Tensile behavior of carbon fiber bundles at different strain rates

Quasi-static and high strain rate tensile tests have been performed on T700 carbon fiber bundles and complete stress-strain curves at the strain rate range of 0.001 s^− 1 to 1300 s^− 1 were obtained. Results show that strain rate has negligible effect on both ultimate strength and failure strain, and T700 carbon fiber can be regarded as strain rate insensitive materials. On the basis of the fiber bundles model and the statistic theory of fiber strength, a damage constitutive model based on Weibull distribution function has been developed to describe tensile behavior of T700 fiber bundles. And the method to determine the statistic parameters of fibers by tensile tests of fiber bundles is established, too.     

Compressive behaviour of closed-cell aluminium foams at high strain rates

It has been well established that ALPORAS® foams is a strain rate sensitive material. However, the strain rate effect is not well quantified as it is not unusual for strain rate to vary during high speed compression. Moreover, according to previous research, aluminium foams, especially ALPORAS® foams, behave differently at low and high strain rates. Therefore, different plastic deformation mechanisms are expected for low and high strain rate loadings as a result of micro-inertia of cell walls. In this paper, the strain rate effect on the energy dissipation capacity of ALPORAS® foam was investigated experimentally by using a High Rate Instron Test System, with cross-head speed up to 10 m/s. The compressive tests were conducted over strain rates in the range of 1 × 10^?3 to 2.2 × 10² s^?1, with each test being at a fairly constant strain rate. An energy efficiency method was adopted to obtain the densification strain and plateau stress. The effect of strain rate and the foam density was well presented by empirical constitutive models. The experimental data were also discussed with reference to the recent results by other researchers but with different range of strain rates. An attempt has been made to qualitatively explain the observed decrease of densification strain with strain rate.     

Strain rate effects on the compressive property and the energy-absorbing capacity of aluminum alloy foams

The strain rate sensitivity of various relative densities, open-cell aluminum alloy foams fabricated by a powder metallurgical method is investigated under compression loading. Their response to strain rate has been tested over a wide range of strain rates, from 10⁻³ to 2600 s⁻¹ at room temperature. Within this range, the experimental results show that the yield strength and the energy absorbed increase with an increase of strain rate. However, the yield strength of higher relative density foams increases bilinearly with the logarithm of strain rate, and the yield strength of lower relative density foams shows only a linear increase. The compaction strain slightly decreases with an increase of strain rate. The higher relative density aluminum alloy foams are more sensitive to strain rate than the lower relative density foams.     

Compressive behaviour of aluminium foams at low and medium strain rates

Compressive behaviour of CYMAT aluminium foams with relative densities ranged from 5% to 20% has been studied experimentally in this paper. An MTS machine is employed to apply a compressive load at strain rates of 10⁻³–10⁺¹ s⁻¹ to these closed-cell aluminium foams. It has been found that the plateau stress is insensitive to the strain rate and is related to the relative density by a power law. Deformation is not uniform over the whole sample: it first occurs in the weakest band, followed by the next weakest bands after the first one has been completely crushed.     

闭孔泡沫铝的动态压缩性能试验研究

为了研究闭孔泡沫铝动态压缩性能的应变率效应,采用改进的INSTRON高速动力加载系统,对不同应变率下闭孔泡沫铝试件进行动态压缩试验研究。首先利用正向试验和反向试验技术对不同厚度的闭孔泡沫铝试件在同一加载速率下的动态压缩性能进行了研究,得到了在一定速率下消除泡沫铝动态压缩试验中惯性效应的合理试件厚度。进一步开展了闭孔泡沫铝试件在不同加载速率下的高速压缩试验,研究了其动态压缩性能随应变率的变化规律。结果表明在高速压缩下,闭孔泡沫铝的应力-应变曲线与准静态条件相同,具有明显的弹性段、平台段及压实段的3阶段特征。闭孔泡沫铝的平台应力具有明显的应变率效应,而致密应变在不同的应变率下表现出了不同的变化趋势,初步解释为泡沫铝孔壁塑性变形机制的改变以及波动效应的相互影响。闭孔泡沫铝的吸能能力随应变率的增加而明显提升。     

