The effects of foam filling on compressive response of hexagonal cell aluminum honeycombs under axial loading-experimental study

In this paper the effects of foam filling of honeycomb panels on their plastic behavior and mechanical properties are studied experimentally. Five types of Al 5052-H39 honeycombs in bare and foam filled conditions are subjected to quasi-static axial compressive loading. The panels are selected so that the effects of parameters such as the cell size, the cell walls thickness and the panel thickness on the mean crushing strength, energy absorption capacity and the wavelength of the folds could be investigated. Tests show that foam filling of panels increases their mean crushing strength and energy absorption capacity up to 300% and the less the honeycomb density the greater the effect of foam filling. Furthermore, mean crushing strength of foam filled panels is larger than the sum of the mean crushing strengths of bare honeycomb and foam alone. The wavelength of folds and densification strain in foam filled panels are smaller than those of bare honeycombs. These tests also showed that unlike the theoretic formula the panel thickness influences the mean crushing strength of honeycomb.     

Hybrid carbon fiber composite lattice truss structures

Carbon fiber reinforced polymer (CFRP) composite sandwich panels with hybrid foam filled CFRP pyramidal lattice cores have been assembled from linear carbon fiber braids and Divinycell H250 polymer foam trapezoids. These have been stitched to 3D woven carbon fiber face sheets and infused with an epoxy resin using a vacuum assisted resin transfer molding process. Sandwich panels with carbon fiber composite truss volumes of 1.5–17.5% of the core volume have been fabricated, and the through-thickness compressive strength and modulus measured, and compared with micromechanical models that establish the relationships between the mechanical properties of the core, its topology and the mechanical properties of the truss and foam. The through thickness modulus and strength of the hybrid cores is found to increase with increasing truss core volume fraction. However, the lattice strength saturates at high CFRP truss volume fraction as the proportion of the truss material contained in the nodes increases. The use of linear carbon fiber braids is shown to facilitate the simpler fabrication of hybrid CFRP structures compared to previously described approaches. Their specific strength, moduli and energy absorption is found to be comparable to those made by alternative approaches.     

Evaluation of sandwich panels with various polyurethane foam-cores and ribs

The objective of this study was to evaluate three potential core alternatives for glass fiber reinforced polymer (GFRP) foam-core sandwich panels. The proposed system could reduce the initial production costs and the manufacturing difficulties while improving the system performance. Three different polyurethane foam configurations were considered for the inner core, and the most suitable system was recommended for further prototyping. These configurations consisted of high-density polyurethane foam (Type 1), a bidirectional gridwork of thin, interconnecting, GFRP webs that is in-filled with low-density polyurethane foam (Type 2), and trapezoidal-shaped, low-density polyurethane foam utilizing GFRP web layers (Type 3). The facings of the three cores consisted of three plies of bidirectional E-glass woven fabric within a compatible polyurethane resin. Several types of small-scale experimental investigations were conducted. The results from this study indicated that the Types 1 and 2 cores were very weak and flexible making their implementation in bridge deck panels less practical. The Type 3 core possessed a higher strength and stiffness than the other two types. Therefore, this type is recommended for the proposed sandwich system to serve as a candidate for further development. Additionally, a finite element model (FEM) was developed using software package ABAQUS for the Type 3 system to further investigate its structural behavior. This model was successfully compared to experimental data indicating its suitability for parametric analysis of panels and their design.     

Study on quasi-static compressive properties of aluminum foam-epoxy resin composite structures

Closed cell aluminum foam (AF) has extensive application prospects due to its extended plateau stress region and high energy absorption capacity. As one of the most important manufacturing routes for aluminum foams, the gas injection method still does not guarantee an excellent energy absorption performance. In order to improve the energy absorption capacity while remaining the plateau region extended, epoxy resin (ER) was infiltrated into the aluminum foams in various composite forms. In this paper, different AF-ER composite structures were designed and their uniaxial quasi-static compressive behaviors were investigated. The experimental results indicate that the plateau stress and energy absorption capability of the AF-ER composite structures increase with increasing amount of epoxy resin. Additionally, both the stress fluctuation and the peak stress in the plateau region become insignificant, which is beneficial for energy absorption applications. The composite form is also confirmed to have great effect on the compressive property of the AF-ER composite structures. At last, the Young's modulus of the composite structure is theoretically deduced while the plateau stress and the energy absorption capacity are fitted with the composite parameters by considering the contribution of aluminum foam, epoxy resin and the reciprocity of these two materials. The present model is found to have good agreement with experimental data.     

