A novel indirect tensile test method to measure the biaxial tensile strength of concretes and other quasibrittle materials

A novel indirect tensile test method, the biaxial flexure test (BFT) method, has been developed to measure the biaxial tensile strength of concretes. The classical modulus of rupture (MOR) test has been generalized to three dimensions. In this method, we use a circular plate as the new test specimen. This plate is supported by an annular ring. We apply an external load to this specimen through a circular edge. The centers of the specimen, the loading device and the support are identical. The biaxial tensile strength measured by this new method is about 19% greater than the uniaxial tensile strength obtained from the classical modulus of rupture test as reported by other researchers. However, at the same time, we also found that the stochastic deviation of the biaxial tensile strength is about 63% greater than the uniaxial strength.     

Investigating direct tensile behavior of high performance fiber reinforced cementitious composites at high strain rates

This research reported the process of measuring direct tensile stress versus strain response of high performance fiber reinforced cementitious composites (HPFRCCs) at high strain rates between 10 s^− 1 and 40 s^− 1. High rate tensile tests were performed using a strain energy frame impact machine (SEFIM) built by authors. The stress history of HPFRCC at high rates was estimated from two strain gauges attached on two sides of a transmitter bar while the strain history was obtained by analyzing the sequential images recorded using a high speed camera. HPFRCCs exhibited strong rate sensitivity, i.e., their tensile parameters, including post cracking strength, strain capacity, peak toughness, and number of cracks, were significantly enhanced as the strain rate increased although the enhancement was different according to the types of fiber. The source of the dynamic enhancements in the tensile parameters of HPFRCCs was discussed.     

Compressive mechanical properties of HTPB propellant at low,intermediate, and high strain rates

Low, intermediate, and high strain rate compression testing (1.7 × 10^?4 to 2500 s^?1) of the hydroxyl‐terminated polybutadiene (HTPB) propellant at room temperature, were performed using a universal testing machine, a hydraulic testing machine, and a split Hopkinson pressure bar (SHPB), respectively. Results show that the stress linearly increases with strain at each condition; the increasing trend of stress at a given strain with the logarithm of strain rate changes from a linear to an exponential form at 1 s^?1. By combining these characteristics, we propose a rate‐dependent constitutive model which is a linearly elastic component as a base model, then multiplied by a rate‐dependent component. Comparison of model with experimental data shows that it can characterize the compressive mechanical properties of HTPB propellant at strain rates from 1.7 × 10^?4 to 2500 s^?1. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43512.     

Direct tensile behavior of ultra high performance fiber reinforced concrete (UHP-FRC) at high strain rates

This experimental study investigates the direct tensile behavior of ultra-high performance fiber reinforced concrete (UHP-FRC) at strain rates ranging from 90 to 146/s. The tests are conducted using a recently developed impact testing system that uses suddenly released strain energy to generate an impact pulse. Three fiber types were considered, a twisted fiber and two other types of straight fibers. Specimen impact response was evaluated in terms of first cracking strength, post-cracking strength, energy absorption capacity and strain capacity. The test results indicate that specimens with twisted fibers generally exhibit somewhat better mechanical properties than specimens with straight fibers for the range of strain rates considered. All UHP-FRC series tested showed exceptional rate sensitivities in energy absorption capacity, generally becoming much more energy dissipative under increasing strain rates. This characteristic highlights the potential of UHP-FRC as a promising cement based material for impact- and blast-resistant applications.     

Laminated connections under tensile load at different temperatures and strain rates

