Constant True Strain Rate Research Articles

High temperature deformation behaviour and microstructure evolution in Cu containing High Strength Low Alloy (HSLA) steel

ABSTRACT The hot deformation behaviour of a Cu containing high strength low alloy (HSLA) steel has been evaluated by carrying out hot compression tests over a temperature range of 900 °C to 1100 °C and constant true strain rates of 0.0003 to 1 s−1. Although single peak flow curve is observed predominantly, a tendency to exhibit multiple peaks at low strain rates (0.0003 and/or 0.001 s−1) at temperature greater than 950 °C. The important parameters pertaining to dynamic recrystallisation (DRX), such as critical, peak, steady state stress and strain were ascertained, and the DRX state diagram depicting the variation of critical parameters governing DRX with Zener-Hollomon parameter Z was established. The volume fraction of recrystallised grains, computed from flow stress, as expected, follows a sigmoidal trend indicating higher kinetics at low strain rates for a given temperature. Reconstruction of prior austenite grain boundaries (PAGB) based on electron back scattered diffraction (EBSD) technique for a few select specimens clearly elucidated the effect of temperature and strain rate on the DRX behaviour of the steel. The DRX state diagram established here clearly demarcates the deformation space i.e. temperature-strain rate-stain space into regimes where the material exhibits full DRX, hardening/softening + DRX and only hardening (no DRX) behaviour. This would be useful for designing suitable forging sequence to achieve the desired microstructure and mechanical properties.

A comparative study on the interfacial characteristics and tensile behaviors of natural rubber composites reinforced by carbon and boron nitride nanotubes

AbstractThe reinforcing effect of carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) on the interfacial characteristics and tensile behaviors of natural rubber (NR) composites were comparatively investigated using molecular dynamics simulations. A pull‐out simulation was performed to study the interfacial characteristics between CNTs, BNNTs and NR matrices. The results showed that increases of about 56.63% and 90.14% in the interfacial frictional force and interfacial shear strength were obtained for the composites by the incorporation of BNNTs than those by the incorporation of CNTs. A uniaxial tensile process was fulfilled by using the constant true strain rate method to study the mechanical properties of the NR composites reinforced by CNTs and BNNTs. The results indicated that significant increases of 22.34% and 26.59% in the Young's modulus, 4.34% and 19.28% in the yield stress, and 11.07% and 8.01% in the tensile strength were respectively achieved by the incorporation of CNTs and BNNTs as reinforcements. To deeply reveal the deformation mechanisms of the NR composites, the mean square displacement and root‐mean‐square radius of gyration of the NR chains, the fractional free volume in the NR composites, and the interfacial interaction energy were calculated and discussed accordingly.

Theoretical instructions for experimenting controllable Hopkinson pressure bar

Hopkinson bar loading technique needs theoretical instruction for desired stress waves, to achieve controllable deformation under constant high strain rate loading, for it is the indispensable access while investigating material's rate-dependence and deformation mechanism under transient loading. In this work, theoretical and experimental studies are focused on the involved issues. Firstly, the theoretical deduction based on stress wave theory is performed for constant true strain rate loading, and the equation is formulated of an ideal incident strain wave for different materials. The formulated waves are further clarified by case studies of specimens with various mechanical behaviors, and they are found to present concave characteristics under large deformation. Secondly, an efficient methodology is proposed for single stress wave loading, and the target requirement is deduced theoretically. It is easily achieved experimentally by performing specific strain-controlled experiments: prior to and beyond peak stress of epoxy. Finally, some methods are proposed for achieving the formulated stress waves, and experimental verifications are also conducted on various materials.

