Strength Of Composite Sandwich Panels Research Articles

The effect of flame treatment on the compressive strength of composite sandwich panels made of pure epoxy resin H‐shaped structures and hollow glass bead/epoxy resin composite material

AbstractAt present, syntactic foam composites (HGB/EP) made of epoxy resin (EP) and hollow glass beads (HGBs) have received extensive attention in the research field of lightweight materials because of their high compression strength‐to‐density ratio. However, the deep‐sea environment has put forward higher requirements for the compressive properties of materials. The traditional sandwich structure with HGB/EP as the core has gradually failed to meet the requirements for the mechanical properties of equipment deeper in the ocean. This paper proposes a sandwich structure comprising a pure EP H‐shaped structural component as the rigid skeleton and an HGB/EP syntactic foam composite as the lightweight filling component. The pure EP H‐shaped structural component was subjected to flame treatment using a butane flame jet at different distances. The compression resistance of the resulting sandwich structure was tested, and the results showed varying degrees of improvement in compressive strength after flame treatment. The effects of flame treatment on the surface adhesion performance of the pure EP were characterized using compression‐shear tests, optical microscopy, contact angle tests, infrared spectroscopy, and flame temperature tests. The results demonstrated that flame treatment at a distance of 0 cm significantly increased the oxygen‐containing functional groups on the surface of the pure EP, improved the wettability of the pure EP surface, increased the adhesive strength between the pure EP H‐shaped structural component and HGB/EP, and enhanced the compressive strength of the sandwich structure. The shear strength of the shear test specimens improved by 33.0%, while the compressive strength of the sandwich structure specimens increased by 17.5%.Highlights The sandwich structure composed of pure epoxy resin H‐shaped structure and foam composite material was prepared. The distance of flame treatment affects the surface chemical groups of epoxy resin. Flame treatment significantly improved the surface wettability of epoxy resin.

Temperature effects on the compressive properties and failure mechanisms of composite sandwich panel with Y-shaped cores

The effects of temperature on the mechanical properties and failure mechanisms of composite sandwich panel with Y-shaped cores under out-of-plane compression have been studied in this paper. The carbon fiber composite sandwich panel with Y-shaped cores was manufactured by a hot-press method using unidirectional carbon/epoxy prepregs. The out-of-plane compression tests of composite sandwich panels were conducted at temperatures ranging from 20 °C to 200 °C. Then the stress-strain curves, failure modes, compressive stiffness and strength of composite sandwich panel at high temperatures were analyzed and compared with those ones at room temperature. The results have shown that temperature had a significant effect on mechanical properties and failure behaviors of composite sandwich panel. And high temperature resulted in the decreasing of compressive stiffness and strength. Finally, the analytical expressions were presented to predict the compressive stiffness and strength of composite sandwich panel and compared with experimental results at different temperatures.

The compressive responses of glass fiber composite pyramidal truss cores sandwich panel at different temperatures

A new method for fabricating glass fiber composite sandwich panel with pyramidal truss cores was developed based on the vacuum assisted resin transfer molding technology. The microstructure and organizations of fabricated sandwich panels were examined by the scanning electron microscope. The out-of-plane compressive tests of composite sandwich panels were performed throughout the temperature range from −60°C to 125°C. Then the effects of temperature on the compressive strength, compressive modulus and failure mechanism were investigated and analyzed. Our results indicated that cryogenic temperature resulted in the increasing of the compressive modulus and strength, while high temperature caused the degradation of the compressive modulus and strength. The effect of temperature on failure mode of composite sandwich panel was also observed. Analytical expressions were presented to predict the compressive modulus and strength of composite sandwich panels at different temperatures.

Experimental strength of composite sandwich panels with cores made of aluminum honeycomb and foam

Composite sandwich panels with face sheets made of carbon/epoxy fabric and carbon/epoxy uni-directional prepreg were tested. The sandwich cores made by aluminum honeycomb and foam were cured with the face sheet at 180 °C. The aluminum core sandwich failed at the 40% failure strain of the face skin in the compression test. On the other hand, the foam core sandwich failed at the 100% failure strain of the face skin in the compression test. In the case of the aluminium core sandwich, nonhomogenous load distributions and fiber contents may affect the lower failure strain. Therefore, the specific compression strength of the foam core was 72% higher than the compression strength of the aluminum core. Failure mode analysis of sandwich panels made of aluminium honeycomb cores shows that shear failure leads to final failure. However, the specific flexural strength of the sandwich panels with aluminum cores had a slightly higher value than that of the sandwich panels with foam cores. Because the sandwich panel with aluminium cores had higher critical stress for the face wrinkling than the sandwich panel with foam cores due to higher compression modulus of aluminium cores than foam cores.

