Total Axial Strain Research Articles

Experimental study on plastic deformation of alluvial silty sand under traffic cyclic loading

With the development of highway construction, alluvial silt is gradually used as roadbed filler, but its dynamic characteristics are less studied. Based on the highway section from Luanzhou to Qingtuoying of G508, the plastic strain and dynamic stress-dynamic strain development law of alluvial silty sand under cyclic loading are analyzed by GDS dynamic triaxial test. The results show that under the action of a small range of dynamic stress amplitude, the soil sample is not destroyed, and the number of cycles is 1000 times, which is the critical point at which the axial total strain gradually tends to be stable. When the dynamic stress amplitude is 300 kPa, the total axial strain increases linearly, and the soil sample is destroyed. The cumulative plastic strain of the alluvial silt is the smallest when the optimum water content is 8.0 %, the confining pressure is 80 kPa, and the frequency is 1Hz.At this time, the cyclic load has a dense effect on the soil, and the soil structure does not fail. The dynamic stress-strain relationship of alluvial silty sand conforms to the R. L. Kondner hyperbolic model. The greater the confining pressure and the closer the moisture content to the optimal moisture content, the greater the dynamic strength. The research results are helpful to provide reference for other similar projects.

Impact of Notches on Additively Manufactured Inconel 718 Tensile Performance.

This study was completed in effort to characterize the notch sensitivity of additively manufactured (AM) Inconel 718 produced by laser powder bed fusion (L-PBF). Three different root radii on V-notched test specimens and smooth specimens were evaluated under tensile conditions for specimens built in vertical and horizontal orientations. Both the total axial strain and localized notch diametral strain were measured. Finite element analysis (FEA) was completed on each specimen geometry to confirm the actual strain measurements near the notch. Test results showed the tensile strength of the notched specimens were larger than the tensile strength values of the smooth specimens. These tensile results equate to a notch-sensitivity ratio (NSR) greater than one, indicating that the L-PBF Inconel 718 material is a notch-strengthened material. It is suspected that the notch strengthening is a result of increased triaxial stress produced near the notch tip causing added material constraints, resulting in higher strength values for the notched specimens. Fractography analysis was completed on the various fracture surfaces and identified a dominate ductile failure mode within all of the specimens; however, the amount of ductility reduced with smaller notch root radii. While this study provides the initial notch responses of the L-PBF Inconel 718, further research must be completed in regard to the impact of notches on more complex loading behaviors, such as fatigue and stress-rupture conditions.

Enhanced critical axial tensile strain limit of CORC® wires: FEM and analytical modeling

Conductor on Round Core (CORC®) cables and wires are composed of spiraled high-temperature superconducting (HTS) rare-earth barium copper oxide (REBCO) tapes, wound in multiple layers, and can carry very high currents in background magnetic fields of more than 20 T. They combine isotropic flexibility and high resilience to electromagnetic and thermal loads. The brittle nature of HTS tapes limits the maximum allowable axial tensile strain in superconducting cables. An intrinsic tensile strain above about 0.45% will introduce cracks in the REBCO layer of straight HTS tapes resulting in irreversible damage. The helical fashion at which the REBCO tapes are wound around the central core allows tapes to experience only a fraction of the total axial tensile strain applied to the CORC® wire. As a result, the critical strain limit of CORC® wires can be increased by a factor of more than 10 that of REBCO tapes. Finite element (FE) and analytical models are developed to predict the performance of CORC® wires under axial tensile strain. A parametric analysis is carried out by varying the winding angle, the Poisson’s ratio of the CORC® wire core, the core diameter, and the tape width. The results show that a small variation in winding angle can have a significant impact on the cable’s axial tensile strain tolerance. While the radial contraction of the helically wound tapes in a CORC® wire under axial tensile strain depends on its winding angle, it is mostly driven by the Poisson’s ratio of the central core, affecting the tape strain state and thus its performance. Contact pressure from multiple layers within the CORC® wire also affects the CORC® wire performance. The FE model can be used to optimize the cable design for specific application conditions, resulting in an irreversible strain limit of CORC® cables and wires as high as 7%.

