Pronounced Strain Hardening Research Articles

The softening effect of welding on the mechanical properties of cold-worked stainless steel

Cold-working can occur during production of flat sheet products and during fabrication of structural cross-sections. In both cases, when the material is cold-worked, plastic deformations result in material strength enhancements. These strength enhancements are particularly significant for materials such as stainless steel, which exhibit rounded stress-strain behaviour and pronounced strain hardening, and for hollow structural sections, where the strength increases arise in both the corner and flat regions of the cross-sections. Numerous studies have shown the importance of utilising this strength enhancement for efficient structural design, and predictive models have been derived for harnessing these enhancements. However, if cold-worked material is welded, some of the enhanced strength can be lost due to partial softening in the heat affected zone (HAZ). The extent of this strength loss is investigated experimentally in the present study. The experimental programme comprised ten tensile coupon tests on 1 mm thick austenitic (Grade 1.4301) stainless steel sheet material, with central welds parallel and transverse to the direction of cold-rolling, as well as twelve full cross-section tensile tests on 80×60 mm and 60×60 mm hollow sections with thicknesses varying between 2 and 4 mm. Digital image correlation was utilised to determine the local constitutive properties of the base metal, the heat affected zone, and the weld metal of each specimen. The hardness and microstructure of the welded samples, along with the widths of the weld and heat affected zone, were also characterised.

X-ray diffraction study of strain-induced crystallization of hydrogenated nitrile-butadiene rubbers: Effect of crosslink density

The structure and the mechanical and thermal properties of a hydrogenated acrylonitrile-butadiene rubber (HNBR) with acrylonitrile content of 44 wt% and residual unsaturation content of 9% are investigated as a function of the crosslink density. The analysis is focused on specimens that have not been subjected to any prior deformation. All samples are amorphous at room temperature and show glass transition temperatures that increase with increasing crosslink density. The values of the Young's modulus of the vulcanized samples are three times higher than that of the non-vulcanized counterpart and tend to increase with increasing the crosslink density. The tensile strength increases with increasing crosslink density, whereas the deformation at break decreases. All samples are amorphous at low deformations and whereas the non-vulcanized and the weakly and medium crosslinked samples undergo strain-induced crystallization (SIC) above a critical value of deformation, the highly crosslinked samples show only faint SIC and rupture at deformations slightly higher than that at SIC onset. It is shown that the deformation at SIC onset for the crosslinked samples tends to increase with increasing the crosslink density. This is in contrast with the prediction of Flory's theory for a homogeneous network. Instead, in agreement with Flory's predictions, the degree of orientation of the amorphous phase, increases with increasing deformation and reaches a plateau at SIC onset, indicating that the onset of crystallization is associated with a relaxation of the amorphous chains connected to the crystalline stems. The remarkable tensile strength of the crystallizing HNBR samples at high deformations is attributed to the strong alignment of the crystals in the stretching direction formed by SIC, causing pronounced strain hardening.

Fundamental influences of propylene‐based elastomer on the foaming properties of high melt strength polypropylene based on extrusion foaming

AbstractHigh melt strength polypropylene (HMS‐PP)/propylene‐based elastomer (PBE) microcellular foams were prepared by the extrusion foam method. The fundamental mechanism of PBE improving the foaming properties of HMS‐PP was investigated. PBE 6102, with a low melt flow rate and high ethylene content, enables HMS‐PP/PBE blends to form a microphase‐separated structure and maintain good melt strength and pronounced strain hardening, which would promote cell nucleation and hold cell structure at relatively high temperatures. Furthermore, PBE 6102 postpones HMS‐PP crystallization and decreases the optimum foaming temperature, which helps balance melt elasticity and viscosity near the foaming window to avoid cell rupturing and merging. The introduction of PBE 6102 does not reduce the crystallization rate of HMS‐PP nor the crystallinity of the foamed samples, fixing the cell structure to prevent collapse during the cell solidification stage. Microcellular HMS‐PP/PBE 6102 foams with a high expansion ratio, small cell size, and large cell count were successfully fabricated.

