Strain Hardening Effect Research Articles

Introduction of a constitutive material model considering variable carbon content for cutting simulation

Carbon steels show complex strain hardening and dynamic strain aging effects under high plastic strains, high strain rates, high temperatures and high heating rates, and thus a sufficiently complex constitutive model is needed to capture this behavior. Carbon content in such steels (e.g., AISI 1018, 1045, 1075) influences strength and hardening as well. A recently-developed material model for 1045 steels was therefore modified to capture the effect of carbon content on the material response. The model can also capture time-dependent plastic response associated with interrupted austenite transformation under short (sub-second) heating times, for the simulation of machining operations.

Phenomenological model of hardening and flow for Ti-6Al-4 V titanium alloy sheets under hot forming conditions

Hardening is the core factor to determine deformation uniformity in sheet metal forming. Hot deformation of titanium alloy sheets encounters the coupled effects of strain hardening, strain rate hardening and softening, which makes the determination of forming parameters aiming for an enhanced hardening very difficult in practical processes. This paper presents a new model to quantify the hardening of Ti-6Al-4 V titanium alloy sheets under hot forming conditions based on the underlying correlation between uniform strain and hardening. Firstly, to precisely determine hot deformation characteristics of titanium alloy sheets, hot tensile uniaxial tests using Gleeble systems at various strain rates of 0.01–1 s−1 and temperatures of 973–1123 K were performed systematically. A newly developed volume-based correction method for stress–strain curves of Gleeble thermo-mechanical testing was proposed to eliminate the damaging effect of temperature gradients on strain calculations, which enables the strain hardening, strain rate hardening and softening to be determined precisely. Then, a simple unified formula of hardening components (n, m and s) was proposed to predict the achievable uniform strain at certain conditions efficiently. Using which, occupation of each hardening can be quantified and compared to facilitate the determination of process parameters. Finally, a phenomenological model based on the hardening and softening components was developed to predict the hot flow behaviour. The proposed quantitative model can provide an efficient and useful approach for process designers to design process parameters driven by the objective of enhancing hardening to maximize uniform deformation during hot forming of titanium alloy sheets.

Origin of strain softening in a nanograined Al alloy

One-step and multi-step nanoindentation experiments were conducted to investigate the mechanical response of an Al-Mg-Li alloy. Opposite to the strain hardening effect observed in the coarse-grained (CG) alloy, the nanograined (NG) alloy exhibits strain softening. Compared with multi-step nanoindentation experiments, the ultralow dislocation density and the grain boundary (GB) segregation promote a higher stress level during one-step nanoindentation experiments. Residual perfect dislocations induced by a previous loading and unloading during multi-step nanoindentation experiments lower the stress required for further plastic deformation. In addition, the storage of partial dislocations also serves to render plastic flow to commence at lower stresses. This work provides new insights for the fabrication and property optimization of NG alloys.

Linear friction welding of Ti60 near-α titanium alloy: Investigating phase transformations and dynamic recrystallization mechanisms

Linear friction welding (LFW) is a solid-state welding process used primarily in manufacturing of aeroengine blisks. Ti60 near-α titanium alloy was developed for blisks operating at 600 °C, and shows excellent thermal stability and high-temperature oxidation resistance. In order to promote the wider use of such blisks, the mechanisms of microstructure evolution including phase transformation and dynamic recrystallization of welded Ti60 joint were studied. The effects of phase transformation and dynamic recrystallization on microhardness and tensile strength were investigated as well. Results show that the microstructure of the thermo-mechanically affected zone (TMAZ) deforms severely along the friction direction, forming {0001}[11 2 ¯ 0] α texture. β → metastable β then → α + β transformations occur, leading to lamellar α precipitating from the deformed β. Sufficient continuous dynamic recrystallization makes the WZ almost composed of fine equiaxed grains of 10 μm size. In addition, α → β with the subsequent αˊ transformation occurs in the weld zone (WZ), producing extensive distribution of acicular αˊmartensite inside the recrystallized grains. Under the effects of strain hardening, precipitation strengthening and fine grain strengthening, the hardness in WZ increases significantly (i.e., 381 HV) compared to base metal (BM) (335 HV), with the tensile strength of the joint (943 MPa) also increasing over that of the BM (914 MPa). However, because of inhomogeneous deformation, the elongation of the joint (13.3%) is slightly lower than that of BM (14.1%). • β → metastable β then → α + β transformations occur in TMAZ, leading to lamellar α precipitating from deformed β; α → β with the subsequent αˊ transformation occurs in WZ, producing extensive distribution of acicular αˊmartensite. • Adequate continuous dynamic recrystallization occurs in weld zone, accompanying by violent dislocation motion and formation of many subgrains and low-angle grain boundaries. • The tensile strength of the joint (943 MPa) increases to that of the base metal (914 MPa), but the elongation of the joint (13.3%) is slightly lower than that of the base metal (14.1%) because of inhomogeneous deformation.