In the last years, a novel typology of adhesive connections for structural glass application has emerged, known as laminated adhesive connections, which makes use of the transparent ionomer SentryGlas® (SG) from Kuraray and the Transparent Structural Silicon Adhesive (TSSA) from Dow Corning. Despite being used in several projects, limited information is available in literature on their mechanical behaviour and on the effects of strain rate and temperature. In this work the behaviour of laminated connections under tensile loading is studied by means of experimental, analytical and numerical analyses. The experimental investigations show that temperature and strain rate variations have important effects on the mechanical response of the connections. Two main interesting phenomena are also observed: the whitening phenomenon in TSSA and the development of bubble within the SG adhesive. The analytical studies of the stress state show that confinement state of the adhesive induces a non-uniform three-dimensional stress distribution in the adhesive with a dominant hydrostatic component of the stress tensor, which is observed to be in agreement with the experimental results. Three-dimensional finite numerical analyses show that the stress field deviates from the uniform distribution with a large gradient of hydrostatic and deviatoric stresses over the adhesive area. The output of the finite numerical model are then compared with the observations of the experimental campaigns. Herein, the full set of numerical results is synthetized by the definition of so-called stress factors. The latter allow to derive the three-dimensional stress state in the adhesive at different temperatures and to compute the stress peak in the non-linear stress field distribution. Finally, prediction models are proposed for the tensile resistance of TSSA and SG laminated connections. A logarithmic law is proposed for the strain rate effects for both TSSA and SG connections. Linear and inverse hyperbolic-tangent-based laws are instead proposed for the TSSA and SG temperature effects, respectively.     

The compressive and tensile behavior of a 0/90 C fiber woven composite at high strain rates

An investigation into the compressive and tensile behavior of a carbon fiber reinforced resin matrix composite at high strain rates is carried out using a split Hopkinson bar. All the dynamic tests are performed under the condition of stress equilibrium and constant strain rate. The results of the compressive tests show that the failure strength and strain of the composite increase with the increase of strain rate. A plateau is observed in a typical stress–strain curve which prompts further study into the failure mechanism by monitoring the failure process with a high-speed camera. The three-phase failure mechanism of on-impact compression, crack-induced unloading, and crack deviation-caused further condensation, is found to have greatly increased the strength and toughness of the composite. In the tensile tests, an increase of strain rate produces a reduced fracture angle and extended crack path. In this process, more failure energy is absorbed, thus the failure strength and strain of the composite are improved. The Cowper–Symonds model of strain rate dependency indicates that the material has a higher tensile strength than compressive strength, and the strain rate sensitivity is more noticeable at high stain rates than quasi-static conditions.     

Compressive behavior of biaxial spacer weft knitted fabric reinforced composite at various strain rates

The in‐plane and out‐of‐plane compressive properties of biaxial weft knitted E‐glass fabric reinforced vinyl ester composite at quasi‐static strain rate of 0.001/s and high strain rates from 700/s to 2200/s were tested to investigate the strain rate effect on the compressive behavior. The compressive tests were conducted on split Hopkinson pressure bar at high strain rate and on MTS 810.23 system at quasi‐static state. The experimental results indicated the strain rate sensitivity of compressive stiffness, failure stress, and strain of the composite in both out‐of‐plane and in‐plane compressive direction. The compressive stiffness and failure stress linearly increased with the increase of strain rate. The failure strain linearly decreased with the increase of strain rate. As the strain rate increased, the main failure mode at out‐of‐plane compression is the interlaminar shear failure and at in‐plane direction is the delamination. At the high strain rate of 2200/s, the composite coupon was compressed into debris with the shear or delamination failure. POLYM. COMPOS., 28:224–232, 2007. © 2007 Society of Plastics Engineers     

Model for tensile fracture of concrete at high rates of loading

Mechanisms of tensile fracture of concrete are described. A model is developed for an idealized material. The amount of simultaneous cracking and the path of each crack depend on the rate of stressing. The fracture energy and the tensile strength have been determined as functions of the rate of loading. The results of earlier experiments on concrete under impact tensile loading can be explained by this model.     

低易损性固体推进剂钝感特性及评估试验方法研究进展

总结了低易损性固体推进剂的钝感特性及国内外在低易损性固体推进剂研究方面的主要差距,分析了低易损性推进剂的危险性与钝感特性的相关性,并阐述了固体推进剂的钝感特性评估试验方法,提出了完善低易损性固体推进剂、钝感弹药的安全试验方法和评估标准,展望了低易损性弹药的发展方向。     

Direct tension test and tensile strain capacity of concrete at early age

The tensile strain capacity of concrete under uniaxial tension is investigated using the direct tension test method. The adopted method of testing improves the weak bond strength between the embedded bar and concrete and reduces the stress concentration at the end of the embedded bar. The method has overcome the difficulties in centralizing and aligning the two embedded bars in the specimens. Seven mixes of concrete were designed to study the effects of age, compressive strength and mineral admixture on the tensile strain capacity. The investigation shows that the tensile strain capacity of concrete is a relatively independent parameter. The average tensile strains at failure and at 90% failure load are 120 and 100 μ, respectively. The corresponding characteristic tensile strain values at failure and at 90% failure load are 86 and 78 μ, respectively.     

