Core Crushing Research Articles

Low velocity impact behaviour of glass fabric/epoxy honeycomb core sandwich composites

Abstract In this paper, the low velocity impact characteristics of honeycomb sandwich panels with varying cell sizes such as the maximum load absorbed, absorbed energy and failure modes were investigated. Low-velocity impact tests were carried out at different energy levels using an instrumented drop-weight apparatus as per ASTM D3029-95. Honeycomb core sandwich composite panels were made with glass fabric/epoxy honeycomb core with different cell sizes i.e., 8, 12, 16 and 20 mm using the vacuum bag molding technique. Tests were conducted at impact energies that ranged ranging from 7 to 50 J. The energy absorption mechanism and the damage process are analysed with parameters derived from the load-time and energy-time curves. It is observed that at low energy levels, the core buckles while it gets crushed at higher levels. This buckling of the core is due to damage to the epoxy resin that allows the glass fiber to bend freely, and core crushing is due to the breakage of the glass fiber reinforcement within the core. It is inferred that for a given core height, as the cell size increases, the peak load at which cracking occurs in the face sheet decreases drastically. This is attributed to the fact that a core with a smaller cell size has fewer load paths to distribute the absorbed energy. It is evident from the post impact damage analysis that three types of failure modes occur viz., crack initiation, cracking of the face sheet along with debonding, and the crushing of cells. The common damage pattern observed on the front facing has a square shape which is the same as the shape of the indentor.

Root Causes Analysis of Disbonds and Unidentifiable Ultrasonic Indications on Composite Materials

To ensure the structural integrity of composite materials, honeycomb sandwich panels must be inspected for defects with through-transmission ultrasonic testing. This study focused on the root causes analysis of disbonds and unidentifiable ultrasonic indications on composite materials. Disbonds is a condition that lack of adhesion between ply and core. Unidentifiable ultrasonic indications define as an unknown defect which can only identified after remove the peel ply. In this study, Minitab 16 and SPSS 16 are used in analyzing the potential causes that influences the disbonds and unidentifiable ultrasonic indications. The root causes of disbonds are core crush and core crack. Core crush and core crack may caused by high localized loads or high compaction pressure. For unidentifiable ultrasonic indications, the root cause is adhesive flood at core. Imperfect compaction pressure leads to the uneven flow of adhesive. For the area have lower pressure, probability for adhesive to accumulate is higher. Pleated or dog ear method can be implemented in bagging process to ensure better compaction mechanism. Operators must be trained and educated for better performance.

Damage Behaviors of Foam Sandwiched Composite Materials Under Quasi-Static Three-point Bending

This paper reports the quasi-static three-point bending damage behaviors of foam sandwiched composites in finite element analyses (FEA) and experimental. Finite element calculations were performed to characterize the static response of foam sandwich composites with different ply angle face sheets. Quasi-static three-point bending tests were conducted with a MTS materials testing system to obtain the load–displacement curves and energy absorption under quasi-static bending. A crushable foam model was used in order to explore the mechanical behaviors of core materials, while the Hashin criterion was employed to predict the failure of the face sheets. The load–displacement curves show a satisfactory agreement between the experimental and numerical results. The finite element calculations can also be used to obtain the failure mode included the core damage, face sheet damage and face-core interface damage. It can be observed that the damage at the core material can be classified as either core cracking or core crushing. The damage of the face sheet was through matrix cracking and delamination, with fiber breakage. The significant indentation occurs as a result of the fiber breakage. The face-core interface crack was typically induced by the cracks initiated from the tensile side and propagated to the compressive side.

Foam-core, curved composite sandwich panels under blast

An elastic-plastic model was developed for predicting the blast response of a foam-core, curved composite sandwich panel. A multi-layered approach was used to distinguish facesheets and core deformations. Core compressibility and transverse shear through the thickness were accounted for using linear displacement fields through the thickness. The predicted solution from the elastic-plastic model was shown to compare well with FEA from ABAQUS Implicit. A parametric study showed that blast resistance of the sandwich shell is increased by allowing cores to undergo plastic crushing. Very thick and shallow shells derive much of their resistance to blast from core crushing. Strong, dense foam cores did not increase the blast resistance of the curved sandwich panel but allowed facesheets to fracture while the core remained elastic.

