Hardness Stress Research Articles

Dynamic Uniaxial Compression of HSLA-65 Steel at Elevated Temperatures

In the present study, the dynamic response of a high-strength, low alloy Grade 65 (HSLA-65) steel, used by the United States Navy for ship hull construction, is investigated under dynamic uniaxial compression at temperatures ranging from room temperature to 1000 °C using a novel elevated temperature split-Hopkinson pressure bar. These experiments are designed to probe the dynamic response of HSLA-65 steel in its single α-ferrite phase, mixed α + γ-austenite phase, and the single γ-austenite phase, as a function of temperature. The investigation is conducted at two different average strain rates—1450 and 2100/s. The experimental results indicate that at test temperatures in the range from room temperature to lower than 600 °C, i.e. prior to the development of the mixed α + γ phase, a net softening in flow strength is observed at all levels of plastic strain with increase in test temperatures. As the test temperatures are increased, the rate of this strain softening with temperature is observed to decrease, and at 600 °C the trend reverses itself resulting in an increase in flow stress at all strains tested. This increase in flow stress is understood be due to dynamic strain aging, where solute atoms play a distinctive role in hindering dislocation motion. At 800 °C, a (sharp) drop in the flow stress, equivalent to one-half of its value at room temperature, is observed. As the test temperature are increased to 900 and 1000 °C, further drop in flow stress are observed at all plastic strain levels. In addition, strain hardening in flow stress is observed at all test temperatures up to 600 °C; beyond 800 °C the rate of strain hardening is observed to decrease, with strain softening becoming dominant at temperatures of 900 °C and higher. Moreover, comparing the high strain rate stress versus strain data gathered on HSLA 65 in the current investigation with those available in the literature at quasi-static strain rates, strain-rate hardening can be inferred. The flow stress increases from 700 MPa at 8 × 10−4/s to 950 MPa at 1450/s and then to 1000 MPa at 2100/s at a strain of 0.1. Optical microscopy is used to understand evolution of microstructure in the post-test samples at the various test temperatures employed in the present study.

Heat stress hardening of oriental armyworms is induced by a transient elevation of reactive oxygen species during sublethal stress.

Pre-exposure to mild heat stress enhances the thermotolerance of insects. Stress hardening is a beneficial physiological plasticity, but the mechanism underlying it remains elusive. Here we report that reactive oxygen species (ROS) concentrations were quickly and transiently elevated in the armyworms, Mythimna separata, by exposing them to 40°C, but not other tested temperatures. Larvae exposed to 40°C had subsequently elevated antioxidant activity and the highest survival of all tested heating conditions. The elevation of ROS after lethal heating at 44°C for 1 h was approximately twofold compared to heating at 40°C. Injection of an optimal amount of hydrogen peroxide (H2 O2 ) similarly caused sequential elevation of ROS and antioxidant activity in the test larval hemolymph, which led to significantly enhanced survival after lethal heat stress. The H2 O2 -induced thermotolerance was abolished by coinjection of potent antioxidants such as ascorbic acid or N-acetylcysteine. Both preheating at 40°C and H2 O2 injection enhanced expression of genes encoding superoxide dismutase 1, catalase, and heat shock protein 70 in the fat body of test larvae, indicating the adequate heat stress induced a transient elevation of ROS, followed by upregulation of antioxidant activity. We infer that thermal stress hardening is induced by a small timely ROS elevation that triggers a reduction-oxidation signaling mechanism.

Grain size effect on deformation behaviour of Mg–Sc alloys

The effect of grain size on the flow stress and work hardening in binary Mg–Sc alloys has been investigated under conditions of uniaxial compression at 78 K and 298 K. The yield stress depends on the grain size and obeys Hall–Petch relationship. Yielding and post–yielding deformation is accomplished by both slip and twinning, organized in the form of a deformation front propagating throughout the sample. The results show similar volume fraction of twins in fine- and coarse–grained samples, suggesting no significant effect of the grain size on twinning. Modelling of mechanical properties indicates that Mg–Sc alloys deform predominantly by basal, prismatic <a>, pyramidal <c+a> slip and extension twinning. Small influence of the grain size on work hardening is interpreted due to the enhancement of <c+a> slip. Analysis of σΘ characteristics shows that an increase of Sc content leads to the decrease of σΘ at corresponding flow stress. This behaviour is interpreted in terms of the combined effect of Sc on the relative activity of slip systems and stacking fault energy (SFE) of Mg–Sc alloys.