Hot deformation of cast AZ31 and AZ80 magnesium alloys – Influence of Al content on microstructure and texture development

The present research studies and compares the hot deformation behaviour of the AZ31 and AZ80 magnesium alloys. Uniaxial compression tests using a Gleeble® 3500 thermal-mechanical simulation testing system were conducted at 400 °C and constant true strain rates in the range of 0.001 s−1 to 0.1 s−1. Samples were deformed to various levels of strain to assess the progression of dynamic recrystallization. Detailed microstructure and texture characterization were performed using optical microscopy, XRD macrotexture measurements, and electron backscatter diffraction (EBSD) microtexture measurements. Both the starting materials showed coarse dendritic microstructures and random texture. Both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) occurred in both the alloys during deformation, although the AZ31 alloy showed a higher tendency towards CDRX compared to the AZ80 alloy, while reverse was true for DDRX. The AZ80 alloy showed initiation of DDRX at lower deformation strain levels, had coarser DDRX grain size and a higher DDRX% compared to the AZ31 alloy. These features resulted in the development of a homogeneous fine-grained microstructure in the AZ80 alloy for deformation to an equivalent strain of 1.0, while the microstructure of the AZ31 alloy sample became bimodal. The choice of the alloy also affected the texture development during the deformation, with the AZ31 alloy showing DDRX texture randomization, while the AZ80 alloy showing deformation texture preservation by the DDRXed grains. The study highlights the substantial effect the Al content in the α-Mg solid solution has on the microstructure and texture development during hot deformation of the AZ alloys.

Study on the Hot Deformation Characterization of Borated Stainless Steel by Hot Isostatic Pressing.

Borated stainless steel (BSS) specimens have a boron content of 1.86 wt%, and are prepared by hot isostatic pressing (HIP) conducted at different temperatures, ranging from 1000 to 1100 °C and a constant true strain rate (0.01, 0.1, 1 and 10 s−1). These tests, with observations and microstructural analysis, have achieved the hot deformation characteristics and mechanisms of BSS. In this research, the activation energy (Q) and Zener–Hollomon parameter (Z) were contrasted against the flow curves: Q = 442.35 kJ/mol. The critical conditions associated with the initiation of dynamic recrystallization (DRX) for BSS were precisely calculated based on the function between the strain hardening rate with the flow stress: at different temperatures from 1000 to 1100 °C: the critical stresses were 146.69–254.77 MPa and the critical strains were 0.022–0.044. The facts show that the boron-containing phase of BSS prevented the onset of DRX, despite the saturated boron in the austenite initiated DRX. The microstructural analysis showed that hot deformation promoted the generation of borides, which differed from the initial microstructure of HIP. The inhomogeneous distribution of elements in the boron-containing phase was caused by hot compression.

Design of a hot deformation processing map for a Ni-free, N-bearing austenitic stainless steel

The hot deformation characteristics of a FeCrMnN austenitic stainless steel containing 0.28 wt.% nitrogen (N) was investigated by hot compression tests using a Gleeble simulator in the temperature range of 800−1200 °C and at constant true strain rates of 0.01–10 s−1 with all specimens deformed to ∼0.9 true strain. The influence of deformation conditions on microstructural mechanisms and phase transformations was characterized. A processing map based on dynamic materials modelling (DMM) was designed and interpreted for predicting the domain of stable flow for safe, defect-free hot deformation. The results revealed the occurrence of dynamic recrystallization (DRX) in a domain extending over the temperature and strain rate ranges of 1100−1200 °C and 0.1–1 s−1, respectively, with the efficiency of power dissipation (η) of 45–55 %. Decreasing temperature and increasing strain rate led to a reduction in DRX grain size following microstructural reconstitution. Another small deterministic domain of 820−1000 °C and 0.01−0.05 s−1 was identified showing occurrence of partial DRX in shear bands leading to formation of a mixed microstructure. The instability criteria delineated the regime of unstable flow covering a large part of the processing map extending over low temperatures (800−950 °C) and high strain rates (0.1–10 s−1) that must be avoided during processing.

A Conical Striker Bar to Obtain Constant True Strain Rate for Kolsky Bar Experiments

Many split Hopkinson pressure bar (SHPB) or Kolsky bar techniques use pulse shaping methods to obtain constant engineering strain rate for the specimen response. However, constitutive models for numerical simulations use the axial rate of deformation which is the true axial strain rate. In this study, we present an equation for the incident bar strain produced by a truncated conical striker bar. These incident bar strains are shaped so that we can obtain constant true strain rate for the specimen.

