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.     

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.     

Optimization of composite sandwich panel against impact using genetic algorithm

The present paper deals with the optimization of composite sandwich panels subjected to low-velocity impact using advanced genetic algorithm (GA). Equivalent multi degree-of-freedom system is used to predict the contact force history and deformation response of the sandwich panels. Also, analytical solutions are used to determine the impact force and velocity at damage initiation. The GA's objective is to maximize the strength of panel by varying the ply angles of the facesheets. The results show that the stacking sequence of facesheets plays an important role in the strength of the composite sandwich panels subjected to impact loading.     

Indentation study of Z-pin reinforced polymer foam core sandwich structures

Z-pin reinforced foam core sandwich panels with composite face sheets, supported on a rigid base and subjected to quasi-static indentation using spherical indenter was studied in this paper. The effects of configurations of Z-pin, including inclination angle and pinning density, on the load–indentation response were studied, and the resulting damage modes were investigated. The effect of inclination angle of pin on the load–indentation behavior is not notable compared with those of Z-pinning density and Z-pin configuration. The collapse of Z-pinned foam core is due to the buckling of pin, and the pin buckling is significantly dependent on the location of indenter. An approximate solution was developed based on the principle of minimum potential energy to simulate the indentation damage response of Z-pin reinforced foam core sandwich. The analytical predictions compare well with the experimental results.     

“Segment-wise model” for theoretical simulation of barely visible indentation damage in composite sandwich beams: Part II – Experimental verification and discussion

When localized transverse loading is applied to a sandwich structure, the facesheet locally deflects and the core crushes. A residual dent induced by the core crushing significantly degrades the mechanical properties of the sandwich structure. In a previous paper, the authors established a “segment-wise model” for theoretical simulation of barely visible indentation damage in honeycomb sandwich beams with composite facesheets. Honeycomb sandwich beam was divided into many segments based on the periodic shape of the honeycomb and complicated through-thickness characteristics of the core were integrated into each segment. In this paper, the new model is validated by experiments using specimens with different types of honeycomb cores. In addition, the damage growth mechanism under indentation load was clarified from the viewpoint of the reaction force from the core to the facesheet. The applicability of the model to other types of core materials is also discussed.     

Compression-after-Impact Strength of Sandwich Panels with Core Crushing Damage

Compression-after-impact (CAI) strength of foam-cored sandwich panels with composite face sheets is investigated experimentally. The low-velocity impact by a semi-spherical (blunt) projectile is considered, producing a damage mainly in a form of core crushing accompanied by a permanent indentation (residual dent) in the face sheet. Instrumentation of the panels by strain gauges and digital speckle photography analysis are used to study the effect of damage on failure mechanisms in the panel. Residual dent growth inwards toward the mid-plane of a sandwich panel followed by a complete separation of the face sheet is identified as the failure mode. CAI strength of sandwich panels is shown to decrease with increasing impact damage size. Destructive sectioning of sandwich panels is used to characterise damage parameters and morphology for implementation in a finite element model. The finite element model that accounts for relevant details of impact damage morphology is developed and proposed for failure analysis and CAI strength predictions of damaged panels demonstrating a good correlation with experimental results.     

Collapse of clamped and simply supported composite sandwich beams in three-point bending

Composite sandwich beams, comprising glass–vinylester face sheets and a PVC foam core, have been manufactured and tested quasi-statically. Clamped and simply supported beams were tested in three-point bending in order to investigate the initial collapse modes, the mechanisms that govern the post-yield deformation and parameters that set the ultimate strength of these beams. Initial collapse is by three competing mechanisms: face microbuckling, core shear and indentation. Simple formulae for the initial collapse loads of clamped and simply supported beams along with analytical expressions for the finite deflection behaviour of clamped beams are presented. The simply supported beams display a softening post-yield response, while the clamped beams exhibit hardening behaviour due to membrane stretching of the face sheets. Good agreement is found between the measured, analytical and finite element predictions of the load versus deflection response of the simply supported and clamped beams. Collapse mechanism maps with contours of initial collapse load and energy absorption are plotted. These maps are used to determine the minimum mass designs of sandwich beams comprising woven glass face sheets and a PVC foam core.     

