Effects of density and strain rate on properties of syntactic foams

Syntactic foams are characterized for high strain rate compressive properties using Split-Hopkinson Pressure Bar (SHPB) technique in this study. The results at high strain rates are compared to quasi-static strain rate compressive properties of the same material. Four different types of syntactic foams are fabricated with the same matrix resin system but different size microballoons for testing purpose. The microballoons have the same outer radius. However, their internal radius is different leading to a difference in their density and strength. The volume fraction of the microballoons in syntactic foams is maintained at 0.65. Such an approach is helpful in isolating and identifying the contribution of matrix and microballoons to the dynamic compressive properties of syntactic foams. Results demonstrate considerable increase in peak strength of syntactic foams for higher strain rates and increasing density. It is also observed that the elastic modulus increases with increasing strain rate and density. Scanning electron microscopy is carried out to understand the fracture modes of these materials and the density effect on high strain rate properties of syntactic foam.     

Statistical properties of microcracking in polyurethane foams under tensile test, influence of temperature and density

We report tensile failure experiments on polyurethane (PU) foams. Experiments have been performed by imposing a constant strain rate. We work on heterogeneous materials for whom the failure does not occur suddenly and can develop as a multistep process through a succession of microcracks that end at pores. The acoustic energy and the waiting times between acoustic events follow power-law distributions. This remains true while the foam density is varied. However, experiments at low temperatures (PU foams more brittle) have not yielded power-laws for the waiting times. The cumulative acoustic energy has no power-law divergence at the proximity of the failure point which is qualitatively in agreement with other experiments done at imposed strain. We notice a plateau in cumulative acoustic energy that seems to occur when a single crack starts to propagate.     

5CrMnMo应变率和温度相关力学性能实验研究

在较大的温度(25℃-537℃)和应变率(10-4s-1-10-2s-1)范围内对5CrMnMo进行了拉伸实验,获得了相应的应力应变曲线.试验结果表明在室温和试验的应变率范围内(10-4s-1-10-2s-1),5CrMnMo的力学性能是应变率无关的.随着温度的升高,出现了模量E、屈服强度σs和抗拉强度σb的应变率强化效应和温度弱化效应;还出现了加工硬化倾向减小的机制和蠕变效应增大机制,且温度越高这两种机制越强,应变率越高这两种机制越弱.在这两种机制作用下,温度越高失稳应变εb越小,断后伸长率δ50越大;但应变率越高δ50越小.当试验温度较高且应变率较低时,伴随有马氏体板条向拉伸方向偏转的细观特征.     

Effects of temperature and strain rate on the mechanical properties of lead-free solders

Recently studies of mechanical properties of lead-free solders show that large reductions in stiffness, yield stress, ultimate strength, and strain to failure (up to 35%) were found after 2 months of aging at room temperature. It also shows that the tensile properties of both lead-free solders and Sn–Pb solders tend to become relatively stable after 10 days of aging at room temperature. In this study, in order to minimize any room temperature aging contribution in the investigation of the dependence of temperature and strain rates, all specimens tested were preconditioned after 10 days of aging at room temperature. All tests were conducted under the same conditions. Testing has been performed at three strain rates, 10⁻³, 10⁻⁴, and 10⁻⁵ s⁻¹, in temperature range from −40 to 150 °C. A linear relationship was found between the temperature and the tensile properties (elastic modulus, yield stress, and ultimate stress), while a power law relationship was found between strain rate and tensile properties. Constitutive models have also been developed based on the experimental data with multiple variables of strain rate and temperature for both lead-free and lead content solders. With the obtained constitutive models, tensile properties of lead-free solders can be predicted at any testing strain rate and temperature.     

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.     

Microscale mechanical properties dependent on the strain rate and temperature of cured isotropic conductive adhesive

Mechanics of Time-Dependent Materials - In this research, the microscale strain rate sensitivity and high-temperature mechanical properties of cured isotropic conductive adhesive (ICA) were...     