The Annealing Effect of Welding on the Strength of Cold‐worked Stainless Steel

AbstractCold‐working can occur during the production of flat sheet products and the fabrication of structural cross‐sections. In both cases, when the material is cold‐worked, plastic deformations result in material strength enhancements. These strength enhancements are particularly significant for materials such as stainless steel, which exhibit rounded stress‐strain behaviour and pronounced strain hardening, and for hollow cross‐sections, where the strength increases arise in both the corner and flat regions of the cross‐section. Numerous studies have shown the importance of utilising this strength enhancement in design, and expressions to predict the strength enhancements have been established. However, if cold‐worked material is welded, some enhanced strength can be lost due to partial annealing in the heat affected zone (HAZ). The extent of this strength loss is investigated experimentally herein. The experimental programme comprised ten tensile coupon tests, with central welds parallel and transverse to the direction of cold‐rolling, as well as twelve full cross‐section tensile tests. Digital imaging correlation was utilised to determine the local constitutive properties of the base metal, the HAZ, and the weld metal. The microhardness and microstructure of the welded coupons, along with the widths of the weld and HAZ, were also characterised.

Superior elevated-temperature strength of Mg–Y–Sn alloys with thermostable multi-scale precipitates and grain structure

The newly developed Mg–Y–Sn extrusions are prepared by adding Sn (0.6 and 1.5 wt%) to Mg–6Y. The optimized Mg–6Y-1.5Sn exhibited superior elevated-temperature strength up to 300 °C, charactered by the pronounced strain hardening. Specifically, the yield stress and ultimate tensile strength of this alloy at 300 °C are 186 ± 3 MPa and 328 ± 7 MPa, respectively, which are 64% and 93% higher than that of the highly RE-alloyed WE54. Sn addition to Mg–6Y introduces a large number of thermal-stable micron/nano-scale Sn3Y5 precipitates. Mg–6Y-1.5Sn, which contained more nano-scale precipitates, exhibited better elevated-temperature strength than that of Mg–6Y-0.6Sn. The grain structure transitioned from a uniformed equiaxial grain in Mg–6Y to a thermal-stable multiscale grain structure in Mg–6Y-0.6/1.5Sn, which consisted of fiber textured un-DRXed area with high dislocation density and randomly orientated DRXed grains with low dislocation density. The exceptional elevated-temperature strength strongly correlated to the combined thermal-stable multiscale precipitates and grain structures.

Local formability of medium-Mn steel

• Local formability of medium-Mn steel is described by forming limits at fracture. • Ductile fracture occurs prior to plastic localization in MMnS. • Large uniform elongation indicates superior global formability of MMnS. • Local formability of MMnS is not enhanced by pronounced strain hardening. Global formability and local formability are critical in different metal forming processes. Edge cracking, controlled by the local formability, is a dominant factor limiting the application of advanced high strength steels (AHSS) in automotive industries. The local formability of a medium-Mn steel (MMnS), a promising candidate of the third generation of AHSS, is evaluated based on forming limit curves at fracture and compared with a dual-phase DP1000 steel using the damage mechanics approach. The superior tensile properties of the investigated MMnS, high tensile strength, pronounced strain hardening, large uniform and total elongation, lead to a very good global formability, which is an indicator of necking resistance. However, the local formability of the investigated MMnS, which is an indicator of fracture resistance and quantified by the plastic strain at fracture under different stress states, is worse than the DP1000 steel. By comparing the local and global formability of the two AHSS, it is confirmed that ductile fracture is the dominant failure mode in the MMnS and the onset of localized necking occurs prior to ductile fracture in the DP1000 steel. To achieve an accurate determination of the local formability, the effects of stress states need to be considered, which cannot be derived explicitly from uniaxial tensile tests. In addition to tensile properties, more attention should be paid to the local formability of new AHSS to assess their potential application in automotive industries.

