing of two-layer composite pyramidal-core sandwich panels

Quasi-static uniform compression tests and low-velocity concentrated impact tests were conducted to reveal the failure mechanisms and energy absorption capacity of two-layer carbon fiber composite sandwich panels with pyramidal truss cores. Three different volume-fraction cores (i.e., with different relative densities) were fabricated: 1.25%, 1.81%, and 2.27%. Two-layer sandwich panels with identical volume-fraction cores (either 1.25% or 2.27%), and also stepwise graded panels consisting of one light and one heavy core, were investigated under uniform quasi-static compression. Under quasi-static compression, load peaks were identified with complete failure of individual truss layers due to strut buckling or strut crushing, and specific energy absorption was estimated for different core configurations. In the impact test, the damage resulting from low-velocity concentrated impact was investigated. Our results show that compared with glass fiber woven textile truss cores, two-layer carbon fiber composite pyramidal truss cores have comparable specific energy absorptions, and thus could be used in the development of novel light-weight multifunctional structures.     

Flexural impact response of textile-reinforced aerated concrete sandwich panels

Mechanical response of textile-reinforced aerated concrete sandwich panels was investigated using an instrumented three-point bending experiment under static and low-velocity dynamic loading. Two types of aerated concrete: autoclaved aerated concrete (AAC) and polymeric Fiber-Reinforced Aerated Concrete (FRAC) were used as the core material. Skin layer consisted of two layers of Alkali Resistant Glass (ARG) textiles and a cementitious binder. Performance of ductile skin-brittle core (TRC–AAC) and ductile skin-ductile core (TRC–FRAC) composites was evaluated in terms of flexural stiffness, strength, and energy absorption capacity. The effect of impact energy on the mechanical properties was measured at various drop heights on two different cross-sections using energy levels up to 40 J and intermediate strain rates up to 20 s^− 1. The externally bonded textile layers significantly improved the mechanical properties of light-weight low-strength aerated concrete core under both loading modes. Dynamic flexural strength was greater than the static flexural strength by as much as 4 times. For specimens with larger cross-sections, unreinforced-autoclaved AAC core had a 15% higher apparent flexural capacity. With 0.5% volume of polypropylene fibers in the core, the flexural toughness however increased by 25%. Cracking mechanisms were studied using high speed image acquisition and digital image correlation (DIC) technique.     

The low velocity impact response of foam-based sandwich panels

This paper describes the results of a combined experimental/numerical study to investigate the perforation resistance of sandwich structures. The impact response of plain foam samples and their associated sandwich panels was characterised by determining the energy required to perforate the panels. The dynamic response of the panels was predicted using the finite element analysis package ABAQUS/Explicit. The experimental arrangement, as well as the FE model were also used to investigate, for the first time, the effect of oblique loading on sandwich structures and also to study the impact response of sandwich panels on an aqueous support.     

Dynamic models for low-velocity impact damage of composite sandwich panels – Part A: Deformation

Equivalent single and multi degree-of-freedom systems are used to predict the low-velocity impact response of rigidly supported, two-sided clamped, simply supported and four-sided clamped composite sandwich panels. The composite sandwich panels have orthotropic facesheets and are symmetric. Analytical solutions for the transient deformation response of the sandwich panels are presented in this paper, and analytical predictions of impact damage initiation are given in a companion paper. Equivalent masses are derived by assuming velocity distributions and calculating average kinetic energies (KEs) in terms of the amplitude of the top facesheet indentation and the global panel deflection. Equivalent spring and dashpot resistances are derived from the static load–indentation response and adjusted with dynamic material properties of the facesheet and core. Analytical predictions of the impact force compare well with experimental values from three independent studies.     

Experimental evaluation of the crush energy absorption of triggered composite sandwich panels under quasi-static edgewise compressive loading

An experimental evaluation of the energy absorption of constrained triggered sandwich structures is presented. Four configurations of embedded ply-drop triggering mechanisms were analysed and compared. A composite pi-joint was then developed to provide a constraint fixture that is representative of an in-service integrated energy absorbing structure. The specimens tested within the pi-joints obtained slightly lower specific energy absorption compared to the equivalent specimens tested in a rigid test fixture. The potential of an integrated energy absorbing triggered sandwich structure contained within a composite pi-joint was demonstrated. The interface between the sandwich panel and the pi-joint was not bonded, and further research will focus on the development of a bonded joint configuration suitable for structural applications.     

Modelling of aluminium foam core sandwich panels under impact perforation

Aluminium foam core sandwich panels are good energy absorbers for impact protection applications, such as light-weight structural panels, packing materials and energy absorbing devices. In this study, the high-velocity impact perforation of aluminium foam core sandwich structures was analysed. Sandwich panels with 1100 aluminium face-sheets and closed-cell A356 aluminium alloy foam core were modelled by three-dimensional finite element models. The models were validated with experimental tests by comparing numerical and experimental damage modes, output velocity, ballistic limit and absorbed energy. By this model the influence of foam core and face-sheet thicknesses on the behaviour of the sandwich panel under impact perforation was evaluated.     