Damage and failure mode maps of composite sandwich panel subjected to quasi-static indentation and low velocity impact

In this contribution, modes of damage and modes of failure of composite sandwich panel subject to quasi-static indentation and low velocity impact were investigated. This study was conducted through the use of the finite element analysis, existing formulae and experiments. The damage and failure modes of composite sandwich panels were analyzed with some deliberation. With these results, damage and failure mode maps were compiled to facilitate the design of composite sandwich structures. Existing formulae that predict limit loads for damage modes which includes matrix shear/tension damage and core buckling, were reviewed and compiled to plot the damage mode map. The failure modes of facesheet cracking, overall core crushing and core shear fracture, were investigated to configure the failure mode map. Preferred damage and failure modes for impact resistance were also discussed in term of energy absorption. Extensive quasi-static indentation and low-velocity impact tests were carried out to verify these modes. The results of the finite element analysis are also included to complement the work reported in this contribution.

Failure mechanisms of sandwich composites with orthotropic integrated woven corrugated cores: Experiments

To enhance the skin–core debonding resistance of sandwich composites, a new integrated woven corrugated sandwich composite (IWCSC) was designed and fabricated. Quasi-static compression, shearing and three-point bending tests were carried out to reveal the mechanical property and the failure mechanism of the IWCSC. In compression, gradual core crushing and contacting with the skin induced ductile load–displacement curves featured by a long deformation plateau. The anisotropic core structure leads to orthotropic anti-shearing resistances in the warp and weft directions. The difference is even at the level of a magnitude. IWCSC is an anisotropic bending dominated light weight material. Anisotropy of the core structure leads to different failure mechanisms of IWCSC panels in bending. Bended in the warp direction failure mode of the beam is core shearing while in the weft direction is indentation. High skin–core debonding resistance has been testified by the experiments. Behaviors of IWCSC panels were predicted and the predictions are close to the tested data.

Dynamic lateral crushing of empty and sandwich tubes

The dynamic lateral crushing behaviour of short empty and sandwich circular tubes is examined in this paper. Unlike the conventional impact method, the specimens were placed on the bottom platen of an Instron machine with a constant upwards velocity and then the tube collided with the fixed upper rigid platen. Load-deflection curves of empty tubes were first obtained and analysed. From the viewpoint of deformation modes and plastic strain energy absorbed by the tube quadrants around the proximal surface and distal surface, a critical velocity of impact is determined which corresponds to a mode change. A relationship is found to exist between the critical velocity and thickness-to-diameter ratio as well as the yield stress and density of the material. To understand the dynamic crushing behaviour of short aluminium foam-filled sandwich tubes by two rigid platens, further tests and corresponding finite element analysis were performed, respectively. Similar to the observations in the quasi-static tests, the mode of dynamic collapse is bending, with the formation of plastic bending zones accompanied with core crushing. Corresponding FE models for ABAQUS/Explicit were developed and validated against the experimental observations. Detailed deformation features and energy absorption characteristics during the crushing process were identified. It was found that increasing the compression velocity leads to an increase in the total internal plastic energy dissipation for both empty and sandwich tube. The propagation of plastic bending in the form of double-moving-hinges is the main mechanism of energy dissipation, as opposed to the low velocity impact which involves stationary plastic deformation zones.

Effect of core density on deformation and failure in sandwich composites subjected to underwater impulsive loads

The response of sandwich structures to underwater blast loading is analyzed. The analysis focuses on the effect of varying structural attributes on energy dissipation and deformation. The structures analyzed are simply-supported sandwich structures with PVC foam cores and fiber-reinforced polymer composite facesheets. For the analysis carried out, the material properties of the sandwich cores are varied and the total mass is kept constant. In conjunction with experiments, simulations account for underwater blast loading on structures in air-backed and water-backed conditions. Core crushing is accounted for through the Deshpande and Fleck model and facesheet failure is accounted for using the Hashin damage model. Results reveal a significant difference between the response of air-backed and water-backed/submerged structures. In general, thick and low-density cores provide superior blast mitigation and failure resistance. Scaling relations are developed to quantify the responses. These relations can be used to optimize the design of sandwich structures in critical parts of ships like keel, turbine-blades and rudders which involve different contact conditions with water.