Analysis of Mechanical Properties and Slope Stability of Red Bed Soft Rock: A Case Study in Xinjiang Irrigation Diversion Channel

The channel slope in the red‐bed soft rock area is prone to instability and collapse due to the influence of the channel flow movement, rainfall, weathering, and other factors. Under long‐term operation conditions, the sediment stripped from the channel side wall is liable to silt along the process of water flow transportation in the channel, which seriously leads to elevation of the channel bottom, increase of channel width, and reduction of the horseway, which affect the normal water conveyance and operation safety of channels. In view of the collapse and instability of Zhetang diversion irrigation channel project in Xinjiang hydraulic project, through on‐site sampling and indoor rock mechanics test, the strength mechanical parameters of slope rock samples are obtained, and the indoor rheological test is carried out, and the triaxial rheological law is obtained. Under different levels of stress, the axial creep strain accounts for more than 50% of the total axial strain, indicating that the rheological effect of sample rock is obvious. Based on the strength parameters of rock samples, a numerical model is established. The stability of red‐bed soft rock channel bank slope is studied by using the strength reduction method of finite element method. The seepage field of channel slope under conventional working conditions is analyzed, and the slope stability under different operating conditions is compared and calculated. The results show that the slope stability safety factor of Zhetang diversion irrigation channel slope is 1.60 under conventional working conditions and 1.33 under check working conditions, which are greater than the recommended value in the specification.

Axial strain accumulation projection model for sand in cyclic loading

Strain accumulation analysis is important to understand the deformation mechanism of soil structure subjected to cyclic loading. An empirical model is a superior choice for the description of strain accumulation. However, the ambiguity and sensitivity for the physical meaning of the parameters in the empirical model hindered the understanding of the strain accumulation mechanism. Compared with conventional empirical models by directly fitting experimental data, this study proposed a new approach to formulate an empirical model of strain accumulation using the intrinsic relationship between the reloading process in cyclic loading and the monotonic loading process. All axial strain accumulation increments of silica powder during cyclic loading with constant cyclic stress amplitude were projected onto the corresponding monotonic loading stress-strain curve, and the evolution of strain accumulation was described by the projection modulus, which was regarded as an intrinsic parameter of the projection stress-strain curve. An empirical formulation for describing the connection between projection modulus and stress levels was proposed. In addition, the prediction model of total axial strain in cyclic loading was presented by combining the accurate representation of elastic and plastic moduli during primary loading. Finally, discussions on the intrinsic relation mentioned above were enhanced by presenting more experimental evidence. This paper provided novel insight into the mechanism of strain accumulation in cyclic triaxial conditions by linking strain accumulation with a unified stress-strain relation. However, limited by the triaxial stress conditions in this study, it is recommended to extend this projection method to other cyclic load tests.

Influences of initial static shear stress on the cyclic behaviour of over consolidated soft marine clay

In offshore engineering, soft marine clays which are overconsolidated due to surcharge pre-loading beneath embankments may be subjected to initial static shear stress. To experimentally investigate the influences of the initial static shear stress on the cyclic behaviour of overconsolidated soft marine clay, a series of triaxial tests are performed covering various values of the initial static shear stress ratio (SSR) and overconsolidated ratio (OCR). Several useful conclusions are obtained. First, the total axial strain increases as the SSR increases, whereas an increase in the OCR reduces the rate of increase in axial strain. Second, the area encompassed by the stress–strain loop decreases with an increasing OCR, whereas it increases with an increasing SSR. Third, a threshold SSR is proposed. Two quantitative relationships between the permanent axial strain and SSR are then established based on the proposed threshold and verified against the experimental results.

