Analytical and experimental studies on mechanical behavior of composites under high strain rate compressive loading

An analytical method is presented for the prediction of compressive strength at high strain rate loading for composites. The method is based on variable rate power law. Using this analytical method, high strain rate compressive stress–strain behavior is presented up to strain rate of 5000 s⁻¹ starting with the experimentally determined compressive strength values at relatively lower strain rates. Experimental results were generated in the strain rate range of 472–1957 s⁻¹ for a typical woven fabric E-glass/epoxy laminated composite along all the three principal directions. The laminated composite was made using resin film infusion technique. The experimental studies were carried out using compressive split Hopkinson pressure bar apparatus. It was generally observed that the compressive strength is enhanced at high strain rate loading compared with that at quasi-static loading. Also, compressive strength increased with increasing strain rate in the range of parameters considered. Analytically predicted results are compared with the experimental results up to strain rate of 1957 s⁻¹.     

Influence of emulsifier content on cement hydration and mechanical performance of bitumen emulsion mortar

The objective of this research was to investigate the influence of a cationic emulsifier on the kinetics of cement hydration and on the overall mechanical performance of bitumen emulsion mortar, as an essential structural component of cold asphalt mixtures, within a 28-day curing period. This influence was investigated as a function of emulsifier content and bitumen grade, selected to cover the whole range of their applicability. The hydration kinetics was evaluated by heat release of cement pastes and bitumen emulsion mortars. The residual w/c was determined at characteristic points during curing. The emulsifier showed a very powerful effect on the complex relation between hydration kinetics, emulsion rheology, and water binding. The presence of the emulsion initially accelerated the hydration, with no main peaks and dormant period observed, but had almost no effect on the ultimate hydration degree. The rate of hydration during the rest of the curing period was critically dependent on the residual w/c which was decisively influenced by the content of emulsifier. The cement could be expected to reach a very high degree of hydration only in mixtures with the highest emulsifier content, considering the specimens as a whole. Together with emulsion breaking at early stage, cement hydration later on was directly reflected on indirect tensile properties of the mortars, which performed from very ductile to very brittle. In conclusion, the emulsifier was recognised as a key factor for a fundamental understanding of the mechanical performance of cold asphalt mixtures.     

Texture evolution and dynamic mechanical behavior of cast AZ magnesium alloys under high strain rate compressive loading

In this study, texture and compressive mechanical behavior of three cast magnesium alloys, including AZ31, AZ61 and AZ91, were examined over a range of strain rates between 1000 and 1400 s⁻¹ using Split Hopkinson Pressure Bar. Texture measurements showed that after shock loading, initial weak texture of the cast samples transformed to a relatively strong (00.2) basal texture that can be ascribed to deformation by twinning. Furthermore, increasing the aluminum content in the alloys resulted in increase in the volume fraction of β-Mg₁₇Al₁₂ and Al₄Mn phases, strength and strain hardening but ductility decreased at all strain rates. Besides, it was found for each alloy that the tensile strength and total ductility increased with strain rate. By increasing the strain rate, the maximum value of strain hardening rate occurred at higher strains. Also, it is suggested that a combination of twinning and second phase formation would affect the hardening behavior of the cast AZ magnesium alloys studied in this research.     

Effect of strain rate and saturation on uniaxial dynamic compressive behaviours of mortar

The effect of moisture content on the compressive mechanical behaviours of cement mortar under different high strain rates is studied in this paper. The rapid impact testing, i.e. the strain rates of 80, 100, 150, 200 and 250 s^? 1 by Split Hopkinson pressure bar, on number of specimens with special water/cement ratio of 0.50 and saturations as 0%, 25%, 50%, 75% and 100%, respectively, was executed. The dynamic compressive behaviours were analysed in terms of the maximum stresses, elastic modulus, critical strain at maximum stresses and ultimate strains at failure. Results indicated that similarity existed in the shape of strain–stress curves of mortars with different moisture subjected to different strain rates of impact loading, i.e. the upward section presented bilinear characteristics, while the descending stage was almost linear. As strain rate increases, the dynamic compressive strength, elastic modulus and critical strain at maximum stress increase which can be ascribed to the dynamic fracture effect and the microscope inertia effect. Besides, it was shown that desiccation provokes an increase in mortar strength and deformation behaviour of the studied mortar with different saturation caused by capillary depression and microcracking. Drying effect has to be considered in modelling of the coupling between desiccation and mechanical behaviour of the mortar. Finally, the multi-parametric statistical analysis of water content and strain rate on the mechanical behaviours of cement mortar subjected to dynamic loading is detailed.     

