Dynamic compression of cellular cores: temperature and strain rate effects

Cross-linked polyvinyl chloride closed-cell foams were examined under quasi-static and high strain rate compression loading using a servo-hydraulic testing machine and a modified split Hopkinson pressure bar apparatus consisting of polycarbonate bars for strain rates up to 1900 s⁻¹. Three foam densities were examined viz. 75, 130, and 300 kg/m³. Each core density has been subjected to compressive loading at room and elevated temperatures. A reverse trend in failure modes was observed when moving from room to elevated temperatures at high strain loading, which was not found in quasi-static testing at elevated temperatures. Accordingly, post-impact tests were conducted to evaluate the residual strength of the foam cores subject to elevated temperatures and HSR. Results of the post-impact test revealed that the foam cores are still capable of taking some loading. The residual strength of cores was fairly constant regardless of temperature therefore recovery of volume does not signify an increase in residual strength of cores.     

Dynamic compression behavior of ultra-high performance cement based composites

In order to investigate the dynamic compression behavior of Ultra-high performance cement based composites (UHPCC) used in defense works, UHPCC with 200 MPa compressive strength is prepared by replacing a large quantity of cement by industrial waste residues such as silica fume, fly ash and slag; and substituting ground fine quartz sand (≤600 um in diameter) with natural sand (2.5 mm in diameter). Split Hopkinson pressure bar (SHPB) is performed on UHPCC with different fiber volume fraction to investigate the dynamic compression behavior. Results show that impact resistance of UHPCC is improved with an increase of fiber volume fraction. The dynamic compressive strength of UHPCC is also increased with an increase of strain rate. In addition, the finite element method (LS-DYNA) is employed to simulate the whole impact process of UHPCC. Numerical simulations demonstrate that the Johnson_Holmquist_Concrete material constitutive model can be used for the dynamic compression of concrete. The numerical values are in good agreement with experimental results.     

被动围压条件下岩石材料冲击压缩试验研究

为研究煤矿岩石材料被动围压条件下动态力学性能和变形破坏规律,利用Ø50mm变截面分离式Hopkinson压杆(SHPB)试验装置,对45#钢质套筒环向约束状态下煤矿岩石试件进行了不同加载速率冲击压缩试验。试验结果表明：被动围压条件下SHPB试验中,岩石试件的材料延性和抗破坏能力均得到增强,试件轴向应力是采用同种加载条件无围压SHPB试验时的1.2倍,破坏应变比无围压SHPB试验提高2～3倍,且径向应力随轴向应变增大总体呈上升趋势,试件破坏为压剪破坏模式,与无围压SHPB试验有所不同。     

Effect of stitching and weave architecture on the high strain rate compression response of affordable woven carbon/epoxy composites

In this study, experimental investigations on stitched and unstitched woven carbon/epoxy laminates under high strain rate compression loading are discussed. Stitched/unstitched laminates are fabricated with aerospace grade plain and satin weave fabrics with room temperature curing SC-15 epoxy resin using affordable vacuum assisted resin infusion molding process. The samples are subjected to high strain rate loading using modified compression split Hopkinson’s pressure bar at three different strain rates ranging from 320 to 1149 s⁻¹. Results are discussed in terms of unstitched/stitched configuration, fabric type and loading directions. Dynamic compression properties are compared with those of static loading. Failure mechanisms are characterized through optical and scanning microscopy.     