Controlling Damage Evolution in Geometrically Identical Cold Forged Parts by Counterpressure

Abstract It is investigated to what extent the evolution of ductile damage in cold forging can be controlled without changing the geometry of the produced part. Besides the effects of strain hardening and residual stresses, damage, which is the nucleation, growth and coalescence of voids on microscopic level, affects product properties of the manufactured components such as fatigue strength, impact strength, or elastic stiffness. Former investigations have shown that the load path-dependent damage evolution in forward rod extrusion, and thus, the performance of produced parts can be controlled by the process parameters extrusion strain and shoulder opening angle. As these parameters also affect the geometry of extruded parts, design requirements of components might be violated by varying these. Thus, counterpressure is used to superpose purely hydrostatic stresses to forward rod extrusion in order to decrease triaxiality in the forming zone without causing geometric variations in the produced parts. The counterpressure is either introduced by a counterpunch or by modified process routes. The achieved improvements in product performance are in agreement with results obtained by variation of extrusion strain and shoulder opening angle as described in the literature. In addition, it is observed in tensile tests that damage in cold extruded parts does not significantly affect flow stress. All advancements in product performance are realized without affecting the products’ geometries.

Improved energy absorption capacity of Al/Al2O3 foams by the ductile/brittle hybrid deformation mode

In the present study, Al/Al 2 O 3 foams were prepared to increase the energy absorption capacity by enlarging the densification strain. Analysis of the mechanical characteristics was performed and the energy absorption ability was calculated on the samples. The deformation process of the inner and outer-layer cells was investigated on the micro- and macro-scales to clarify the optimisation mechanism of the densification strain. The stress–strain curves demonstrated that the ceramic/aluminium foam exhibited an extended plateau region (densification strain of 0.63), resulting in high energy absorption capacity and efficiency. Analysis of the deformation process illustrated that the oxide layer played a dominant role in the brittle deformation of the outer-layer cells. The evolution of crack formation, propagation, and coalescence in cells provided randomly distributed “defects” in foams, resulted in the scattered plastic deformation zones in inner cells. The strain hardening effect of the cell wall was suppressed based on this typical deformation mode of foam, which resulted in increased densification strain and improved energy absorption capacity. Moreover, the Al/Al 2 O 3 foams exhibited superior corrosion resistance compared to the Al foams.

Variations in dynamic recrystallization behavior and mechanical properties of AZ31 alloy with extrusion temperature

This study investigates the effects of extrusion temperature on the dynamic recrystallization (DRX) behavior of a Mg–3Al–1Zn–0.3Mn (AZ31, wt%) alloy during hot extrusion and on the microstructural characteristics and mechanical properties of materials extruded at 350 and 450 °C. An increase in the extrusion temperature causes a decrease in the amount of strain energy accumulated in the material during extrusion, because of promoted activation of pyramidal <c + a> slip and dynamic recovery. This reduced strain energy weakens the DRX behavior during extrusion, which eventually results in a decrease in the area fraction of recrystallized grains of the extruded material. The material extruded at 450 °C has coarser grains and a stronger basal fiber texture than that extruded at 350 °C. As the extrusion temperature increases from 350 to 450 °C, the tensile yield strength (TYS) of the extruded material increases from 191.8 to 201.5 MPa, whereas its compressive yield strength (CYS) decreases from 122.5 to 111.0 MPa; consequently, its tension–compression yield stress ratio (CYS/TYS) decreases from 0.64 to 0.55. The increase in the TYS is attributed mainly to the stronger texture hardening and strain hardening effects of the extruded material, and the decrease in the CYS is attributed to the reduced twinning stress resulting from grain coarsening and texture intensification. The microstructural and textural evolutions of the materials during extrusion and the deformation and hardening mechanisms of the extruded materials are discussed in detail.

