Tensile Flow Research Articles

High Strain Rate Tensile Behavior of 1180MPa Grade Advanced High Strength Steels

<div class="section abstract"><div class="htmlview paragraph">Tensile behavior of advanced high strength steel (AHSS) grades with strengths up to 980 MPa has been extensively studied. However, limited data is found in literature on the tensile behavior of steels with tensile strengths of the order of 1180 MPa, especially at nominal strain rates up to 500/s. This paper examines tensile flow behavior to fracture of four different 1180 MPa grade steels at strain rates of 0.005/s, 0.5/s, 5/s, 50/s and 500/s using an experimental methodology that combines a servo-hydraulic tester and high speed digital image correlation. Even though the strength increase with the strain rate is consistent between the four different materials, the total elongation increase with the strain rate varies widely. Some insights as to why this occurs from examination of the steel microstructure and variation of retained austenite with strain are offered.</div></div>

Ductile fracture behavior and flow stress modeling of 17-4PH martensitic stainless steel in tensile deformation at high temperature

The ductile fracture behavior of 17-4PH martensitic stainless steel at high temperature was investigated based on the experimental data of hot uniaxial tensile test. The elongation and reduction of area indicated that the ductility was mainly enhanced during the deformation at the temperature 1150 °C–1200 °C with high strain rate more than 0.1 s−1. The hot flow stress curves were obtained from the tensile experiments, and the true strain of necking initiation was determined by analyzing the contours of fractured specimens. Then, the true stress-true strain data of the deformations before necking were adopted to establish the tensile flow stress model. In order to express the combined influences of the strain, strain rate and temperature on flow stress, a modified Johnson-Cook model was proposed to describe the hot constitutive behavior of the presented steel. It indicates that reasonable agreements between the model-predicted results and the experimental data were achieved.

Construction of tensile stress-strain curves for solid bars pre-deformed by gradient shear strain

Cai Chen, Beygelzimer Y., Kulagin R., Davydenko O. Construction of tensile stress-strain curves for solid bars pre-deformed by gradient shear strain. Material working by pressure. 2020. № 1 (50). P. 114-118.
 Strain hardening curves were derived for the first time for tension of cylindrical solid bars in the presence of a linear strain gradient produced by severely pre-straining by torsion. An analytical formula was obtained, which enables constructing the local stress-strain curve. Data obtained for pure copper samples pre-twisted to different levels of shear strain were used. A saturation stage not known hitherto was found for large strains.
 One particularity of the present results is that for low tensile strains the tensile flow stress after torsion agrees well with the flow stress in monotonic tension.
 Another particularity of the results is that the tensile flow stress is constant after torsion at large strains, starting from about 1.5 strain. This effect was observed for the first time because the present technique is the first one to provide access to tensile flow stress after large strain torsion.

Soundness Discrimination in Ferrite Ductile Irons Through Tensile Data Analysis

Ferritic ductile iron (DI) GJS 400 and high-silicon strengthened ductile irons (HSSDIs) with 3.5 and 4.5 wt% of silicon contents were produced with different cooling rates, and the microstructures were analyzed to find the main microstructure parameters. The Y-blocks GJS 400 presented good nodularity, while the heavy section GJS 400 presented some graphite degeneracy with a lower nodularity. The 3.5 wt% HSSDI shown a good nodularity, while with increasing silicon content to 4.5 wt%, significant graphite degeneracy occurred with the appearance of chunky graphite. Samples were tensile tested and the tensile data were analyzed through the physical-based constitutive Voce equation, and the Voce parameters were plotted to produce a matrix assessment diagram (MAD). Y-blocks and heavy section GJS 400 data lied on two distinct lines in MAD, and they had, however, positive and negative intercepts which meant indeed two contradictory plastic behaviors. The intercept of the best fitting line of the GJS 400 Y-blocks data was positive and so consistent with the physical meaning of Voce equation, while the intercept of the best fitting line of the GJS 400 heavy section data was negative, which meant an unexpected opposite plastic behavior. Same behavior was reported for the investigated HSSDIs, resulting in positive intercept of the best fitting line in MAD for the 3.5 wt% HSSDI data and negative intercept for the 4.5 wt% HSSDI data. The uniform strain energy density (SEDU) that is the area below the tensile flow curve up to the uniform strain, i.e., the strain where necking begins, was also investigated. SEDU resulted to be almost constant for all the GJS 400 Y-blocks tensile flow curves and 3.5 wt% HSSDI, which was typical of a sound material, while in GJS 400 heavy section and 4.5 wt% HSSDI, SEDU changed significantly in a systematic way, indicating that metallurgical defects had affected the plastic behavior. So it was concluded that in the MAD the best fitting line of the tensile data with positive intercept was a possible indication of the material soundness, while the negative intercept was indication of defected material. The results suggested that the MAD analysis produced from tensile Voce parameters can be a useful and easy tool for industry not only to classify the production routes of DIs (Si content mainly and heat treatments), but also to identify possible microstructure poorness within a single DI grade.