High strain rate compression response of carbon/epoxy laminate composites

Composite materials exhibit excellent mechanical properties over metallic materials and hence are increasingly considered for high technology applications. In many practical situations, the structures are subjected to loading at very high strain rates. Material and structural response vary significantly under such loading as compared to static loading. A structure that is expected to perform under dynamic loading conditions, if designed with the static properties, might be too conservative. Hence, it is necessary to characterize the advanced composites under high strain rate loading. In the current investigations, the response of carbon/epoxy laminated composites under high strain rate compression loading is considered using a modified split Hopkinson Pressure Bar (SHPB) setup at three different strain rates of 82, 164 and 817 s⁻¹. The laminates were fabricated using 32 plies of a DA 4518 unidirectional carbon/epoxy prepreg system. Both unidirectional and cross-ply laminates were considered for the study. In the case of cross-ply laminates, the samples were tested in the thickness as well as in the in-plane direction. The unidirectional laminate samples were subjected to loading along 0° and 90° directions. Dynamic stress–strain plot was obtained for each sample and compared with the static compression test result. The results of the study indicate that the dynamic strength (with the exception of through the thickness loading of cross-ply laminates) and stiffness exhibit considerable increase as compared to the static values within the tested range of strain rates.     

Using split Hopkinson pressure bars to perform large strain compression tests on polyurea at low,intermediate and high strain rates

The strain rate sensitivity of polyurea is characterized using a modified split Hopkinson pressure bar (SHPB) system. The device is composed of a hydraulic piston along with nylon input and output bars. In combination with an advanced wave deconvolution method, the modified SHPB system provides an unlimited measurement time and thus can be used to perform experiments at low, intermediate and high strain rates. A series of compression tests of polyurea is performed using the modified SHPB system. In addition, conventional SHPB systems as well as a universal hydraulic testing machine are employed to confirm the validity of the modified SHBP technique at low and high strain rates. The analysis of the data at intermediate strain rates shows that the strain rate is not constant due to multiple wave reflections within the input and output bars. It is demonstrated that intermediate strain rate SHPB experiments require either very long bars (>20 m) or very short bars (<0.5 m) in order to achieve an approximately constant strain rate throughout the entire experiment.     

Elastomeric epoxy nanocomposites: Nanostructure and properties

In polymer layered silicate nanocomposites, significant differences have been reported between the effects of the nano-reinforcement on rigid and elastomeric nanocomposites. In this paper, we have studied elastomeric nanocomposites based upon DGEBA epoxy resin filled with montmorillonite (MMT) and cured with a long-chain polyoxypropylene diamine, for comparison with analogous rigid nanocomposites. Ultrasonic mixing was used to disperse the MMT in the matrix to improve homogeneity and decrease the agglomerate size. Two different methods of nanocomposite preparation were used in which the MMT was first swollen with either the curing agent or the epoxy before the addition of, respectively, DGEBA or diamine. A better dispersion of the nanoclay in the matrix and a greater amount of intercalation occurred when the MMT was first swollen with the diamine. The effect of MMT concentrations up to 8 wt.% on the mechanical behaviour of the epoxy/MMT nanocomposites was investigated. It was found that the addition of MMT increased the tensile strength and modulus, although SAXS and TEM indicated that a significant fraction of the clay layers were not exfoliated. Nevertheless, the addition of the clay resulted in changes in the fracture surfaces, as indicated by SEM, consistent with the tensile results and indicative of toughening.     

High strain rate mechanical behavior of epoxy under compressive loading: Experimental and modeling studies

Investigations on high strain rate behavior of epoxy LY 556 under compressive loading are presented. Compressive Split Hopkinson Pressure Bar (SHPB) apparatus was used for the experimental investigations. The studies are presented in the strain rate range of 683-1890 per second. It was generally observed that the compressive strength is enhanced at high strain rate loading compared with that at quasi-static loading. During SHPB testing of the specimens, it was observed that the peak force obtained from the strain gauge mounted on the transmitter bar is lower than the peak force obtained from the strain gauge mounted on the incident bar. Further, an analytical method is presented based on variable rate power law for the prediction of compressive strength at high strain rate loading for epoxy LY 556. Using the analytical method, high strain rate compressive stress-strain behavior is presented up to strain rate of 10,000 per second.     