Mechanical model for the full range behaviour of bolted T-stubs

A new mechanical model for the full range behaviour of bolted T-stubs is proposed in this paper on the basis of a new failure mode classification system which also captures an important failure mode characterised by the rupture of the bolts combined with a moderate level of plasticity in the flange. The proposed model incorporates the abovementioned failure mode in the classic T-stub calculation approach set out in EN 1993-1-8, and also allows for the strain hardening effect of the T-stub flange as well as the strain hardening and necking-induced softening effects of the bolts. Hence, the model is able to accurately capture the full-range behaviour including ductility of the T-stubs featuring any failure mode, using only the geometric and material properties of the T-stubs. The new model is applied to experimental tests on 31 T-stubs, showing good agreement over the full-range of behaviour. Its accuracy is notably higher than the prior models, especially for the ultimate resistance and ductility.

Dynamical Models of Plasticity with Nonmonotonic Deformation Curves for Nanomaterials

Nanomaterials are widely used in different fields, such as microelectronics, industry, and nanocomposites, and they can exhibit unstable deformation behaviour depending on the strain rates. Under strain rates of 10−4–10−1 s−1, the deformation of nanomaterials, unlike the quasi-static deformation of micromaterials, is characterized by the presence of the rate sensitivity as a possible scale phenomenon in dynamic plasticity. In this paper, the relaxation model of plasticity for the prediction of deformation curves at different strain rates is used. It allows us to comprehensively study the effects of strain hardening in a wide range of deformation conditions for coarse-grained materials and nanomaterials. Considering the plastic deformation of the nanosized samples in the early stages, dynamical softening, associated with a generation of new defects, and dynamic hardening, are crucial. The proposed model, using one parameter or the classical hardening law as an example of nanosized gold whisker crystals, tungsten single-crystal pillars, and single-crystalline Au-Ag alloy nanowires, is verified. Calculated sets of parameters of characteristic time, as a parameter of rate sensitivity of a material, and hardening parameters for different nanomaterials are compared. It is shown that the characteristic relaxation times for the single-crystal nanomaterials (100–103 s) are greater than for the nanostructured materials (10−6–10−4 s). Despite the manifestation of dynamics at different strain rates of nanomaterials, single crystal and nanostructured materials, the proposed model can be successfully applied to materials with different degrees of hardening or softening.

Unusual phase transformation and novel hardening mechanisms upon impact loading in a medium entropy alloy with dual heterogeneous structure

Dynamic shear properties and corresponding deformation mechanisms of a CrCoNi-based medium entropy alloy (MEA) with both coherent nanoprecipitate and heterogeneous grain structure were investigated utilizing hat-shaped specimens in Hopkinson-bar and microstructure characterization. The present MEA with dual heterogeneous structure is found to have extraordinary dynamic shear properties, as compared to the other advanced metals and alloys, including the CrCoNi MEA with similar heterogeneous grain structure while with single FCC phase. A formula for considering effects of strain hardening, strain rate and thermal softening was utilized to calculate the critical shear strain for onset of adiabatic shear band for the present MEA, and the experimental results were found to be well predicted by the theoretical estimations. The formations of microscopic shear bands with low-angle grain boundary type, nano-grains, deformation twins, stacking fault network, and Lomer-Cottrell locks were found to be responsible for the multiple strain hardening mechanisms during dynamic shear deformation for both unaged and aged samples. An unusual phase transformation (γ→σ) was observed for only aged samples under dynamic shear loading, which was not found under quasi-static conditions before, should have significant contribution on strain hardening due to the precipitation by-passing hardening by the hard σ nano-particles.

