Showing papers on "Stress–strain curve published in 2016"

Cracking and Stress–Strain Behavior of Rock-Like Material Containing Two Flaws Under Uniaxial Compression

Abstract: This paper investigates the cracking and stress–strain behavior, especially the local strain concentration near the flaw tips, of rock-like material containing two flaws. A series of uniaxial compression tests were carried out on rock-like specimens containing two flaws, with strain gauges mounted near the flaw tips to measure the local strain concentration under the uniaxial compressive loading. Four different types of cracks (wing cracks, anti-wing cracks, coplanar shear cracks and oblique shear cracks) and seven patterns of crack coalescences (T1 and T2; S1 and S2; and TS1, TS2 and TS3) are observed in the experiments. The type of crack coalescence is related to the geometry of the flaws. In general, the crack coalescence varies from the S-mode to the TS-mode and then to the T-mode with the increase of the rock bridge ligament angle. The stress–strain curves of the specimens containing two flaws are closely related to the crack development and coalescence process. The strain measurements indicate that the local tensile strain concentration below or above the pre-existing flaw tip causes wing or anti-wing cracks, while the local compressive strain concentration near the flaw tip is related to the shear crack. The measured local tensile strain shows a jump at the initiation of wing- and anti-wing cracks, reflecting the instant opening of the wing- and anti-wing crack propagating through the strain gauge. During the propagation of wing- and anti-wing cracks, the measured local tensile strain gradually increases with few jumps, implying that the opening deformation of wing- and anti-wing cracks occurs in a stable manner. The shear cracks initiate followed by a large and abrupt compressive strain jump and then quickly propagate in an unstable manner resulting in the failure of specimens.

A Novel Mogi Type True Triaxial Testing Apparatus and Its Use to Obtain Complete Stress–Strain Curves of Hard Rocks

Abstract: A true triaxial apparatus (TTA) was designed and fabricated at Northeastern University, Shenyang, China, by modifying the original Mogi type testing apparatus to emulate three-dimensional stress paths in deep mining and tunneling excavations. Such an apparatus can be used to investigate deformation and brittle failure behaviors of hard rocks as well as the cause of rockbursts. The novel TTA can capture the post-peak behavior of a 50 × 50 × 100 mm3 specimen. Technical improvements such as a considerable increase of the stiffness of the loading frames were implemented to deal with difficulties in TTA testing. The accuracy of the volume change measurement was improved and a combined pneumatic and hydraulic technique was applied to create a “floating” vertical loading frame. The end friction effect and the loading gap effect were evaluated using a series of tests. Repeatability tests, brittle failure tests in a loading stress path and an unloading stress path (unloading of σ 3) were carried out on granite specimens to verify the performance of the TTA. The test results show that the apparatus achieves its original design goal.

Investigation on Mechanical Behaviors of Sandstone with Two Preexisting Flaws under Triaxial Compression

Abstract: Triaxial compression experiments on sandstone samples with two preexisting closed non-overlapping flaws were performed to investigate the deformation and strength behaviors. Three types of preexisting closed flaw pair in sandstone samples, i.e., parallel low-dip (type B), parallel high-dip (type C), and composite high- and low-dip (type D), were considered as the typical arrangements of the non-overlapping crack pair. A general rule has been found that the arrangement of the flaw pair has greater impact on the rock deformation, strength, and crack coalescence pattern than the confining pressure (5–20 MPa). Experimental results showed that, compared with intact sandstone samples, the postpeak stress–strain curves of flawed samples distinctly demonstrate stress fluctuation. In particular, the unique prepeak stress–strain curves of the specimens with a low-dip flaw pair (type B) present oblique Z-shape with a double-peak stress. The stress for crack initiation σ ci, the critical stress of dilation σ cd, and the peak strength σ c of precracked sandstone samples are significantly lower than those of intact rock. The present numerical study, which is an extension of the test analysis, focuses on identifying the crack nature (tensile or shear) and coalescence process. These simulated crack coalescence patterns are in good agreement with the laboratory test results. The cracks of the precracked samples that contained flaws with small inclination angle (associated with either type B or type D) generally initiate at the inner flaw tips and eventually lead to simple direct shear coalescence. However, complex indirect shear coalescence appears in the model containing a steep preexisting flaw pair (associated with type B specimen), even though no coalescence occurs when σ 3 = 5 MPa.

