Relative performance of metal and polymeric foam sandwich plates under low velocity impact

Highlights

• Low-speed Impact tests on sandwich plates were performed using Instron^R Dynatup apparatus.

• Numerical models for simulating impact were developed in LS-Dyna commercial finite element software.

• Relative performance of PVC and aluminum alloy foams is studied.

• Effect of core grading on energy and peak forces is evaluated.

• Peak loads from analysis are in good agreement with experiments and numerical models.

Abstract

Relative performance of metal and polymeric foam cored sandwich plates is studied under low velocity impact loading. The metal and polymeric foam sandwich plates are constructed using a core of 40 mm thickness (with two layers of 20 mm each) and aluminum faceplates. Metal foam sandwich plates are constructed using aluminum alloy foam (ALPORAS) core while polymeric foam sandwich plates are constructed using polyvinyl chloride (Divinycell H80 and H250) foam core. Impact experiments are conducted with a hemispherical punch of mass 8.7 kg at a nominal velocity of 5.8 m/s. The effect of stepwise core grading on the maximum dynamic penetration force as well as energy absorption is studied. To maximize the energy absorption or to minimize the mass of the sandwich plate for a given penetration force, alternatives to Alporas foam are chosen based on either equivalent density (H250) or through-thickness compressive yield strength (H80). The increase in penetration force and energy absorption resulting from the choice of H250 in place of Alporas for the same density of the foam as well as the effect of decrease in mass of the sandwich panel by choosing H80 foam in place of Alporas for the same compressive strength of the foam is discussed. Numerical models were developed in LS-Dyna to predict the impact response (force-displacement history) and failure modes. Upperbound analysis is used to estimate the maximum penetration force. Peak force, energy absorption values and failure mode patterns obtained by analytical estimates, experimental measurements and numerical predictions all agree well.

Introduction

Naturally available cellular materials such as balsa wood, bone have high specific strength and stiffness. Recently both engineering alloys and polymers have been foamed to a range of relative densities by a variety of manufacturing processes [1], [2]. These foams are utilized as cores in sandwich construction with strong and stiff faceplates designed against static indentation (loading rates of less than10^–3 s⁻¹), low (5–10 s⁻¹) or high (10²–10⁵ s⁻¹) velocity impact and blast loading conditions [3], [4], [5], [6].

Mines et al. [7] conducted a series of impact tests on two types of sandwich plates (woven glass vinyl ester skin/Coremat core and woven glass epoxy skin/honeycomb core) and concluded that perforation energy can be increased by using ductile skins and use of multiple layers of the core. Zhao et al. [8] have modified the split-Hopkinson pressure bar to measure the impact response of aluminum alloy foam (CYMAT) sandwich plates up to a velocity of 50 m/s. Akil Hazizan and Cantwell [9] conducted impact tests on sandwich plates with woven glass phenolic resin faceplate and three types of polymeric foams as cores (viz., linear polyvinyl chloride, PVC; polyetherimide or PVC/Polyurethane, PUR foam). They observed shear cracking of core, fiber buckling close to impact and delamination in the top faceplate as dominant failure patterns. More recently, Zhou et al. [10] conducted normal and oblique impact tests on a range of densities of bare foams and sandwich plates (with linear PVC, cross-linked PVC or polyethylene terephthalate, PET foams from Airex AG). They found that the cross-linked PVC foams offered a higher perforation resistance than linear PVC foams at low densities, whereas the converse is true for higher density foams. Experimental observations on bare foams revealed a combination of pure core shear and conoid failure modes.

Expensive experimental works can be replaced by approximate analytical models for design purposes. Hoo-Fatt and Park [11] provided an upperbound estimates for the peak load of sandwich plates subjected to low velocity impact. The authors considered face failure by shear (under flat punch) or tensile failure of face (under hemispherical punch), core shear and bottom faceplate failure as dominant failure modes. Olsson [12] proposed a method for predicting the small/large mass impact response and damage of composite sandwich plates accounting for local core crushing, delamination and large face sheet deflections without any empirical constants in the contact law. Semi-analytical methods are used to predict the force-displacement history under large mass impact, in which contact between plate and impactor is modeled by a linear or nonlinear spring [13], [14].

Experimental evaluation of impact response on a variety of sandwich plates is costly and time consuming, and approximations in analytical models can be avoided by using detailed numerical models. Appropriate numerical models can accurately predict the impact force-displacement histories and failure modes. A detailed description of the numerical modeling strategies available in literature is provided in our previous work [15].

It is important to investigate the relative performance of the foams so as to minimize the weight of the sandwich panel for a given energy absorption capacity or to maximize the energy absorption for a given mass under prescribed loading conditions. Relative performance of sandwich plates with Alporas^® and Divinycell^® PVC foams is studied by Compston et al. [16]. Faceplates for Alporas and PVC foam sandwich plates are constructed using lamina of 754 gsm (plain weave glass fiber/polypropylene prepreg) and 450 gsm (plain weave glass fiber/vinylester resin matrix using wet lay-up), respectively. However, the overall fiber content in each sandwich structure is maintained approximately the same and the authors concluded that PVC foam sandwich plates have smaller overall deflection against Alporas sandwich plates. Relative performance of motorcycle helmet with aluminum alloy foam (Alulight) shell against acrylonitrile butadiene styrene, ABS polymeric shell is studied by Pinnoji et al. [17] and concluded that the resultant force on the head is small in case of low density metal foam helmet. Crupi et al. [18] investigated the relative performance of the GFRP/PVC foam (75 kg/m³) sandwich plate against two types of aluminum alloy sandwich plates (from Schunk GmbH and Alulight GmbH) and concluded that PVC foam sandwich plates absorb high energy compared to aluminum alloy sandwich panels. However, in the attempts reported in literature for relative performance evaluation, the sandwich panels do not consists of same type of faceplate materials.

In contrast to the conventional sandwich construction with single core layer, several layers of foams are often used in recent literature to increase the energy absorption capacity. Zeng et al. [19] have investigated the perforation of two core gradient varieties (viz., monotonically ascending density and monotonically descending density) in sandwich plate using four different densities of polyepoxide hollow spheres layers. Authors concluded that the ascending density configuration has shown higher energy absorptions with low damage initiation force against descending density configuration. Yang and Qiao [20] have used two layers of aluminum honeycomb core sandwich structures to protect highway bridge girders under low velocity impact loading. Gardner et al. [21] have used stepwise graded core with two layers, three layers, and four layers of foam core gradation to monotonically increase the acoustic wave impedance of sandwich plate in testing the blast resistance of sandwich structures using shock-tube apparatus. Experimental results showed that monotonic increase in number of graded layers increases the blast resistance of the structure. To the best of authors knowledge, a comprehensive relative performance evaluation and failure modes comparison between PVC (Divinycell H grade) and aluminum alloy foam (Alporas) and effect of core grading have not been reported in literature. Hence, a detailed evaluation of relative performance and effect of Alporas and Divinycell PVC foam grading under low velocity impact is the focus of the present work using experimental and numerical studies.

In the present work, relative performance of sandwich plates consisting of Divinycell PVC and Alporas (ALP) foam core with aluminum faceplates is studied under low velocity impact. For a meaningful comparison, H80 and H250 are selected with yield strength and density equal to that of Alporas foam: H80 has same yield strength as that of Alporas foam and H250 has same mass density as that of Alporas. Effect of core grading on maximum dynamic penetration force and energy absorption capacity is also investigated by interchanging the layers of H80, H250 and Alporas foams. Using numerical models, failure modes and impact force-displacement response are predicted and compared with that of experiments. Upperbound analytical calculations were used to estimate the penetration force.