Low Cycle Fatigue Performance and Failure Analysis of Reinforcing Bar

Low cycle fatigue (LCF) behaviour and associated failure mechanisms of a thermo-mechanically treated Fe 500D steel rebar have been experimentally evaluated with a view to assess its performance under seismic condition. The total axial strain-controlled LCF tests have been performed at five different strain amplitudes (± 0.30 to ± 1.00%) at ambient temperature until failure maintaining a constant true strain rate of 1 × 10− 3 s− 1 and a fixed strain ratio of − 1. Fatigue data have been analysed following both strain–life and plastic strain energy–life relationships; while, macro as well as micro features of the failed specimens, have been critically examined. These are supplemented by microstructural characterizations in addition to tensile and hardness measurements. Significant reduction of yield strength is recorded under dynamic loading which is responsible for considerable cyclic softening of rebar till failure in all strain amplitudes indicating the deterioration of seismic resistance property. Both strain–life and plastic strain energy–life relationships are found to accurately predict the cyclic plastic behaviour of the selected rebar. A near Masing behaviour is established by three different analytical approaches. The fatigue crack is always found to initiate at the transverse rib root and primarily propagates alongside the rim region.

Hot Deformation Characteristic and Strain Dependent Constitutive Flow Stress Modelling of Ti\u2009+\u2009Nb Stabilized Interstitial Free Steel

An effort has been made to establish a relation between Zener–Hollomon parameter, flow stress and dynamic recrystallization (DRX). In this context, the plastic flow behavior of Ti + Nb stabilized interstitial free (IF) steel was investigated in a temperature range of 650–1100 °C and at constant true strain rates in the range 10−3–10 s−1, to a total true strain of 0.7. The flow stress curves can be categorized into two distinct types, i.e. with/without the presence of steady-state flow following peak stress behavior. A novel constitutive model comprising the strain effect on the activation energy of DRX and other material constants has been established to predict the constitutive flow behavior of the IF steel in both α and γ phase regions, separately. Predicted flow stress seems to correlate well with the experimental data both in γ and α phase regions with a high correlation coefficient (0.982 and 0.936, respectively) and low average absolute relative error (7 and 11%, respectively) showing excellent fitting. A detailed analysis of the flow stress, activation energy of DRX and stress exponent in accord with the modelled equations suggests that dislocation glide controlled by dislocation climb is the dominant mechanism for the DRX, as confirmed by the transmission electron microscopy analysis.Graphic

Effect of temperature on the hot deformation behavior of AZ80 magnesium alloy

The present research studies the hot forgeability of a cast AZ80 alloy by means of uniaxial compression tests using a Gleeble® 3500 thermal-mechanical simulator. Compression tests were conducted for temperatures ranging from 300 °C to 450 °C, and constant true strain rates ranging from 0.001 s−1 to 1 s−1, while the characterization of deformed samples for microstructure and texture was performed only for the samples deformed within 300 °C–400 °C, at a strain rate of 0.001 s−1. The samples were deformed to true strains of 0.05, 0.15, 0.4 and 1.0, to study the microstructure and texture evolution with deformation strain, while the characterization was performed using optical microscopy, SEM-EDX, and EBSD. Analysis of the flow stress data suggested a transition in the rate controlling deformation mechanism from dislocation cross-slip at 300 °C to dislocation climb at 400 °C, with a corresponding transition in the dominant DRX mechanism from continuous dynamic recrystallization (CDRX) to discontinuous dynamic recrystallization (DDRX). The thermodynamic stability of the Mg17Al12 phase (which is the dominant secondary phase in the AZ80 alloy) varied over the test temperature range, and significantly impacted the microstructure and texture evolution at different test temperatures. At 300 °C, multiple morphologies of the Mg17Al12 precipitates were present in the microstructure during deformation, and affected the DRX behavior of the material differently. At 400 °C, the precipitates rapidly dissolved in the α-Mg matrix, while the DRX took place by the grain boundary bulging mechanism, with the newly formed DRXed grains preserving the deformation texture. The study shows that the effect of temperature on the hot deformation behavior of the AZ80 alloy is especially pronounced due to a changing amount of the Mg17Al12 phase in the microstructure.