Effect of strain rate on the dynamic compressive mechanical behaviors of rock material subjected to high temperatures

Strain rate is not only an important measure to characterize the deformation property, but also an important parameter to analyze the dynamic mechanical properties of rock materials. In this paper, by using the SHPB test system improved with high temperature device, the dynamic compressive tests of sandstone at seven temperatures in the range of room temperature to 1000 °C and five impact velocities in the range of 11.0–15.0 m/s were conducted. Investigations were carried out on the influences of strain rate on dynamic compressive mechanical behaviors of sandstone. The results of the study indicate that the enhancement effects of strain rates on dynamic compressive strength, peak strain, energy absorption ratio of sandstone under high temperatures still exist. However, the increase ratios of dynamic compressive strength, peak strain, and energy absorption ratio of rock under high temperature compared to room temperature have no obvious strain rate effects. The temperatures at which the strain rates affect dynamic compressive strength and peak strain most, are 800, and 1000 °C, respectively. The temperatures at which the strain rates affect dynamic compressive strength and peak strain weakest, are 1000 °C, and room temperature, respectively. At 200 and 800 °C, the strain rate effect on energy absorption ratio are most significant, while at 1000 °C, it is weakest. There are no obvious strain rate effects on elastic modulus and increase ratio of elastic modulus under high temperatures. According to test results, the relationship formula of strain rate with high temperature and impact load was derived by internalizing fitting parameters. Compared with the strain rate effect at room temperature condition, essential differences have occurred in the strain rate effect of rock material under the influence of high temperature.     

Dynamic compressive behaviour of Ti-6Al-4V alloy processed by electron beam melting under high strain rate loading

This paper documents an investigation into the compressive deformation behaviour of electron beam melting(EBM) processing titanium alloy(Ti-6A1-4V) parts under high strain loading conditions.The dynamic compression tests were carried out at a high strain rate of over1×10~3/s using the split Hopkinson pressure bar(SHPB)test system and for comparison the quasi-static tests were performed at a low strain rate of 1×10~3/s using a numerically controlled hydraulic materials test system(MTS) testing machine at an ambient temperature.Furthermore,microstructure analysis was carried out to study the failure mechanisms on the deformed samples.The Vickers micro-hardness values of the samples were measured before and after the compression tests.The microstructures of the compressed samples were also characterized using optical microscopy.The particle size distribution and chemical composition of powder material,which might affect the mechanical properties of the specimens,were investigated.In addition,the numerical simulation using commercial explicit finite element software was employed to verify the experimental results from SHPB test system.     

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.     

氧化铝陶瓷动态压缩强度的高压和高应变率效应

平面冲击压缩下材料由弹性到非弹性变形转变的临界值对应着材料的动态压缩强度,高压和高应变率强烈影响材料的强度。采用粘塑性比拟法建立了材料发生非弹性变形的动态屈服条件,并引入Drucker-Prager静态屈服准则,可以联合考虑动态压缩强度的高压和高应变率效应,据此提出了新的Hugoniot弹性极限表征形式。利用轻气炮进行了氧化铝陶瓷平板冲击实验,VISAR测试了样品的自由面速度历程,讨论了氧化铝陶瓷动态压缩强度的测定和存在的不确定性问题。     

Energy absorption capability of carbon nanotubes dispersed in resins under compressive high strain rate loading

The focus of the present study is on energy absorption capability (E_A) of carbon nanotubes (CNTs) dispersed in thermoset epoxy resin under compressive high strain rate loading. Toward this objective, high strain rate compressive behavior of multi-walled carbon nanotube (MWCNT) dispersed epoxy is investigated using a split Hopkinson pressure bar. The amount of MWCNT dispersion is varied up to 3% by weight. Calculation methodology for the evaluation of E_A of individual CNTs and CNTs dispersed in resins/composites is presented. Quantitative data on E_A of individual CNTs and CNTs dispersed in resins under quasi-static and high strain rate loading is given.     

