High strain rate compression of epoxy based nanocomposites

This work aimed to investigate the strain-rate effect (0.001–3000 s⁻¹) on compressive properties of the highly cross-linked epoxy and the epoxy sample filled with 10 wt% sol-gel-formed silica nanoparticles. As the strain rate increased, the compressive modulus and transition strength of both samples went up distinctly, the strain at break and ultimate strength decreased more or less, while the strain energy at fracture nearly did not change. Adding the sol-gel-formed silica nanoparticles can improve effectively the compressive modulus, transition strength as well as strain energy at fracture of the epoxy polymer owing to their homogeneous dispersion in epoxy matrix.     

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.     

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.     

Mechanical properties of 3D printed interpenetrating phase composites with novel architectured 3D solid-sheet reinforcements

Interpenetrating phase composites (IPCs) are novel types of multifunctional composite materials. This work focuses on investigating experimentally and computationally the mechanical behavior of novel types of three-dimensional (3D) architectured two-phase IPCs. The current IPCs are architectured using several morphologies of the fascinating and mathematically-known triply periodic minimal surfaces (TPMS) that promote several multifunctional attributes. Specifically, the second hard reinforcing phase takes the architecture of one of the 3D non-intersecting and continuous TPMS-based solid sheets. The mechanical response of the 3D printed polymer-based IPCs is measured under uniaxial compression where the effect of varying the second-phase architecture and volume fraction is explored. Anisotropy induced by the 3D printing is also investigated. 3D finite element analysis has been performed and validated for predicting elastic properties of the various types of TPMS-based IPCs. The most effective TPMS architecture in enhancing the mechanical properties and damage-tolerance has been identified.     

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.     

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 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.     

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.     

温度和应变率对原位合成钛基复合材料动态压缩性能及破坏机制的影响

使用分离式Hopkinson压杆（SHPB）系统,在温度293~973 K、应变率6 000~10 000 s-1下,对原位合成TiC颗粒和TiB晶须混合增强钛基复合材料（TMCs）的动态压缩性能进行了研究。试验结果表明:在373~573 K、673~773 K和873~973 K范围内TMCs流变应力随温度的增加而显著减小;在较低温度（低于373 K）和较低应变率（6 000~8 000 s-1）下,TMCs呈现小幅的应变率硬化特征,而在较高温度（573 K及以上）时各应变率下TMCs均存在应变率软化特征,且温度越高材料应变率软化效应越明显。材料失效/断裂机制分析表明:应变率软化机制主要是绝热软化及其产生的绝热剪切带（ABS）中微裂纹的形成和扩展的综合作用;在较高的应变率和较大应变下ABS中会产生微裂纹,温度较低时TMCs塑性不足以抑制或阻碍微裂纹的扩展,从而导致TMCs在宏观上迅速破坏;材料破坏时以钛合金基体塑性断裂为主,但在局部伴随部分增强相脆性断裂。      

Mechanical behaviour of glass and carbon fibre reinforced composites at varying strain rates

The choice of composite materials as a substitute for metallic materials in technological applications is becoming more pronounced especially due to the great weight savings these materials offer. In many of these practical situations, the structures are prone to high impact loads. Material and structural response vary significantly under impact loading conditions as compared to quasi-static loading. The strain rate sensitivity of both carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) are studied by testing a single laminate configuration, viz. cross-ply [0°/90°] polymer matrix composites (PMC) at strain rates of 10⁻³ and 450 s⁻¹. The compressive material properties are determined by testing both laminate systems, viz. CFRP and GFRP at low to high strain rates. The laminates were fabricated from 48 layers of cross-ply carbon fibre and glass fibre epoxy. Dynamic test results were compared with static compression test carried out on specimens with the same dimensions. Preliminary compressive stress–strain vs. strain rates data obtained show that the dynamic material strength for GFRP increases with increasing strain rates. The strain to failure for both CFRP and GFRP is seen to decrease with increasing strain rate.     

