Foam Core Material Research Articles

Impact response of sandwich beams with various curvatures and debonds

This article presents experimental results of the impact response and failure characteristics of curved composite sandwich beam with debond. The objective of this study is to determine how the curvature and debond affects the contact force, displacement, absorbed energy, and damage type under impact loading. An instrumented drop-weight impact test are performed on sandwich beams made of carbon/epoxy face sheets and polyurethane foam core material with four different radii of curvature and debonds between the top and bottom interface of face sheet and foam core. Impact energy was kept constant of 4 J. The contact force and displacement histories of curved sandwich composites with debond are measured to determine the impact response. The results are compared with those of flat beams with the same dimensions and no debond. They indicate that the value of the contact force increases as the curvature angle increases for the beam with no debond. Opposed to the beams with no debond, the contact force decreases as the curvature angle increases for the beam with debond. These results show that curvature angle and debond are important mechanical parameters that must be considered in the design of structures.

Numerical analysis of splice-type crack arrester with a filler under mode-I type loading

A new type of crack arrester with a configuration suitable for an airframe structure was numerically evaluated for use in the splice of a foam core sandwich panel to suppress interfacial crack growth and to enhance the structural integrity of the foam core sandwich panel. A tapered butt joint configuration was selected for the core-core splice. Carbon fiber reinforced plastic (CFRP) prepreg was installed as the crack arrester between the cores and the tapered core edge, where the foam core material was removed and replaced with CFRP prepreg. This type of arrester is named a splice-type crack arrester with a filler. The decrease in the energy release rate at the crack tip owing to this type of arrester was analytically confirmed by finite element (FE) analysis and crack closure integrals. A quantitative estimation of the crack suppression effect revealed that a tapered core edge with higher stiffness had a strong effect on crack suppression.

Finite element investigation of the influence of material properties on the crushing characteristics of in-plane loaded composite sandwich panels

In the present paper, the explicit finite element code LS-DYNA3D was used to investigate the influence of selected material properties in the crash energy absorption characteristics of composite sandwich panels subjected to in-plane compressive loading. The first step in this investigation was to simulate as accurate as possible representative tests corresponding to the collapse modes that occurred in a series of static edgewise compression tests performed in the National Technical University of Athens (NTUA) using various types of composite sandwich panels. These sandwich panels were candidate materials for use in the new type of composite front-end bumper of a transportation vehicle. Subsequent to the precise reproduction of the collapse modes, a step-by-step approach was followed in order to examine the influence of selected faceplate and foam core material properties on the crash energy absorption characteristics of the in-plane loaded sandwich panels. More specifically, several series of finite element models were created, by altering the value of only one material parameter per time of the initial FEA models used for the simulation of the sandwich panels collapse modes. The results from the processing of these series of FEA models were used to create figures that graphically depict the influence of the material properties on the energy characteristics. More over the simulation results were analysed in order to express by means of simple linear or polynomial functions the dependence of the crash energy absorption characteristics on the sandwich material parameters. The findings of this series of investigations were recorded aiming to be used as reference to the design of composite sandwich panels in various crashworthiness applications.

The Sensitivity of the Foam-Core Parameters under Impact Loading

This study presents the numerical investigation of the low-velocity impact for the foam-cored sandwich composites. Firstly, the proposed FEA model is validated by comparing the results between simulation and test. The user subroutine VUMAT and the crushable foam model are chosen to describe the damage of the face sheets and the characteristics of the foam material, respectively. The detailed damage process of the sheets and the foam is clearly shown. The sensitivity of seven parameters related to foam-core material are studied. It is shown that the yield strength, the fracture strain and the fracture displacement have significant effects on the impact-resistance of the foam-cored sandwich composites.

Microvascular based self-healing polymeric foam

A self-healing microvascular polymeric foam has been developed to improve the resilience of rigid foam core materials for sandwich structures. We investigated the healing of brittle polyisocyanurate (PIR) foam after mode-I crack separation in a 3-point single edge notch bend (SENB) specimen. A two-part healing chemistry based on a commercially available polyurethane (PUR) foam formulation is employed to rebond the interface. Both components are initially sequestered in separate channels in a vascularized SENB geometry. Upon loading and subsequent crack propagation through the network, the healing agents are released and polymerize on contact to create new foam material in the crack plane. An attractive feature of this system is the volumetric expansion of the healing chemistry, demonstrating the ability to repair macro-scale damage. The foaming reaction occurs on the order of minutes at room temperature, enabling rapid in-situ healing. Furthermore, by using a vascular delivery technique, multiple damage–recovery cycles are achieved at consistently high healing efficiencies. Through repeated mechanical testing, we have demonstrated over 100% recovery in fracture toughness for this new class of bioinspired, self-healing cellular materials.