The effects of cyclic loading on the reconsolidation behaviours of marine sedimentary clays under intermittent drainage conditions

The reconsolidation settlement of marine sedimentary clay subgrade subjected to cyclic dynamic loading under intermittent drainage conditions needs to be further studied. In this study, a series of triaxial tests was conducted on natural Wenzhou soft clays to investigate the reconsolidation responses induced by staged cyclic loading under intermittent drainage conditions. The dynamic behaviours of the test specimens, such as the evolution of excess pore water pressure, variations in stress-strain hysteresis loops and resilient modulus, and cumulative strain development, are analysed and discussed. The experimental results show that during the intermittent quiescent drainage period, the dissipation of the excess pore pressure generated during the cyclic loading period results in reconsolidation of the test specimen. Hardening occurs throughout the tests, as determined by observing the variations in the hysteresis loops and resilient modulus. Throughout the tests, a higher percentage of the axial plastic strain in the quiescent period to the total axial plastic strain indicates that the reconsolidation deformation of the soft clay subgrade caused by intermittent drainage cannot be ignored. These conclusions have significant value for performing in-depth research on the dynamic behaviours of marine sedimentary clays subjected to cyclic traffic loading and predicting the settlement of roads in engineering practice.

Features of Inelastic Behavior of a Composite Under Cyclic Loading. Experimental and Theoretical Investigations

For a carbon-fiber-reinforced plastic made of an ELUR-P unidirectional tape and an ХТ-118 cold-cure binder, specimens with a ±45° stacking sequence were tested for cyclic tension. The total axial strain was considered as the sum of nonlinearly elastic, viscoelastic, and irreversible creep strains. Using the Abel creep kernel, it was found that, at instants of time shifted by values multiples of the loading period, the ratios between the viscoelastic strain components depended neither on the period of cyclic loading, nor the loading amplitude, nor the parameter determining the degree of viscosity of the material, but only on the parameter determining the decrease in the rate of the viscoelastic strain. To find parameters of the creep kernel, an identification method based on the use of a hypothesis that allows one to separate reversible and irreversible creep strains, as well as on the properties of Abel creep kernel was developed. The method is illustrated by an example of processing experimental results.

Low-cycle fatigue crack initiation and propagation from controlled surface imperfections in nuclear steels

The impact of a controlled surface imperfection on the low-cycle fatigue life in two different steels is investigated. After introduction of a single imperfection with a depth varying from 50 to 350 µm in cylindrical samples, fatigue tests were conducted under fully-reversed total axial strain control in air at ambient temperature. It is shown that the fatigue life is significantly reduced, even in presence of small imperfections. In addition, the use of the potential drop methods provided an assesment of the initiation life and the determination of crack growth rates. Two characteristic crack growth domains were thus identified.

Simple approach for solution of the quasi-plane-strain problem in a circular tunnel in a strain-softening rock mass considering the out-of-plane stress effect

The out-of-plane stress is sometimes the major or intermediate principal stress in a circular tunnel opening. The influences of the out-of-plane stress and axial strain are often neglected in the stability analyses of tunnel excavation, which can induce significant errors in the determination of surrounding rock deformations. In this paper, the use of a simple approach is proposed to solve the quasi-plane-strain problem of circular tunneling considering the effect of the out-of-plane stress, which is deformation-dependent and influenced by the in situ stress. As the intermediate principal stress is deformation-dependent, to obtain the numerical solution of the intermediate principal stress, the quasi-plane-strain problem is defined based on assumptions that the initial axial total strain is a nonzero constant (ε0) and that the axial plastic strain is nonzero. With the numerical solution for the plastic strain, obtained using the plastic potential functions based on the three-dimensional failure criteria, the formula for the intermediate principal stress can be derived using Hooke’s law. The proposed approach can be utilized to obtain the numerical solution for the intermediate principal stress, which is deformation-dependent, and the numerical results can be simplified as the solution presented by Pan and Brown. The proposed approach can also be used to obtain the solution for the strain softening of the surrounding rock. To verify its validity and accuracy, the results obtained using the proposed approach are compared with the solution of Pan and Brown. In addition, parametric studies are performed to address the influences of the out-of-plane stress on the stress and displacement in the circular tunnel.