Mechanical behaviour at large strain of polycarbonate nanocomposites during uniaxial tensile test

The present study focuses on the mechanical behaviour of polycarbonate nanocomposites reinforced by alumina or silica nanoparticles at low levels of incorporation. More particularly, we present an experimental approach, specific to large strain measurements by using a Digital Image Correlation (DIC) technique. The mechanical mechanisms involved during uniaxial tensile test of polycarbonate nanocomposites were studied at two-dimensional scales. First, elastic properties of the nanocomposites were determined at macro-homogeneous scale and compared to values obtained by continuum-based elastic micromechanical models. Then the in-plane kinematics measurements at the central part of a double-edge notched sample is analysed locally. The evolution during the test of the volumetric strain and axial strain profiles were studied before and after the yield stress: this analysis revealed the existence of several damage processes during the test up to rupture and put in relief the influence of fillers. Finally, the necking phenomenon was statistically studied and the shape of the neck in terms of strain intensity and localisation was analysed.     

Stress-strain behaviour of concrete and mortar at high rates of tensile loading

The Split-Hopkinson-Bar technique was used in the investigation on tensile stress-strain behaviour of concrete and mortar at high stress rates (5–30 N/mm²ms).The single loading tests showed that the impact tensile strength was higher than the static one, and that impact strains at the maximum stress were larger than static strains.The impact fatique tensile tests indicated an increase of strains in the course of fatigue loading and increasing fatique life with decreasing maximum stress level.These phenomena are discussed with the aid of fracture mechanics concepts and explanations for differences in the behaviour of concrete and mortar are suggested.     

Confined compression of elastic adhesives at high rates of strain

Elastic adhesives are used in composite armours to bond the ceramic front face and the metallic backing plate. The mechanical behaviour of different elastic adhesives under impact loads have been studied by means of dynamic compression tests performed in a split Hopkinson pressure bar. In this experiments, the stress-strain curve of confined materials at high strain rates and the capability of transmitting and reflecting the impact energy have been determined. The influence of thickness and ageing on the response of the adhesive layer have been also considered.     

Fracture energy of ultra-high-performance fiber-reinforced concrete at high strain rates

The fracture energy of ultra-high-performance fiber-reinforced concrete (UHPFRC) at high strain rates (5–92 s^− 1) was investigated, and specimens with 1–1.5% fibers exhibited very high fracture energy (28–71 kJ/m²). Evaluation of the rate effects on the UHPFRC fracture resistance, including fracture strength (f_t), specific work-of-fracture (W_S), and softening fracture energy (W_F), indicated that f_t and W_S were highly sensitive to strain rate, whereas W_F was not. The effects of fiber type, volume content, specimen shape and fiber blending on the fracture resistance at high and static strain rates differed significantly: 1) smooth fibers exhibited higher f_t and W_S at high rates than twisted fibers; 2) higher fiber volume content did not clearly generate higher W_S and W_F at high rates; 3) notched specimens generally exhibited higher fracture resistance than un-notched samples at both static and high rates; and 4) UHPFRC blending two fibers produced higher W_S and W_F than UHPFRC with mono fiber at high rates.     

Combustion of a composite solid propellant under conditions of static mechanical tensile stresses

Mechanical stresses have a progressive effect on the combustion characteristics of composite solid propellants. On the basis the kinetic theory for the durability of polymers within their composition the mechanism is studied for the effect of stress-strain state on steady combustion rate. Chemical bonds of the polymer matrix are activated as a result of an applied stress and there is an increase in the rate of their thermal destruction. It is shown that this is an important reason for an increase in combustion rate with uniaxial tension. An analytical equation is obtained which expresses the dependence of combustion rate on the magnitude of a prescribed stress or that measured by experiment.Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 4, pp. 20–28, July–August, 1993.     