A quantitative study on the influence of compressive stress on crack-tip opening displacement

To fill the research gap of estimating crack-tip opening displacement (CTOD) under compressive portion within a cyclic loading, a numerical study is conducted in this paper to quantify the influence of compressive stress on CTOD, and a calculation model of CTOD is established for center-cracked plates under tension-compression loading. The residual CTOD and the variable quantity of CTOD are introduced. Based on a series of finite element analyses, a simple relationship between the variable quantity of CTOD and compressive stress is obtained, which considers the influences of plate width, crack length, stress level (the maximum tensile and compressive stresses) and material property (yield stress, elastic modulus and material hardening). Finally, an example is given and the method in this paper is validated. The proposed calculation model of CTOD has a wide application range and is applicable for finite plates of different materials under tension-compression cyclic loading.

Stress softening and hardening during compression and tensile consecutive cyclic loading of Mn18Cr18N austenitic stainless steel

The compression and tensile consecutive deformation behavior and microstructure evolution of Mn18Cr18N steel were investigated by cyclic loading tests at room temperature and strain amplitude from 0.005 to 0.15. The results indicated that the cyclic loading stress-strain curves of the steel show plastic deformation characterized by almost the same working hardening rates without yield plateaus at various strain amplitudes. At the lower strain amplitudes from 0.005 to 0.01, the stress amplitudes and subsequent yield surface radius decreased with cycles, which suggest that stress softening occurs during cyclic loading. At the strain amplitudes from 0.02 to 0.15, the stress amplitudes and subsequent yield surface radius increased with cycles, which imply that stress hardening occurs during cyclic loading. With the increasing of strain amplitudes, the cyclic hardening coefficients monotonically increased, while the cyclic softening coefficients rapidly decreased. The maximum flow stress increased up to 1549.6MPa after three cycles at the strain amplitude of 0.15, which increased by 395.4MPa compared to the maximum uniaxial tensile flow stress. It was also demonstrated that the cyclic loading stress-strain behaviors (cyclic softening and hardening) depend on internal stress component rather than effective stress component. TEM observation shows that planar slip with parallel substructures along one direction at high number cycles evolved from various dislocation configurations at low number cycles occurs at the lower strain amplitude of 0.005; double planar slip with grid substructures along two directions at high number cycles evolved from parallel substructures along one direction at low number cycles occurs at the higher strain amplitude of 0.04. Internal stress could be released by dislocation rearrangement and activation of cross slips during cyclic loading process, which induce stress softening at lower strain amplitudes; while slipping and twining alone two intersectional directions caused stress hardening at higher strain amplitudes.

Effects of hot/cold deformation on the microstructures and mechanical properties of ultra-low carbon medium manganese quenching-partitioning-tempering steels

The mechanical properties and designed microstructure evolution have been investigated in ultra-low carbon medium Mn steels for enhanced strength, plasticity and toughness, after an innovative quenching-partitioning-tempering (QPT) treatment with different initial conditions, cold rolling (CR) and hot rolling (HR). The transmission electron microscopy (TEM) combined with 3D atom probe tomography (APT) observed plenty of block austenite (30%) with dispersed precipitation in CR-QPT steels, while less amounts of film austenite (18%) with a higher density of nanoprecipitates were found in HR-QPT steels. The controlled multiphase microstructure evolution strongly depends on the Mn diffusion and segregation process. The overall strength-ductility combinations of two QPT-steels from the contribution of combined nanoprecipitation hardening and transformation-induced plasticity (TRIP) effects, are strongly influenced by the varying austenite mechanical stability connected with the volume fraction, grain size, morphologh and dislocation density of CR and HR-QPT samples. The unloading-reloading tests reveal the respective roles of precipitation hardening and TRIP effect in the overall mechanical properties according to the Baushinger effect (BE): the nanoprecipitation results in a higher back stress strengthening, while the deformation-induced martensite transformation in a wide strain regime degrades the large stress concentration in grain boundaries (GB), leading to a back stress softening but effective stress hardening in the later deformation stage. In addition, CR-QPT samples show a significant higher value of impact toughness than HR-QPT samples. The QPT treatment of CR-QPT steels can not only eliminate the susceptible prior austenite grain boundaries, but also drive Mn enrichment at the phase boundaries diffusing into the pre-existing austenite interior due to a low migration rate of austenite/ferrite interfaces impeded by the nanoprecipitations in ferrite, contributing to a homogeneous Mn distribution and removing the grain boundary embrittlement.

