Dynamic models for low-velocity impact damage of composite sandwich panels – Part B: Damage initiation

Equivalent single and multi degree-of-freedom systems are used to predict low-velocity impact damage of composite sandwich panels by rigid projectiles. The composite sandwich panels are symmetric and consist of orthotropic laminate facesheets and a core with constant crushing resistance. The transient deformation response of the sandwich panels subjected to impact were predicted in a previous paper, and analytical solutions for the impact force and velocity at damage initiation in sandwich panels are presented in this second paper. Several damage initiation modes are considered, including tensile and shear fracture of the top facesheet, core shear failure, and tensile failure of back facesheet. The impact failure modes are similar to static indentation failure modes, but inertial resistance and high strain rate material properties of the facesheets and core influence impact damage loads. Predicted damage initiation loads and impact velocities compare well with experimental results.     

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.     

Low-velocity impact damage initiation in graphite/epoxy/Nomex honeycomb-sandwich plates

Low-velocity impact and static indentation tests on sandwich plates composed of 4- to 48-ply graphite/epoxy cross-ply laminate facesheets and Nomex honeycomb cores have been performed to characterize damage initiation as a function of facesheet thickness and loading rate. Force histories during low-velocity impact are measured by using an instrumented impactor and integrated to produce energy histories. Energy histories are shown to reveal damage initiation. Static indentation tests show damage that is similar to that produced by low-velocity impact. The force at which damage initiates is shown to be lower for static tests than for low-velocity impact tests, and differences between equilibrium curves for the two types of loading are discussed. The difference between static and low-velocity impact tests is greater for plates with thicker facesheets. This may indicate a limitation of the applicability of the common assumption that low-velocity impact is a quasi-steady process.     

The low velocity impact response of an aluminium honeycomb sandwich structure

The low velocity impact response of two aluminium honeycomb sandwich structures has been investigated by conducting drop-weight impact tests using an instrumented falling-weight impact tower. Initially, the rate-sensitivity of the glass fibre reinforced/epoxy skins and aluminium core was investigated through a series of flexure, shear and indentation tests. Here, it was found that the flexural modulus of the composite skins and the shear modulus of the aluminium honeycomb core did not exhibit any strain-rate sensitivity over the conditions investigated here. In addition, it was found that the indentation characteristics of this lightweight sandwich structure can be analysed using a Meyer indentation law, the parameters of which did not exhibit any sensitivity to crosshead displacement rate.

The impact response of the aluminium honeycomb sandwich structures was modelled using a simple energy-balance model which accounts for energy absorption in bending, shear and contact effects. Agreement between the energy-balance model and the experimental data was found to be good, particularly at low energies where damage was localised to the core material immediate to the point of impact. The energy balance was also used to identify energy partitioning during the impact event. Here, it was shown that the partition of the incident energy depends strongly on the geometry of the impacting projectile.     

Simulation of hyper-velocity impact on double honeycomb sandwich panel and its staggered improvement with internal-structure model

The double honeycomb sandwich panel, which was formed by inserting an intermediate facesheet into single honeycomb core, showed better capability than single honeycomb panel in shielding hyper-velocity impact from space debris. Shielding structures with double honeycomb cores are thoroughly investigated with material point method and point-based internal-structure model. The front honeycomb core and the rear honeycomb core are staggered to obtain better shielding effect. It is found that staggered double honeycomb cores can fragment the debris and lessen impact threats much more than original double honeycomb cores. The sizes of the holes on the rear facesheet are greatly reduced, and the panels are not perforated for some impact velocities. Staggered double honeycomb panels can be adopted as novel effective shielding structures for hyper-velocity impacts.     

Behaviour of sandwich panels subjected to intense air blast – Part 1: Experiments

Tests that investigate the inelastic response of blast-loaded sandwich structures, comprising mild steel plates and aluminium alloy honeycomb cores, are reported. The “uniform” loading was generated by detonating a disc of explosive and directing the blast through a tube towards the target. Localised blast loading was generated by detonating discs of explosive in very close proximity to the test structure. The sandwich panels responded in a more efficient manner to the uniformly distributed loading, and hence the majority of the paper is concentrated on uniform loading response. The honeycomb sandwich results are compared to test results on structures with air as the core. The failure modes and interaction between the components are discussed. Three phases of interaction are identified for each sandwich structure, based upon deformation, contact, crushing and tearing responses of the sandwich components. The compromise between load transfer through the core and improved energy absorption is discussed.     

Experimental and numerical study of compression after impact of sandwich structures with metallic skins

A finite element model is proposed to determine the residual print of sandwich structures with Nomex honeycomb core and metallic skins indented by a spherical indenter and to simulate its behavior when this indented structure is subjected to lateral compressive loading (known as CAI/ Compression after impact). The particularities of this model rely on representing the honeycomb with a grid of non-linear springs which its behavior law calibrated from uniform compression test. This simple model, after integrating the cycle behavior law of honeycomb, allows predicting the geometry of residual print with a good precision. This model is then developed to propose a complete computation from indentation, residual print geometry to lateral compressive loading after indentation (CAI). This model also allows predicting numerically the residual strength of structure in CAI and the elliptical evolution of residual print geometry during CAI loading. A good correlation with test results is obtained except for the very small residual print depth.     