Dynamic tests of cemented paste backfill: effects of strain rate,curing time,and cement content on compressive strength

This article investigates the compressive strength of cemented paste backfill (CPB) under dynamic loading. To accommodate the low impedance CPB, a modified split Hopkinson pressure bar (SHPB) system is adopted. In contrast to traditional solid steel transmitted bar, a hollow aluminum transmitted bar is introduced to reduce the impedance. With this system, the dynamic stress equilibrium is achieved, which guarantees the valid dynamic material testing condition. The dynamic tests are conducted for CPB with different cement contents and curing time. It is observed that: (1) for CPB with the same curing time and cement content, the dynamic strength increases with the strain rate, (2) for CPB with the same cement content, the dynamic strength increases with the curing time, and (3) for CPB with the same curing time and tested under similar strain rate, the dynamic strength increases with the percentage of cement. This observation can be understood by considering the hydration process of cements.     

Modeling of structures subjected to impact: concrete behaviour under high strain rate

This paper deals with a viscoplastic model which is the natural way to take into account the rate effect. Consideration of viscosity averts the habitual mesh sensitivity when strain softening is introduced by preserving the well-posedness of the initial boundary value problem. Modeling can constitute an alternative to experimentation not in order to predict the material response, but to try to understand and to evaluate the rate effect. Numerical simulation of the split test Hopkinson pressure bar gives an idea of dynamic concrete behaviour: forces of inertia, inertial confinement, structural effect and rate effect. Finally, the model is used to simulate a reinforced concrete beam submitted to impact.     

A physical phenomenon which can explain the mechanical behaviour of concrete under high strain rates

The free water in concrete underlies the various physical mechanisms that shape the mechanical behaviour of the material. In this article we attempt to show, through an experimental observation and a theoretical assumption, that the mechanical behaviour of concrete under high strain rates could be explained by a coupling between one of these physical effects and the process of cracking in the material. The assumption about the physical mechanism involved is made more to understand what happens inside the material than to lead to quantitative predictions.

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Resume L'eau libre au sein du béton est à l'origine de différents mécanismes physiques qui interviennent dans le comportement mécanique du matériau. Dans le cadre d'une coopération européenne, l'Université de Technologie de Delft et le LCPC avaient étudié le comportement dynamique du béton (c'est à dire, dans le cas présent, sous de grandes vitesses de déformation), en s'intéressant essentiellement à l'influence, sur ce type de comportement, de l'humidité interne du matériau. Les essais étaient réalisés sur la barre de Hopkinson de Delft qui permet de solliciter, en traction directe, des éprouvettes de béton à des vitesses de déformation pouvant aller jusqu' à 10 s⁻¹. Un micro-béton était testé dans deux conditions d'hygrométrie interne: complètement sec, et humide. On a constaté qu'en ce qui concernait le béton humide, la résistance à la traction directe augmentait avec la vitesse de déformation, phénomène connu pour la plupart des matériaux, alors que cette dépendance n'existait pas pour le béton sec. Dans cet article on propose une hypothèse quant au mécanisme physique qui permettrait d'expliquer cette augmentation de la résistance à la traction du béton avec la vitesse de déformation. Il s'agit de l'effet Stefan qui peut être décrit succinctement de la manière suivante: l'existence d'un mince film visqueux (eau ou huile par exemple) entre deux cales parfaitement planes et parallèles, et distantes d'une certaine longueurh, conduit à la création d'une force de rappel lorsque l'on tente d'écarter les deux cales avec une vitesse h. Plus la vitesse est grande, plus cette force de rappel le sera. Dans un béton, les cales séparées par un ménisque d'eau ne sont autres, par analogie, que les parois des micropores et des capillaires.