An Improved Particle Impact Model by Accounting for Rate of Strain and Stochastic Rebound

Abstract A methodology for informing physics-based impact and deposition models through the use of novel experimental and analysis techniques is presented. Coefficient of restitution (CoR) data were obtained for Arizona Road Dust (ARD), AFRL02 dust, and each component of AFRL02 impacting a Hastelloy X plate at a variety of flow temperatures (295–866 K), surface temperatures (295–1255 K), particle velocities (0–100 m/s), and impact angles (0–90 deg). High speed particle shadow velocimetry (PSV) allowed individual impact data to be obtained for more than 8 million particles overall, corresponding to 20 combinations of particle composition, flow temperature, and surface temperature. The experimental data were applied to an existing physics-based particle impact model to assess its ability to accurately capture the physics of particle impact dynamics. Using the experimental data and model predictions, two improvements to the model were proposed. The first defined a velocity-dependent “effective yield strength,” designed to account for the effects of strain hardening and strain rate during impact. The second introduces statistical spread to the model output, accounting for the effect of randomizing variables such as particle shape and rotation. Both improvements were demonstrated to improve the model predictions significantly. Applying the improved model to the experimental data sets, along with known temperature-dependent material properties such as the elastic modulus and particle density, allowed the temperature dependence of the effective yield strength to be determined. It was found that the effective yield strength is not a function of temperature over the range studied, suggesting that changes in other properties are responsible for differences in rebound behavior. The improved model was incorporated into a computational simulation of an impinging flow to assess the effect of the model improvements on deposition predictions, with the improved model obtaining deposition trends that more closely match what has been observed in previous experiments. The work performed serves as a stepping stone towards further improvement of physics-based impact and deposition models through refinement of other modeled physical processes.

NUMERICAL SIMULATION OF DYNAMIC MECHANICAL PROPERTIES OF GRAPHENE-REINFORCED ALUMINUM MATRIX COMPOSITES

The effects of the strain rate of dynamic loading, volume fraction, shapes and orientation of graphene on the dynamic mechanical properties, deformation and damage of graphene/7075Al composites were simulated using the finite element method, and the damage mechanism of the composites under dynamic load was revealed. The flow strain-stress curves were obtained. The effect of strain rates on the flow stress of the composites was analyzed. The results show that graphene/7075Al composites exhibit a significant strain rate effect. With an increase in the volume fraction of graphene, the yield strength of the composites increases and the composites have significant stress-softening characteristics at a strain rate of 10000 s–1, leading to a reduced strain-hardening effect. The shape and orientation of graphene have an important influence on the dynamic mechanical properties and the extent of damage made to the composites. The flow stress of the composites increases with varying the graphene shape and orientation in the following order, respectively: prism &lt; cylinder &lt; platelet &lt; disc and 0°&lt; 3D &lt; 90° &lt; 45°. The extent of the damage to the composites increases in the following order: disc &lt; platelet &lt; cylinder &lt; prism and 0° &lt; 3D &lt; 90° &lt; 45°. The dynamic failure mechanism of graphene/7075Al composites is interface damage.

Effects of homogenization temperature on microstructure and mechanical properties of high-speed-extruded Mg–5Bi–3Al alloy

This study investigates the effects of billet homogenization temperature on the dynamic recrystallization behavior during high-speed extrusion and resultant microstructure and tensile properties of the Mg–5Bi–3Al (BA53, wt%) alloy. Two billets homogenized at 350 and 450 °C (350H and 450H billets) are extruded at a high speed of 69 m/min. The 350H billet has a relatively smaller grain size and a higher abundance of fine Mg 3 Bi 2 particles compared to the 450H billet. During extrusion of the 350H billet, enhanced dynamic recrystallization occurs as a result of its finer grains and abundance of particles, while the growth of recrystallized grains is suppressed by the grain-boundary pinning effect of particles. Ultimately, the extruded 350H material is characterized by smaller grains, relatively greater number of Mg 3 Bi 2 particles, and a higher internal strain energy than the extruded 450H material. The tensile strength of the extruded 350H material is higher than that of the extruded 450H material owing to stronger grain-boundary hardening, particle hardening, and strain hardening effects. The extruded 350H material also exhibits a higher tensile elongation as its smaller grains inhibit the formation of crack-inducing undesirable twins during tension. The results from this study demonstrate that a decrease in the homogenization temperature from 450 to 350 °C leads to improved strength and ductility in the high-speed-extruded BA53 material.