Effect of temperature and strain rate on tensile mechanical properties of ARALL-1 laminates

The combined effect of temperature and strain rate of the mechanical properties for unidirectional 3/2 ARALL^®-1 laminates was studied. In this paper, the effect of strain rates from 0.00083–0.833 min^–1 on tensile behaviour at temperatures up to 250°F (121 °C) has been conducted. It is demonstrated that tensile strength, tensile modulus, and fracture strain are found to depend on temperature and strain rate. However, the effect of strain rates at 75 °F (24 °C) and 180 °F (82 °C) was found to be insignificant except the lowest strain rate at 180 °F. It was also observed that the tensile yield strength decreased as the strain rate decreased. The tensile properties were moderately reduced at high temperatures and were higher at high strain rates than at low strain rates. The temperature effect is more significant than that due to the strain rate. Scanning electron photomicrographs of the fracture surfaces observed in the aramid/epoxy layer of ARALL-1 laminates at the lowest strain rate are shown to be significantly different only at 250 °F (121 °C). But this phenomenon is not obvious when the highest strain rate is employed.     

Effect of fibre concentration, strain rate and weldline on mechanical properties of injection-moulded short glass fibre reinforced thermoplastic polyurethane

The effect of fibre concentration, strain rate and weldline on tensile strength, tensile modulus and fracture toughness of injection-moulded thermoplastic polyurethane (TPU) reinforced with different concentration levels of short glass fibres was investigated. It was found that tensile strength, σ_c, of single-gated mouldings increased with increasing volume fraction of fibres, ϕ_f, according to a second order polynomial function of the form and increased linearly with natural logarithm of strain rate (). Tensile modulus and fracture toughness (at initiation) of single-gated mouldings increased linearly with increasing ϕ_f (rule-of-mixtures) and . A linear dependence was obtained between fibre efficiency parameter for composite modulus, η_E, and . The presence of weldline in double-gated mouldings reduced tensile strength, tensile modulus and fracture toughness of TPU composites but had no significant effect upon properties of the TPU matrix. All the aforementioned properties increased with increasing fibre concentration and showed a linear dependence with respect to . Weldline integrity factor for all three properties decreased with increasing fibre concentration showing no strain-rate effect of any significance. Results indicated that tensile strength was more affected by the presence of weldline than tensile modulus or fracture toughness. It was noted that composite properties in the presence of weldline were still much greater than those for the unweld matrix. Weldline integrity values close to unity indicated that measured properties for the matrix were not significantly affected by the weldline.     

Change of mechanical properties of rigid poly(vinylchloride) during photochemical ageing

Thermally stabilised, unpigmented, rigid PVC samples, were exposed to accelerated photoageing at 40°C, 55°C and 70°C. The concentration profiles of photoproducts were determined on microtome slices ( 20 m) parallel to the irradiated surface using IR (carbonyls) and UV (polyenic double bonds) spectrophotometry and by steric exclusion chromatography (Mn and Mw). They indicate that carbonyls and chain scissions predominate only in a thin superficial layer whereas polyconjugated double bonds and crosslinks predominate in a subcutaneous layer (300–400 m). Tensile measurements show that the ultimate elongation decreases after an induction period whose duration is a decreasing function of temperature. The change of mechanical behaviour can be described in terms of a ductile-brittle transition shift mechanism in which crosslinking plays an important role. A tentative explanation of a such crosslinking induced transition is proposed in the discussion.     

Effects of strain rate on the mechanical properties of tricalcium phosphate/poly(l-lactide) composites