Deformation and fracture of impulsively loaded sandwich panels

Light metal sandwich panel structures with cellular cores have attracted interest for multifunctional applications which exploit their high bend strength and impact energy absorption. This concept has been explored here using a model 6061-T6 aluminum alloy system fabricated by friction stir weld joining extruded sandwich panels with a triangular corrugated core. Micro-hardness and miniature tensile coupon testing revealed that friction stir welding reduced the strength and ductility in the welds and a narrow heat affected zone on either side of the weld by approximately 30%. Square, edge clamped sandwich panels and solid plates of equal mass per unit area were subjected to localized impulsive loading by the impact of explosively accelerated, water saturated, sand shells. The hydrodynamic load and impulse applied by the sand were gradually increased by reducing the stand-off distance between the test charge and panel surfaces. The sandwich panels suffered global bending and stretching, and localized core crushing. As the pressure applied by the sand increased, face sheet fracture by a combination of tensile stretching and shear-off occurred first at the two clamped edges of the panels that were parallel with the corrugation and weld direction. The plane of these fractures always lay within the heat affected zone of the longitudinal welds. For the most intensively loaded panels additional cracks occurred at the other clamped boundaries and in the center of the panel. To investigate the dynamic deformation and fracture processes, a particle-based method has been used to simulate the impulsive loading of the panels. This has been combined with a finite element analysis utilizing a modified Johnson–Cook constitutive relation and a Cockcroft–Latham fracture criterion that accounted for local variation in material properties. The fully coupled simulation approach enabled the relationships between the soil-explosive test charge design, panel geometry, spatially varying material properties and the panel's deformation and dynamic failure responses to be explored. This comprehensive study reveals the existence of a strong instability in the loading that results from changes in sand particle reflection during dynamic evolution of the panel's surface topology. Significant fluid–structure interaction effects are also discovered at the sample sides and corners due to changes of the sand reflection angle by the edge clamping system.

Experimental impact damage study of a z-pinned foam core sandwich composite

This paper assesses the impact damage and post-impact compression properties of a foam core sandwich composite reinforced with through-thickness z-pins. Low-speed flat-wise compression tests performed on the sandwich composite revealed that z-pins improved the elastic modulus, crush strength and absorbed energy capacity (by 260–300%). However, these property improvements do not necessarily translate into a reduction in the amount of damage suffered by the z-pinned sandwich composite under localised (point) impact loading. There was no reduction to the impact damage area or an improvement to the post-impact compression properties of the z-pinned sandwich composite at low-impact energies (when damage was confined to the impacted face skin). Z-pins were only marginally effective at reducing the impact damage when the impact energy was high enough to cause core crushing. Z-pins absorbed high-impact energy via splitting, microbuckling and fragmentation during core crushing which reduced slightly the amount of impact damage to the sandwich composite. However, this did not cause a significant improvement to the post-impact compressive stiffness and strength for most energy levels.

Geometry effect on the behaviour of single and glue-laminated glass fibre reinforced polymer composite sandwich beams loaded in four-point bending

The research investigated the behaviour of single and glue laminated glass fibre reinforced polymer (GFRP) composite sandwich beams considering different spans and beam cross sections. The composite sandwich beams with different thicknesses (1, 2, 3, 4, and 5 sandwich layers) have been tested in four-point static flexural test with different shear span to depth ratio (a/d). The a/d ratios showed a direct effect on the flexural and shear behaviour. The capacity of the beam decreased with increasing a/d. Various failure modes were observed including core crushing, core shear, and top skin compression failure. The failure mode map developed based on the experimental finding and analytical prediction indicated that the failure mode is affected by the a/d with the number of glue laminated panels.

Interaction effects of multiple damage mechanisms in composite sandwich beams subject to time dependent loading

Theoretical models are formulated to explain evolution and interaction of the damage mechanisms for multiple delamination of the face-sheet and core crushing in composite sandwich beams subjected to dynamically applied out-of-plane loading and continuously supported by rigid planes. The models are based on simplified one-dimensional formulations and describe the impacted face of the sandwich as a set of Timoshenko beams joined by cohesive interfaces and resting on a nonlinear Winkler foundation, which approximates the response of the core; the dimensionless formulation highlights the material/structure groups that control the mechanical response. The characteristic features of the problem and transitions in damage progression are explored on varying geometrical parameters and material properties and magnitude and duration of the applied load. For quasi-static loading and low velocity impact, core/face-sheet interactions generate energy barriers to the propagation of delaminations; the efficacy of the barriers in controlling damage in the face-sheets depends on the relative stiffnesses of face-sheet and core and on the foundation yielding strength. For dynamic loading conditions, significant dynamic effects arise in certain regimes and cause substantial changes in behavior: shielding of the crack tip stress fields provided by the foundation is reduced, especially after the load is removed when important delamination openings occur; core plasticity generally opposes this behavior and limits damage in the face-sheet.