Morphology and Plastic Yielding of Ultrahigh Molecular Weight Polyethylene.

The structure, morphology, and mechanical properties of two compression-molded grades of ultrahigh-molecular-weight polyethylene (UHMWPE) and, for comparison, one conventional linear polyethylene (HDPE) were studied. Compression molding resulted in some preferred orientation of lamellae in the compression direction in UHMWPE samples, while no preferred orientation in HDPE. The mean crystal thickness estimated from the size distribution agrees better with those obtained from small-angle X-ray scattering (SAXS) and mechanical yield data than the thickness determined from the melting peak temperature. Microscopic examination of microtomed and etched UHMWPE samples showed that the lamellae are in the form of platelets with the width and length in the range of 300–700 nm. The lamellae radiate from primary nuclei forming small embryonal spherulites; their radial growth ends at 0.3–0.7 μm from the center. There is no evidence of branching and secondary nucleation from those primary lamellae. Because the lamellae are radially ordered, there is no parallel stacking of lamellae. Samples were subjected to deformation by plane-strain compression at a constant true strain rate. In axial UHMWPE samples, where lamellae were preferentially oriented along the loading direction, the second yield was clearly observed. The second yield was found to be related to the deformation instability leading to kinking of lamellae oriented initially along the loading direction. Kinking was clearly shown by SAXS and microscopic observation of microtomed and etched samples. No cooperativity of kinking was observed because the lamellae are arranged in small spherulites and not parallel in stacks. The stress–strain curves were fitted with model curves assuming crystal plasticity and network elasticity in the amorphous component. The effective density of the molecular network within the amorphous phase depended on the molecular weight of UHMWPE.

Elucidation of deformation mechanisms and construction of processing maps for a Ti+Nb stabilized IF steel

Deformation characteristics of a Ti + Nb stabilized IF steel have been investigated in the present study to clarify the associated strain hardening and/or dynamic restoration mechanisms. Single-hit hot compression experiments were performed in a Gleeble thermomechanical simulator in the temperature range 650–1100 °C at various constant true strain rates in the range 10−3-10 s−1 to a true strain of 0.7. The ‘safe’ and ‘unsafe’ deterministic domains were identified through the construction of processing maps and verification of mechanisms through the examination of evolved microstructures, regardless of the phase type (austenite and/or ferrite). Among different materials models employed, the ‘safe’ workable region at austenite and ferrite phase, predicted by a modified dynamic materials model (DMM) superimposed with Poletti instability criteria has been identified to be appropriate in accord with the experimental data. Development of a fully recrystallized fine equiaxed ferrite grain structure is ascribed to the occurrence of dynamic recrystallization (DRX). The characteristics of the onset of DRX mechanism have been further analyzed in terms of critical strain and stress required. Moreover, a kinetic model has been established to evaluate the fractional transformation through DRX. In addition, industrial optimization of hot working schedule of the IF steel could be ascertained from the processing map.

High-Strain Low-Cycle Fatigue Behavior of Thermomechanically Treated Rebar

Low-cycle fatigue (LCF) behavior of a thermomechanically treated Fe 500 grade rebar has been evaluated under high-strain ranges to understand its seismic performance. The total strain-controlled LCF tests have been performed at five different strain (1.0, 2.0, 3.0, 4.0 and 5.0%) ranges at ambient temperature until failure maintaining a constant true strain rate of 1 × 10−3 s−1 and a fixed strain ratio of − 1. Fatigue data have been analyzed following strain-life relationship; while, macro- and micro-features of the failed specimens have been critically examined under SEM. Evaluation of fatigue behavior has been supplemented by microstructural characterization apart from measurements of hardness and tensile property. Continued cyclic softening has been observed till failure, and the cyclic yield strength is found to be significantly lower than the monotonic one. Cross-sectional microhardness measurements of rebar before and after fatigue reveals significant reduction in hardness specifically in the rim region with tempered martensite microstructure. The strain-life relationship accurately predicts the cyclic plastic behavior of the selected rebar which also exhibits non-Masing behavior. The fatigue crack is always found to initiate from the transverse rib root and propagates along the rim region following the rib.