Mechanical Behavior and Failure Mechanisms of Carbon Fiber Composite Pyramidal Core Sandwich Panel after Thermal Exposure

An attempt has been made here to evaluate the effect of thermal exposure on the mechanical behavior and failure mechanisms of carbon fiber composite sandwich panel with pyramidal truss core under axial compression. Analytical formulae for the collapse strength of composite sandwich panel after thermal exposure were derived. Axial compression tests of composite laminates and sandwich panels after thermal exposure were conducted at room temperature to assess the degradation caused by the thermal exposure. Experimental results showed that the failure of sandwich panel are not only temperature dependent, but are time dependent as well. The decrease in residual compressive strength is mainly attributed to the degradation of the matrix and the degradation of fiber–matrix interface, as well as the formation of cracks and pores when specimens are exposed to high temperature. The measured failure loads obtained in the experiments showed reasonable agreement with the analytical predictions.

Shear response of carbon fiber composite pyramidal truss structures after high-temperature exposure

Shear behavior and residual strength at room temperature of carbon fiber composite sandwich panel with pyramidal truss cores exposed to different temperatures for different time were studied experimentally and analytically. Experimental results showed that the residual shear modulus and strength of composite sandwich panels decreased with increasing exposure temperature and time. The decreasing of shear modulus and strength after thermal exposure was mainly attributed to the degradation of the matrix property and fiber-matrix interface property. The effect of high-temperature exposure on failure mode was revealed as well. The failure was characterized by node rupture when sandwich panels were exposed to temperature in a range of 20–250°C, while node rupture and face sheet delamination were observed when specimens were exposed to 280°C. The analytical formulae were also presented to describe residual shear modulus and strength after thermal exposure.

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.

Self-healing sandwich panels: Restoration of compressive strength after impact

Impact damage can significantly reduce the strength of composite sandwich panels, giving rise to large factors of safety in design. A self-healing sandwich panel was designed and manufactured. Specimens were tested in edgewise compression-after-impact to prove the concept. The self-healing system consists of vascular networks carrying the two liquid components of an epoxy resin system. After damage and autonomous self-healing, the specimens could be restored to their undamaged strength. Key influences on healing efficiency, and the need for further improvements in reliability were identified. A simple specific strength analysis suggested that the mass penalty of self-healing could, in some cases, be offset by the improved design allowables permitted.

A fibre microbuckling model for predicting the notched compressive strength of composite sandwich panels

Cohesive-zone models have been successfully applied to predicting the damage from notches in engineering materials loaded intension. They have also been used to determine the growth of fibre microbuckling from a hole in a composite laminate under compression. The usual strategy is to replace the in elastic deformation associated with plasticity or microbuckling with a line crack and to assume some form of stress-displacement bridging law across the crack faces. This paper examines recent published experimental data for notched glass-fibre epoxy/honey comb sand wich panels loaded in uniaxial compression. A plastic fibre kinking analysis and a linear softening cohesive-zone model are used for the prediction of the unnotched and open-hole compressive strength and the theoretical results are found to be in a good agreement with experimental data.

Compression after impact strength of composite sandwich panels

Two types of sandwich panels with carbon epoxy skins and aluminium honeycomb core were subjected to low velocity impacts and then the damaged panels tested for their compression-after-impact (CAI) strength. One type of panel was found to be a very robust energy absorber, i.e. a thick-skin thin-core option. The other panels with their thin skins and thick core were found to penetrate easily whereupon the impactor forced the back-face to debond massively. These panels then had the worst CAI strength. Both the impact damage and the CAI behaviour––to failure––were simulated using a finite element model of skins and core. The model gave very respectable agreements with the impact tests even though the damage in skin and core were extremely complex mechanisms, involving progressive fibre fracture in the skin and elasto-plastic deformation in compression and shear in the honeycomb core. The model for CAI strength predicted well the failure load and the mode of propagation from the damaged zone.

Scaling and Curvature Effects on the Damage Tolerance of Impacted Composite Sandwich Panels

An experimental program was conducted to investigate the effects of specimen width and specimen curvature on the residual compression strength of composite sandwich panels after low-energy impact. Flat and curved (1.1 m radius) specimens of three widths (83, 152, and 305 mm) were manufactured from thin-gage (four plies) graphite/epoxy facesheets and 25.4-mm-thick Nomex core. A drop mechanism was used to create barely visible impact damage on one facesheet of the panels (convex face of the curved panels) maintaining the energy level as constant in all cases. All specimens were then loaded axially in compression to failure. Above a certain panel width, the panel width and curvature had noeffect on the ultimate strength of the panel. Although width and curvature increased the size of the damage caused by an identical energy impact, the indentation growth was delayed with respect to farfield strain (stress). Because indentation growth is chiefly a local phenomenon, the delay was probably due to the variations in the damage state. The baseline specimen (flat, 83 mm in width) serves as an adequate coupon-level test even though it is conservative when compared to wider plates.