Silicone rubber matrix composites with shear thickening fluid microcapsules realizing intelligent adaptation to impact loadings

In this paper, silicone rubber matrix composites with shear thickening fluid microcapsules (SR-STF) with intelligent adaptation to impact loadings are fabricated for advanced impact protective structure. The composites exhibit more flexibility at low strain rates but higher stiffness at high loading rates, indicating significant strain-rate sensitivity. In addition, the initial flexibility and the strain-rate sensitivity of the composite increase with increasing the mass fraction of the STF microcapsules, which should be ascribed to the shear thickening and compressive jamming of the STF microcapsules in the composite during impulse loadings. Based on experimental results, a hyper-viscoelastic constitutive model is developed, in which the influence of the mass fraction of STF microcapsules, the strain-rate sensitivity, and the strain-hardening effect are taken into account. The paper develops a practical strategy for applications of STF in soft protective structures and provides a reliable prediction of the dynamic mechanical behavior of the SR-STF composites.

Deformation and fracture initiation of Ni3Al intermetallic single crystal turbine blade

The present study deals with the determination of fracture properties of a cracked Ni3Al-based intermetallic single crystal turbine blade subjected to tensile load. The analysis has been carried out by accounting the effect of microstructure, strain hardening and damage development under monotonic loading. A continuum damage criterion combined with crystal plasticity model incorporating non-Schmid effects was employed to evaluate material damage owing to highly localised deformation. The damage evolution was defined as the function of a microstructure variable. The present coupled model was used to replicate the stress-strain behaviour of Ni3Al single crystals until material softening due to damage after necking. The model parameters were ascertained by a close match of the computed data with the available experimental data. Using the model, damage evolution near crack tip of a cracked Ni3Al turbine blade was assessed for randomly varying secondary orientations, while maintaining primary orientations same as that of along the solidification direction. The analysis helped to calculate the fracture initiation toughness (Ji) of turbine blade for various crystallographic orientations. Combined by damage formulation, analysis was continued to estimate the Ji values for different crack lengths in all the cases of orientation. Finally, triaxiality quotient (q) was computed to characterise the estimated Ji values as a function of crack tip constraints. The approach provides a useful way to analyse orientation dependent fracture behaviour of single crystal components with crack under mechanical loading.

Mechanistic Study of Rail Gouging during Hypersonic Rocket Sled Tests

Gouging—an obstacle to the development of hypersonic rocket sled test techniques—was mechanistically investigated through experimental and theoretical analyses. Typical gouges were analyzed using macroscopic and microscopic experiments to investigate the evolution of gouging. Quasistatic compression and Hopkinson bar experiments were performed to systematically study the thermoviscoplastic properties of U71Mn rail steel under wide ranges of the strain rate and temperature. The critical condition for gouging was derived based on a thermoviscoplastic constitutive model supplemented by a three-variable criterion for an adiabatic shear instability. The results showed the following. (1) Adiabatic shear bands (ASBs) form when stress reduction under the combined action of frictional heating and high-speed deformation exceeds the strain-hardening effect of the rail material. (2) The nonuniform deformation of the edges of ASBs leads to the generation of cracks that split the rail surface. As the ASBs expand, the cracks grow and coalesce, eventually causing the material to peel off, forming gouges. (3) The relationships among the temperature, strain rate, and strain at the onset of gouging can be determined based on the critical condition for the formation of ASBs in rail steel.

Effects of hydrostatic preload on strain hardening and strain aging of boiler tubes

Tests were conducted to identify the effects of existing defects on strain hardening and strain aging of boiler tubes. Electron backscatter diffraction (EBSD) and finite element analysis (FEA) were used to quantify the local plastic strain near the pre-existing defects. Strain hardening and strain aging effects depend on the degree of plastic deformation. Any existing defects in the boiler tubes can act as stress raisers during the room temperature hydrostatic test. For the morphology and orientations of the defects, a preload of 95 MPa which is equivalent to the hydrostatic test pressure was enough to induce local plasticity. When heated to boiler service temperature and tested to fracture, the mechanical properties of boiler tubes were affected. The test results justify a recommendation that the hydrostatic pressure to be kept near 110% of maximum allowable operating pressure (MAWP) to avoid significant alteration of mechanical properties at stress concentration points in defect-containing boiler membranes.

