Effects of temperature and thermal cycles on the mechanical properties of expanded polystyrene foam

Understanding the effects of high temperature and thermal cycles on the mechanical properties of expanded polystyrene (EPS) foam is critical for its use in sandwich panels. This paper presents an experimental investigation of these effects in typical environmental conditions that exist in construction applications. A total of 117 small specimens were cut from metal-faced sandwich panels with EPS core and were exposed to different numbers of thermal cycles and/or sustained high temperatures. The specimens were then loaded under compression, tension, and four-point bending for evaluating the degradation of the mechanical properties of the foam. The thermal cycles reflect typical surface temperature during daily summer conditions, with a period of 24 h each and with a temperature varying between 24°C to 80°C. The results show that the modulus of elasticity of EPS foam in compression reduced by about 38% after exposure to thermal cycles for 45 days, whereas the tensile and shear moduli reduced by about 5.7% and 13.8%, respectively. Exposure to sustained high temperature after thermal cycles led to larger degradation of the elastic and shear moduli in the range of 38%–50%. These degradations can lead to early failures in applications that rely on the EPS foam as a structural component like in insulating sandwich panels.

Analysis on fracture behaviour of stitched foam sandwich composites using interlaminar tension test

Sandwich composites are susceptible to interfacial delamination, owing to the mismatches in the material properties between the face sheets and core. Previous studies have shown that stitching can improve the performance of sandwich composites. In this study, an analysis approach was developed to investigate the fracture behaviour of stitched foam sandwich composites. The stitched foam sandwich composites were manufactured by a vacuum-assisted resin transfer moulding process. Interlaminar tension tests revealed the effects of the linear thread density on the failure mechanisms of the stitched foam sandwich composites. Asymmetric double cantilever beam tests were performed to investigate the influences of the stitch thread reinforcement on the fracture behaviour. An analytical approach combining extended finite element method and nonlinear spring elements was proposed to predict the failure behaviour of the stitched sandwich composites. Experiment and simulation approaches were employed to investigate the influences of the stitch parameters (stitch pitch and linear thread density) on the ultimate load and energy absorption. The results show that stitched method can significantly enhance the mechanical properties of sandwich composites. The energy absorption and ultimate load values of the specimens tend to increase with an increase in the linear thread density or a decrease in the stitch pitch.

Vibration response of glass fiber composite multi-layer graded corrugated sandwich panels

Based on the criteria of equal total height and relative density, a series of glass fiber composite multi-layer graded corrugated sandwich panels (MLGCSPs) with different configurations is designed and fabricated by an in-house hot-molding secondary molding method. The effects of arrangement modes, graded corrugated cores and corrugated topologies on the vibration behavior of the present MLGCSPs are comprehensively investigated by the vibration shaker tests. The results reveal that the arrangement modes, graded arrangement and topologies of the corrugated cores all have significant influences on the frequency responses, and vibration reduction of the present MLGCSPs. The arrangement mode II and III generally have significantly higher resonant frequencies and vibration attenuation performance in the low frequency range, but the arrangement mode I shows better vibration reduction and isolation performance in the higher frequency range. It is possible to simultaneously achieve higher fundamental frequency and lower amplitude of the structures by optimizing the arrangement mode and gradient configuration. Subsequently, finite element simulation is carried out to systematically analyze the vibration responses of the composite MLGCSPs with different configurations. The consistency between numerical and experimental results is quite well. Finally, the effects of structural parameters on the vibration characteristics of the present structures are also revealed. Some conclusions are obtained, which can provide some meaningful guidelines for the vibration reduction design of such type of multi-layer structures.