An investigation on the dynamic response of polymeric,metallic, and biomaterial foams

Mechanical characterization of foams at varying strain rates is indispensable for the selection of foam as core material for the proficient sandwich structure design at dynamic loading application. Both servo-hydraulically controlled Material Testing System (MTS) and Instron machines are generally considered for quasi-static testing at strain rates on the order of 10⁻³ s⁻¹. Split Hopkinson pressure bar (SHPB) with steel bars is typically utilized for characterizing metallic foams at high strain rates, however modified SHPB with polycarbonate or soft martial bars are used for characterizing polymeric and biomaterial foams at high strain rates on the order of 10³ s⁻¹ for impedance match between the foam specimens and bars. This paper reviews the effect of strain rate of loading, density, environmental temperature, and microstructure on compressive strength and energy absorption capacity of various closed-cell polymeric, metallic, and biomaterial foams. Compressive strength and energy absorption capacity increase with the increase in both strain rate of loading and density of foams, but decrease with the increase in surrounding temperature. Foams of same density can have different strength and can absorb unequal amount of energy at the same strain rate of loading due to the variation of microstructure.     

Compressive and tensile behaviour of aluminum foams

The uniaxial compressive and tensile modulus and strength of several aluminum foams are compared with models for cellular solids. The open cell foam is well described by the model. The closed cell foams have moduli and strengths that fall well below the expected values. The reduced values are the result of defects in the cellular microstructure which cause bending rather than stretching of the cell walls. Measurement and modelling of the curvature and corrugations in the cell walls suggests that these two features account for most of the reduction in properties in closed cell foams.     

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.     

Compressive properties of closed-cell aluminum foams with different contents of ceramic microspheres

In this paper, closed-cell aluminum foams with different kinds and contents of ceramic microspheres are obtained using melt-foaming method. The distribution and the effects of the ceramic microspheres on the mechanical properties of aluminum foams are investigated. The results show that both kinds of ceramic microspheres distribute in the foams uniformly with part in the cell wall matrix, some in adhere to the cell wall surface and part embed in the cell wall with portion surface exposed to the pores. Ceramic microspheres have an important effect on the yield strength, mean plateau stress, densification strain and energy absorption capacities of aluminum foams. Meanwhile, the content of ceramic microsphere in aluminum foams should be controlled in order to obtain good combination of compressive strength and energy absorption capacity. The reasons are discussed.     

The dependence of damping property of fly ash reinforced closed-cell aluminum alloy foams on strain amplitude

Closed-cell aluminum alloy foams with 1.5, 3.0 wt.% fly ash particles have been manufactured by molten body transitional foaming process. The room temperature damping properties of fly ash reinforced aluminum alloy foams were measured at different strain amplitude in two directions using the forced vibration mode. The results show that the damping properties of fly ash reinforced aluminum alloy foams increase with FA content. A critical strain amplitude ε_crit was observed and ε_crit decreases with increasing FA content. Moreover, the damping property in the transverse direction is higher than that in the longitudinal direction. The related mechanism has been discussed.     

Compressive properties of nanoparticle modified epoxy resin at different strain rates

Epoxy resins often exhibit high strength yet are often brittle, especially at high strain rates. Block copolymer modified epoxy resins have generated significant interest since it was demonstrated that the combination could lead to nanostructured thermosets through the self-assembly of the block copolymer. Such nanostructured epoxies exhibit increased ductility without the significant loss in yield strength exhibited by traditional rubber-modified epoxies. In this study, the effect of different nanoscale additives on the compressive yield strength of a model epoxy resin has been studied. In the first case, a block copolymer styrene-b-butadiene-b-polymethylmethacrylate (SBM) was added to the model epoxy resin. In the second case, carbon nanotubes (CNTs) were added. In the final case, both additives were mixed simultaneously with the epoxy resin. The compressive mechanical behavior of these materials has been investigated over a wide range of strain rates (0.001–3500 s⁻¹). The yield behavior was found to fit the cooperative yield model proposed by Fotheringham and Cherry.     