Prediction of flow stress behaviour by materials modelling technique for Inconel 718 alloy at elevated temperature

ABSTRACT Tensile flow behaviour attracts a continue attention in enhancing material processing and ensuring secure performance during metal forming processes. In present study, tensile deformation behaviour of Inconel 718 alloy have been investigated from room temperature (RT) to 700°C, strains (from 0.01 to 0.3 with interval of 0.01) at strain rates of 0.0001-0.01 s−1. From experimental observation, it is observed that flow stress behaviour was considerably influences by temperature and strain rate variation. Additionally, constitutive models, mainly Khan–Huang–Liang (KHL) and modified Fields–Backofen (mFB) models have developed to predict flow stress behaviour. Statistical parameters, mainly average absolute error (Δ), correlation coefficient (R) and root mean square error (s), have been evaluated the prediction capability of models. KHL model has given highest coefficient of correlation (R = 0.9182); this signifies suitability of KHL model for flow prediction of Inconel 718 alloy.

The mechanism of dynamic strain aging for type A serrations in tensile flow curves of Fe-18Mn-0.55C (wt.%) twinning-induced plasticity steel

To elucidate the mechanism of dynamic strain aging (DSA) causing serrations in the tensile curves of an fcc austenitic Fe-18Mn-0.55C (wt.%) twinning-induced plasticity (TWIP) steel, many tensile tests were performed by varying both tensile temperature (203−323 K) and initial strain rate (ε˙ini = 1 × 10−2 − 1 × 10−4/s). At the ranges of tensile temperature and ε˙ini adopted in this study, only type A serrations appeared, and a critical engineering strain (ec) for serrations decreased with increasing tensile temperature and with decreasing ε˙ini. For the short-range diffusion model based on C-Mn complex, the activation energy value (QreC) for the reorientation of C in C-Mn complex was calculated by ab-initio simulation. The QreC value was ~2.4 eV for the fcc austenite matrix, and 0.60 eV and 0.18 eV for the hcp stacking fault depending on diffusion path. There was no intersection between staying time (ts) and reorientation time (tre) calculated using the QreC values. This indicates that DSA is not caused by the reorientation of C-Mn complexes. In addition, ec revealed no dependency on the concentration of vacancy (Va). Therefore, DSA causing type A serrations is not explained by short-range diffusion models based on C-Mn and C-Va complexes. DSA is elucidated by the dislocation arrest model, which belongs to the long-range diffusion model. The measured activation energy (0.85 eV) corresponding to the activation energy (0.57−1.00 eV) for dislocation pipe diffusion of C and C-segregated dislocations in atom probe tomography (APT) maps support the occurrence of long-range C diffusion.

Effect of Silica on Hot Mix Asphalt Mixture

The hot asphalt mixture is the main layer in flexible paving as it is exposed to traffic loads and environmental factors which require high resistance, high durability and good stability to avoid structural deformities that leading to functional and structural failure. In the past few years. Several tests have been carried out on the prepared and compact specimens such as density testing, indirect tensile strength (exposed to freezing and thawing conditions and not exposed), stability and flow as well as non-destructive test using sound waves. The results of the tests showed that the specimens, containing silica fume as apart from filler aggregate (Portland cement), have high Marshall stability-flow and extra resistance to tensile strength than conventional specimens that contain just ordinary Portland cement. All results found to be satisfied the Iraqi specifications adopted for roads and bridges.Calculation of optimum silica fume content has achieved for all tests in order to find overall optimal content which was 4.2%.

Constitutive Modeling of 17-4PH Stainless Steel Sheet at Elevated Temperature and Statistical Optimization

The tensile flow behavior of 17-4PH stainless steel sheet was investigated from 650 °C to 850 °C at strain rates of 10−4 to 10−1 s−1. The result indicates that work hardening and dynamic recovery are exhibited in the tensile test in the temperature lower than 750 °C, and a steady stage of stress occurs higher than 750 °C because of dynamic recrystallization. After a step-by-step procedure for data analysis in the hot tensile test, the original constitutive model was given. Considering the strain compensation, several revised constitutive models of Arrhenius-type were constructed with statistical optimization and multi-strain correction. The comparison shows perfect accurate flow predictions through multi-strain modified models. Meanwhile, the flow behavior of random conditions is predicted accurately by a single correction model with Tikhonov regularization.

Comparison between ductility examination and a new approach based on strain hardening analysis to support the determination of proper austempering times

ABSTRACTAustempered Ductile Irons (ADIs) with different contents of nickel and copper were austenitized at the same conditions, and austempered for 60, 90 and 180 min to investigate the austempering evolution. Besides the conventional ductility analysis to assess the best austempering times, strain hardening analysis of tensile flow curves was carried out, since it has been found reliable for this assessment if dislocation-density-related constitutive equations are used. Through plotting the Voce equation parameters 1/εc vs. ϴo found from the strain hardening analysis, Matrix Assessment Diagrams (MADs) were drawn, and austempering conditions closest to the best austempering times were identified. Austempering times found with ductility-criterion and MAD approach did not match. Mg–Cu particles found in copper-containing ADIs affected the reliability of tensile ductility that could not be ascribed to ausferrite stability only, while the austempering times found with MAD approach seemed to be consistent with solid-state diffusion transformations.