Complementary and normalized energies during static and dynamic uniaxial deformation of single and multi-layer foam-filled tube

This article investigates the quasi-static compressive behavior and the drop weight impact tests during the crashing of energy-absorbing structures such as aluminum foam-filled tubes. The closed-cell Al and A356 Alloy foams were cast and, after cutting, inserted into the Al thin wall tube as axial fillers of single-, double- and quad-layer structures. Then, the specific energy absorption (SEA), complementary energy (CE), normalized energy (NE), and specific normalized energy (SNE) are calculated based on static and dynamic test results under uniaxial loading. In this new method, values of NE and SNE are always between 0 and 1. Results show that the SEA-strain curves obtained from crashing the foam-filled tubes were linear and overlapping under static and dynamic loading. However, NE curves for dynamic tests were cyclic and in the static tests were asymptotic non-linear, and utterly separable. Results indicated that the SNE for Al, A356 single layer, Al-A356 double-, and Al-A356-Al-A356 quad-layer foam-filled tubes during dynamic tests were 0.25, 0.29, 0.31, and 0.31, while for the static tests, 0.14,0.15, 0.17, and 0.14 were recorded. It was found that CE and NE energies were better than the SEA energy for recognizing plastic deformation and crushing behavior.

Influence of geometric parameters on free vibration behavior of an aluminum honeycomb core sandwich beam using experimentally validated finite element models

The hexagonal honeycomb core sandwich panels used in the satellite structure are subjected to severe vibration during launch. Therefore, the amounts of natural frequencies of these panels are of great importance for design engineers. Three-dimensional finite element modeling of the core considering all geometric parameters (i.e., a high-fidelity model) to achieve accurate results is not cost-effective. The honeycomb core is traditionally equivalent to a homogenized continuum core (i.e., a low-fidelity model) using simple analytical relations with ignoring the adhesive layer at the double cell-walls and radius of inclined cell-wall curvature. In this study, analytical formulations are first presented for the prediction of the equivalent elastic properties of a hexagonal aluminum honeycomb with considering all geometric parameters including adhesive layer thickness, cell-wall thickness, inclined cell-wall length, radius of inclined cell-wall curvature at the intersection, internal cell-wall angle, and honeycomb height. Then, two aluminum honeycomb core sandwich beams with free-free boundary conditions are modeled and analyzed in Abaqus finite element software, one with 3D high-fidelity core and the other with 3D low-fidelity core. In order to validate the results of the equivalent model, the modal analysis test was performed and the experimental natural frequencies were compared. The obtained results show a good agreement between the 3D low-fidelity and high-fidelity finite element models and experimental results. In addition, the influence of the above-mentioned geometric parameters has been investigated on the natural frequencies of a sandwich beam. [Formula: see text]

Mechanical behaviour in shear and compression of polyurethane foam at elevated temperature

This paper presents an experimental investigation about the effect of elevated temperature on the mechanical properties of two polyurethane (PUR) foams, with densities of 40 kg/m3 and 93 kg/m3. The experimental campaign included shear and compressive tests over a temperature range of 20°C–200°C, performed to assess the degradation of the mechanical properties of the PUR foams with temperature. To validate the diagonal tension shear test method adopted in this investigation, a numerical study was also performed, namely to assess the (shear) stress state developed within the foam. The results obtained validated the adopted test procedures, showing that the compressive and shear responses are strongly affected by elevated temperature, due to the softening of the polymeric material when it undergoes the glass transition process. For the temperature range considered in this study, both strength and modulus in shear and compression present an approximately linear reduction with temperature.

Vibro-acoustical responses of a sandwich panel consist of aluminum honeycomb core and fabric-reinforced graphite facings

This research presents a numerical study on vibro-acoustic and sound transmission loss behavior of an aluminum honeycomb core sandwich panel with fabric-reinforced graphite (FRG) composite face sheets. The sandwich theory, which assumes the honeycomb core as an orthotropic structural layer, is applied to investigate the free and forced vibration behavior of the panel. The radiated sound power from the panel is quantified by Rayleigh integral method, and the random diffuse field as an incident sound source is derived based on finite element method with the employment of ACTRAN. A validation between the simulated results and the experimental data published is carried out to demonstrate the accuracy and reliability of the present approach. The comparison between different materials of honeycomb sandwich structures illustrates the advantages of the fabric-reinforced graphite honeycomb sandwich structure over the other types of sandwich structures considered. The effects of different boundary conditions and honeycomb structural geometry properties on the acoustical performance of the stiffness of the FRG panel are also investigated. The approach of the present research provides useful guidance for evaluating and selecting the other honeycomb sandwich panels when the vibratory and acoustic behaviors of the panels are considered.