Micromechanics constitutive model for predicting the stress–strain relations of particle toughened bulk metallic glass matrix composites

Based on the secant modulus and extended Mori-Tanaka method for dual ductile phases, a micromechanics model is proposed to predict the monotonic mechanical behaviors of bulk metallic glass matrix composites (BMGCs) toughened by particles. In this model, the deformation behaviors of the BMG matrix and particles are described by the use of the free volume model and the simple Ludwik flow equation, respectively, and Weng's homogenization frame is adopted to bridge the constituents and the composites. As compared to the existing relevant models, the present model is much more convenient for applying, and more readily to be extended. The developed model is applied with stain-controlled loading, and is verified by modeling the monotonic stress–strain relations of particle toughened BMGCs. The predictions were in good agreement with the experiments from the literature, which confirms that the developed analytical model is capable of successfully describing the mechanical properties, such as yield strength, stress hardening and strain softening elongation, of composites.

A new expression for crack opening stress determined based on maximum crack opening displacement under tension–compression cyclic loading

AbstractThe maximum crack opening displacement is introduced to investigate the effect of compressive loads on crack opening stress in tension–compression loading cycles. Based on elastic–plastic finite element analysis of centre cracked finite plate and accounting for the effects of crack geometry size, Young's modulus, yield stress and strain hardening, the explicit expression of crack opening stress versus maximum crack opening displacement is presented. This model considers the effect of compressive loads on crack opening stress and avoids adopting fracture parameters around crack tip. Besides, it could be applied in a wide range of materials and load conditions. Further studies show that experimental results of da/dN − ΔK curves with negative stress ratios could be condensed to a single curve using this crack opening stress model.

Interplay of coarsening, aging, and stress hardening impacting the creep behavior of a colloidal gel

We explore the dynamical and mechanical characteristics of an evolving gel in diffusing wave spectroscopy (DWS) and rheometry, aiming to assess how the gel evolution impacts the creep response of the system. Our gel is formed by inducing the aggregation of thermosensitive colloids by a variation in temperature. We find experimental evidence that the long time evolution of this gel is due to two distinct processes: A coarsening process that involves the incorporation of mobile particles into the network structure and an aging process that triggers intermittent rearrangement events. While coarsening is the main process governing the evolution of the elastic properties of the gel, aging is the process determining structural relaxation. The combination of both processes in addition to stress hardening governs the creep behavior of the gel, a creep behavior that is determined by three distinct contributions: an instantaneous elastic, a delayed elastic, and a loss contribution. The systematic investigation of these contributions in recovery experiments provides evidence that losses and delayed elastic storage have a common origin, both being due to intermittent local structural relaxation events.

Numerical stress analysis in resistance spot-welded nugget due to post-weld shear loading

The magnitude and state of residual stress has a great influence on the fatigue life of sheet metal joints used in the automobile and aircraft industries. The present study developed a parametric 3D finite element model for simulation of welding and post-weld loading on the specimen. Electro-thermo-mechanical analysis is utilized to estimate the residual stress distribution at the nuggets and their surroundings at different stages of welding and post-weld mechanical loading. The results show that at the end of post-weld loading, the maximum tensile residual stress location moves nearer to the edge of the weld nugget and the residual stress magnitude and distribution change. Also, residual stress have sufficiently effects on stress distribution at the weld joint due to mechanical loading. To accurately estimate the fatigue life of resistance spot-welded joints, moreover external stresses, residual stress and local strain hardening of the material near the edge must be considered.

Plasticity and crack extension in single-crystalline long-period stacking ordered structures of Mg85Zn6Y9 alloy under micro-bending

Micro-bending tests were performed on single-crystalline cantilever beams of long-period stacking ordered (LPSO) structures of Mg85Zn6Y9 alloy with and without a notch to study crystal plasticity and cracking behaviour under basal and non-basal slip conditions. The activation of basal slip induced load drops during the deformation of a plain cantilever beam with the bending moment perpendicular to the basal plane. A cantilever sample oriented for prismatic slip exhibited high yield stress and moderate strain hardening. The apparent critical resolved shear stress (CRSS) for prismatic slip was determined to be ∼360 MPa, significantly higher than 30 MPa CRSS determined for basal slip. A crystal plasticity finite element analysis of the micro-bending test on the sample oriented for prismatic slip was performed to verify the CRSS values determined. In addition, the intrinsic fracture resistance for crack growth along the <a> direction on a prismatic plane is higher than that for crack growth along the <a> direction on a basal plane. In combination, these results indicate that inhibition of basal slip activity contributes not only to high strength but also to high intrinsic toughness observed in this LPSO structure.