Strain controlled isothermal low cycle fatigue life, deformation and fracture characteristics of Superni 263 superalloy

The total axial strain controlled low-cycle fatigue (LCF) behaviour of a wrought nickel-base superalloy Superni 263 has been evaluated at 298 K, 673 K, and 923 K employing the strain amplitudes in the range ± 0.25% to ± 0.80%. All the tests were performed using a triangular wave form at a constant strain rate of 10−3 s−1. The Superni 263 alloy was subjected to a solutionizing treatment at 1373 K/1.5 h followed by water quenching. An ageing treatment of 1073 K/8 h was given to the solutionized samples. The microstructure of the alloy is composed of γʹ in the intragranular regions and discrete M23C6 precipitates on the grain boundaries. Few undissolved (Ti, Mo)C and TiN precipitates were also noticed in the matrix. The strain amplitude and temperature played a significant role in the evolution of macroscopic cyclic stress response and the development of deformation substructure. The alloy displayed serrated flow in the plastic portions of stress-strain hysteresis loops in the tests conducted at 673 K and 923 K, predominantly at strain amplitudes higher than ±0.40%. Several manifestations depicting the occurrence of dynamic strain ageing (DSA) were derived by analysing both the macroscopic and micro-mechanistic information generated. The comparative evaluation of monotonic and cyclic stress-strain curves was conducted and the factors responsible for rapid cyclic hardening during DSA were identified. The fatigue life decreased drastically with increasing temperature, particularly at low strain amplitudes. The strain-life data could be represented well by using Basquin and Coffin-Manson relationships. The slope in Coffin-Manson relationship revealed more negative values under the influence of DSA. The crack initiation and propagation modes at all the test conditions remained transgranular without the indications of interference from creep and oxidation effects. The deformation substructure at elevated temperatures was characterized by the occurrence of co-planar slip, stacking faults, micro twinning and high dislocation density in between the slip bands.

Deformation Features and Models of [±45]2s Cross-Ply Fiber-Reinforced Plastics in Tension

Series of experiments on [±45]2s cross-ply carbon-fiber-reinforced plastic specimens were carried out in tension with various loading programs. In analyzing stress–strains relations, the material was considered homogeneous. The total axial strain is presented as the sum of instantaneous residual (irreversible), nonlinear reversible, irreversible creep, and reversible creep strains. To separate the last two components, the hypothesis that their rates at different instants of time are different is used. Together with a generalized Kachanov hypothesis, this allowed us first to obtain equations for increments of only the viscoelastic strain. Further, equations in which only the viscoplastic strain is unknown are written, and only then the secant elastic modulus is determined. Questions of the choice of relations for describing strain components and the problem on identification of parameters of the relations are considered. Experimental data and results of their processing are presented, and they testify to the acceptability of the assumptions used and the efficiency of the approaches proposed.

Characteristics of Deformation and Damping of Cement Treated and Expanded Polystyrene Mixed Lightweight Subgrade Fill under Cyclic Load

To investigate the deformation and damping characteristics of cement treated and expanded polystyrene (EPS) beads mixed lightweight soils, this study conducted a series of triaxial shear tests cyclic loading for different confining pressures, cement contents, and soil categories. Through repeated loading and unloading cycles, axial accumulative strain, resilient modulus, and damping ratio versus axial total strain were analyzed and the mechanical behavior was revealed and interpreted. Results show that the resilient modulus increases with increasing confining pressure and cement content. A decreasing power function can be used to fit the relationship between the resilient modulus and the axial total strain. Although sandy lightweight specimens usually own higher resilient modulus than silty clay lightweight specimens do, the opposite was also found when the axial total strain is larger than 8% with 50 kPa confining pressure and 14% cement content. For damping ratio the EPS beads mixed lightweight soil yields a weak growth trend with increasing axial total strain and a small reduction with higher confining pressure and cement content. For more cementations, the damping ratio of the sandy lightweight soil is always smaller than the silty clay lightweight soil. Nonetheless, the differences of damping ratios that were obtained under all of the test conditions are not significant.