Refined plastic hinge method for steel frames with local–global interactive buckling

In the conventional refined plastic hinge model, the section is assumed to be compact and the strength deterioration by local buckling is not considered. In this paper, a refined plastic hinge method for welded normal strength steel frames with H and box sections is developed to account for the local–global interactive buckling behaviour. Based on the exact stability solution of a beam–column subjected to axial force, shear force and bending moment, beam–column element with arbitrarily located plastic hinges is developed. Residual stresses and strain hardening effect are considered in the refined plastic hinge model. Accounting for the interaction between flange and web, a simplified design method for the post-buckling capacities of columns and beams is proposed and experimentally verified. The local buckling effect is incorporated into the beam–column element formula and the refined plastic hinge method considering the interaction of local–global buckling is then established. As validated by existing experimental results, the proposed refined plastic hinge method that only needs one element per member can predict the bearing capacity of welded normal strength steel columns and frames accurately and be easily employed into design practice.

A predictive model for strain hardening and inertia effect of aluminum tubes filled with aluminum foam

Studies on strain hardening and inertia effect of foam-filled tube (FFT) remain limited. Therefore, this work has conducted mesoscopic finite element analyses on FFTs. The 7075-T651 aluminum alloy is chosen for tube material and the 1100-H14 aluminum is employed for the base material of closed-cell foam. The deformation mechanism of the interaction between tube and foam, and the inertia effect on compressive response of FFTs, are both revealed. Results show that due to the interaction, FFT has a higher compressive stress compared with the sum of empty tube and individual foam under the same mass. As more material is involved, the load-bearing capacity, strain hardening, and interaction stress of FFTs all increase with the relative density of infilled foam. A predictive model is established to predict the compressive stress and strain hardening of FFTs. The load-bearing capacity of tube, the plastic hardening of foam, and the interaction effect between tube and foam are all considered in this model. Moreover, due to the inertia effect, the stress at the impact end is significantly higher than that at the support end under high-velocity impact. The one-dimensional shock wave theory is added to the model, enabling the prediction under high-velocity impact.

Effects of Angle and Rate of Twist via Pre-Torsional Deformation on Tensile Properties and Hardness of Hot-Rolled, Low-Alloy Steel Rods

The effects of twist angle and twist rate during the pre-torsional deformation on tensile and hardness properties of hot-rolled SCM415 low-alloy steel were investigated. Hot-rolled SCM415 round tensile specimens were pre-torsioned using a free-end torsion machine to 36, 72, 144 and 288 degrees at 2 different twist rates of 0.1 and 30 rpm. As the angle and the rate of twist increased, yield strength and ultimate tensile strength increased with a tradeoff in a reduction in ductility. The relationship of true stress and true strain within the uniform plastic deformation were well described by a power law. The strength coefficient and strain hardening exponent obtained from the power-law curve fitting decreased with increasing twist angle and higher twist rate, suggesting that higher shear strain and shear strain rate led to a decrease in work hardening capacity. The local hardness after pre-torsional deformation showed a positive gradient from specimens’ centers to their surfaces along the radial direction. Also, the local hardness at the same radial position increased with the increasing angle and rate of twist. The average grain sizes were uniform along the radial direction and were comparable for all pre-torsional conditions. The strengthening mechanism via pre-torsional deformation of hot-rolled SCM415 was shown to be dominated by the strain hardening effect.&#x0D; HIGHLIGHTS&#x0D; &#x0D; The effects of twist angle and twist rate during the pre-torsional deformation using a free-end torsion tester on tensile and hardness properties of hot-rolled SCM415 low-alloy steel were investigated&#x0D; Within uniform plastic deformation region, the power-law parameters, namely strength coefficient (K) and strain hardening exponent (n) decreased with increasing amount of shear plastic strain and shear strain rate during the pre-torsional deformation&#x0D; Local Vicker hardness showed a positive gradient from the central region to the surface of pre-torsioned specimens, and the local hardness at the same distance from the center increased with increasing torsional angle and twist rate&#x0D; The average grain sizes of pre-torsioned specimens were found to be unaffected by the amount of torsional angle or the rate of twist&#x0D; The strengthening of hot-rolled SCM415 steel via pre-torsional deformation is predominated by strain hardening effect&#x0D; &#x0D; GRAPHICAL ABSTRACT