Damage prediction in composite sandwich panels subjected to low-velocity impact

The paper illustrates the application of a finite element tool for simulating the structural and damage response of foam-based sandwich composites subjected to low-velocity impact. Onset and growth of typical damage modes occurring in the composite skins, such as fibre fracture, matrix cracking and delaminations, were simulated by the use of three-dimensional damage models (for intralaminar damage) and interfacial cohesive laws (for interlaminar damage). The nonlinear behaviour of the foam core was simulated by a crushable foam plasticity model. The FE results were compared with experimental data acquired by impact testing on sandwich panels consisting of carbon/epoxy facesheets bonded to a PVC foam. Good agreement was obtained between predictions and experiments in terms of force histories, force–displacement curves and dissipated energy. The proposed model was also capable of simulating correctly nature and size of impact damage, and of capturing the key features of individual delaminations at different depth locations.     

Finite element modelling of low velocity impact of composite sandwich panels

This paper outlines a finite element procedure for predicting the behaviour under low velocity impact of sandwich panels consisting of brittle composite skins supported by a ductile core. The modelling of the impact requires a dynamic analysis that can also handle non-linearities caused by large deflections, plastic deformation of the core and in-plane degradation of the composite skins. Metal honeycomb, frequently used as a core material, is anisotropic and requires a non-standard approach in the elasto-plastic part of the analysis. A suitable yield criteria based on experimental observations is proposed. Comparisons of experimental and finite element responses are shown for sandwich panels with carbon fibre skins and aluminium honeycomb cores.     

Ballistic limit for oblique impact of thin sandwich panels and spaced plates

Ballistic perforations of monolithic steel sheets, two-layered sheets and lightweight sandwich panels were investigated both experimentally and numerically. The experiments were performed using a short cylindrical projectile with either a flat or hemispherical nose that struck the target plate at an angle of obliquity. A total of 170 tests were performed at angles of obliquity 0–45°. The results suggest that during perforation by a flat-nosed projectile, layered plates cause more energy loss than monolithic plates of the same material and total thickness. There was no significant difference in the measured ballistic limit speed between monolithic plates and layered plates during oblique impact perforation by a hemispherical-nosed projectile.     

明胶鸟弹撞击复合材料蜂窝夹芯板试验

开展明胶鸟弹撞击复合材料蜂窝夹芯板试验,研究夹芯结构在软体高速冲击下的损伤形式,分析相关因素对结构动态响应结果的影响。通过CT扫描对复合材料蜂窝夹芯板内部进行检测可知,面板出现分层、基体开裂、纤维断裂、凹陷、向胞内屈曲等损伤形式,蜂窝芯出现芯材压溃、与面板脱粘的损伤形式;分析复合材料蜂窝夹芯板后面板的动态变形过程及撞击中心处位移-时间数据可知,复合材料蜂窝夹芯板在撞击过程中出现由全局弯曲变形主导和局部变形主导的两种变形模式;通过对比不同工况下的复合材料蜂窝夹芯板损伤程度可知,复合材料蜂窝夹芯板损伤程度随鸟弹撞击速度的增加而增大;蜂窝芯高度为10 mm的复合材料蜂窝夹芯板较蜂窝芯高度为5 mm的复合材料蜂窝夹芯板的损伤程度大;初始动能较大的球形鸟弹较圆柱形鸟弹对复合材料蜂窝夹芯板造成的冲击损伤程度更大。      

Ballistic impact experiments of metallic sandwich panels with aluminium foam core

Metallic sandwich structures with aluminium foam core are good energy absorbers for impact protection. To study their ballistic performance, quasi-static and impact perforation tests were carried out and the results are reported and analysed in this paper. In the experiments, effects of several key parameters, i.e. impact velocity, skin thickness, thickness and density of foam core and projectile shapes, on the ballistic limit and energy absorption of the panels during perforation are identified and discussed in detail.     

Dynamic response of aluminum honeycomb sandwich panels under foam projectile impact

The dynamic response of honeycomb sandwich panels under aluminum foam projectile impact was investigated. The different configurations of panels were tested, and deformation/failure modes were obtained. Corresponding numerical simulations were also presented to investigate the energy absorption and deformation mechanism of sandwich panels. Results showed that the deformation/failure modes of sandwich panels were sensitive to the impact velocity and density of aluminum foam. When the panel was impacted by the aluminum foam projectile with the back mass of nylon, the “accelerating impact” stage can be produced and may lead to further compression and damage of the sandwich structures.     

Energy absorption of SMC/balsa sandwich panels with geometrical triggering features

The influence of triggering topologies on the peak load and energy absorption of sandwich panels loaded in in-plane compression is investigated. Sandwich panels with different geometrical triggering features are manufactured and tested experimentally. Damage initiation in panels with grooves is investigated using finite element models.