High strain rate effects on direct tensile behavior of high performance fiber reinforced cementitious composites

Direct tensile behavior of high performance fiber reinforced cementitious composites (HPFRCCs) at high strain rates between 10 s⁻¹ and 30 s⁻¹ was investigated using strain energy frame impact machine (SEFIM) built by authors. Six series of HPFRCC combining three variables including two types of fiber, hooked (H) and twisted (T) steel fiber, two fiber volume contents, 1% and 1.5%, and two matrix strengths, 56 MPa and 81 MPa, were investigated. The influence of these three variables on the high strain rate effects on the direct tensile behavior of HPFRCCs was analyzed based on the test results. All series of HPFRCCs showed strongly sensitive tensile behavior at high strain rates, i.e., much higher post cracking strength, strain capacity, and energy absorption capacity at high strain rates than at static rate. However, the enhancement was different according to the types of fiber, fiber volume content and matrix strength: HPFRCCs with T-fibers produced higher impact resistance than those with H-fibers; and matrix strength was more influential, than fiber contents, for the high strain rate sensitivity. In addition, an attempt to predict the dynamic increase factor (DIF) of post cracking strength for HPFRCCs considering the influences of fiber type and matrix strength was made.     

High strain rate behavior of 4-step 3D braided composites under compressive failure

The out-of-plane and in-plane compressive failure behavior of 4-step 3D braided composite materials was investigated at quasi-static and high strain rates. The out-of-plane and in-plane direction compressive tests at high strain rates from 800/s to 3,500/s were tested with the split Hopkinson pressure bar (SHPB) technique. The quasi-static compressive tests were conducted on a MTS 810.23 tester and compared with those at high strain rates. The comparisons indicate that the failure stress, failure strain and compressive stiffness both for out-of-plane and in-plane loading directions are rate sensitive. For example, the failure stress, failure strain and stiffness are 55.19 MPa, 6.70% and 1.35 GPa respectively as opposed to 145.00 MPa, 1.21% and 13.50 GPa respectively for strain rate of 2,500 s⁻¹ under in-plane compression. The 3D braided composites have higher values of failure stress and strain for out-of-plane than for in-plane compression at the same strain rate; however, the in-plane compression stiffness is higher than that of out-of-plane compression at high strain rates. The compressive failure mode of 3D braided composites in the out-of-plane direction is mainly shear failure at various strain rates, while for the in-plane direction it is mainly cracking of matrix.     

Evaluation of stress–strain curve estimates in dynamic experiments

Accurate measurements of the forces and velocities at the boundaries of a dynamically loaded specimen may be obtained using split Hopkinson pressure bars (SHPB) or other experimental devices. However, the determination of a representative stress–strain curve based on these measurements can be challenging. Due to transient effects, the stress and strain fields are not uniform within the specimen. Several formulas have been proposed in the past to estimate the stress–strain curve from dynamic experiments. Here, we make use of the theoretical solution for the waves in an elastic specimen to evaluate the accuracy of these estimates. It is found that it is important to avoid an artificial time shift in the processing of the experimental data. Moreover, it is concluded that the combination of the output force based stress estimate and the average strain provides the best of the commonly used stress–strain curve estimates in standard SHPB experiments.     

Damage characterisation of glass/textile fabric polymer hybrid composites in sea water environment

The results of the experimental analysis carried out on the glass/textile fabric reinforced hybrid composites under normal condition and sea water environments have been reported. The critical stress intensity factor, interlaminar shear strength and impact toughness have been evaluated, both in interlaminar and translaminar directions. The specimen preparation and the experimentations were carried out according to the ASTM standards. Results have revealed that the damage in hybrid composite under sea water environment is entirely different. The characterising parameters have shown changes in their magnitudes with the variation in immersion time. The nature of fracture as a function of the reinforcement volume, loading and environmental conditions has been analyzed with the aid of scanning electron microscopy. The SEM analysis has shown that the fibers pull out, matrix cracking and also the nature of crack growth is different in sea water environment. The fracture in individual fiber has also been identified.     

Stress–strain behavior of composites under high strain rate compression along thickness direction: Effect of loading condition

Investigations are carried out on the behavior of typical plain weave E-glass/epoxy; plain weave carbon/epoxy; satin weave carbon/epoxy; and satin weave carbon – plain weave E-glass and epoxy hybrid composites under high strain rate compressive loading along thickness direction. Compressive split Hopkinson pressure bar apparatus was used for the studies. Two loading cases, namely, specimen not failed and specimen failed during loading are investigated. The special characteristics of specimen not failed case are presented. For this case, the specimens are under compressive strain initially and are under tensile strain during the later part of loading. The induced tensile strain is higher than the induced compressive strain. This could lead to failure of specimen/structure under tensile strain even though the applied load is compressive.     