Low-velocity impact response of a thin sandwich plate with a metallic sheared dimple core subjected to a free boundary condition

► Excessive crushing and shear of core commence when core compressive strain is larger than 0.39. ► Energy absorption characteristics of the sandwich plate are influenced by the core compressive strain. ► Specific dissipated energy of the sandwich plate is 13.2–24.8% larger than that of DP780. ► Benefits of the sandwich plate can be maintained as bending dominant deformation is induced. Various studies of the impact and crushing characteristics of sandwich plates with metallic structured cores have been undertaken to apply sandwich plates to structural materials of transport vehicles. The goal of this paper is to investigate into low-velocity impact responses of a thin sandwich plate with metallic sheared dimple cores subjected to a free boundary condition through drop impact experiments. In order to examine the influence of the impact parameter, including the impact energy and the nose diameter of the impact head, on the deformation, failure and energy absorption characteristics of the thin sandwich plate in a free boundary condition, drawing type drop impact experiments have been performed. The results of the experiments have been shown that excessive extension of the sheared section and significant crushing of the core as well as excessive deformation of the front face-sheet in the vicinity of the impacted region commence when the nose diameter is 20 mm and the impact energy is larger than 65.3 J. In addition, it has been noted that the energy absorption rate of the thin sandwich plate is influenced by the core compressive strain and that the core crushing should be reduced to improve the energy absorption characteristics of the thin sandwich plate. The performance and benefits of the thin sandwich plate have been discussed through the comparison of the thin sandwich plate with a DP780 monocoque plate. From the results of the comparison, it has been shown that the thin sandwich plate can improve energy absorption characteristics in spite of the reduced weight and weaker properties.

Experimental analysis of the through-thickness compression properties of z-pinned sandwich composites

This paper presents an experimental investigation into the flat-wise compression properties, strengthening mechanisms and failure modes of sandwich composite materials reinforced with orthogonal z-pins. The compression modulus of the sandwich composite increases rapidly with the volume content of z-pins due to their high longitudinal stiffness, however acoustic emission monitoring and X-ray computed tomography reveal that some z-pins are damaged during elastic loading. The compression stress to induce core crushing is increased greatly by z-pinning (up to nearly 700%), although a large percentage of the z-pins fail close to the elastic stress limit by longitudinal splitting and/or kinking. The total absorbed compressive strain energy of the sandwich composite is also improved greatly by z-pinning (more than 600%) due to the z-pins resisting core crushing, even though they are severely damaged. The results and observations presented in this paper have implications on the mechanical modelling of sandwich materials reinforced with brittle z-pins.

The Effects of Air and Underwater Blast on Composite Sandwich Panels and Tubular Laminate Structures

The resistance of glass-fibre reinforced polymer (GFRP) sandwich panels and laminate tubes to blast in air and underwater environments has been studied. Procedures for monitoring the structural response of such materials during blast events have been devised. High-speed photography was employed during the air-blast loading of GFRP sandwich panels, in conjunction with digital image correlation (DIC), to monitor the deformation of these structures under shock loading. Failure mechanisms have been revealed by using DIC and confirmed in post-test sectioning. Strain gauges were used to monitor the structural response of similar sandwich materials and GFRP tubular laminates during underwater shocks. The effect of the backing medium (air or water) of the target facing the shock has been identified during these studies. Mechanisms of failure have been established such as core crushing, skin/core cracking, delamination and fibre breakage. Strain gauge data supported the mechanisms for such damage. These studies were part of a research programme sponsored by the Office of Naval Research (ONR) investigating blast loading of composite naval structures. The full-scale experimental results presented here will aid and assist in the development of analytical and computational models. Furthermore, it highlights the importance of support and boundary conditions with regards to blast resistant design.