On the constitutive relationship between solidification cells and the fatigue behaviour of IN718 fabricated by laser powder bed fusion

IN718 combines excellent mechanical properties with a good weldability and is therefore an ideal alloy for laser powder bed fusion (LPBF). Knowledge of the relationship between its as-built microstructure, particularly solidification cells, and its fatigue properties is needed to better utilise additively manufactured microstructures and guide their further optimisation. This study presents a comprehensive investigation of the as-built microstructure and the associated monotonic and fatigue properties of LPBF IN718 aimed at highlighting the influential effect of solidification cells on monotonic and cyclic plasticity. In monotonic tension, cells induced pronounced strain hardening and good ductility by acting as strong yet not impenetrable obstacles to dislocation slip. In fatigue loading, cyclic hardening followed by cyclic softening was linked to the stability of the as-built solidification cells, the high initial dislocation densities and the subsequent rearrangements of such dislocations during cyclic loading using the similitude relation and the evolution of friction and back stresses. By thoroughly investigating the evolution of the cyclic response of samples printed using two different scanning patterns, the relationship between process (scanning line length and thus local substrate temperature), microstructure (dislocation cell size and their spatial arrangement) and mechanical properties (cyclic hardening and softening responses) was comprehensively discussed.

Hysteretic performance of stainless steel double extended end-plate beam-to-column joints subject to cyclic loading

This paper focuses on the hysteretic performance of stainless steel beam-to-column joints with double extended end-plate connections under cyclic loading. Six full-scale joint specimens were fabricated, including three interior column joints and three exterior column joints. Two stainless steel grades — austenitic grade EN 1.4301 and duplex grade EN 1.4462, and one carbon steel grade Q345B were considered. A4-80 stainless steel bolts and grade 10.9 high strength steel bolts were employed to assemble the joint specimens. The cyclic loading tests were conducted using the loading protocol set in Chinese standard JGJ/T 101. The force versus displacement (F-Δ) hysteresis curves were recorded, and the failure modes, the ductility, the strength and stiffness degradation, and the energy dissipation capacity of the tested joints were analysed and discussed in detail. Empirical hysteresis models were developed and calibrated against the recorded hysteretic curves and energy dissipation capacities of the tested joints. The experimental results involving the initial rotational stiffness values and the moment resistances were further utilised to evaluate the accuracy of current design provisions codified in EN 1993-1-8, ANSI/AISC 358 and GB 51022. The comparison shows that the development of accurate design guidance accounting for the pronounced strain hardening of stainless steels is warranted.

Evolving asymmetric yield surfaces of quenching and partitioning steels: Characterization and modeling

Quenching and partitioning (Q&P) steel plays a significant role in automotive lightweighting on the merits of high strength and good ductility. Yield behavior of Q&P steels with strength grades of 1180 MPa (QP1180) and 980 MPa (QP980) and one dual-phase steel (DP980) were characterized under various proportional loading, i.e., uniaxial tension, uniaxial compression, simple shear and biaxial tension utilizing cruciforms with arms reinforced by laser deposition, i.e., an additive manufacturing process. Results show that QP1180 and QP980 exhibit pronounced strain hardening, strength differential effect and anisotropic hardening rather than DP980. The Karafillis and Boyce (1993) yield criterion was modified and further employed to construct a multiplicative yield criterion under non-associated flow rule. Anisotropic hardening is captured by directly employing the stress vs. strain curves under uniaxial tension, uniaxial compression and equi-biaxial tension. Besides, a user-friendly calibration strategy was proposed to identify the shape-dominating parameters in the developed yield criterion utilizing the flow stresses under plane strain and simple shear. Finally, evolving asymmetric yield surfaces of the two investigated Q&P steels were accurately described by the developed yield criterion.

Film Blowing of Linear and Long-Chain Branched Poly(ethylene terephthalate).

Film blowing of Poly(ethylene terephthalate) (PET) is challenging due its inherently low melt viscosity and poor melt strength. In this study, it is shown how the rheological properties of a commercial PET can be altered by reactive extrusion using either pyromellitic dianhydride (PMDA) or a multifunctional epoxy (Joncryl® ADR 4368) as chain extender, in order to improve the processing behavior during film blowing. The modified materials were characterized by shear and elongation rheometry and relevant processing characteristics, like melt pressure, bubble stability, and film thickness uniformity, were used to assess the influence of the type of modifier on processing and product performance. It is shown that PMDA is useful to increase the melt strength which leads to an improved bubble stability, while epoxy modified PET shows a reduced drawability that can cause problems at high take-up ratios. On the other hand, the epoxy modifier indicates a pronounced strain hardening during elongational deformation, and therefore leads to a better film thickness uniformity compared to the neat PET and the PET modified with PMDA. The differences with respect to processing performance are discussed and ascribed to the molecular structure of the materials.