Uniaxial Compression Behavior of Ultra-High Performance Concrete with Hybrid Steel Fiber

Abstract: This study investigated the effects of hybrid steel fiber of different sizes (6 and 13 mm) on the uniaxial compression performance of ultra-high performance concrete (UHPC). The failure pattern, stress–strain relationship, and toughness under the uniaxial compression of a reference UHPC mixture with no fiber and five UHPC mixtures with mono or hybrid fiber content of 2%, by volume of concrete, were studied. The results indicated that the incorporation of hybrid fiber significantly improved compression properties, with increased strength and toughness. An UHPC mixture with 1.5% long and 0.5% short fiber showed the best compressive behavior, whereas those with 2% short fiber showed the worst properties. Based on the obtained peak stress, strain at peak stress, elastic modulus, and compression toughness index, an analytical model was proposed to generate the complete stress–strain relationship. The proposed model agreed well with the experimental results.

Mechanical Characterization of the Tensile Properties of Glass Fiber and Its Reinforced Polymer (GFRP) Composite under Varying Strain Rates and Temperatures.

Abstract: Unidirectional glass fiber reinforced polymer (GFRP) is tested at four initial strain rates (25, 50, 100 and 200 s-1) and six temperatures (-25, 0, 25, 50, 75 and 100 °C) on a servo-hydraulic high-rate testing system to investigate any possible effects on their mechanical properties and failure patterns. Meanwhile, for the sake of illuminating strain rate and temperature effect mechanisms, glass yarn samples were complementally tested at four different strain rates (40, 80, 120 and 160 s-1) and varying temperatures (25, 50, 75 and 100 °C) utilizing an Instron drop-weight impact system. In addition, quasi-static properties of GFRP and glass yarn are supplemented as references. The stress⁻strain responses at varying strain rates and elevated temperatures are discussed. A Weibull statistics model is used to quantify the degree of variability in tensile strength and to obtain Weibull parameters for engineering applications.

Mean field modelling of dynamic and post-dynamic recrystallization during hot deformation of Inconel 718 in the absence of δ phase particles

Abstract: Dynamic and post-dynamic recrystallization kinetics were investigated during hot-working of Inconel 718 as a function of temperature, strain and strain rate in the single phase domain. The post-dynamic evolution was found to be extremely fast and impacting significantly the grain size. A two-site mean field model including both dynamic and post-dynamic evolution was used to predict the recrystallized fraction, the average grain size and the stress–strain curves in different thermomechanical conditions.

Twinning effects on strength and plasticity of metallic materials

Abstract: Twins are domain crystals inside their parent crystals, where they share some of the same crystal lattice points in a symmetrical manner. The formation and growth of twins result in substantial evolution of microstructures and properties in a large variety of metallic materials. Twin boundaries that separate two crystals effectively strengthen the material by impeding mobile dislocations, and increase the ductility and work-hardening capability of metallic materials. The articles in this issue of MRS Bulletin overview the synthesis and mechanical behavior of nanotwinned metallic materials, as well as plasticity dominated by mechanical twinning.

Dynamic impact factors of strain hardening UHP-FRC under direct tensile loading at low strain rates

Abstract: Enhanced matrix packing density and tailored fiber-to-matrix interface bond properties have led to the recent development of ultra-high performance fiber reinforced concrete (UHP-FRC) with improved material tensile performance in terms of strength, ductility and energy absorption capacity. The objective of this research is to experimentally investigate and analyze the uniaxial tensile behavior of UHP-FRC under various strain rates, ranging from 0.0001 to 0.1 1/s. A direct tensile test set up is used. The experimental parameters encompass three types of steel fibers, each in three different volume fractions at four different strain rates resulting in 36 test series. Elastic and strain hardening tensile parameters, such as, cracking stress, elastic and strain hardening modulus, composite tensile strength and strain, energy absorption capacity, and crack spacing of the UHP-FRC specimens, are recorded and analyzed. Explanation of the material’s strain rate sensitivity is mainly based on the inertia effect of matrix micro cracking. Potential contributions of other mechanisms include viscosity of water within nanopores and confinement effects. Dynamic impact factor (DIF) formulas are provided based on the experimental data to illustrate the relationship between DIF and strain rate for UHP-FRC.