Bioactive ceramic/bioresorbable plastic composites have been expected as materials for the bone fracture fixations which have more biocompatibility than monolithic bioresorbable plastics. Many studies have been conducted on these materials. Most studies, however, focused on the mechanical properties under static loading. In the actual usage, these materials are loaded dynamically. In this study, effects of strain rate on the mechanical properties of tricalcium phosphate/poly(l-lactide) (TCP/PLLA) composites were investigated experimentally and analytically. The TCP/PLLA composites containing three different TCP contents (5, 10 and 15 wt.%) were prepared by injection molding. In order to characterize the mechanical properties, tensile and compressive tests were conducted. The results of tensile tests indicated that the Young’s moduli of composites increased with increasing TCP contents. For each TCP contents, tensile Young’s modulus kept constant up to strain rate of 10⁻¹/s. On the other hand, tensile strength increased with increasing strain rate. The effect of strain rate became larger with decreasing TCP contents, which means the strain rate dependency of the PLLA is more effective than that of TCP. From the results of compressive tests, similar results with tensile tests were obtained. That is, compressive Young’s modulus kept constant up to strain rate of 10⁻¹/s and the 0.2% proof stress increased with increasing strain rate. In order to predict the mechanical behavior of TCP/PLLA composites, the micro-damage mechanics was proposed. In this analysis, 3-phases particle reinforced composites, which include the intact particles, damaged particles and matrix, are assumed. The elastic constants are calculated with micromechanics based on the analyses by Eshelby and Mori and Tanaka. Only the debonding between particle and matrix are assumed as the damage. The nonlinearity in the stress-strain behavior of matrix PLLA is also considered. The debonding particles are assumed as voids. Void formation is calculated based on the energy criterion. The energy release rate associated with void formation was estimated by fitting the analytical results with the experimental results of the composites with 15 wt.% TCP contents for each strain rate. Then the analytical results for the composites with 5 and 10 wt.% TCP contents were compared with the experimental results. The analytical tensile stress-strain curves are in good agreement with experimental results. It is also clarified that the energy release rate associated with void formation increased with increasing strain rate.     

多因素加速老化对硬质聚氨酯泡沫塑料压缩力学性能的影响研究

对硬质聚氨酯泡沫塑料进行了温度、湿度、载荷多因素加速老化试验研究,对老化前后的压缩性能进行了检测。采用方差分析法初步探讨了各老化因素对材料力学性能的影响,并对诱发材料力学性能劣化的物理机制进行了探讨。试验结果表明,硬质聚氨酯泡沫塑料在温湿度无载加速老化试验后,其压缩性能变化不明显,而经历温湿度负载加速老化后,其压缩性能出现了明显的降低。分析认为,蠕变损伤的形成演化和累积或是造成这一现象的关键机制。     

应变率和温度对耐碱玻璃纤维织物增强水泥基复合材料弯曲力学行为的影响

为研究不同应变率和温度下耐碱玻璃织物增强水泥基复合材料的弯曲力学行为,采用美特斯(MTS)万能试验机和INSTRON落锤冲击系统对其试样分别进行室温(25℃)下准静态三点弯曲(应变率为3.33×10-5 s-1)和不同应变率(4、8、12、16和18s-1)及温度(-50、0、25、50和100℃)下的动态三点弯曲试验,静态和动态三点弯曲试验采用一套弯曲夹具。同时考虑了增强织物层数对其弯曲力学性能的影响。试验结果表明:室温下,随应变率的增加,弯曲强度提高,弯曲峰值应变和韧性先减小后增大,弯曲模量先增大后减小;应变率为12s-1时,随着温度的升高,弯曲强度、弯曲模量和韧性整体上减小,弯曲峰值应变变化不明显;纤维织物为六层时,对混凝土的增韧效果较明显。应变率、温度和织物层数均能对试样的弯曲性能产生较大影响。     

Molecular dynamics evaluation of strain rate and size effects on mechanical properties of FCC nickel nanowires

Based on molecular dynamics method, an atomistic simulation scheme for damage evolution and failure process of nickel nanowires is presented, in which the inter-atomic interactions are represented by employing the modified embedded atom potential. Extremely high strain rate effect on the mechanical properties of nickel nanowires with different cross-sectional sizes is investigated. The stress–strain curves of nickel nanowires at different strain rates subjected to uniaxial tension are simulated. The elastic modulus, yield strength and fracture strength of nanowires at different loading cases are obtained, and the effect of strain rate on these mechanical properties is analyzed. The numerical results show that the stress–strain curve of metallic nanowires under tensile loading has the trend identical to that of routine polycrystalline metals, and the yield strain of nanowires is independent of the strain rate and cross-sectional size. Based on the simulation results, a set of quantitative prediction formulas are obtained to describe the strain rate sensitivity of nickel nanowires on the mechanical properties, and the resulting formulas of the Young’s modulus, yield strength and fracture strength of nickel nanowires exhibit a linear relation with respect to the logarithm of strain rate. Furthermore, some comprehensive correlation equations revealing both the strain rate and size effects on mechanical properties of nickel nanowire are proposed through the numerical fitting and regression analysis, and the mechanical behaviors observed in this study are consistent with those from the experimental and available numerical results.     