Factors in the Compressive Strength of Composite Sandwich Panels with Thin Facesheets

An experimental investigation was conducted to investigate the contribution of various factors to the compressive strength of composite sandwich panels with thin facesheets. The main factors investigated were the laminate configuration and the inclusion of notches, the latter to provide information on the damage tolerance of such configurations. Tests were conducted on sandwich panels with 25.4 mm thick Nomex honeycomb core and secondarily-bonded facesheets made from AWl 93PW/3501-6 graphite/epoxy fabric. Two different facesheet configurations, both 0.35 mm thick and consisting of two plies of the fabric, were investigated: (0/90) and (45). Unnotched specimens and specimens with holes ranging from 3.18 mm to 9.53 mm in diameter were tested monotonically to failure in compression. In a number of applications of thin graphite/epoxy facesheets in sandwich configurations, it has been reported that the existence of facesheet wrinkling played a significant role in the response and failure of these panels. This includes a reduction in the failure load from that anticipated. The possibility of the existence of face wrinkling and the factors involved were investigated by placing strain gages on both facesheets in a number of locations. This included strain gages within the region of influence of the hole for these specimens. Specimen failure was seen to be a result of compressive failure of the facesheets. Evidence of any local instability of the facesheets possibly indicative of wrinkling was observed in the strain response for only one particular configuration. However, the failure mechanisms did not indicate any instability in all cases. The failure mechanism was dependent on the facesheet configuration as was response to the presence of notches. The (0/90) facesheet configuration showed an almost linear stress-strain curve up to failure. Failure occurred with a neat fracture perpendicular to the applied load. This case showed typical sensitivity to the presence of the notches for hole diameters larger than 3.18 mm and was adequately correlated using available models for composite laminates. For the (45) facesheet configuration, a highly stress-strain behavior was observed much as in tension tests. Damage patterns at failure were characterized by fractures developing along the +45 and -45O directions and at the interply

Structural Response of Composite Sandwich Panels Impacted With and Without Compression Loading

The results of an experimental study of the impact damage characteristics and residual strength of composite sandwich panels impacted with and without a compression loading are presented. Results of impact damage screening tests conducted to identify the impact-energy levels at which damage initiates and barely visible impact damage occurs in the impacted facesheet are discussed. Parametric effects studied in these tests include the impactor diameter and dropped-weight vs airgun-launched impactors. Residual strength results of panels tested in compression after impact are presented and compared with results of panels that are subjected to a compressive preload prior to being impacted.

Strength of composite sandwich panels containing debonds

A method for determining the critical debond size between the facesheet and the core in composite sandwich panels under in-plane compression is described. The approach uses fracture mechanics together with a buckling criterion for a debonded faceskin. The technique yields predictions for the critical in-plane compressive load for debond propagation as a function of core-to-faceskin debond size, faceskin thickness, lay-up, composite material properties, and honeycomb properties and geometries. A computer program, developed in this work, calculates the critical buckling load and facesheet deformed shape by solving an eigenvalue problem. An experimental study was conducted to determine the onset of delamination buckling in composite sandwich panels containing such flaws. Sandwich panel specimens of graphite/epoxy faceskins and aluminum honeycomb core were constructed with embedded delaminations and with varying faceskin thicknesses and core sizes. Four-point bending tests were conducted such that the faceskin containing the debond was under in-plane compression. The predicted critical fracture loads, computed using the proposed theoretical models which were solved using a numerical computational scheme, closely followed the experimental measurements.

Optimization of the bending stiffness and strength of composite sandwich panels

The minimum weight criterion of a sandwich panel with respect to a given bending stiffness and strength is verified. For this purpose a number of sandwich constructions consisting of a foamed PVC core and glass fibre reinforced epoxy faces are tested in a four-point bending test. An optimum is found for both the bending stiffness and strength as design parameters.

Compressive Strength of Composite Sandwich Panels After Impact Damage: An Experimental and Analytical Study

The effect of low speed impact on the compressive strength of graphite/epoxy sandwich panels was examined experimentally and analytically. Various impactor sizes and impact energy levels were used. Impact damage was found to decrease the compressive strength of panels in this investigation by up to 33%. The use of film adhesive as an interply layer was found to reduce the extent of damage and increase the residual compressive strength. A one-parameter model to predict compressive failure of impact damaged panels is presented. The theoretical predictions are in good agreement with experimental results. Some of the factors affecting the panel behavior are also discussed.