Initiation and propagation of fatigue cracks from surface imperfections on 304L austenitic stainless steel

Given the stringent requirements of high levels of safety in nuclear components, stakeholders of French nuclear industry must anticipate the presence of residual surface imperfections in these components. Such imperfections could be introduced during manufacturing or maintenance operations. The incidence of surface irregularities on the fatigue strength of metallic components must be considered. Meanwhile, nuclear components are generally loaded under low-cycle fatigue and large-scale plasticity conditions. The present work aims at assessing the impact of a controlled surface irregularity on the fatigue life of typical nuclear materials. The influence of characteristic parameters under low-cycle fatigue conditions is also investigated. A 304L austenitic stainless steel used in components of French nuclear power plants was studied. Fatigue tests were conducted under fully-reversed total axial strain control in air at ambient temperature with total strain amplitudes ranging from Δεt/2=0.2% to Δεt/2=0.6%. Surface irregularities, whose depth varies between 100 and 350 micrometers, have been introduced on polished cylindrical samples. The direct current potential drop method has been used to monitor the crack propagation, and thereafter, to derive crack growth rate data. Experimental markings of the crack front for different cycle numbers have been carried out to calibrate the variation of the potential as a function of crack depth. It appears that the fatigue life is strongly reduced in presence of a surface irregularity.

Twinning-induced strain hardening in dual-phase FeCoCrNiAl0.5 at room and cryogenic temperature

A face-centered-cubic (fcc) oriented FeCoCrNiAl0.5 dual-phase high entropy alloy (HEA) was plastically strained in uniaxial compression at 77K and 293K and the underlying deformation mechanisms were studied. The undeformed microstructure consists of a body-centered-cubic (bcc)/B2 interdendritic network and precipitates embedded in 〈001〉-oriented fcc dendrites. In contrast to other dual-phase HEAs, at both deformation temperatures a steep rise in the stress-strain curves occurs above 23% total axial strain. As a result, the hardening rate associated saturates at the unusual high value of ~6 GPa. Analysis of the strain partitioning between fcc and bcc/B2 by digital image correlation shows that the fcc component carries the larger part of the plastic strain. Further, electron backscatter diffraction and transmission electron microscopy evidence ample fcc deformation twinning both at 77K and 293K, while slip activity only is found in the bcc/B2. These results may guide future advancements in the design of novel alloys with superior toughening characteristics.

Deformation Behavior of Sandstones From the Seismogenic Groningen Gas Field: Role of Inelastic Versus Elastic Mechanisms

AbstractReduction of pore fluid pressure in sandstone oil, gas, or geothermal reservoirs causes elastic and possibly inelastic compaction of the reservoir, which may lead to surface subsidence and induced seismicity. While elastic compaction is well described using poroelasticity, inelastic and especially time‐dependent compactions are poorly constrained, and the underlying microphysical mechanisms are insufficiently understood. To help bridge this gap, we performed conventional triaxial compression experiments on samples recovered from the Slochteren sandstone reservoir in the seismogenic Groningen gas field in the Netherlands. Successive stages of active loading and stress relaxation were employed to study the partitioning between elastic versus time‐independent and time‐dependent inelastic deformations upon simulated pore pressure depletion. The results showed that inelastic strain developed from the onset of compression in all samples tested, revealing a nonlinear strain hardening trend to total axial strains of 0.4 to 1.3%, of which 0.1 to 0.8% were inelastic. Inelastic strains increased with increasing initial porosity (12–25%) and decreasing strain rate (10−5 s−1 to 10−9 s−1). Our results imply a porosity and rate‐dependent yield envelope that expands with increasing inelastic strain from the onset of compression. Microstructural evidence indicates that inelastic compaction was controlled by a combination of intergranular cracking, intergranular slip, and intragranular/transgranular cracking with intragranular/transgranular cracking increasing in importance with increasing porosity. The results imply that during pore pressure reduction in the Groningen field, the assumption of a poroelastic reservoir response leads to underestimation of the change in the effective horizontal stress and overestimation of the energy available for seismicity.