A new test to determine the tensile strength of brittle balls—The notched ball test

A new strength test for ceramic spheres (balls) is presented. A long narrow notch is cut in the equatorial plane of the ball and the ball is then loaded in compression perpendicular to the notch. This causes tensile stresses in the outer surface region of the ball opposite to the notch, which are analysed carefully with finite element (FE) methods. The tensile stress amplitude depends on the bending moment in the notch ligament – given by the applied force – and on details of the notch geometry. The stress state in the highly stressed surface is almost uniaxial showing only a slight influence of Poisson's ratio. Numerical solutions for balls with quite different notch geometries are given.Strength tests have been performed on commercial silicon nitride balls of 5 mm diameter. Two sets of specimens having notches of different length have been tested. Although the typical fracture loads in both sets of data are quite different, the tensile strengths are closely similar. This indicates the validity of the data evaluations. Experimental details are discussed and an analysis of the experimental uncertainties on the test results is made. For balls with 5 mm the uncertainties are estimated to be less than ±3% (of the measured value). For balls having a diameter of 10 mm or more the uncertainties are less than ±1%.     

Flow stress of high density polyethylene and nylon 66 at high rates of strain

Measurement of the flow stress of high density polyethylene (HDPE) and nylon 66 at strain rates of 10³ s^?1 using a split Hopkinson pressure bar technique is discussed. The flow stress at a strain of 10% has been determined for both polymers at 20°C. The intrinsic errors involved in this technique are briefly reviewed. The results indicate that the flow stress of HDPE and nylon 66 were 50MPa and 150MPa, respectively, at strain rates of about 10³s^?1.     

Markedly compressible behaviors of gellan hydrogels in a constrained geometry at ultraslow strain rates

The fracture behaviors of gellan hydrogels under compression remarkably depend on the strain rate as well as the boundary conditions for lateral expansion. In the geometry with no constraint for lateral expansion (conventional uniaxial compression), the gels rupture at relatively small strains independently of the compression rates. In contrast, when the gels are compressed extremely slowly (at a strain rate of ca. 10⁻⁵ s⁻¹) in the geometry prohibiting the lateral expansion at their top and bottom surfaces, they are remarkably compressible down to 2% of the initial height without macroscopic fracture and they are accompanied by a large amount of water release. In such markedly compressed gels, many microscopic cracks are formed around the central layer, where strain concentration occurs due to the nonuniform deformation arising from the constrained geometry. In the highly compressible case, the formation of macroscopic cracks is prevented by the localization of microscopic cracks as well as the enhancement in mechanical toughness by a significant increase in polymer concentration due to water release.     

A simultaneous SAXS/WAXS and stress-strain study of polyethylene deformation at high strain rates

An experimental system has been commissioned which allows the collection of time-resolved simultaneous two-dimensional small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) and stress-strain data with a temporal resolution of 40 ms during the drawing of synthetic polymers. X-ray data collection is achieved via two CCD-based area detectors which are positioned in order to maximize the amount of scattering that can be observed from a particular sample. True stress-strain measurements are obtained from video extensometer and load cell measurements. The system was used at the Daresbury SRS to study the structural changes which occur in a sample of isotropic high-density polyethylene when subjected to an overall strain rate of ≈ 3 s⁻¹. It is shown that the onset of both a partial stress-induced crystal phase change and micro-voiding in the sample can be directly correlated to the point of yield in the true stress-strain curve. © 1997 Elsevier Science Ltd.     

Compressive response of PMMA microcellular foams at low and high strain rates

Microcellular foams are widely applied in various applications in both civil and military applications for barriers and energy absorption materials. Poly(methyl methacrylate) microcellular foams were fabricated via supercritical foaming method. Field emission scanning electron microscopy, differential scanning calorimetry, and mechanical test machine were used to visualize the foam structure and test the quasi‐static compression properties. Moreover, Split Hopkinson Bar (SHPB) setups were adopted to explore the dynamic compression properties. The experimental results show that the microcellular foams have homogeneous cell size distribution and exhibit superior compressive behavior at both quasi‐static and high strain rates. The mechanical properties depend on both foam density and strain rate. Strain rate effects are clearly observed. At quasi‐static strain rate and 7500 S^?1 regime, cell wall bucking and folding are the main failure mechanism. However, at high strain rate regime, softening phenomenon is observed. By roughly calculating the energy absorbed and the temperature rise, the temperature of the foams will rise up to as high as 130 °C after conducting high strain rate compression, and it is postulated that the generated heat will destroy the cell structure of the foams. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46044.