Energy Absorption Characteristics of Web-Core Sandwich Composite Panels Subjected to Drop-Weight Impact

The objective of the study was to characterise the energy absorption of composite panels with tied cores, subjected to a drop weight impact test. Numerical simulations based on explicit finite element analysis have successfully modelled low velocity impact tests carried out on sandwich panels with web-core structure and plastic foam. The numerical model has been validated in terms of the failure behaviour of the panel and the variation of the contact force after the initial peak load corresponding to flexural failure. The numerical model is used for a better interpretation of the test results and of the failure mechanisms within the structure. The contribution to the overall energy absorption of the different parts composing the panels has been studied, with the aim of evaluating the feasibility of using low density foam in combination with web-core reinforcement in structural applications.     

The influence of correlating material parameters of gradient foam core on free vibration of sandwich panel

For the sandwich panel with mass density gradient (DG) foam core, the Young's modulus of the core varies with the mass density along the thickness direction. To characterize the correlative effect of Young's modulus and mass density of the DG closed-cell foam material, a simplified formula is presented. Subsequently, based on a high-order sandwich plate theory for sandwich panel with homogeneous core, a new gradient sandwich model is developed by introducing a gradient expression of material properties. Finite element (FE) simulation is carried out in order to verify this model. The results show that the proposed model can predict well the free vibration of composite sandwich panel with the gradient core. Finally, the correlating effects of material parameters of the DG foam core on the natural frequencies of sandwich panel are investigated. It is found that the natural frequencies of sandwich panels decrease as the gradient changes of the DG foam cores increase under the condition of that the core masses keep constant.     

The effect of pin reinforcement upon the through-thickness compressive strength of foam-cored sandwich panels

Titanium and carbon fibre pins have been inserted into the polymethacrylimide foam core of a sandwich panel (with carbon fibre face sheets) in order to increase the through-thickness strength. The elevation in compressive strength has been measured both quasi-statically and dynamically using a direct Kolsky bar, and the sensitivity of strength to the relative density and thickness of foam have been determined. An X-ray CAT scan machine was used to examine the deformed shape of the pins during interrupted compression testing of the sandwich specimens. It was found that the foam core stabilises the pins against elastic buckling, and the pin-reinforced core has a strength and energy absorption capacity in excess of the individual contributions from the foam and unsupported pins. It is shown that the compressive strength is governed by elastic buckling of the pins, with the foam core behaving as an elastic Winkler foundation in supporting the pins. The peak strength of the pin-reinforced core is increased by a factor of about four when the speed of loading is increased from the quasi-static rate of about 10⁻⁶ ms⁻¹ to the dynamic value of 10 ms⁻¹; it is concluded that the micro-inertia of the pins stabilises them against elastic buckling and leads to the observed elevation in strength.     

钢板夹泡沫铝组合板抗爆性能研究

鉴于泡沫铝材料优异的吸能特性和夹层结构在强度、刚度上的优势,提出了分层结构为钢板-泡沫铝芯层-钢板的抗爆组合板。对厚度为10 cm、7 cm和5 cm的组合板进行了5组不同装药量的爆炸试验,考察了各板在不同装药量爆炸条件下的变形及破坏情况,并对变形破坏过程进行了理论分析。研究表明:组合板承受爆炸冲击荷载时,通过局部压缩变形和整体弯曲变形吸收能量。钢板相同时,适当增大泡沫铝芯层厚度,增强面板与芯层间连接,可提高该组合板的抗爆性能,防止组合板发生剥离,减小其承受爆炸冲击荷载时产生的变形。     

碳纤维增强环氧树脂复合材料金字塔点阵夹芯假脚结构在竖向载荷下的力学性能

对碳纤维增强树脂复合材料金字塔点阵夹芯假脚结构在竖向载荷下的力学性能进行研究。制备了三种不同相对密度的假脚,并进行了竖向载荷压缩试验。结果表明,相对密度对结构力学性能的影响显著,载荷-位移曲线呈非线性,峰值载荷和刚度值随相对密度的增加而增大,三种相对密度的破坏模式均为节点的失效和面板的皱曲,结构具有一定的能量吸收能力。建立了金字塔点阵夹芯假脚结构的理论强度预报模型,给出了结构在竖向载荷作用下的挠度响应,获得了四种失效模式和临界破坏载荷。对比了理论计算与试验的峰值载荷、破坏模式和挠度,得到较好的一致性。给出假脚结构参数(面板厚度、杆件角度和杆件直径)对破坏模式和破坏临界载荷的影响,并绘制了结构失效机制图。      

Three-point bending of sandwich beams with aluminum foam-filled corrugated cores

Sandwich panels having metallic corrugated cores had distinctly different attributes from those having metal foam cores, the former with high specific stiffness/strength and the latter with superior specific energy absorption capacity. To explore the attribute diversity, all-metallic hybrid-cored sandwich constructions with aluminum foam blocks inserted into the interstices of steel corrugated plates were fabricated and tested under three-point bending. Analytical predictions of the bending stiffness, initial failure load, peak load, and failure modes were obtained and compared with those measured. Good agreement between analysis and experiment was achieved. Failure maps were also constructed to reveal the mechanisms of initial failure. Foam insertions altered not only the failure mode of the corrugated sandwich but also increased dramatically its bending resistance. All-metallic sandwich constructions with foam-filled corrugated cores hold great potential as novel lightweight structural materials for a wide range of structural and crushing/impulsive loading applications.     