Blast Performance of Marine Foam Core Sandwich Composites at Extreme Temperatures

The dynamic behavior of sandwich composites for marine applications, made of E-glass Vinyl Ester skin and Corecell™ M100 foam core material, was studied at different temperatures using a shock tube apparatus. High-speed photography coupled with the optical technique of Digital Image Correlation was used to record the real time deformation of the specimen during shock wave loading. Sandwich composites under a high temperature environment demonstrated a significant amount of fiber breakage, fiber-matrix delamination as well as core compression when subjected to a shock wave loading. When subjected to low temperature environment and shock loading, the core material showed brittle behavior resulting in core-cracking in the sandwich composite. The dynamic response and failure mechanisms demonstrated by the sandwich composites at these temperatures were in correspondence with the high strain rate behavior of the individual constituents.

Comparative Study the Effect of Different Raw Powder on Foaming Process during Sandwich Rolling

An element powder-Al plate sandwich rolling processing is proposed for preparation of aluminum alloy foam sandwich with a metallurgy bonding interface. By comparing the deformation characteristic and it’s effect on foaming processing by using Al, Si elements and AlSi alloy powder, and to explore the effect of different powder as raw material on preparing foam aluminum sandwich. It was found that the densification of precursor using element powder is higher than using alloy powder, and rolling deformation resistance is smaller. And it was more suitable to manufacture different types of foam core material by using element powder as raw material.

Determination of mechanical properties of PVC foam using a modified Arcan fixture

The design and development of a modified Arcan fixture (MAF) is described. The purpose of the fixture is to characterise polymer foam materials with respect to their tensile, compressive, shear and bidirectional mechanical properties. The MAF enables the application of pure compression or high compression to shear bidirectional loading conditions that is not possible with conventional Arcan fixtures. The tensile and shear behaviour to failure of a cross-linked Divinycell H100 PVC foam core material are studied using Digital Image Correlation (DIC). A detailed investigation of the parasitic effects of the fixture and misalignment of the fixture and loading machine are discussed. Thermoelastic Stress Analysis (TSA) is used to directly examine and validate the uniformity and symmetry of the stress fields obtained for both tensile and shear specimens. To account for the inhomogeneity of the strain field across the specimen cross sections, a “correction factor” for the measured “gauge section” surface strains has been determined using nonlinear finite element analysis (FEA). The outcome is a set of validated mechanical properties that are in excellent agreement with material property measurements conducted using conventional test tensile and shear test fixtures.

The Influence of Construction Detailing on the Fire Performance of Polystyrene Insulated Panels

Concerns exist both in New Zealand and internationally about the performance in fire of insulated panel, particularly relating to the combustibility of polymer foam core materials. The focus of this paper is the most commonly used panel product in New Zealand, polystyrene insulated panel (PIP), with its core of expanded polystyrene. A series of experimental research projects were conducted to investigate the fire performance of the material. The objective of the research programme was to quantify the fire performance, using oxygen depletion calorimetry, of different construction details. In addition, laboratory experiments also investigated the influence of fire retardant treatment and the issue of fire spread in panel cavities. The major conclusion of the research was that maintaining the integrity of the metal skins of PIP had a quantifiable and significant beneficial impact on the fire performance of PIP.

Effects of Seawater and Low Temperatures on Polymeric Foam Core Material

Polymeric composite sandwich structures, often manufactured using a thick foam core material and thin composite facings, are of significant interest in naval applications. This paper summarizes the coupled effect of sea water and low temperature on the mechanical properties of closed cell polymeric H100 foam core material. The study considers the effects of harsh sea environmental conditions on the fracture and deformation behavior of such a foam material under complex loading conditions that include tension, torsion, compression, and true-triaxial stress paths. Mechanical testing techniques are developed using coupon samples of suitable geometry that minimize grip effects on these low density complex foam materials, along with information associated with the observed cross-anisotropic behavior. Interfacial delamination fracture response for the sandwich structures due to the combined effects of sea water and low temperature are evaluated and the associated degradation in critical energy release rate for delamination is found to be substantial. Experimental data for H100 foam cores associated with moisture induced expansional strains are also included.