Spall damage of a mild carbon steel: Effects of peak stress, strain rate and pulse duration

Abstract: We investigate spall damage of a mild carbon steel under high strain-rate loading, regarding the effects of peak stress, strain rate, and pulse duration on spall strength and damage, as well as related microstructure features, using gas gun plate impact, laser velocimetry, and electron backscatter diffraction analysis. Our experiments demonstrate strong dependences of spall strength on peak stress and strain rate, and its weak dependence on pulse duration. We establish numerical relations between damage and peak stress or pulse duration. Brittle and ductile spall fracture modes are observed at different loading conditions. Damage nucleates at grain boundaries and triple junctions, either as transgranular cleavage cracks or voids.

Cross-Sectional Unification on the Stress-Strain Model of Concrete Subjected to High Passive Confinement by Fiber-Reinforced Polymer.

Abstract: The stress-strain behavior of concrete can be improved by providing a lateral passive confining pressure, such as fiber-reinforced polymer (FRP) wrapping. Many axial stress-strain models have been proposed for FRP-confined concrete columns. However, few models can predict the stress-strain behavior of confined concrete columns with more than two specified cross-sections. A stress-strain model of FRP-confined concrete columns with cross-sectional unification was developed in this paper based on a database from the existing literature that includes circular, square, rectangular and elliptical concrete columns that are highly confined by FRP jackets. Using the database, the existing theoretical models were evaluated. In addition, the ultimate stress and strain models with cross-sectional unification were proposed using two parameters: the cross-sectional aspect ratio and corner radius ratio. The elliptical cross-section can be considered as a rectangular one with a special corner radius for the model calculations. A simple and accurate model of the equivalent corner radius ratio for elliptical columns was proposed. Compared to the other existing models and experimental data, the proposed models show good performance.

Stress–Strain Relationship, Critical Strain (Stress) and Irreversible Strain (Stress) of IBAD-MOCVD-Based 2G HTS Wires Under Uniaxial Tension

Abstract: The mechanical behavior of a REBCO-coated conductor wire under uniaxial tension is largely determined by the two thickest component layers in the architecture, namely, the substrate and the stabilizer. A rather complicated stress–strain relationship is often observed when the composite conductor is under uniaxial tension, which is the result of the differences in elastic modulus and yield stress between the Hastelloy substrate and the electroplated Cu stabilizer. In this paper, the stress–strain relationships of a free standing Cu stabilizer and a bare REBCO wire, i.e., a tape without any stabilizer, were measured. The results are utilized for the calculation of the stress–strain relationships of electroplated Cu stabilized wires using a two-component composite model. Given the thicknesses of the substrate and stabilizer, the calculated results agree well with the measured stress–strain curves, which can be well fitted with the Ramberg–Osgood equation. The tensile strain (stress) dependence of the critical current $(I_{c})$ was measured at 77 K in liquid nitrogen. Depending on whether the critical current was measured with the wire under a stress or after unloading, a critical strain (stress) and an irreversible strain (stress) were determined, respectively.

Effects of Foot Strike and Step Frequency on Achilles Tendon Stress During Running

Abstract: Achilles tendon (AT) injuries are common in runners. The AT withstands high magnitudes of stress during running which may contribute to injury. Our purpose was to examine the effects of foot strike pattern and step frequency on AT stress and strain during running utilizing muscle forces based on a musculoskeletal model and subject-specific ultrasound-derived AT cross-sectional area. Nineteen female runners performed running trials under 6 conditions, including rearfoot strike and forefoot strike patterns at their preferred cadence, +5%, and -5% preferred cadence. Rearfoot strike patterns had less peak AT stress (P .05) between step frequencies within each foot strike condition. Our results suggest that a rearfoot pattern may reduce AT stress, strain, and strain rate. Increases in step frequency of 5% above preferred frequency, regardless of foot strike pattern, may also lower peak AT stress and strain.

Mechanical and damage evolution properties of sandstone under triaxial compression

Abstract: To study the mechanical and damage evolution properties of sandstone under triaxial compression, we analyzed the stress strain curve characteristics, deformation and strength properties, and failure process and characteristics of sandstone samples under different stress states. The experimental results reveal that peak strength, residual strength, elasticity modulus and deformation modulus increase linearly with confining pressure, and failure models transform from fragile failure under low confining pressure to ductility failure under high confining pressure. Macroscopic failure forms of samples under uniaxial compression were split failure parallel to the axis of samples, while macroscopic failure forms under uniaxial compression were shear failure, the shear failure angle of which decreased linearly with confining pressure. There were significant volume dilatation properties in the loading process of sandstone under different confining pressures, and we analyzed the damage evolution properties of samples based on acoustic emission damage and volumetric dilatation damage, and established damage constitutive model, realizing the real-time quantitative evaluation of samples damage state in loading process.