Response of thin-skinned sandwich panels to contact loading with flat-ended cylindrical punches: Experiments,numerical simulations and neutron diffraction measurements

The response of aluminium foam-cored sandwich panels to localised contact loading was investigated experimentally and numerically using flat-ended cylindrical punch of four varying sizes. ALPORAS and ALULIGHT closed-cell foams of 15 mm thickness with 0.3 mm thick aluminium face sheets (of 236 MPa yield strength) were used to manufacture the sandwich panels. Face sheet fracturing at the perimeter of the indenter, in addition to foam cells collapse beneath the indenter and tearing of the cell walls at the perimeter of the indenter were the major failure mechanisms of the sandwich panels, irrespective of the strength and density of the underlying foam core. The authors employed a 3D model in ABAQUS/Explicit to evaluate the indentation event, the skin failure of the face sheets and carry out a sensitivity study of the panel's response. Using the foam model of Deshpande and Fleck combined with the forming limit diagram (FLD) of the aluminium face sheet, good quantitative and qualitative correlations between experiments and simulations were achieved. The higher plastic compliance of the ALPORAS led to increased bending of the sheet metal and delayed the onset of sheet necking and failure. ALULIGHT-cored panels exhibited higher load bearing and energy absorption capacity, compared with ALPORAS cores, due to their higher foam and cell densities and higher yield strength of the cell walls. Additionally, they exhibited greater propensity for strain hardening as evidenced by mechanical testing and the neutron diffraction measurements, which demonstrated the development of macroscopically measurable stresses at higher strains. At these conditions the ALULIGHT response upon compaction becomes akin to the response of bulk material with measurable elastic modulus and evident Poisson effect.     

Improving the bending strength and energy absorption of corrugated sandwich composite structure

The bending strength, stiffness and energy absorption of corrugated sandwich composite structure were investigated to explore novel designs of lightweight load-bearing structures that are capable of energy absorption in transportation vehicles. Key design parameters that were considered include fibre type, corrugation angle, core-sheet thickness, bond length between core and face-sheets, and foam inserts. The results revealed that the hybridization of glass fibres and carbon fibres (50:50) in face-sheets was able to achieve the equivalent specific bending strength as the facet-sheets made entirely of carbon fibre composites. Increasing the corrugation angle and the core sheet thickness improved the specific bending strength of the sandwich structure, while increasing the bond length led to a reduction in the specific bending strength. The hybrid composite coupons with foam insertion showed medium energy absorption, ranging between the glass fibre and the carbon fibre composite coupons, but the highest crush force efficiency among all designs.     

Hierarchical lattice composites for electromagnetic and mechanical energy absorptions

A new hierarchical radar absorbing structure with multifunction of ultra-light weight, anti-crushing and radar absorption was designed and made by glass fiber reinforced lattice composites filled with radar absorbing foams. Experiments were performed to reveal the electromagnetic absorption and anti-crushing behaviors. Mechanisms of the composite in electromagnetic absorption and anti-crushing were analyzed. The newly designed composite lattice displays excellent performances in absorbing both microwave and mechanical energies at ultra-light weight. To balance the anti-crushing and radar absorption behaviors, key factors of the composite lattices, including the panel thickness, the relative density of the lattice, the cell dimension and the geometry, were revealed based on the analysis and experiments.     

钢板夹泡沫铝组合板抗弯性能研究

试验设计了6块钢板夹泡沫铝组合板,其中无侧板组合板与有侧板组合板各为3块,侧板材料与面板相同,泡沫铝芯层厚度分别为40 mm、60 mm和90 mm。对组合板进行抗弯试验,绘制了组合板跨中荷载-位移（P-δ）曲线,记录了组合板变形失效过程。基于Gibson模型最大承载力公式建立了无侧板组合板的失效模式图。推导了有侧板组合板最大承载力计算公式,建立了失效模式图。结果表明:泡沫铝芯层厚度越大,组合板承载力越高,加载刚度越大。建立的失效模式图可以较好预测组合板的失效模式。与无侧板组合板相比,仅增加侧板,可以显著提高组合板的承载能力和加载刚度,有效限制泡沫铝开裂后裂缝的进一步开展。通常无侧板组合板每种失效模式仅独立对应失效模式图中一块区域,而有侧板组合板失效模式图被划分为四块区域,且表皮屈服失效模式独立对应两块区域。     