Effect of temperature and strain rate on tensile behavior of ultrafine-grained aluminum alloys

Ultrafine-grained Al–4Y–4Ni and Al–4Y–4Ni–0.9Fe (at.%) alloys were synthesized by the consolidation of atomized powders and subsequent hot extrusion. The mechanical behavior of these two alloys has been studied by performing uniaxial tension tests ranging from room temperature to 350 °C. These alloys, with high volume fraction of second-phase particles, exhibited ambient temperature tensile strength ranging from 473 to 608 MPa and plastic elongation ranging from 6.7 to 9.6% at an initial strain rate of 1 × 10⁻³ s⁻¹. However, lower ductility was observed with decreasing strain rate at the intermediate temperature ranging from 150 to 250 °C for Al–Y–Ni–Fe alloys due to limited work hardening.     

Elastic and electric properties of closed-cell aluminum foams: Cross-property connection

Foamed aluminum (AlMg1Si0.6) in the porosity range 0.45–0.85 produced by the powder metallurgy method is analyzed with regard to its elastic and electric properties. Various predictive models for the electrical conductivity and Young's modulus of closed-cell metal foam are assessed based on the experimental measurements. It is shown that the differential scheme provides the best predictions of the electrical conductivity in the porosity range 0.7–0.85, while Mori–Tanaka's scheme gives the best results for the Young's modulus. Comparing the two sets of the experimental data, cross-property coefficient that connects changes in the Young's modulus and electrical conductivity of a material due to pores was determined. A non-trivial finding is that the best prediction of the cross-property coefficient is obtained in the framework of non-interaction approximation.     

Quasi-static and high strain rate response of aluminum matrix syntactic foams under compression

Aluminum alloy matrix syntactic foams were produced by inert gas pressure infiltration. Four different alloys and ceramic hollow spheres were applied as matrix and filler material, respectively. The effects of the chemical composition of the matrix and the different heat-treatments are reported at different strain-rates and in compressive loadings. The higher strain rates were performed in a Split-Hopkinson pressure bar system. The results show that, the characteristic properties of the materials strongly depends on the chemical composition of the matrix and its heat-treatment condition. The compressive strength of the investigated foams showed a limited sensitivity to the strain rate, its effect was more pronounced in the case of the structural stiffness and fracture strain. The failure modes of the foams have explicit differences showing barreling and shearing in the case of quasi-static and high strain rate compression respectively.     

Evaluation of low strain rate constitutive equation of 7075 aluminium alloy at high temperature

Abstract

The 7075 aluminium alloy is one of the most important engineering alloys utilised extensively in aircraft and transportation industries due to its high specific strength. In the present research, the flow behaviour of this alloy has been investigated using hot compression test at strain rates of 0·001, 0·01, 0·1 and 1 s^?1 and temperatures of 350, 400 and 450°C. The results reveal that dynamic softening occurred in these temperatures and strain rates. The activation energy, strain rate sensitivity and two constitutive equations (hyperbolic sine law and the power law) are derived from the results. It is shown that the hyperbolic sine law has a better agreement with the experimental results.     

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.     