复合材料蜂窝夹芯板低速冲击损伤研究

本文对蜂窝夹芯板试件进行了低速冲击试验, 然后用X 光技术、热揭层技术、断面显微技术和外观检测等对冲击后板的损伤进行了较为全面的研究, 讨论了表面布的作用, 分析了外观损伤、面板损伤、蜂窝损伤等与冲击能量的关系。     

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

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 Nomex™ 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 nonlinear spring located between the impactor and the structure. Thus, it is possible to predict the dynamic structural response in the case of low-velocity/low-energy impact on metal-skinned sandwich structures. A good correlation with dynamic experimental tests is achieved.     

Elastic–plastic response of structural foams subjected to localized static loads

Structural foams are increasingly used in engineering applications where high strength and low weight are important. They are used also as energy absorbers. Sandwich structures are a typical area for application of structural foams (as core materials). In a sandwich structure, the core transfers the transverse forces as shear stresses and supports the face sheets against buckling and wrinkling. The structural foams are notoriously sensitive to failure by the application of localized surface loads. Thus, the proper design requires an understanding of the mechanical response of the foam materials to localized external loads.In this paper, the elastic–plastic behavior of closed-cell cellular foams subjected to point and line loads is investigated both experimentally and numerically. Two types of Divinicell foam (H60 and H100) are studied. A finite element modeling procedure is developed using the ABAQUS package. Both plane and axisymmetric formulations for local indentations by rigid bodies are considered. The plastic behavior is described using the *CRUSHABLE FOAM HARDENING material model. This model is calibrated using experimental curves from uniaxial compression tests. Geometrical non-linearity is also taken into account. Both indentation and unloading phases are modeled. Static indentation tests of foam panels and beams are performed using spherical and cylindrical indentors, respectively. A comparison of indentation response obtained from the numerical analysis and from the tests is carried out. A good agreement between the modeling and the experimental data is achieved. In perspective view, the present investigation can contribute towards the development of a damage tolerance methodology for rigid foams.     

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.     

Fracture mechanics approach to facesheet delamination in honeycomb: measurement of energy release rate of the adhesive bond

Two types of experiments were designed and performed to evaluate the adhesive bond in honeycomb sandwich panels. The tensile bond strength between the facesheet and the core was determined through the flatwise tension test. The fracture toughness of the bond line was measured through the double cantilever beam test. Fracture toughness values varied for different facesheet thicknesses and core materials. Toughness was also different for the bag and tool sides of the panels for all specimen types.     

夹层结构复合材料低速冲击试验与分析

设计了聚甲基丙烯酰亚胺(PMI)泡沫、 交联聚氯乙烯(X-PVC)泡沫、 NOMEX蜂窝、 缝合PMI以及开槽PMI泡沫等形式的玻璃布面板夹层结构复合材料, 研究了芯材种类和厚度、 面板玻璃布层数以及缝合和开槽等因素对夹层结构低速冲击性能的影响。结果表明, PMI泡沫芯较X-PVC泡沫芯和NOMEX蜂窝芯具有更高的冲击破坏载荷和吸收能量。随着泡沫密度及面板厚度的增加, 夹层结构复合材料的冲击破坏载荷和破坏吸收能量增大。合理的缝合和开槽, 能够增加PMI泡沫夹层结构的强度、 刚度及界面性能, 提高冲击承载能力。     

复合材料夹芯板低速冲击后弯曲及横向静压特性

对低速冲击后的复合材料Nomex 蜂窝夹芯板进行了纯弯曲和准静态横向压缩实验, 用X 光技术、热揭层技术和外观检测等对板内的损伤进行测量, 分析了被冲击面在受压情况下蜂窝夹芯板的弯曲破坏特点, 对比了横向静压与低速冲击所造成的板内损伤, 讨论了不同横向压缩速度时接触力P-压入位移$h 的变化规律和损伤情况。结果表明: 低速冲击可使蜂窝夹芯板的弯曲强度大幅度降低; Nomex 蜂窝夹芯板对低速冲击不敏感。      

Simulation of delamination–migration and core crushing in a CFRP sandwich structure

Following the onset of damage caused by an impact load on a composite laminate structure, delaminations often form propagating outwards from the point of impact and in some cases can migrate via matrix cracks between plies as they grow. The goal of the present study is to develop an accurate finite element modeling technique for simulation of the delamination–migration phenomena in laminate impact damage processes. An experiment was devised where, under a quasi-static indentation load, an embedded delamination in the facesheet of a laminate sandwich specimen migrates via a transverse matrix crack and then continues to grow on a new ply interface. Using data from this test for validation purposes, several finite element damage simulation methods were investigated. Comparing the experimental results with those of the different models reveals certain modeling features that are important to include in a numerical simulation of delamination–migration and some that may be neglected.     