高压水射流破碎胎面胶的脆化效应与胶粉形成

采用霍布金森压杆试验模拟子午线轮胎胎面胶的破碎回收过程,分析高压水射流冲击下胎面胶材料受力及响应状态,试验表明材料存在韧脆转变现象,进而发生脆性断裂。橡胶断口与胶粉微观形貌表明,裂纹扩展区呈现典型的放射状脆性断面形貌,并形成大量与胶粉尺寸匹配的平整光滑区域,直接验证了脆性断裂的存在并阐述其发生过程。然后利用应力波传播判据和脆断力学分析解释了胎面胶材料出现脆化效应的原因。对材料韧脆转变的影响因素进行分析后可知,高压水射流冲击过程中,材料质点变形速度远大于韧脆转变临界速度,在力学性能上表现为断裂应力小于屈服应力,致使材料发生脆性断裂并形成精细胶粉。     

成型压力对0-3型锆钛酸铅/水泥压电复合材料的影响

以快硬硫铝酸盐水泥为基体,以锆钛酸铅(PZT)为功能相,用压制成型法制备出0-3型PZT/水泥基压电复合材料。分析成型压力对PZT/水泥基压电复合材料的压电性和介电性的影响,结果表明:不同粒径PZT颗粒作为功能相的水泥基压电复合材料,成型压力对其压电性和介电性有不同的影响。在30~90 MPa压力范围内,成型压力越大,PZT/水泥压电复合材料的压电应变常数d33和相对介电常数εr均显著提高,这是由于气孔率随压力增大而减少,而压电电压常数g33的变化则与功能相的粒径有关。机电耦合系数也有着不同的变化趋势,对于6μm和126μm PZT/水泥压电复合材料,其机电耦合系数Kt和Kp随压力增大缓慢下降,而对于430μm PZT/水泥压电复合材料则呈上升趋势。当压力达到150 MPa时,其压电性和介电性均急剧减小。     

编织角和承载方向对三维四向编织复合材料动态压缩性能的影响

利用分离式霍普金森压杆（SHPB）装置对三维四向编织碳纤维增强树脂基复合材料的动态压缩性能进行了研究。通过对编织角为20°、30°和45°的试验件分别进行沿纵向、横向和厚度方向的动态压缩试验,得到材料在800~2 000/s应变率范围内的应力-应变曲线,并与准静态压缩试验结果进行对比,研究了应变率、压缩方向及编织角对材料极限强度和弹性模量的影响。结合高速摄影记录的动态压缩过程,进一步分析了不同情况下材料的破坏模式与破坏过程。结果表明：应变率越高,材料的极限强度和弹性模量越大,材料在受压的三个方向上均具有一定的应变率强化效应,且高应变率下表现出比准静态压缩时更明显的脆性;编织角的改变对材料在三个方向上的动态压缩性能均有影响,其中对纵向的影响最为明显;不同方向受压时材料的失效形式不同,且准静态和高应变率下的失效形式也有区别。     

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.     

0-3/1-3混合连通型铁电陶瓷-铁电聚合物的热释电性

推导出了0-3/1-3混合连通型铁电陶瓷-铁电聚合物的热释电系数表达式, 并详细讨论了陶瓷体积分数 v₂和陶瓷颗粒粒径与膜厚比(G/t)分别对复合材料的热释电系数p_c及p_c/ε_c(ε_c为复合材料的介电常数)的影响。结果表明: 当G/t≥0.5时, 不同陶瓷体积分数复合材料的热释电系数p_c均趋向于某个定值, 说明此时G/t对热释电系数的影响可忽略不计; 另外当陶瓷体积分数为0.1时, 随着G/t的增大, p_c /ε_c出现一个极大值, 且当G/t=0.9时, p_c /ε_c比纯PZT陶瓷的大8倍, 表明此条件为最佳工艺条件。而且, 在低陶瓷体积分数和低G/t比时, 理论曲线与实验数据符合较好。