Deformation mechanism of strain localization in 2D numerical interface tests

Heterogeneity arises in soil subjected to interface shearing, with the strain gradually localizing into a band area. How the strain localization accumulates and develops to form the structure is crucial in explaining some significant constitutive behaviors of the soil–structural interface during shearing, for example, stress hardening, softening, and shear-dilatancy. Using DEM simulation, interface shear tests with a periodic boundary condition are performed to investigate the strain localization process in densely and loosely packed granular soils. Based on the velocity field given by grains’ translational and rotational velocities, several kinematic quantities are analyzed during the loading history to demonstrate the evolution of strain localization. Results suggest that tiny concentrations in the shear deformation have already been observed in the very early stage of the shear test. The degree of the strain localization, quantified by a proposed new indicator, α, steadily ascends during the stress-hardening regime, dramatically jumps prior to the stress peak, and stabilizes at the stress steady state. Loose specimen does not develop a steady pattern at the large strain, as the deformation pattern transforms between localized and diffused failure modes. During the stress steady state of both specimens, remarkable correlations are observed between α and the shear stress, as well as between α and the volumetric strain rate.

Numerical and experimental investigation of low cycle fatigue behaviour in P91 steel

Abstract Fatigue influences steel structure and material properties and thus significantly influences degradation mechanism and lifetime of steel. The determination of fatigue curves is expensive, time consuming, difficult to implement the experiment, especially in reactor environment (high or low temperatures, acid or alkaline environments, etc.). Therefore finite element method can be helpful for material investigation to evaluate the behaviour of steel specimen during fatigue. The aim of this work is to propose a numerical tool able to compute low cycle fatigue and developing a finite element (FE) model for investigation of fatigue behaviour. The numerical simulation can be used to determine some points for construction of fatigue curve and to understand the mechanics of fatigue mechanism in more detail. In order to validate the model and to calibrate the model parameters, the experimental investigation of steel P91 LCF at 550 °C in strain control was carried out. P91 strain controlled high temperature low cycle fatigue tests were carried out and numerical investigations were performed using finite element code Caste3M. The plastic model with isotropic stress hardening was chosen for the description of material. As a result, the stress vs. strain hysteresis loops and stress vs. number of cycles curves at different loading were obtained. The numerical simulation results were compared with the experimental tests.

Flow stress and work hardening of Mg-Y alloys

Flow stress and work hardening behaviour of Mg-Y binary alloys have been studied under uniaxial tension and compression at 4 K, 78 K and 298 K. Electron microscopy observations show that Yttrium forms multi-atomic clusters distributed randomly in Mg matrix. During thermomechanical processing, Y accumulates at the grain boundaries and controls recrystallization texture, which acquires lower intensity of the basal component and higher degree of randomness in comparison to polycrystalline Mg. Tensile and compressive yield stress of the alloys is temperature dependent and decreases as temperature increases from 4 K to 298 K. The grain size and solution strengthening contributions to the yield stress have been analyzed for all alloys and deformation temperatures. Critical resolved shear stress (CRSS) of individual deformation modes has been obtained from the modelling of the flow stress. These results show that Y strengthens most the <c+a> slip system, followed by pyramidal I, prismatic, twinning and basal systems. All slip systems exhibit monotonic increase of CRSS with Y concentration, but at different rates. Analysis of the solution strengthening component of the yield stress suggests that it is determined by activity of the basal slip participating in yielding. A correlation between the athermal hardening rate, Θh, and the amount of slip in individual deformation modes leads to the conclusion that higher athermal hardening rates are produced by a balanced contribution of basal and non-basal slip, whereas lower Θh is associated typically with less activity of <c+a> slip. Improved ductility of Mg-Y alloys is reflected in the type of dislocation microstructure developed during plastic flow and the character of the fracture surfaces. These alloys' characteristics are linked to the mechanical property data to obtain better understanding of the mechanisms responsible for the enhanced plasticity of Mg-Y alloys.

Effect of technological heredity on the fatigue strength in the manufacture of gas turbine engine blades

In the study, the task of researching of the finishing-strengthening machining stage of gas turbine engine compressor blades manufactured of titanium and nickel-chromium alloys in order to extend their service life was solved. The application of electrochemical pulse machining as a technological heredity barrier was substantiated since this method allows a considerable decrease of the residual stress and surface layer work hardening. To ensure the extended service life of blades, the conditions for the subsequent finishing-strengthening machining were identified.