Structural performance of sandwich wall made of concrete cast in between GFRP facings

The structural performance of concrete walls reinforced with pultruded glass fibre-reinforced polymer (GFRP) ribbed plates on either side has been experimentally investigated. The GFRP plates were used as a stay-in-place (SIP) structural formwork replacing internal steel reinforcement. The pultruded flat plates incorporated 51 mm deep T-shape ribs on one side, spaced at 100 mm, which provided interlocking with concrete. Six 3000 × 616 mm panels, either 150 or 200 mm thick, were tested in bending ( M), under axial compression ( N) and under combined loads to establish the completed ( M- N) failure envelope of the wall. The effect of surface treatment of the GFRP forms was also investigated. It resulted in full composite action with no concrete slip, reaching 30% higher flexural strength than untreated panels. The effect of reinforcement ratio was studied by varying wall thickness. In all panel tests, diagonal concrete shear cracking occurred and propagated into a horizontal delamination above the GFRP ribs. Slenderness effect and secondary moments were accounted for in developing the ( M- N) interaction curve. Initially, M increased by 25% as N increased from zero to 17% of pure axial strength. Then, M reduced linearly to zero at pure N as concrete crushing occurred when the GFRP compression plate separated from the ribs and buckled outwards at midspan. A simplified design approach is also presented.

On the use of a trilinear traction-separation law to represent stitch failure in stitched sandwich composites

Modern aircraft employ the use of lightweight engineering materials such as sandwich composites to increase the flexural rigidity of their structural components. These sandwich composites are limited by their low interfacial strength between the outer facesheets and internal core, which can result in facesheet-core debonding at relatively low out-of-plane loads. In this study, sandwich composites that are reinforced with through-the-thickness stitching are considered. Stitched sandwich composite specimens, fabricated from 110 kg/m3 perforated foam core with cross-ply carbon/epoxy facesheets, were manufactured with different combinations of stitch densities (0.0016–0.01 stitches/mm2) and linear thread densities (400–1200 Denier) of through-the-thickness reinforcement. Single cantilevered beam (SCB) tests were performed to characterize the facesheet-core debonding within the stitched sandwich composites. Unique fracture morphologies were observed that exhibit dependency on stitch processing parameters. A discrete cohesive zone modeling approach is used to simulate the separation of the facesheet from the core. Three-dimensional finite element analysis reveals crack curvature near the stitching. Good agreement between predicted and experimental measurements were obtained.

A novel graded auxetic honeycomb core model for sandwich structures with increasing natural frequencies

A novel angle graded auxetic honeycomb (AGAH) core is designed for sandwich structures in the present study. The angle of the cells is varied through the thickness of the AGAH core using linear functions. Therefore, the thickness of the cell walls is kept constant along the gradation of the cell angle, and the length of the cell walls is changed through the core thickness as the result of angle variation. New analytical relations are proposed to predict the equivalent elastic properties of the AGAH core. The performance of the new proposed core is analytically assessed for the vibrational behavior of a sandwich plate. The governing equations are deduced adopting Hamilton’s principle under the assumption of quasi-3D exponential plate theory. Three-dimensional finite element (3D-FE) simulation is accomplished to verify the analytical results of the vibrational response of the sandwich structure. The influence of variation of the cell wall, the cell angle and cell aspect ratio of AGAH core, and geometric parameters of the sandwich structure are investigated on the vibration response of the sandwich panel. The present graded design of the auxetic honeycomb enhances the specific stiffness (i.e., stiffness to density ratio) and consequently increases the natural frequencies of sandwich structures with this type of core.