New materials tackle weight, strength and cure time issues for wind turbine blades

Bernd Hoevel, Application Technology Development Leader, Alternative Energy, and Alain Sagnard, Senior R&D Engineer, at The Dow Chemical Company describe how the company's foam core material and epoxy resin systems can help manufacture lighter, stronger wind turbine blades.

A Methodology for Obtaining Material Properties of Polymeric Foam at Elevated Temperatures

A new methodology to characterise the elastic properties of polymeric foam core materials at elevated temperatures is proposed. The focus is to determine reliable values of the tensile and compressive moduli and Poisson’s ratio based on strain data obtained using digital image correlation (DIC). In the paper a detailed coverage of the source of uncertainties in the experimental procedure is provided. The uncertainties include those associated with the load introduction, the measurement and the data processing. The design of the specimens and loading jigs are developed and assessed in terms of the introduction of uniform strain. It is shown that due to the mismatch in stiffness between the jig material and the foam the introduction of a uniform strain through the cross section of the specimens is difficult to obtain. A means for correcting for the non-uniform strain across the specimen cross section is developed. To validate the methodology, tests are firstly conducted at room temperature on Divinycell PVC H100 foam. It is shown that the material is highly anisotropic with a stiffness of 50% less in the plane of the foam sheet compared to the through-thickness direction. It is also shown that because of the compliance of the foam, jig misalignment causes large errors in the measurement, and a means for correcting for this is defined. Tests are then conducted in a temperature controlled chamber at elevated temperatures ranging from 20°C to 90°C. A nonlinear reduction in Young’s modulus is obtained with significant degradation occurring after 70°C. The Poisson’s ratio remains fairly stable at different temperatures. A strong theme in the paper is the accuracy and precision of the DIC data and the factors which introduce scatter in the data, along with the uncertainties that this introduces. Particular attention is paid to the affect of the correlation parameters on the derived strain data.

Comparison of foam core materials in innovative lightweight wood-based panels

In the present paper, lightweight wood-based foam core panels were produced in a novel continuous process. Expandable microspheres (MS) and expandable polystyrene (EPS) were used as core materials. The influence of surface thickness and core materials on bending strength, internal bond and specific strength of the produced panel were investigated. With increasing the surface thickness, bending strength was increased in both core types of panels. The internal bond value of the panels with expandable microspheres was steadily raised while the surface thickness was increased. The internal bond for the expandable polystyrene with increasing face thickness was reduced. In comparison with conventional particleboards, the specific bending strength and internal bond were increased. In addition, FE-SEM-microscopy and gamma-ray densitometry were used to characterize the panels.

Noise mitigation and wave number characterization in sandwich structures.

Sandwich structures are the ideal materials to use for many engineering applications since they are lightweight, strong, and stiff. While sandwich structures are mechanically advanced, their acoustic properties are not ideal since they are excellent sound radiators. Therefore by studying their wave number characteristics, the acoustic properties of sandwich structures can be improved to help mitigate noise propagation. This study used several different combinations of carbon fiber-epoxy, glass fiber-epoxy, and aluminum face sheets with honeycomb and Rohacell foam core materials. These beams were vibrated with an electro-dynamic shaker, while the vibration response was measured at numerous equidistant points along the beam with a micro-accelerometer. Results showed the frequency response functions, which were transformed to the wave number/frequency domain yielding dispersion plots. From this plot one can calculate the coincidence frequency of the sandwich structure, which determines how and when the structure will radiate noise. Experimental results showed that thin aluminum face sheets with Rohcell 51 WF foam core had the greatest noise mitigation properties with a coincidence frequency of approximately 4500 Hz. Furthermore, it is shown that the experimental wave speed values agree with analytical methods. [Work supported by The Boeing Company.]