Perforation of clamped,woven E-glass/polyester panels

Analytical solutions to predict the perforation of fully clamped, woven E-glass/polyester panels subjected to static indentation by a blunt cylinder were derived. Three panel aspect ratios of 12.5, 25 and 50 were considered in order to show how modes of failure vary with panel flexibility. The analytical solutions for the deformation and interlaminar shear stresses compared well to finite element predictions using ABAQUS. It was found that the panel with an aspect ratio of 50 underwent large deformation and deformed in a membrane phase before fracture due to tensile necking. Failure of panels with aspect ratios of 12.5 and 25 was predicted using ply-by-ply failure analysis and the Tsai–Wu failure criterion.     

Localized indentation of sandwich panels with metallic foam core: Analytical models for two types of indenters

Sandwich structures with metallic foam core are sensitive to local indentation because of the low strength of the core and low bending stiffness of the thin face sheets. In this paper, local indentation response of sandwich panels with metallic foam core under a flat/spherical indenter was analyzed. The composite sandwich is modeled as an infinite, isotropic, plastic membrane on a rigid-plastic foundation. For simplicity, a quadratic polynomial displacement field was employed to describe the deformation of the upper face sheet. By using the principle of minimum work, explicit solutions for the indentation force and the sizes of the deformation regions were derived. The analytical results were verified by those from simulation by using the ABAQUS code, and they are in close agreement. Distribution of radial tensile strain of the upper face sheet and the ratio of energy dissipation of foam core to that of the upper face sheet were analyzed.     

Quasi-static indentation tests on aluminium foam sandwich panels

Mechanical response and energy absorption of aluminium foam sandwich panels subjected to quasi-static indentation loads were investigated experimentally. These sandwich panels consisted of two aluminium face-sheets and a closed cell aluminium foam core (ALPORAS®). Quasi-static indentation tests were conducted using an MTS universal testing machine, with sandwich panels either simply supported or fully fixed. Force–displacement curves were recorded and the total energy absorbed by sandwich panels was calculated accordingly. Videos and photographs were taken to capture the deformation of top face-sheets, foam cores and bottom face-sheets. Effects of face-sheet thickness, core thickness, boundary conditions, adhesive and surface condition of face-sheets on the mechanical response and energy absorption of sandwich panels were discussed.     

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.     

High Temperature Residual Properties of Carbon Fiber Composite Sandwich Panel with Pyramidal Truss Cores

A study on the mechanical property degradation of carbon fiber composite sandwich panel with pyramidal truss cores by high temperature exposure is performed. Analytical formulae for the residual bending strength of composite sandwich panel after thermal exposure are presented for possible competing failure modes. The composite sandwich panels were fabricated from unidirectional carbon/epoxy prepreg, and were exposed to different temperatures for different time. The bending properties of the exposed specimens were measured by three-point bending tests. Then the effect of high temperature exposure on the bending properties and damage mechanism were analyzed. The results have shown that the residual bending strength of composite sandwich panels decreased with increasing exposure temperature and time, which was caused by the degradation of the matrix property and fiber-matrix interface property at high temperature. The effect of thermal exposure on failure mode of composite sandwich panel was observed as well. The measured failure loads showed good agreement with the analytical predictions. It is expected that this study can provide useful information on the design and application of carbon fiber composite sandwich panel at high temperature.     

Enhanced mechanical performance of foam core sandwich composites with through the thickness reinforced core

The purpose of this study is to improve the mechanical performance of the foam core sandwich composites with a rather simpler method of core reinforcement. With this aim; sandwich composite panels are manufactured using only-perforated foam and perforated-stitched foam as the core with multi-axial glass fabrics as the facesheet materials by vacuum infusion method using epoxy resin. Sandwich composites with perforated core, stitched core and plain core have been compared in terms of compressive, bending, shear and impact performances. It was seen that newly proposed perforated core specimens and stitched core specimens with relatively insignificant weight increase have superior mechanical performances than plain core specimens. Thus reinforcing foam core with perforation and stitching is proposed as simpler but very effective method in performance improvement for the sandwich composites.     

The contact behavior between a foam core sandwich plate and a rigid indentor

The purpose of this paper is to determine the indentation produced by a rigid indentor falling on a sandwich plate with a height–density closed cellular foam core. Two different indentor shapes are considered: a rigid sphere and a cylindrical rigid punch. By assuming that the time of complete indentation of the indentors is much larger than the time required by the elastic waves to propagate from the point of first contact to the boundary of the plate, a static analysis is performed. A distribution of surface pressures reproducing the contact law of the rigid indentor on an elastic half-space is introduced and the sandwich plate theory proposed by Dundrová, Kovarı́k and Slapák (1970) is adopted; the explicit solutions with pre-assigned surface pressures are obtained and the contact force–indentation relations, before the sandwich plate is damaged, are found. The contact laws are obtained for both a simply supported and a clamped circular sandwich plate and the relevant influence of boundary conditions on the elastic response of the sandwich plate is shown.     