Parameter study of variable order fractional model for the strain hardening behavior of glassy polymers

In this work, a novel approach of variable order fractional derivative model of viscoelasticity is provided to describe the strain hardening behavior of amorphous glassy polymers, which has been considered as a combined viscoelastic phenomenon in recent studies. The proposed model contains only four parameters as the order function is assumed to linearly vary with time. To validate the model, experimental tests of constant true strain rate uniaxial compression are conducted on PC to get the stress–strain data at different temperatures and strain rates. Comparison between the model prediction and experimental data show that the model can well describe the strain hardening behavior. It is also indicated by the linearly decreasing order function that strain hardening is a continuous stiffening process of material property because the smaller fractional order means that the property of material is closer to solid. Furthermore, a study on parameter is performed to investigate the physical significance of model parameters. It is shown that the slope of order change determines the rate of strain hardening and the intercept of order function mainly affects the initial stress of strain hardening. Finally, the rule of order function under various temperatures and strain rates reveals that during strain hardening the mechanical property of amorphous glassy polymers is softer but stiffens faster at higher temperatures and smaller strain rates.

Atomistic molecular dynamics simulation study on the mechanical behavior and dispersion of surface functionalized graphene/polypropylene nanocomposites

The effect of methyl surface functionalization of single-layer graphene on the mechanical behavior and interfacial load transfer of polypropylene (PP) nanocomposites is studied by using molecular dynamics simulations. In the molecular model, both pristine and 20 methyl-group-functionalized graphenes are embedded into an amorphous PP matrix to constitute unit cell models of transversely isotropic periodic nanocomposites. The stress–strain curves of nanocomposites under uniaxial tension and shearing are determined based on non-equilibrium ensemble simulations with a constant true strain rate. Due to the degradation of graphene by the methyl groups, the longitudinal Young’s modulus and in-plane shear modulus of the nanocomposites are degraded. On the other hand, the longitudinal shear moduli of the nanocomposites, which depend on the interfacial shear load transfer, are considerably improved upon methyl functionalization. To establish the relationship between the improved interfacial properties in nanocomposites and methyl functionalization, the intrinsic adhesion energy between graphene and PP matrix and the arithmetic mean surface roughness of graphenes are determined and compared. The dispersibility of graphenes are also studied in terms of the solubility parameters.

Evolution of microstructure and microtexture during hot deformation in an advanced P/M nickel base superalloy

In the present study, the effect of hot deformation parameters on the microstructural evolution is investigated for an experimental powder metallurgy nickel superalloy. The hot isostatically pressed (HIPed) alloy is isothermally hot compressed at constant true strain rate over wide temperatures (1000 °C, 1050 °C, 1100 °C, 1150 °C) and strain rates (0.001 s−1, 0.01 s−1, 0.1 s−1, 1 s−1) regimes. Electron Back-Scattered Diffraction (EBSD) characterization of the hot deformed samples is carried out to evaluate the microstructural evolution, grain boundary characteristics, and microtextural behaviour. The strong dependence of dynamic recrystallization characteristics on the hot deformation conditions is divulged through EBSD derived colour inverse pole figure maps endorsed by grain/twin boundary characteristics and misorientation gradient. Dynamic recrystallization (DRX) is maximized at higher deformation temperatures, whereas limited DRX is observed at lower deformation temperatures, especially at higher strain rates. The dominance of discontinuous dynamic recrystallization (DDRX) is manifested through bulged/serrated grain boundaries resulting in the formation of necklace microstructure corroborated by grain boundary cumulative misorientation gradient. Although random microtexture is revealed in both deformed and recrystallized microstructures, the tendency towards the formation of 〈110〉 fiber compression texture is apparently evident.