     

Influence of microstructure and strain rate on the compressive strength of whisker-reinforced ceramic matrix composites

The compressive strength of pyroceramic reinforced with a wide variety of SiC whiskers was characterized at loading rates which range from quasi-static to dynamic. It was found that strength is inversely related to whisker size, and essentially strain rate insensitive. The same strain rate independence was obtained for unreinforced matrix, but the strength of the latter lies below that for small ( 1 m diameter) whisker-reinforced composites, and above that for large ( > 3 m diameter) whisker material. Whisker/crack interaction and (to a lesser extent) whisker pull-out seem to be responsible for the beneficial influence of small whiskers, while the apparently detrimental large whiskers serve as microcrack-nucleating inclusions.     

Behavior of high performance fiber reinforced cement composites under multi-axial compressive loading

Experiments were conducted to better understand the behavior of strain hardening, high performance fiber reinforced cement composites (HPFRCC) when subjected to uniaxial, biaxial, and triaxial compression. The experimental parameters were: type of fiber, fiber volume fraction, and loading path. Two types of commercially available fibers, namely high-strength hooked steel fiber and ultra high molecular weight polyethylene fiber, with volume fractions ranging from 1.0% to 2.0%, were used in a 55-MPa mortar matrix. The selected loading paths consisted of uniaxial compression and tension, equal biaxial compression, and triaxial compression with two levels of lateral compression. The test results revealed that the inclusion of short fibers can significantly increase both strength and ductility under uniaxial and biaxial loading paths, but that the role of volume fraction is rather small for the range of fiber volume contents considered. The results also showed that the confining effect introduced by the fibers becomes minor in triaxial compression tests, where there is relatively high external confining pressure. The experimental information documented herein can serve as input for the development of multiaxial constitutive models for HPFRCCs.     

Relaxation characteristics of cement mortar subjected to tensile strain

Tensile relaxation characteristics are important for the crack resistance of concrete members subjected to restrained contraction, for example bonded overlays and patch repairs. In the experimental research discussed in this paper, relaxation characteristics were measured for mortar specimens subjected to constant strain which corresponded to stresses close to the tensile strength of the mortar. Relaxation was found to relieve a considerable portion of tensile stresses. Ultimate relaxation values ranged from 20 to 45%, depending on w/c ratio and specimen age. As expected, a decrease in specimen age and an increase in w/c ratio resulted in increasing relaxation values. The rate of stress decay was found to be rapid, with approximately 80% of the ultimate relaxation occurring in the first 12?h after loading. A basic equation for the prediction of time development of relaxation is proposed and future research needs are discussed.     

Measurement and prediction of compressive properties of polymers at high strain rate loading

Strain-rate effect is widely recognized as a crucial factor that influences the mechanical properties of material. Despite the acknowledge importance, the understanding of how such factor interact with the sensitivity of the polymers in terms of mechanical properties is still less reported. In this study, an experimental technique, based on the compression split Hopkinson pressure bar, was introduced to perform high strain rate testing, whereas, a conventional universal testing machine was used to perform static compression testing, to experimentally investigate the independent and interactive effects of strain rates towards mechanical properties of various polymers. Based on the experimental results, we parameterized two equation models, which were used to predict the yield behavior of tested polymer samplings. The experimental results indicate that, the yield stress, compression modulus, compressive strength, strain rate sensitivity and strain energy increased significantly with increasing strain rates for all tested polymers. Meanwhile, the yield strain and the thermal activation volume exhibit contrary trend to the increasing strain rates. Interestingly, the proposed constitutive models were almost agreed well with experimental results over a wide range of strain rate investigated. Of the three polymers, polypropylene shows the highest strain rate sensitivity at static and quasi-static region. On the other hand, at dynamic region, polycarbonate shows the highest strain rate sensitivity than that of polypropylene and polyethylene. Overall, both experimental and numerical models proved that the mechanical properties of polymer show significant sensitivity and dependency towards applied strain rates up to certain extent.