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。      

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。      

Mechanical behavior of carbon fiber reinforced polymer composite sandwich panels with 2-D lattice truss cores

Composite sandwich structures with lattice truss cores are attracting more and more attention due to their superior specific strength/stiffness and multi-functional applications. In the present study, the carbon fiber reinforced polymer (CFRP) composite sandwich panels with 2-D lattice truss core are manufactured based on the hot-pressing method using unidirectional carbon/epoxy prepregs. The facesheets are interconnected with lattice truss members by means of that both ends of the lattice truss members are embedded into the facesheets, without the bonding procedure commonly adopted by sandwich panels. The mechanical properties of the 2-D lattice truss sandwich panels are investigated under out-of-plane compression, shear and three-point bending tests. Delamination of the facesheets is observed in shear and bending tests while node failure mode does not occur. The tests demonstrate that delamination of the facesheet is the primary failure mode of this sandwich structure other than the debonding between the facesheets and core for conventional sandwiches.     

Low-velocity heavy-mass impact response of slender metal foam core sandwich beam

The objective of this work is to investigate the dynamic large deflection response of fully clamped metal foam core sandwich beam struck by a low-velocity heavy mass. Analytical solution and ‘bounds’ of dynamic solutions are derived, respectively. Also, finite element analysis is carried out to obtain the numerical solution of the problem. Comparisons of the dynamic, the quasi-static and numerical solutions for the non-dimensional maximum deflection of the sandwich beam with non-dimensional initial kinetic energy of the striker are presented for different cases of mass ratio, impact velocity and location. It is seen that the dynamic solution approaches the quasi-static one as the mass ratio of the striker to the beam is large enough, the quasi-static solution is in good agreement with the numerical results and both solutions lie in the ‘bounds’ of dynamic solutions. The quasi-static and numerical results for the impact force against the maximum deflection of the sandwich beam are obtained. It shows that the quasi-static solution can offer adequate accuracy to predict the low-velocity heavy-mass impact response of fully clamped sandwich beam.     

Experimental and numerical investigation of carbon fiber sandwich panels subjected to blast loading

The objective of this paper is to investigate the structural response of carbon fiber sandwich panels subjected to blast loading through an integrated experimental and numerical approach. A total of nine experiments, corresponding to three different blast intensity levels were conducted in the 28-inch square shock tube apparatus. Computational models were developed to capture the experimental details and further study the mechanism of blast wave-sandwich panel interactions. The peak reflected overpressure was monitored, which amplified to approximately 2.5 times of the incident overpressure due to fluid-structure interactions. The measured strain histories demonstrated opposite phases at the center of the front and back facesheets. Both strains showed damped oscillation with a reduced oscillation frequency as well as amplified facesheet deformations at the higher blast intensity. As the blast wave traversed across the panel, the observed flow separation and reattachment led to pressure increase at the back side of the panel. Further parametric studies suggested that the maximum deflection of the back facesheet increased dramatically with higher blast intensity and decreased with larger facesheet and core thickness. Our computational models, calibrated by experimental measurements, could be used as a virtual tool for assessing the mechanism of blast-panel interactions, and predicting the structural response of composite panels subjected to blast loading.     

The use of a controlled multiple quasi-static indentation test to characterise through-thickness penetration of composite panels

During the quasi-static indentation of thin composite panels, well-defined flaps (sometimes called “petals”) can develop on the exit face as a consequence of through-thickness penetration of the panel; such flaps can also be seen in impact tests. The flaps develop as four triangles, with the apex of each triangle at the point of impact. In this work, thin panels of CFRP with a 0/90 configuration have been subjected to quasi-static indentation tests and the development of the flaps has been monitored. The results show that the dependence of the flap compliance is proportional to the square of the flap length, which is in agreement with theoretical predictions. The determination of the compliance/crack-length relationship enables a toughness value for fracture of the composite panel to be derived that is directly relevant to through-thickness penetration of the panel.