Face/core interface fracture characterization of mixed mode bending sandwich specimens

Debonding of the core from the face sheets is a critical failure mode in sandwich structures. This paper presents an experimental study on face/core debond fracture of foam core sandwich specimens under a wide range of mixed mode loading conditions. Sandwich beams with E-glass fibre face sheets and PVC H45, H100 and H250 foam core materials were evaluated. A methodology to perform precracking on fracture specimens in order to achieve a sharp and representative crack front is outlined. The mixed mode loading was controlled in the mixed mode bending (MMB) test rig by changing the loading application point (lever arm distance). Finite element analysis was performed to determine the mode-mixity at the crack tip. The results showed that the face/core interface fracture toughness increased with increased mode II loading. Post failure analysis of the fractured specimens revealed that the crack path depends on the mode-mixity at the crack tip, face sheet properties and core density.

Experimental and Numerical Investigation of Free Vibrations of Composite Sandwich Beams with Curvature and Debonds

In this paper experimental and numerical results concerning the dynamic response of composite sandwich beams with curvature and debonds are reported. Sandwich beams made of carbon/epoxy face sheets and polyurethane foam core material were manufactured with four different radii of curvature and debonds between the top and bottom interface of face sheet and foam core. Dynamic response was obtained using the impulse frequency response technique under clamped-clamped boundary condition. Experimental results were compared with numerical finite element model results. A combined experimental and numerical FE approach was used to determine the material properties of the skin and foam core materials based on modal vibration and static flexure tests. Results indicate that the fundamental frequency increases with increasing curvature angle, however, for higher frequencies; the natural frequencies are not significantly affected. Also, it is found that face/core debond causes reduction of the natural frequencies due to stiffness degradation.

Derivation of temperature dependent mechanical properties of polymer foam core materials using optical extensometry

A methodology for determining the Young’s modulus and Poisson’s ratio of polymer foams is presented. The approach is based on a non contact measurement technique so that the behaviour of the complaint foam is modified by the attachment of gauges or extensometers. Firstly experiments are conducted at room temperature and then at elevated temperatures in a thermal chamber. Readings are taken through an optical window using a standard digital camera. Digital image correlation is used to obtain the strains

Enhancing specific strength and stiffness of phenolic microsphere syntactic foams through carbon fiber reinforcement

AbstractComposite syntactic foams with different densities were fabricated with hollow phenolic microspheres, a phenolic resin binder, and a small addition of chopped carbon fibers. Compressive, shear, and tensile properties were studied for the syntactic foams with and without carbon fibers. Two fiber layer orientations, either parallel (P‐type) or perpendicular (N‐type) to the loading direction, were considered in the mechanical testing. For N‐type foams, mechanical properties were weakly dependent on foam density. For P‐type foams, the mechanical properties of the foams were strongly dependent on the strength of the supporting matrix. The specific strength and specific stiffness of the P‐type foams were significantly enhanced compared with the neat foams. These findings indicate that fiber reinforcement is an effective way to enhance the mechanical performance of syntactic foams, and the enhanced performance should lead to applications as a foam core material for sandwich structures. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers

Experimental study of the low-velocity impact behaviour of primary sandwich structures in aircraft

The susceptibility of sandwich structures to localised (impact) damage is one of the main reasons why the sandwich concept is not yet used in large primary aircraft structures of airliners. The objective of this work is to experimentally investigate the damage tolerance of representative composite sandwich panels for primary aircraft structures. Instrumented low-velocity impact tests were performed on sandwich specimens consisting of carbon Non-Crimp Fabric/epoxy facings and a Rohacell (PMI) foam core. Both internal and external damage resulting from these impact events was evaluated. The foam core material has a considerable influence on the amount of damage detected by ultrasonic TTU C-scan. CAI tests however showed that this core damage has no significant influence on the residual compressive strength of the specimens.

The Fracture Toughness of PVC / GRP Sandwich Systems in Marine Environment

The aim of this study is to investigate experimentally the fracture toughness of sandwich systems with the PVC foam core in marine environment with a Mode-I Cracked Sandwich Beam (CSB) test arrangement. Five CSB specimens at each condition were tested. To get the values under marine environment, one set of specimen was conditioned in a 5% solution of NaCl for a 120 h period at a constant temperature of 50°C. It has been found that the fracture toughness of the PVC foam core material slightly increases under marine environmental conditions. It can be concluded that under the conditions in this study the system with the PVC core was largely unaffected by the immersion process.