Cold rotary swaging of a tungsten heavy alloy: numerical and experimental investigations

Abstract: A finite element analysis was performed to predict behaviour of sintered tungsten-based heavy alloy during cold rotary swaging, while experimental investigations evaluated mechanical and structure properties in both, sintered and swaged material states. The simulation involved prediction of swaging force, which was subsequently compared with force measured experimentally using own designed force detection system, although other parameters, such as strain, strain rate, stress and temperature were also predicted and subsequently compared to experimental data. The results showed significant hardening and strengthening after swaging; the average ultimate strengths after sintering and swaging, respectively, were 860 MPa and 1680 MPa. This also contributed to very high swaging force of almost 600 kN. The distribution of microhardness across the cross-section confirmed the predicted strain distribution. Texture analyses revealed a notion of cube texture given primarily by the fcc matrix in the sintered state, while several ideal orientations for both the fcc and bcc phases were observed after swaging. As indicated by grains misorientations analyses, swaging introduced residual stress, the distribution of which was in conformity with the predicted stress and strain distributions.

The granular and polymer composite nature of kerogen-rich shale

Abstract: In the past decade, mechanical, physical, and chemical characterization of reservoir shale rocks, such as the Woodford shale, which is kerogen-rich shale (KRS), has moved toward micro- and nanoscale testing and analyses. Nanoindentation equipment is now widely used in many industrial and university laboratories to measure shale anisotropic Young’s moduli, kerogen stiffness, plastic yield parameters, and other isotropic and anisotropic poromechanical and viscoelastic properties. However, to date, failure analyses of KRS and the effects of organic components on the tensile strength have not been observed or measured at the micro- or nanoscales. In this study, preserved kerogen-rich Woodford shale samples manufactured in micro-beam and micro-pillar geometries were mechanically tested and brought to failure in tension and compression, respectively. These tests were conducted in situ using a nanoindenter inside a scanning electron microscope (SEM). The load versus displacement curves of prismatic micro-cantilever beams were analyzed in light of high-resolution images collected during tensile fracture initiation, propagation, and ultimately sample failure. The micro-pillar geometries were subjected to a uniaxial compressive load and were also brought to failure while capturing measurements of stress and strain. It was found that, within just a few hundred microns of the KRS micro-cantilever beams, both brittle and ductile failure modes were observed. In the ductile plastic domain, strain-softening and strain-hardening behaviors were identified and characterized. These were not due to confining stress variations, but due to the volume of the organic matter and the way it is interlaced with the shale minerals in and around the failure planes. The tensile strength characteristics and the large modulus of toughness of kerogen, which is a cross-linked polymer, definitely weigh heavily in our engineering field applications, such as hydraulic fracking, which is a Mode I tensile fracture opening and propagation phenomenon. This practice demands that, due to the complex composite nature of KRS, mechanical characterization be not only for unconfined compressive strength but also for unconfined tensile strength and moduli of ruptures. At the end of this study, the need for nanometer scale mechanical characterization of KRS will become apparent. These nano- and micro-scale shale failure tests reinforce our previous understanding of the heterogeneous composite nature of Woodford KRS and its complex behavior, as well as other source shale reservoir formations.

Towards Understanding the Effect of Deformation Mode on Stress Corrosion Cracking Susceptibility of Grade 2205 Duplex Stainless Steel

Abstract: The effect of bending, rolling, and tensile deformation on stress and strain development in grade 2205 duplex stainless steel has been investigated using x-ray diffraction (XRD) and electron backscatter diffraction (EBSD) analyses. The deformed microstructures were assessed for their stress corrosion cracking (SCC) susceptibility, with highest microstructure propensity observed after bending deformation. Strain localisation occurred in the austenite, independent of applied deformation mode. Cold rolling and bending also resulted in stress development in the austenite, with the ferrite also indicating significantly increased stresses after tensile straining. The austenite phase became more susceptible towards SCC, whereas the ferrite seemed to be more prone towards selective dissolution. Rolling deformation enhanced the propensity to localised corrosion.