Performance of a structural acrylic adhesive for linear glass-metal connections under shear and tensile loading

Glass-metal elements with composite action based on linear adhesive connections show a high potential in terms of structural efficiency. The adhesives considered for such applications need to fulfil a wide range of requirements and generally exhibit a non-linear material behaviour. This is also the case of the two-component structural acrylic adhesive investigated in this article. For this adhesive an experimental programme is carried out to assess its structural performance and its adhesion to selected metal and glass surfaces. Furthermore, the influence of different joint dimensions on the stress-strain properties of the adhesive in glass-metal connections, under shear and tensile loading, is analysed. The best results in terms of adhesion are obtained on stainless steel surfaces grinded with an abrasive paper P150 and on the tin side of float glass, respectively. To predict the mechanical behaviour of such connections, the suitability of material models covering both classic and porous plasticity is studied by means of non-linear finite element simulations. A novel surface fitting equation is introduced to obtain stress-strain data at constant true strain rates from uniaxial tensile test results performed at constant displacement rates. A good agreement between experimental and numerical results for shear loading is achieved based just on classical elasto-plasticity formulations. For tensile loading the numerical results show that voids in the adhesive material have to be considered for a reasonable estimation.

Experimental Investigation and Constitutive Modeling for Hot Deformation Behavior of 2219 Al-Alloy Microalloyed With Silver

2219Al and 2219Al + 0.1 wt % Ag alloys were processed by casting route. The hot compression tests were carried out at constant true strain rates and temperatures in the range of 10−3 to 101 s−1 and 300–500 °C, respectively. Flow stress of the alloy decreases with the addition of silver. The flow stress of both alloys increases with the decrease in deformation temperature and the increase in strain rates. Constitutive models correlating the peak flow stress with deformation temperature and strain rates for the two alloys were developed using hyperbolic–sine relationship. The activation energy for hot deformation of 2219 Al alloy decreases with the addition of silver. Comparison of the predicted and experimental values of peak flow stress reveals that 92% of the data could be predicted within a deviation error of ±10% indicating good predictive capability for the developed constitutive relationships.

Relationship of compressive stress-strain response of engineering materials obtained at constant engineering and true strain rates

In this study, a Johnson–Cook model was used as an example to analyze the relationship of compressive stress-strain response of engineering materials experimentally obtained at constant engineering and true strain rates. There was a minimal deviation between the stress-strain curves obtained at the same constant engineering and true strain rates. The stress-strain curves obtained at either constant engineering or true strain rates could be converted from one to the other, which both represented the intrinsic material response. There is no need to specify the testing requirement of constant engineering or true strain rates for material property characterization, provided that either constant engineering or constant true strain rate is attained during the experiment.

Parameter Identification of GTN Model Using Response Surface Methodology for High-Strength Steel BR1500HS

In order to investigate the damage evolution of ultra-high-strength steels at different temperatures, a series of the uniaxial tensile tests were carried out at different temperatures (20-800 °C) with a constant true strain rate (1 s−1). And then, two different deformation mechanisms were employed to describe the flow behaviors of BR1500HS. It is found that dynamic recrystallization (DRX) occurs during the deformation process at high temperatures (600-800 °C), while the tensile flow behaviors exhibit a very long work hardening period before a short flow softening stages without DRX at lower temperatures of 20-400 °C Furthermore, the Gurson-Tvergaard-Needleman (GTN) damage model was employed in this work to evaluate the ductile damage phenomena of BR1500HS. To determine the four vital parameters in GTN model, several numerical simulations were designed by central composite design and conducted by finite element simulation, and then the error evaluation functions (R) were established using four GTN parameters. Thereafter, the four parameters of GTN model were determined along with four minimum values of R by use of response surface methodology (RSM) and least square method. The results show that temperature affects the microvoid volume fraction significantly. Additionally, the parameters of GTN model were applied in the finite element simulation model and a comparison between the simulation results and the scanning electron microscopic observations was conducted.