Investigation of the microstructure, mechanical properties and tensile behavior of a low carbon nickel-manganese dual-phase transformationinduced plasticity steel by heavy warm rolling

A dual phase (martensite–austenite) low carbon nickel-manganese transformation-induced plasticity (TRIP) steel was fabricated by heavily warm rolling (HWR), and the effect of annealing on the phase fraction, mechanical properties and tensile deformation behavior of the heavily warm rolled (HWRed) steel was investigated. The results showed that the reverse transformation of γ-austenite from α′-martensite occurs and that the γ-austenite volume fraction (VA) decreases from 91% to 55% as the annealing temperature increases from 400 °C to 800 °C, respectively. The HWRed steel annealed at 400 °C exhibits a high strength-high ductility combination with yield strength of 706 MPa, ultimate tensile strength (UTS) of 1573 MPa, total elongation (TEL) of 21.6%, and the product of the strength and elongation (PSE: UTS×TEL) is 34 GPa%. These excellent mechanical properties are principally attributed to the formation of a large volume fraction of austenite (γ) by the reverse transformation and subsequent TRIP effect during tensile deformation. It was found that the HWRed and annealed steels exhibit a special tensile behavior with a large yielding strain followed by pronounced strain hardening. The tensile curve can be readily divided into three obviously different stages. The strain-induced martensite (SIM) transformation (γ -α′) occurs in the early yielding deformation stage and in the intermediate rapidly hardening deformation stage, indicating that the TRIP effect dominates the process of these two stages. However, the retained γ-austenite remains very stable, and no TRIP effect is observed in the final hardening deformation stage. The load-unload reload (LUR) test was performed to evaluate the back stress (σb) hardening effect during tensile testing. It is believed that the pronounced strain hardening behavior after yielding is mainly associated with the σb enhancement induced by the strain partitioning between the soft retained γ-austenite and the hard α′-martensite due to the SIM transformation during tensile deformation.

Phase evolution in mechanical alloying and spark plasma sintering of AlxCoCrCuFeNi HEAs

ABSTRACTAlxCoCrCuFeNi high-entropy alloys were synthesised through mechanical alloying and spark plasma sintering. Different alloys were produced by varying the aluminium content (x = 0.5, 1.5, 2.5 and 4). The influences of the milling duration on the evolution of microstructure, constituent phases and morphology were studied. Increasing milling time resulted in grain refinement and higher solid solution homogenisation characterised by a high internal strain. As a consequence of aluminium addition, the microstructure of materials evolved from face centered cubic (FCC) and body centered cubic (BCC) phases to FCC, BCC and ordered BCC phases. Both mechanical alloying and SPS conditions as well as aluminium content led to grain refinement and variations of mechanical properties. In particular, hardness increased with increasing aluminium content. The aluminium percentage and the evolution of consequent phases are responsible for the microstructural stability at high temperatures. In addition, with Al content increase, the further synergy of strength and ductility along with a more pronounced strain hardening was obtained.

Numerical modelling and shear design rules of stainless steel lipped channel sections

The demand for highly structurally efficient stainless steel is limited to a certain extent by its high initial cost. Therefore, the utilisation of material to the optimum possible level is important. In achieving this, further consideration should be given to enhance the design rules where beneficial effects such as pronounced strain hardening in stainless steel should be taken into account in the design process. In addition to that, a thorough understanding of the structural behaviour of stainless steel sections is also required. However, the shear behaviour and capacity of cold-formed stainless steel lipped channel beams (LCBs) have not been thoroughly investigated previously. Therefore, experimental and detailed finite element (FE) modelling were undertaken to investigate the shear behaviour and strength of stainless steel LCBs. A comprehensive parametric study was also conducted by developing 100 FE models. From the results, the available post-buckling strength in slender stainless steel LCBs was highlighted. Furthermore, the beneficial strength increment due to the strain hardening effect of stainless steel, particularly for compact LCBs in shear, was investigated. Comparisons indicated that current EN1993-1-4 and direct strength method (DSM) shear design rules are too conservative in particularly for compact sections. Thus, existing shear design rules were modified to enhance the overall prediction accuracy for stainless steel LCBs while attention was given to capture the available inelastic reserve capacity.