The impact of the nature of nanofillers on the performance of wood polymer nanocomposites

Wood polymer nanocomposites have been prepared from aspen wood using melamine-urea-formaldehyde (MUF) and montmorillonite nanoclay. The nanoparticles were ground with a ball-mill and mixed with the prepolymer to form suspensions that were subsequently impregnated into the wood and in situ polymerized. The influence of the nature of nanofillers and interphase interactions between nanoparticles and MUF on the physical/mechanical properties of the resulting wood polymer nanocomposites was investigated, using SEM, TEM and Electron Probe Micro-Analysis (EPMA) methods. Significant improvements in wood properties, including surface hardness, modulus of elasticity, dimensional stability and water repellence, were obtained with the addition of hydrophobic nanoparticles into the wood. The improved properties could be ascribed to inherent properties as well as better interphase between MUF and nanoparticles, and their co-reinforcement on the wood. Ball-mill treatment favored the dispersion of the nanoparticles into the wood, but broke down functional groups on the hydrophobic nanoclay surface, which was detrimental for the bonding between the nanoparticles and the MUF matrix.     

Modeling and testing strain rate-dependent compressive strength of carbon/epoxy composites

A technique for testing high modulus fiber-reinforced composites in compression at different strain rates is investigated. The rate-dependent compressive behavior of unidirectional AS4/3501-6 carbon/epoxy composite is characterized by using off-axis specimens. It is found that, in the compression test, a titanium coating applied at the contact ends of the off-axis specimen can greatly reduce contact frictions, allowing a fully developed extension–shear coupling so that a state of uniform stress in the specimen can be achieved. A rate-dependent nonlinear constitutive model and a dynamic compressive strength model (fiber microbuckling model) for the unidirectional AS4/3501-6 composite are established based on the low strain rate off-axis test data. Model predictions and experimental data including high strain rate data are in very good agreement indicating that the constitutive model and compressive strength model obtained with low strain rate data are valid for high strain rates as well. A technique is also developed to extract the longitudinal compressive strength of the composite from those of the off-axis specimens.     

Dynamic mixed-mode I/II delamination fracture and energy release rate of unidirectional graphite/epoxy composites

Mixed-mode open-notch flexure (MONF), anti-symmetric loaded end-notched flexure (MENF) and center-notched flexure (MCNF) specimens were used to investigate dynamic mixed I/II mode delamination fracture using a fracturing split Hopkinson pressure bar (F-SHPB). An expression for dynamic energy release rate G_d is formulated and evaluated. The experimental results show that dynamic delamination increases linearly with mode mixing. At low input energy E_i ? 4.0 J, the dynamic (G_d) and total (G_T) energy rates are independent of mixed-mode ratio. At higher impact energy of 4.0 ? E_i ? 9.3 J, G_d decreases slowly with mixed I/II mode ratio while G_T is observed to increase more rapidly. In general, G_d increases more rapidly with increasing delamination than with increasing energy absorbed. The results show that for the impact energy of 9.3 J before fragmentation of the plate, the effect of kinetic energy is not significant and should be neglected. For the same energy-absorption level, the delamination is greatest at low mixed-mode ratios corresponding to highest Mode II contribution. The results of energy release rates from MONF were compared with mixed-mode bending (MMB) formulation and show some agreement in Mode II but differences in prediction for Mode I. Hackle (Mode II) features on SEM photographs decrease as the impact energy is increased but increase as the Mode I/II ratio decreases. For the same loading conditions, more pure Mode II features are generated on the MCNF specimen fractured surfaces than the MENF and MONF specimens.     

Experimental investigation of the dynamic response of graphite-epoxy composite laminates under compression

In this study, the dynamic stress–strain response of graphite-epoxy composite laminates is investigated. The laminates are interposed in a section of a split Hopkinson apparatus. A quasi-rectangular wave is generated at one end of the incident bar by striking it with another bar of known length. This bar is accelerated using a compressed air gun. Approximate average stresses and strains can be obtained by measuring the incident, reflected and transmitted waves in the split bar. The dynamic behavior is evaluated for a range of impact velocities. The dependence of the response on impact velocity is analyzed and discussed. Three different specimen thicknesses have been used. These are obtained by increasing the repetition factor of a base stacking sequence: (+45°, −45°, 0°, 90°). This process is called sublaminate scaling; it is preferred to ply scaling since it has been shown that the accumulation of layers of the same orientation decreases the failure load to such an extent that residual stresses may crack the specimen before any external load is applied. The laminates considered are: (+45°, −45°, 0°, 90°)_ns, n=2,3,4. The scale effects observed in the experimental response are analyzed and discussed.     