微孔结构对PMI泡沫准静态压缩性能的影响

基于BBC点集建立了聚甲基丙烯酰亚胺(PMI)闭孔泡沫的Kelvin十四面体模型和Laguerre模型,并采用有限元方法研究了其在准静态载荷作用下的压缩性能。分析了孔径大小、泡孔体积离散系数对压缩弹性模量、初始峰值应力和能量吸收能力的影响。结果表明:Kelvin十四面体模型可以较好地预测PMI泡沫的压缩弹性模量和峰值应力;在相同相对密度条件下,小孔径泡沫的初始峰值应力和能量吸收能力均高于大孔径泡沫,而压缩弹性模量则低于大孔径泡沫;随着泡孔体积离散系数的增大,闭孔PMI泡沫压缩弹性模量、初始峰值应力和能量吸收能力均减小。     

Fiber-reinforced composite foam from expandable PVC microspheres

The properties of composite foam based on PVC expandable microspheres reinforced with continuous aramid fibers are described. The foam was fabricated by infiltrating low-density non-woven fiber webbing with PVC microspheres. The assembly was subsequently heated to expand the foam. The resulting composite foam consisted of 10 wt% aramid fibers and had a density of 100 kg/m³. Mechanical properties, crack propagation, and microstructure of composite foams were evaluated and compared with properties of similar unreinforced foam and with commercial PVC foam of comparable density. The influence of fiber concentration, fiber architecture and bonding was investigated also. Properties were measured in tension, shear, compression, and flexure using standard ASTM test methods. The composite foam performance equaled or surpassed the performance of most thermoplastic foams commercially available. The tensile strength and modulus of the composite foam increased by factors of 6 and 8, respectively, and the shear strength and modulus increased by factors of 1.8 and 2.4. The composite foam also exhibited improved strain energy density and damage tolerance, and reduced notch sensitivity.     

Aluminum foam integral armor: a new dimension in armor design

Closed-cell aluminum foam offers a unique combination of properties such as low density, high stiffness, strength and energy absorption that can be tailored through design of the microstructure. During ballistic impact, the foam exhibits significant non-linear deformation and stress wave attenuation. Composite structural armor panels containing closed-cell aluminum foam are impacted with 20-mm fragment-simulating projectiles (FSP). One-dimensional plane strain finite element analysis (FEA) of stress wave propagation is performed to understand the dynamic response and deformation mechanisms. The FEA results correlate well with the experimental observation that aluminum foam can delay and attenuate stress waves. It is identified that the aluminum foam transmits an insignificant amount of stress pulse before complete densification. The ballistic performance of aluminum foam-based composite integral armor (CIA) is compared with the baseline integral armor of equivalent areal-density by impacting panels with 20-mm FSP. A comparative damage study reveals that the aluminum foam armor has finer ceramic fracture and less volumetric delamination of the composite backing plate as compared to the baseline. The aluminum foam armors also showed less dynamic deflection of the backing plate than the baseline. These attributes of the aluminum foam in integral armor system add a new dimension in the design of lightweight armor for the future armored vehicles.     

An experimental study on energy absorbing structures made of fabric composites

Egg-box shaped energy absorbing structures made of fabric composites were fabricated to find out the compressive characteristics and energy absorption capacity. Various stacking sequences and boundary conditions (unconstrained and bonded) were examined to investigate the stress–strain curves during compression. Failure modes of composite egg-box panels were observed and investigated correlating each step of meaningful collapsing behaviour. In order to check out the possibility as an ideal energy absorbers foam filled composite egg-box panels were fabricated and tested. From the test results it was found that the foam filled composite egg-box panels had good energy absorption capacity with smooth stress–strain curves which resembles the ideal energy absorber. The energy absorption per unit mass of composite egg-box panels made of different types of material and stacking sequences was calculated and compared with.     

缝纫泡沫夹芯复合材料的刚度预测与试验验证

基于材料细观结构,建立了缝纫泡沫夹芯复合材料的刚度预测模型,并进行了刚度性能的相关试验验证。其中,对缝纫复合材料层合面板部分,考虑了缝纫角对单胞尺寸和富脂区大小的影响,以及缝纫前后层合面板厚度的变化对复合材料面板纤维体积含量的影响,采用改进的纤维弯曲模型计算了缝纫复合材料层合面板的刚度;对缝纫增强的泡沫夹芯部分,把缝线树脂柱看作是泡沫基体中的增强相,将其简化为特殊的单向增强复合材料,提出了用串并联组合模型来预测其刚度。试验测试了缝纫泡沫夹芯复合材料板试件的刚度。应用本文模型对缝纫层合面板和缝纫泡沫夹芯复合材料板的刚度进行预测,结果均与试验结果吻合较好。采用理论模型系统研究了缝纫参数和结构参数对缝纫泡沫夹芯复合材料刚度的影响。