Rheology of polymer multilayers: Slip in shear, hardening in extension

The nonlinear rheology of multilayer stacks of alternating isotactic polypropylene (PP) and polyethylene (PE) films with N layers was measured in shear and uniaxial extension. We show that the force to extend N − 1 interfaces can lead to strain hardening. We studied three PE/PP pairs: a Ziegler–Natta catalyzed pair and two metallocene pairs, one with high density PE and one with linear low density PE. Interfacial tension was measured on matrix/droplet blends of each pair using small amplitude oscillatory shear measurements fitted with the Palierne model. Multilayer coextrusion was used to fabricate bilayers and sheets with hundreds of layers for each polyolefin pair. Under shear deformation, disentangling of chains in the N − 1 interfaces led to a decrease in the overall shear viscosity, which is interpreted as an interfacial slip velocity and is in agreement with previous studies on multilayer films. Under extensional flows, the multilayers exhibited an increased transient extensional viscosity (tensile stress growth coefficient, η E , M +) and pronounced strain hardening, even when the constituent homopolymers exhibited no strain hardening behavior. A model based on a force summation was developed to predict this behavior with interfacial tension as a fitting parameter; the extracted interfacial tensions agreed reasonably well with those from the Palierne model. The “slip in shear” and “hardening in extension” behaviors highlight the role of interfacial polymer physics during rheological measurements and polymer processing.

Influence of Austenite Conditioning on the Mechanical Properties of a Microalloyed Bainitic Steel

The effects of different hot strip rolling process parameters on the phase transformation and the final properties of a microalloyed bainitic steel are investigated. Thermomechanical tests are conducted to analyze the influence on strength and toughness. Transmission electron microscopy, nanoindentation, and electron probe microanalysis are used to investigate the microstructural features and their impact on the mechanical properties. In particular, it is found that a thermomechanical treatment of the microalloyed steel leads to an increase in strength, strain hardening, and toughness. This is related to the microstructure influenced by the transformation, the carbon partitioning, and partly to strain‐induced precipitations. The pronounced strain hardening is achieved by finely dispersed martensite/austenite islands. In contrast, coarse martensite/austenite constituents reduce the toughness. Further, strain induced titanium precipitations contribute to the high yield strength.

Elastoplastic Flexure of Round Steel Beams. 2. Residual Stress

Residual stress in metals may lead to shaping defects and the failure of metal structures in prolonged operation. It may arise on account of plastic metal flow on shaping (as in forging) or slow irreversible creep at elevated temperature, under the prolonged action of loads. In viscoelastic media, it may be due to ductile strain accumulating when a body is in a deformed state for a long time. Residual stress also affects the microstructure of metals and may be present within and around crystalline grains as residual microstress, sometimes known as latent elastic stress. Residual stress is also described as eigenstress, by analogy with the eigenfunctions introduced by mathematicians to denote functions corresponding to specific parameters (eigenvalues) in a differential equation with specified boundary conditions. The concept of internal stress has been proposed as a general term for stress created by the body itself. Residual stress is then confined to cases in which the internal stress is due to irreversible deformation. Besides the creation of a favorable residual stress system, local increase in strength will be noted in forged metal disks with pronounced strain hardening, as long as the Bauschinger effect does not cancel out such benefits. In the present work, extremal values of the residual stress in a straight cylindrical steel rod (beam) are studied, in the case of flexure.