Creep and Long-Term Permeability of a Red Sandstone Subjected to Cyclic Loading After Thermal Treatments

Long-term experiments were performed on red sandstones after different thermal treatments (25, 300, 700 and 1000 °C) under multi-step loading and unloading cycles and a confining pressure of 25 MPa. Furthermore, to quantitatively analyse the temperature influence on the deformation behaviours of the specimens, the concept of the temperature–strain rate was proposed to describe the relationship between strain and temperature, and the experimental results were corrected to identical temperatures (i.e., 20 °C), to overcome the influence of periodic fluctuations in ambient temperature. The results show that the axial mean temperature–strain rate first increased as temperature increased from 25 to 300 °C and then decreased with increasing temperature, whereas the lateral mean temperature–strain rate decreased with increasing temperature. The total strain was divided into the instantaneous elastic strain, the instantaneous plastic strain, the visco-elastic strain and the visco-plastic strain. The total axial strain increased with increasing deviatoric stress, and the irrecoverable strain increased with increasing loading and unloading history. Furthermore, the total axial strain increased with increasing temperature; specifically, at 1000 °C, it was approximately two times that at 700 °C and three times those at 25 and 300 °C. The instantaneous elastic strain and the instantaneous plastic strain increased approximately linearly with increasing deviatoric stress, whereas the creep strain varied with deviatoric stress in complicated ways at different temperatures. However, under identical deviatoric stress, the instantaneous elastic strain and the instantaneous plastic strain increased slightly as temperature increased from 25 to 700 °C and then increased substantially as temperature reached 1000 °C, whereas the variations in the creep strain, the visco-elastic strain and the visco-plastic strain were dependent on temperature and stress level. Finally, the permeability first decreased slightly as temperature increased from 25 to 300 °C and then increased with increasing temperature.

Defining Relations in Mechanics of Cross-Ply Fiber Reinforced Plastics Under Short-Term and Long-Term Monoaxial Load

For cross-ply fiber reinforced plastics under short- and long-term loading conditions we prove the defining relations. It was shown that in general case total axial strain consists from residual (irreversible) strain of lateral degradation, reversible strain of material’s microrearrangement, and reversible creep strain. We consider issue of experimental determination and problems of identification of mechanical parameters taken into consideration.

An evaluation index for the fracturing effect in shale based on laboratory testing

It is clear that evaluating the fracturing effect with laboratory tests can shed some light on designing fracturing programs for shale gas exploitation. To evaluate the fracturing effect quantitatively, a series of uniaxial cyclic loading tests on shale were conducted. Based on the experimental results, a damage variable derived from the total axial strains of the loading–unloading cycles was established and introduced into a damage evolution equation to analyze damage evolution in the shale. In the end, a new index for evaluating the fracturing effect, $$C_{AN} '$$, was proposed. The results show that $$C_{AN} '$$ has a positive correlation with fracture complexity and can be regarded as an effective index to evaluate fracture networks in shale using laboratory tests. In addition, this study demonstrates that the fracturing effect is also related to the angle between the direction in which the load is applied and the shale’s bedding plane. This can provide a useful reference for shale gas exploitation.

Low cycle fatigue behaviour of Grade 92 steel weld joints

In the present study, low cycle fatigue (LCF) behaviour of tungsten (W) alloyed 9Cr steel (Grade 92 steel) base metal and its weld joints has been investigated. LCF tests on both base metal and weld joint were conducted under fully reversed, total axial strain control mode employing a triangular waveform at 823K, under a constant strain rate of 3×10−3s−1 with strain amplitudes varying from ±0.25% to ±1%. Both base metal and weld joint exhibited continuous softening with the weld joint exhibiting a lower stress response than base metal. At lower strain amplitudes, weld joint exhibited lower fatigue life than that of base metal whereas at higher strain amplitude, both the materials exhibited comparable fatigue life. Weld joints failed in the base metal for all the strain amplitudes at 823K except at the strain amplitude ±0.25% where weld joint failed in inter-critical region. Effect of temperature on LCF behaviour of weld joints and base metal at various temperatures ranging from 300K to 873K under constant strain amplitude of ±0.6% and strain rate 3×10−3 has been investigated. At all the temperatures, weld joints failed in the base metal region. The material exhibited dynamic strain ageing phenomenon in the temperature range 473–673K. Oxidation was found to show more adverse effects on fatigue life beyond 673K.