Experimental and analytical study on homogeneous and layered Al matrix syntactic foams under impact
Introduction
Aluminium matrix syntactic foams (AMSFs) are a novel class of lightweight materials, which use hollow or porous ceramic particles such as alumina cenospheres [1,2], fly ash [3,4] or E-spheres [5] to reinforce the aluminium matrix. The main role of the particles or microballoons is to introduce porosity. AMSFs offer advantages of low weight, high specific stiffness, improved strength and high damage tolerance due to their mechanical energy absorption capabilities. These properties give AMSFs many applications such as cores in sandwich structures, crash protectors and damping panels [6].
The manufacture and mechanical properties of AMSFs with homogeneous structures have been widely studied. AMSFs were normally manufactured by the infiltration casting method and their compressive and energy absorption behaviours were investigated [7]. The effects of Al volume percentage, bimodal ceramic microspheres and Al particle toughening on the compressive and energy absorption properties of AMSFs have been studied [8,9]. Tao and Zhao [10] studied the compressive failure mechanisms of AMSFs through observations of the un-confined and confined compression response. The impact response of AMSFs have also been studied by both experiment and simulation [11,12].
For porous metallic foams, energy absorption function is usually affected by porous structure. While hollow spheres can be divided into different densities and sizes by flotation methods and sieves. Then it is possible to designing specific syntactic foams to meet application demands with the variety of hollow spheres [13].
Functionally graded syntactic foams exhibit a gradual and controlled positional change of at least one property and often have desirable properties meeting the increasing demand in many industries [14], [15], [16]. The traditional way of manufacturing graded syntactic foams is fabricating each layer independently and subsequently bonding them together with an adhesive like epoxy. While fabricating the layers independently can easily tailor the physical and mechanical properties of each layer, layered syntactic foams manufactured from this approach have poor structural integrity between the layers. Graded syntactic foam components fabricated in one process by infiltration casting provide better structural integrity and superior mechanical properties [17].
Many efforts have been made to investigate the mechanical properties of syntactic foams under impact [18], [19], [20]. The majority of these studies, however, are largely confined to experimental characterisation or numerical modelling of stress - strain developments. Mechanistic understanding of the stress - strain evolutions and the role played by impact waves is still very limited. Karagiozova et al. [21] and Zheng et al. [22] proposed analytical models for stress evolutions in density-graded cellular materials based on the propagation of compaction waves and compared the analytical results with numerical simulation results. These models failed to capture one of the most important features of all experimental results - the fluctuation in stress evolution caused by propagation of impact waves. Rousseau et al. [23] made observations of stress fluctuation in syntactic foam samples by placing sensors at different locations. Rostilov and Ziborov [24] demonstrated the two-wave configuration stress evolution in syntactic foam under impact and used the Hugoniot states to describe the shocked states behind the wave front. Up to date, a mechanistic explanation of stress-strain evolutions in syntactic foams, especially with graded or layered structures, under impact is still lacking.
This study investigates the stress-strain evolution in AMSFs with homogeneous and layered structures under impact experimentally. An analytical model is developed to predict the stress and strain evolutions inside the AMSFs during impact. The theoretical predictions are compared with the experimental results.
Section snippets
Experimental
The AMSF samples were produced by infiltration casting [25], as shown schematically in Fig. 1, using a 6082 Al alloy and a hollow ceramic microsphere (CM) powder supplied by Envirospheres Pty Ltd. The CM powder has a composition of ~60% SiO2, ~40% Al2O3 and 0.4-0.5% Fe2O3 by weight and was separated into different particle size ranges. Two subset powders with particle size ranges of 75-150 μm and 250-500 μm, designated as small (S) and large (L), respectively, were used in the experiments. The
Compressive behaviour
Fig. 3 exhibits typical quasi-static compressive stress-strain curves of AMSFs. Specimen L has a much lower strength than specimen S but a better ductility, characterised by a much smoother plateau region. The strengths of the layered specimens LS and LSL fall between those of specimens L and S, with LSL having a slightly higher yield stress than LS. The difference in strength between L and S is due to their different structural characteristics. Ceramic microspheres in L are large and generally
Analytical model
Let us consider an AMSF specimen with an initial height h0, situated on a frictionless flat rigid die and subjected to compression by an upper die of mass M, moving with an initial speed v0, as shown in Fig. 7 (a). Based on the compressive stress-strain curves of the AMSF specimens (Fig. 3), the AMSF can be assumed to be an ideal elastic-linear strain-hardening material with yield stress Y, elastic modulus E and plastic modulus P, as shown schematically in Fig. 8. According to the impact theory
Conclusions
The impact response of AMSFs with both homogeneous and layered structures were studied experimentally. Layered structures LS and LSL provided lower impact peak stress and higher ductility than the average values of L and S. The three-layer structure had higher peak stress and lower ductility than the two-layer structure. The energy absorption capacity of the layered structures is the sum of the energy absorption capacities of the constituent layers.
An analytical model for stress and strain
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
Liang would like to thank the China Scholarship Council and the University of Liverpool for a PhD studentship.
Author statement
Chen conducted the experimental and analytical modelling work and drafted the manuscript. Zhao oversaw the whole project work, provided inputs to the theoretical analysis and assisted in writing the manuscript.
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2022, Composite StructuresCitation Excerpt :Collectively, these models demonstrate the ability and effectiveness of the shock wave theory to capture the dynamic behavior of foam materials of uniform density under impact loadings. In the attempt to exceed the impact resistance capacities of uniform density foam, density-graded foam with enhanced properties has been considered through discrete layered design [21–25]. As demonstrated by Yin et al. [26], the double-layer cellular cladding design can provide better resistance to water blast as compared to single-layer cladding.