Fracture test for double cantilever beam of honeycomb sandwich panels

A modified double cantilever beam (DCB) test geometry is designed to investigate the fracture behavior of honeycomb sandwich panels containing embedded artificial pre-crack, and measure the strain energy release rate of the laminate facesheet/honeycomb cores interface. However, in terms of our DCB fracture test, owing to that the pre-crack does not propagate expectedly along the interface of facesheet/honeycomb core, a new fracture mode, namely IKP (initiation of interlaminar delamination, kinking into facesheet and propagation of interlaminar delamination), has been found.     

Strength evaluations of sinusoidal core for FRP sandwich bridge deck panels

Fiber-reinforced polymer (FRP) sandwich deck panels with sinusoidal core geometry have shown to be successful both in new construction and the rehabilitation of existing bridge decks. This paper is focused on an experimental study of the strength evaluations of a honeycomb sandwich core under out-of-plane compression and transverse shear. The sinusoidal core is made of E-glass Chopped Strand Mat (ChSM) and Polyester resin. The compressive, tensile and shear strengths were first obtained from coupon tests. The out-of-plane compression tests were performed on representative single-cell volume elements of sandwich panels, and the tests included “stabilized” samples to induce compression failure, and “bare” samples to induce local buckling of the core. Finally, four-point bending tests were conducted to study the structural strength behavior under transverse shear. Two types of beam samples were manufactured by orienting the sinusoidal wave either along the length (longitudinal) or along the width (transverse). Both typical shear failure mode of the core material and delamination at the core–facesheet bonding interface were observed for longitudinal samples. The failure for transverse samples was caused by core panel separation. For both single-cell and beam-type specimen tests, the number of bonding layers, i.e., the amount of ChSM contact layer and resin used to embed the core into the facesheet, and the core thickness are varied to study their influence. The experimental results described herein can be subsequently used to develop design guidelines.     

Compressive behavior of C/SiC composite sandwich structure with stitched lattice core

C/SiC composite sandwich structure with stitched lattice core was fabricated by a technique that involved polymer impregnation and interweaving. The mechanical behaviors of C/SiC composite sandwich structure were investigated at room temperature. The out-of-plane compressive strength was 20.97 MPa while modulus was 1473.55 MPa. The microstructural evolution on compression fracture surfaces of the stitching yarns was investigated by scanning electron microscopy, and the damage pattern of fibers on compression fracture surface was presented and discussed. Under an in-plane compression loading, the C/SiC composite sandwich structure displayed a linear-elastic behavior until failure. The peak strength and average modulus are 165.61 MPa and 19.74 GPa, respectively. The failure of the specimen was dominated by the fracture of the facesheet.     

Analysis of debonding fracture in a sandwich plate with hexagonal core

In lightweight applications (as, e.g., aerospace structures) sandwich constructions are very useful and common due to their superior specific bending stiffness and bending strength. In many cases the sandwich consists of an upper and lower laminate facesheet and an intermediate hexagonal cellular aluminum core. Along their interfaces the facesheets and the core are glued together. In order to ensure structural integrity, the facesheet/core bonding is of particular interest. Finite element method has been used to study the cause and the effects of debonding phenomena in between the facesheet and the core of a sandwich plate under in-plane loading. A “unit cell” approach has been followed throughout the study. It has been observed that under an applied in-plane loading, there is a significant stress concentration at the junction of three cell walls and facesheet which easily leads to the generation of cracks and their growth. In order to judge about the tendency of crack initiation and growth, hypothetical interface cracks have been considered and analyzed by fracture mechanics technique. In doing so for various crack length, the energy release rate has been calculated and assessed by means of Irwin’s crack closure integral for a number of different situations. It has been observed that there is a significant amount of energy release rate even in the case of a very small or virtually no crack. This phenomenon indicates that the glue used to attach the facesheet and the cell must withstand a non-zero energy release rate even in the intact situation without any debonding.     

Enhancement of through-thickness thermal conductivity of sandwich construction using carbon foam

As a thermal management system, a sandwich construction was developed to have both superior thermal conductivity and structural integrity. The sandwich construction consists of a carbon foam core and unidirectional graphite/epoxy composite facesheets. An emphasis was put on enhancing the thermal conductivity of each phase of sandwich construction as well as interface between the phases. A commercially-available carbon foam was characterized mechanically and thermally. Property variation and anisotropy were observed with the highly conductive graphitic carbon foam. Co-curing of the composite facesheets with the carbon foam core was demonstrated to minimize the thickness of the adhesive layer between the facesheets and the core to produce the best construction of those tested. Comparison made with an adhesively bonded specimen shows that the co-curing is a more efficient method to enhance the through-thickness conductivity. Parametric studies with an analytic model indicate that degree of enhancement in the overall through-thickness conductivity of the sandwich construction from the enhancement of each component including the foam core, facesheet and the bonding methods.