Failure mechanisms in composite panels subjected to underwater impulsive loads

This work examines the performance of composite panels when subjected to underwater impulsive loads. The scaled fluid–structure experimental methodology developed by Espinosa and co-workers was employed. Failure modes, damage mechanisms and their distributions were identified and quantified for composite monolithic and sandwich panels subjected to typical blast loadings. The temporal evolutions of panel deflection and center deflection histories were obtained from shadow Moiré fringes acquired in real time by means of high speed photography. A linear relationship of zero intercept between peak center deflections versus applied impulse per areal mass was obtained for composite monolithic panels. For composite sandwich panels, the relationship between maximum center deflection versus applied impulse per areal mass was found to be approximately bilinear but with a higher slope. Performance improvement of sandwich versus monolithic composite panels was, therefore, established specially at sufficiently high impulses per areal mass ( I 0/ M¯>170 m s −1). Severe failure was observed in solid panels subjected to impulses per areal mass larger than 300 m s −1. Extensive fiber fracture occurred in the center of the panels, where cracks formed a cross pattern through the plate thickness and delamination was very extensive on the sample edges due to bending effects. Similar levels of damage were observed in sandwich panels but at much higher impulses per areal mass. The experimental work reported in this paper encompasses not only characterization of the dynamic performance of monolithic and sandwich panels but also post-mortem characterization by means of both non-destructive and microscopy techniques. The spatial distribution of delamination and matrix cracking were quantified, as a function of applied impulse, in both monolithic and sandwich panels. The extent of core crushing was also quantified in the case of sandwich panels. The quantified variables represent ideal metrics against which model predictive capabilities can be assessed.

Effect of Core Thicknesses on Impact Performance of Thermoplastic Honeycomb Core Sandwich Structure under Low-Velocity Impact Loading

Low-velocity impact test on sandwich panels composed of aluminum face sheets and thermoplastic honeycomb cores have been performed to characterize the impact performance as a function of core thickness and drop heights. Impact parameters like maximum impact force, impact energy and impact damage area were evaluated and compared. Consequent damages were inspected visually on the impact surface as well as the rear surface. The experimental results found that panels with thicker core exhibited higher impact force than thinner core counterparts, allowing the panel to absorbed more energy. Higher degree of impact damage can be observed at elevated drop heights as most of the damage took the form of local core crushing, face sheet buckling and debonding between the face sheet and core,. Resulting damage area were different according to the core thickness as thicker core prone to absorbed more energy that lead to more damage propagation.

A numerical study of the dynamic response of sandwich plates initially damaged by low-velocity impact

A finite element (FE) model has been developed for the analysis of the dynamic response of sandwich plates with impact-induced damage involving core crushing, face sheet damage and core-to-face sheet debonding. The effect of intermittent contact in fragments detached at the damaged interface due to the impact event is examined. Simulation of the strongly nonlinear transient and steady-state dynamic responses of impact-damaged sandwich plates are carried out with the FE code ABAQUS. The potential of the obtained numerical results for detecting and locating impact-induced damage is discussed.

Mechanical behavior of sandwich panels with hollow Al–Si tubes core construction

A new type of lightweight sandwich panels consisting of vertically aligned hollow Al–Si alloy tubes as core construction and carbon fiber composite face sheets was designed. The hollow Al–Si alloy tubes were fabricated using precision casting and were bonded to the face sheets using an epoxy adhesive. The out-of-plane compression (i.e. core crushing), in-plane compression, and three-point bending response of the panels were tested until failure. The hollow Ai–Si alloy tubes core configuration show superior specific strength under crushing compared to common metallic and stochastic foam cores. Under in-plane compression and three-point bending, the buckling of face sheets and debonding of hollow cores from the face sheets were observed. Simple analytical relationships based on the concepts of mechanics of materials were provided for the compression tests, which estimate the sandwich panels’ strength with high fidelity. For three-point bending, detailed finite element analysis was used to model the response and initial failure of the sandwich panels.

Experimental and Numerical Analysis of Critical Buckling Load of Honeycomb Sandwich Panels

The critical buckling loads for various core densities and materials of honeycomb composite panels are experimentally and numerically investigated in this study. The surface plates of honeycomb composite panels are of polyester/glass fiber composite. Polyester resin-impregnated paper or aluminum is used as the honeycomb core material. Honeycomb panels with different cell sizes, but approximately the same volume, are produced and the effect of the honeycomb core density on the critical buckling load is investigated by compression tests. The critical buckling load of paper core panels is determined to be higher than that of aluminum core panels. It is seen that the buckling strength of the specimens increases by the increase of core density. As the critical buckling load exceeds a certain limit, regional core cell buckling and core crushing are seen in aluminum core panels. In paper core panels, regional cracks are seen, in addition to these failures. The study also calculates the numeric buckling loads of the panels using the ANSYS finite element analysis program. The achieved experimental and numerical results are compared with each other and the results are provided in tables.