变形温度与应变速率耦合作用对TWIP效应Ti-15Mo合金力学性能的影响EI北大核心CSCD

变形温度和应变速率均影响β型钛合金的力学性能,且其影响均关联塑性变形过程中变形方式的变化。利用TEM,EBSD,SEM,XRD,OM和拉伸试验机研究变形温度和应变速率耦合作用对{332}〈113〉孪生诱发塑性效应Ti-15Mo合金力学性能的影响。结果表明:在298 K和573 K下,屈服强度均随应变速率的增加逐渐升高,即依赖于位错热激活过程,且573 K下显著的位错热激活作用使得屈服强度表现出更大的应变速率依赖性。不同于298 K下,Ti-15Mo合金在573 K下通过{332}〈113〉孪生和位错滑移耦合变形;构建的流变应力模型表明位错强化成为其主要强化方式。高应变速率下,塑性变形早期形成的更多孪晶虽然会抑制孪生的进一步产生降低加工硬化率,但同时有效降低位错不均匀分布引起的局部应力集中延缓颈缩的发生;两个方面的共同作用使得Ti-15Mo合金在变形温度和应变速率耦合作用下呈现出更小的应变速率依赖性。     

Comparative study of strain rate effects on mechanical properties of glass fibre-reinforced thermoset matrix composite

The tensile dynamic behaviour of glass fibre-reinforced phenolic and polyester resins has been determined in order to find the influence of strain rate on the mechanical properties of composite materials produced by the resin transfer moulding (RTM) and pultrusion processes. Data and experimental test systems from the literature are analysed. A new specimen design is created and validated using drop-weight dynamic tests. The dynamic elastic modulus and strength tend to increase in an important ratio for the majority of the materials studied. The shear modulus measured with off-axis and ±45° coupons produces different effects as a function of strain rate. The influence of the reinforcement structure is emphasized and shown to be effective.     

Fresh and mechanical properties,and strain sensing of nanomodified cement mortars: The effects of MWCNT aspect ratio,density and functionalization

A comprehensive analysis on the effect of aspect ratio, bulk density and functionalization of multi walled carbon nanotubes (MWCNTs) in the development of nanomodified mortars, reinforced with different types of MWCNTs is presented herein. A structural characterization of the pristine and functionalized carbon nanotubes was carried out with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). A simple one step dispersion method, involving the application of ultrasonic energy and the use of a superplasticizer (SP) was utilized for the preparation of uniformly dispersed MWCNT suspensions. The experimental determination of the fresh and 28d mechanical properties of mortars with w/c = 0.5 and s/c = 3.0, using four different types of well dispersed pristine and functionalized MWCNTs at an amount of 0.1 wt% of cement took place through: (i) flow and time of setting tests; (ii) three point bending experiments on 4 × 4 × 16 cm specimens; and (iii) uniaxial compression on the half prisms of the flexural test specimens (4 × 4 × 8 cm). The piezoresistive behavior of the mortars reinforced with the pristine MWCNTs was experimentally determined using the 4-pole method, and compared with the strain sensing ability of the mortars reinforced with the functionalized MWCNTs. All MWCNT reinforced mortars exhibit a remarkable enhancement in the mechanical properties. However, the 28d flexural strength, young's modulus and energy absorption capability of the mortars reinforced with the mechanically functionalized MWCNTs at an amount of 0.1 wt% increased by 120%, 124%, and 103% respectively. Finally, depending on the procedure of the functionalization, chemical or mechanical, a different effect on the intrinsic properties of MWCNTs was observed. The carboxylic groups attached to the surface of the chemically functionalized MWCNTs indeed provided them with the ability of a uniform and effective dispersion, without the need of a sonication procedure. On the other hand, it was found that functionalized MWCNTs do not always retain the electrical properties of pristine MWCNTs.