Mechanical behaviour of Ti-Nb-Hf alloys

Ti-(24-26)Nb-(2-4)Hf at.% alloys were designed by assuming that hafnium has a similar effect to zirconium in the Ti-Nb-Zr system. Alloy specimens were produced using vacuum arc melting and subsequently hot-rolled. Uniaxial tensile testing was then performed both at ambient temperature and in liquid nitrogen at −196°C. While the alloys showed no obvious superelastic behaviour, they exhibited pronounced strain hardening and could achieve high elongations before failure (>30% engineering strain). Post-mortem examination revealed that the mechanism of strain hardening was extensive {332} and/or {211} deformation twinning. Twinning was found to be more prevalent in alloys with 2 at.% Hf compared to those with 4 at.%. The cryogenic temperature deformation also promoted deformation twinning when compared to ambient temperature results. As is the case with other metastable β-Ti alloys, maintaining control over the precipitation of ω phases was found to be crucial for attaining desirable mechanical behaviour. Further, microstructural engineering and alloying may be used to develop strong, lightweight alloys based on the Ti-Nb-Hf system with beneficial strain hardening characteristics for energy absorption, cryogenic and biomedical applications.

Strain hardening and tensile behaviors of gradient structure Mg alloys with different orientation relationships

Surface mechanical attrition treatment (SMAT) was employed to produce gradient structures in AZ31B Mg alloy samples with two different initial textures. The structure of both samples can be regarded as an integration of two main layers: the severely deformed layer exhibited dramatic grain refinement to nano- and submicron-scale with weakened and randomized textures; the less deformed layer exhibited the inherited coarse grains with increased dislocation density, possessing the similar texture with the sample prior to SMAT. All the samples containing different layer constituents cut from the SMAT alloys showed remarkable increase of strength compared to the original Mg alloy. However, the two integral SMAT samples with different initial textures exhibited marked difference in uniform elongation (UE) during tension. That was attributed to the different strain hardening behaviors influenced by the deformation coordination and strain partitioning between layers with different orientation relationships. The 0° oriented SMAT sample showed no macroscopic strain transfer or slip transmission across layers, which was hard to cause dislocation accumulation throughout the whole thickness of the layered sample. That resulted in a limited strain hardening and UE. However, the favored orientation relationship between layers of the 45° oriented SMAT sample facilitated the strain transfer and slip continuity across the layers. That generated a dynamically migrating interface during tension, which allowed the dislocations accumulating over the whole sample volume. This caused a pronounced strain hardening and sustained UE to an appreciable value.

Phase formation, microstructure and deformation behavior of heavily alloyed TiNb- and TiV-based titanium alloys

The effect of chemical composition on microstructure and mechanical properties of heavily alloyed beta-titanium Ti-Nb(V)-Cu-Co-Al alloys was studied. The alloys were fabricated by casting into a water-cooled copper crucible employing relatively high cooling rates. The microstructure of these alloys consists of primary micrometer-sized bcc-structured (bcc – body centered cubic) dendrites surrounded by a minor amount of intermetallic phases. The morphology and volume fraction of the intermetallic phases are strongly affected by the alloys’ chemical composition. Particularly, the solubility of Cu and Co in the bcc dendrites of Ti-V-Cu-Co-Al is lower compared to that of Ti-Nb-Cu-Co-Al leading to a higher volume fraction of the intermetallic phase in the latter alloy. The high mechanical strength of the Ti-Nb(V)-Cu-Co-Al alloys (yield strength up to 1430 MPa) is mainly attributed to their multiphase nature and solid solution hardening of the supersaturated bcc-structured dendrites. Moreover, the large compressive plastic deformability supported by pronounced strain-hardening reaches several tens of percent. The alloys exhibit a significant strength asymmetry between compressive and tensile loadings, namely, they are weak and brittle under tensile loading. The tensile brittleness is associated with the lattice distortion in the bcc-structured dendrites as well as crack initiation at the interdendritic precipitates.

Low Temperature Superplasticity in Ultrafine-Grained AZ31 Alloy

The mechanical behavior of an AZ31 magnesium alloy processed by high-pressure torsion (HPT) was evaluated by tensile testing from room temperature up to 473 K at strain rates between 10-5 – 10-2 s-1. Samples tested at room temperature and at high strain rates at 373 K failed without any plastic deformation. However, significant ductility, with elongations larger than 200%, was observed at 423 K and 473 K and at low strain rates at 373 K. The high elongations are attributed to a pronounced strain hardening and a high strain rate sensitivity. The results agree with reports for a similar alloy processed by severe plastic deformation. However, the level of flow stress is lower and the strain rate sensitivity and the elongations are larger than observed in this alloy processed by conventional thermo-mechanical processing.