Characterization of epoxy functionalized graphite nanoparticles and the physical properties of epoxy matrix nanocomposites

This work presents a novel approach to the functionalization of graphite nanoparticles. The technique provides a mechanism for covalent bonding between the filler and matrix, with minimal disruption to the sp² hybridization of the pristine graphene sheet. Functionalization proceeded by covalently bonding an epoxy monomer to the surface of expanded graphite, via a coupling agent, such that the epoxy concentration was measured as approximately 4 wt.%. The impact of dispersing this material into an epoxy resin was evaluated with respect to the mechanical properties and electrical conductivity of the graphite–epoxy nanocomposite. At a loading as low as 0.5 wt.%, the electrical conductivity was increased by five orders of magnitude relative to the base resin. The material yield strength was increased by 30% and Young’s modulus by 50%. These results were realized without compromise to the resin toughness.     

High strain rate characteristics of 3-3 metal-ceramic interpenetrating composites

3-3 interpenetrating composites (IPCs) are novel materials with potentially superior multifunctional properties compared with traditional metal matrix composites. The aim of the present work was to evaluate the high strain rate performance of the metal-ceramic IPCs produced using a pressureless infiltration technique through dynamic property testing, viz. the split Hopkinson's pressure bar (SHPB) technique and depth of penetration (DoP) analysis, and subsequent damage assessment. Though the IPCs contained rigid ceramic struts, the samples plastically deformed with only localised fracture in the ceramic phase following SHPB. Metal was observed to bridge the cracks formed during high strain rate testing, this latter behaviour must have contributed to the structural integrity and performance of the IPCs. Whilst the IPCs were not suitable for resisting high velocity, armour piercing rounds on their own, when bonded to a 3 mm thick, dense Al₂O₃ front face, they caused significant deflection and the depth of penetration was reduced.     

Preparation and characterization of water-borne epoxy shape memory composites containing silica

Shape memory silica/epoxy composites were successfully prepared by hydrolysis of tetraethoxysilane (TEOS) within the epoxy matrix via latex, freeze-drying, and hot-press molding method. The silane coupling agent 3-triethoxysilylpropylamine (KH550) was introduced to improve the interfacial properties between the in-situ generated silica particle and epoxy matrix. The morphology structure and the effect of the content of the in-situ formed silica on the mechanical and shape memory properties of the silica/epoxy composites were studied. The experimental results indicated that the silica particles were homogenously dispersed and well incorporated into the epoxy matrix. Significant improvements were achieved in the mechanical property of the organic–inorganic hybrid materials. The silica/epoxy composites exhibited high shape recovery and fixity ratio approximately 100% even after 10 thermo-mechanical cycles.     

Properties of graphene nanopowder and multi-walled carbon nanotube-filled epoxy thin-film nanocomposites for electronic applications: The effect of sonication time and filler loading

Graphene nanopowder (GNP) and multi-walled carbon nanotube (MWCNT)-filled epoxy thin-film composites were fabricated using ultrasonication and the spin coating technique. The effect of sonication time (10, 20 and 30 min) and GNP loading (0.05–1 vol%) on the tensile and electrical properties of GNP/epoxy thin-film composites was investigated. The addition of GNP decreased the material’s tensile strength and modulus. However, among the tested samples, the GNP/epoxy composites produced using 20 min of sonication time had a slightly higher tensile strength and modulus, with a lower electrical percolation threshold volume fraction. The effect of sonication time was supported by morphological analysis, which showed an improvement in GNP dispersion with increased sonication time. However, GNP deformation was observed after a long sonication time. The GNP/epoxy composites at different filler loadings showed higher electrical properties but slightly lower tensile properties compared with the MWCNT/epoxy composites fabricated using 20 min of sonication time.     