Relationship between mechanical properties and microstructural response of 6061-T6 aluminum alloy impacted at elevated temperatures

The high temperature impact properties and microstructural evolution of 6061-T6 aluminum alloy are investigated at temperatures ranging from 100 to 350 °C and strain rates ranging from 1 × 10³ to 5 × 10³ s⁻¹ using a compressive split-Hopkinson pressure bar (SHPB) system. It is found that the flow response and microstructural characteristics of 6061-T6 aluminum alloy are significantly dependent on the strain rate and temperature. The flow stress and strain rate sensitivity increase with increasing strain rate or decreasing temperature. Moreover, the temperature sensitivity increases with both increasing strain rate and increasing temperature. The flow stress–strain response of the present 6061-T6 alloy specimens can be adequately described by the Zerilli–Armstrong fcc model. The grain size and dislocation cell size increase significantly with a decreasing strain rate or an increasing temperature. The higher flow stress is the result of a smaller grain size and smaller dislocation cell size. The stacking fault energy of the deformed specimens has a value of 145.78 mJ/m².     

Analysis of strain rate dependence of tensile elongation for a mechanical milling Al–1.1Mg–1.2Cu alloy tested at 748 K from a dislocation dynamics viewpoint

Tensile deformation was carried out for a mechanically milled and thermo-mechanically treated Al–1.1Mg–1.2Cu (at.%) alloy at 748 K and three nominal strain rates of 10⁻³, 10⁰, and 10² s⁻¹. Despite the prevailing belief that superplasticity occurs by grain boundary sliding which requires slow strain rates at high temperatures, the maximum elongation was observed at the intermediate strain rate of 10⁰ s⁻¹, neither at the lowest nor the highest strain rates. In order to explain this phenomenon, the true stress–true strain behaviors at these three nominal strain rates were analyzed from a viewpoint of dislocation dynamics by computer-simulation with four variables of the thermal stress component σ*, dislocation immobilization rate U, re-mobilization probability of unlocked, immobile dislocations Ω and dislocation density at yielding ρ₀. It can then be concluded that the large elongation (>400% in nominal strain) at the intermediate strain rate is produced by a combination of a very large Ω and a moderate U, resulting in a large strain rate sensitivity m value.     

Compressive response of glass–fiber reinforced polymeric composites to increasing compressive strain rates

The utilization of composite materials instead of traditional materials in structural high-speed applications has induced the need for a proper knowledge of dynamic behavior as well as static behavior of them. The material and structural response vary significantly under dynamic loading as compared to static loading conditions. In order to investigate the dynamic responses of composite materials under dynamic loading at various strain rates, special testing machines are needed. Most of the researches in this field are focused on applying real loading and gripping boundary conditions on the testing specimens.The present study is carried out in order to characterize the compressive properties of unidirectional glass–fiber reinforced polymeric composites using a servo-hydraulic testing apparatus at varying strain rates, ranging from 0.001 to 100 s⁻¹. For performing practical tests, a jig and a fixture are designed and manufactured, which could insure the alignment of axial loads on the specimens. During of tests, the performance of the test jig is evaluated. It is found that the designed jig and the fixture perform very well during the test process. The results of the dynamic tests are compared with the results of the static tests carried out on specimens with identical geometry. Based on the experimental results obtained from the tests, empirical functions for the mechanical properties are proposed in terms of strain rates. The results of the study indicate that strain rate has a significant effect on the material response. It is found that the compressive strength and modulus both increased with increasing the strain rate. Also, the results show that the compressive strain to failure is generally insensitive to strain rate.     

中低应变率下闭孔泡沫铝动态力学性能研究

为探索闭孔泡沫铝的动态力学性能与吸能特性,基于万能材料试验机和高速液压伺服材料试验机在常温下分别对闭孔泡沫铝在准静态和中应变率下(0.001~100s^-1)的动态力学性能进行了测试,分析了不同应变率、不同相对密度和不同泡沫铝基体特性下闭孔泡沫铝的应力应变曲线特征和吸能特性变化。研究结果表明:中低应变率下的纯铝基体泡沫铝并不具备应变率效应,高脆性、相对密度较小的泡沫铝具备更好的吸能特性,塑性和脆性基体泡沫铝变形带分别呈现“V”形和“X”形,脆性基体泡沫铝同样不具备应变率效应。