Microstructure and Mechanical Properties of Ti-Nb-Zr Alloys Prepared by Spark Plasma Sintering

Biomedical Ti-24Nb-2Zr (at.%) alloy is prepared by powder metallurgy method including high energy mechanical milling and spark plasma sintering (SPS). Compact bulk specimens, with density equivalent to arc melted ones, are obtained after SPS for 10 min with sintering temperatures above 850 °C. Higher temperature sintering promotes combination of raw element powders and diffusion of element, and results in a single β phase in the obtained specimens. Microhardness, compressive strength and plasticity increase with rising sintering temperatures. As the specimens are further heat treated at 850 °C for 1h, element distribution becomes much more uniformly together with releasing of internal stress and greater solution hardening. The bulk specimens, obtained by sintering at 1100 °C and 1200 °C, shows higher strength and plasticity, which are suitable for biomedical applications. SPS technology can be used as a rapid fabrication method to prepare Ti alloy products.

Small punch tensile testing of curved specimens: Finite element analysis and experiment

The Small Punch (SP) technique is a miniature test used for characterizing irradiated materials or when a testing material is available only in small quantities. In this work Finite Element (FE) models are developed to support the parametric analysis of SP fuel cladding tube specimens in comparison to standard flat ones. FE analysis shows that there are practically no differences between circular and rectangular flat specimens. The tube specimen results in only slightly higher maximal force (Fm). However, Fm is attained at significantly lower displacements. This is attributed to the curvature of the specimen. The friction linearly increases the Fm and to a lesser extent the displacement at Fm. FE analysis also shows that the yield stress and different hardening, while keeping ultimate tensile strength constant, practically do not affect the Fm. Significant specimen deformation can be expected at already small (0.1mm) puncher deflection, which could limit the applicability of the small punch test to ductile materials. Varying degree of clamping in the experimental procedure can cause large scatter in the yield stress estimates. In all cases good agreement between the simulation and experimental results was obtained, best with a friction coefficient of 0.2.

Relaxation of Hardening in Asphalt Concrete under Cyclic Compression Loading

AbstractThe present experimental study investigated the behavior of hot-mix asphalt (HMA) under repetitive creep and recovery loading. Cylindrical samples of HMA were subjected to trapezoidal and haversine loading followed by unloading. Different amounts of unloading (stress reduction) and relaxation periods (rest periods in complete unloading) were tested. The measured vertical deformation was decomposed into viscoelastic and viscoplastic components. It was observed for cyclic loading that an increase in unloading and/or relaxation period increased the recovery of viscoelastic strain, which in turn increased the accumulated viscoplastic strain in subsequent loading cycles. It appears that repetitive creep and recovery loading caused slight reorientation of the aggregates and changes in the microstructure. This led to relaxation of stress hardening of the materials and resulted in higher viscoplastic strain in subsequent cycles.

Strain rate sensitivity and fracture mechanism of AlSi10Mg parts produced by Selective Laser Melting

Implementation of Additive Manufacturing (AM) parts in the growing applications within the automotive and aerospace industries encourages further investigations of the material behavior under various strain rates, spanning from quasi-static to the high strain rate regimes. Although mechanical properties of AM-Selective Laser Melting (SLM) AlSi10Mg parts under a static regime have been investigated, the strain rate sensitivity of these materials, to the best of our knowledge, has not been discussed in the literature. In this work, the properties of AM-SLM AlSi10Mg material were systematically investigated under a wide range of strain rates, spanning from 2.77×10−6 to 2.77×10−1 S−1. The AM-SLM AlSi10Mg alloy, as opposed to Al alloys fabricated by conventional methods, was found to be strain rate sensitive with significant changes to the flow stress and strain hardening exponents with an increase in strain rate. The fracture mechanisms of these specimens, built in different orientations, are discussed.

Low cycle fatigue life prediction in shot‐peened components of different geometries—part I: residual stress relaxation

AbstractIn this study, the residual stress relaxation behaviour occurring during low‐cycle fatigue in shot‐peened specimens with either a flat or a notched geometry has been studied. A representative low‐pressure steam turbine material, FV448, was used. The residual stress and strain hardening profiles caused by shot peening were measured experimentally and were then incorporated into a finite element model. By allowing for both effects of shot peening, the residual stress relaxation behaviour was successfully simulated using this model and correlated well with the experimental data. Although more modelling work may be required to simulate the interaction between shot peening effects and external loads in a range of notched geometries, the model predictions are consistent with the specimens tested in the current study. The novelty of this study lies in the development of such a modelling approach which can be used to effectively simulate the complex interaction between shot peening effects and external loads in notched regions. Compared with the un‐notched geometry, the notched geometry was found to be more effective in retaining the improvement in fatigue life resulting from shot peening, by restricting the compressive residual stress relaxation during fatigue loading.