SHPB试验岩石试件应力平衡时间预估分析

运用一维应力波理论,对分离式Hopkinson压杆(SHPB)试验中弹性应力波的传播过程进行了分析,得到了试件应力分布相关计算公式,讨论了试件应力平衡时间的影响因素和变化规律。以变截面杆SHPB试验装置对煤矿岩石试件加载为例,计算分析了3种岩石试件在光滑的试验入射波和与其升时相同的理论梯形入射波加载情况下试件应力均匀性和应力平衡时间。发现采用变截面入射杆进行加载,能够实现岩石试件在应力峰值之前达到应力平衡,满足应力均匀性假定要求的有效条件。结果表明,采用理论梯形入射波可以近似代替与其升时相同的试验入射波,预估岩石试件应力均匀性和应力平衡时间,对类似脆性材料的SHPB试验设计具有一定的参考价值。     

Effects of sepiolite clay on degradation and fire behaviour of a bisphenol A-based epoxy

As-received and pre-treated sepiolite/epoxy systems, characterised by an inorganic content from 2 to 10 wt.%, were investigated in order to assess induced-filler effect on degradation and fire behaviour.Thermogravimetrical results show that the thermal stability of the hosting epoxy, is slightly affected by the presence of sepiolite for both typologies; whereas, changes induced in char morphology by the pre-treated clay will significantly affect the fire behaviour of the final nanocomposite.Modelling of thermo-gravimetrical results in air atmosphere, by means of Kissinger procedure, shows a noteworthy reduction of activation energies associated with each degradation steps, especially at highest sepiolite content either by using as-received and pre-treated inorganic filler. This substantially indicates that the presence of sepiolite shorten the whole degradation process on the temperature scale. On the other hand, the different morphology of the char layer during the burning process can have relevant flame retardant effects acting on both condensate and vapour phase. Analysing the cone calorimetric data, a reduction of about 27% of the peak of heat release rate for the highest sepiolite percentage is measured and the burning total period is increased thus confirming that sepiolite when pre-treated represents a valid fire retardant inorganic filler for such a system.     

短切碳纤维增强碳化硅陶瓷基复合材料的高温冲击压缩力学性能

采用先驱体浸渍裂解法(即PIP法)制备出3种不同短切碳纤维(C_(sf))体积分数的圆柱形短切碳纤维增强陶瓷基复合材料(C_(sf)/SiC复合材料)试件,通过高温加热装置和自组装功能的霍普金森压杆装置对试件进行高温和动态荷载耦合作用下的冲击压缩试验,并通过扫描电子显微镜(SEM)观察C_(sf)/SiC复合材料断口形貌,对试件的破坏形态进行分析。试验结果表明,采用PIP法制备的C_(sf)/SiC复合材料试件中C_(sf)分布均匀,在外应力作用下C_(sf)/SiC复合材料试件发生破坏,碳纤维和碳纤维束与SiC基体脱粘被不断拔出。试件的抗压强度随C_(sf)体积含量的增加而呈现先增加后减小的变化趋势,C_(sf)体积含量为21%的试件抗压强度最高,为96.55 MPa。与常温相比,在高温压缩试验中随着复合材料试件平均温度的升高,C_(sf)/SiC复合材料试件破碎后的块度越来越大,整体性越来越好,当温度达到300℃时,C_(sf)体积含量对C_(sf)/SiC复合材料试件抗压强度的影响较小。     

光纤Bragg光栅应变传感器在霍普金森压杆上的冲击试验研究

利用复合材料封装光纤Bragg光栅(FBG)设计制作了用于测量高速碰撞或爆炸与冲击作用下混凝土内部动态应变的FBG应变传感器。在霍普金森压杆上对FBG在封装前的裸光栅和封装后的FBG传感器分别进行了高速冲击试验,试验表明：设计的FBG传感器的瞬态响应上升时间小于20μs,具有良好的动态响应特性,可以用于工程中混凝土结构内高速冲击下的动态应变测试