Hot Tensile Deformation Mechanism and Dynamic Softening Behavior of Ti–6Al–4V Alloy with Thick Lamellar Microstructures

The deformation mechanisms and dynamic softening behavior of a Ti–6Al–4V alloy with thick lamellar microstructures are studied by uniaxial hot tensile tests. The true stress first rapidly reaches a peak value because of the rapid dislocations proliferation and tangle, and then gradually decreases due to the dynamic softening with raising the deformation amount. The dynamic softening behavior is mainly induced by the severe dynamic globularization of lamellar α phases. The globularization of α phase is always accompanied by the formation of high‐angle grain boundaries (HAGBs). The fraction of globularized α phases increases with the increasing deformation temperature. The fracture mechanism is microvoid coalescence, i.e., in the initial stage of tensile deformation, the microvoids begin to emerge. With the increasing deformation amount, the number of microvoids increases, and the microvoids accumulate and accelerate to propagate until the final fracture. According to the experimental flow stress curves, a modified Arrhenius‐type equation and a Hensel–Spittel (HS) model are developed to forecast the flow stresses. The average absolute relative error of the established strain‐compensated Arrhenius‐type (SCAT) and HS models are 6.108% and 3.385%, respectively. Both models can be well used to describe the hot tensile flow characteristics of the Ti–6Al–4V alloy with thick lamellar microstructures.

Effect of hydrogen on tensile flow and failure mechanism of low nickel-type 316L austenitic stainless steel

An investigation of the effects of hydrogen on tensile flow, strain-hardening and fracture characteristics of low nickel-type 316L austenitic stainless steel was quantified by means of slow strain rate tensile (SSRT) tests. Both thermal precharged and gaseous hydrogen conditions will be considered. The strain-hardening parameters were derived for an idealized true-stress/true-strain curve expressed by Hollomon’s equation. It was found that hydrogen affects the strain-hardening rate and reduces the overall strain-hardening exponents with a similar tendency for diminution of reduction area and fracture strain. The severity of hydrogen embrittlement was observed to be greater in a gaseous hydrogen environment than when tested in the hydrogen pre-charged specimen. Moreover, the fracture mechanism caused by hydrogen pre-charged and gaseous hydrogen environments were different. Hydrogen-induced cracks in hydrogen were found to appear at the interior of the grain boundary, in which there was a strain-induced martensite transformation phase, and crack formation at the grain boundary was predominant gaseous hydrogen specimens.

Monotonic and cyclic anisotropies of an extruded Mg–Al–Ca–Mn alloy plate: Experiments and crystal plasticity studies

Mechanical anisotropies of an extruded Mg–Al–Ca–Mn alloy plate were investigated under both monotonic and cyclic conditions through a combination of experiments and crystal plasticity finite element simulations. To this end, monotonic tensile, compression as well as fully-reversed strain-controlled and load-controlled fatigue tests were conducted on specimens aligned either in the extrusion or transverse direction. Plastic activities of the deformation mechanisms for the different loading conditions were quantitatively analyzed based on the results of numerical simulations. The asymmetrical mechanical response between tension and compression along the two loading directions was found to be associated with the lack of axi-symmetry of the relatively strong basal texture. In addition, the peculiar and lower tensile flow stress in the transverse direction was explained by the activation of deformation twinning in grains with their c-axis aligned along the loading direction. Under low-cycle fatigue conditions, a lower asymmetry of stress–strain hysteresis curves was observed in the transverse direction due to the occurrence of twinning/detwinning in both the tensile and compressive portions. High-cycle fatigue experiments revealed lower fatigue lives in the transverse direction than in the extrusion direction but similar fatigue limits. Crystallographic analyses of multiple secondary cracks revealed crack initiation from basal slip, twin bands, grain boundaries and non-metallic inclusions. The evaluation of slip-induced and twin-induced fatigue criteria from the results of fatigue simulations of explicitly modeled polycrystals suggested that the most detrimental crack initiation mechanism was crack initiation from basal slip as it predicted a significantly lower initiation life in the transverse direction than in the extrusion direction, as observed experimentally.

Warm deformation and strain-hardening behaviors of fine-grained ZK60 alloy sheet

Tensile flow behaviors of the fine-grained ZK60 alloy sheet are investigated from 298 K to 523 K with strain rates from 10−1 s−1 to 10−3 s−1. The apparent activation energy tested at 523 K is estimated to be 103 kJ mol−1, indicating that grain-boundary sliding (GBS) associated with grain boundary diffusion is the dominant deformation mechanism of the fine-grained ZK60 alloy. The effects of deformation temperature and strain rate on the strain-hardening and strain-softening behaviors are analyzed by the K-M approach. The results show that the dislocation proliferation contributes to the hardening process but DRX play an important role in the softening process.

The Potential u se of Crumb R ubber in H ot A sphalt M ixes in E gypt u sing Dry P rocess

Since 1960 Using crumb rubber modifier (CRM) in hot asphalt mixtures has become a frequent practice in road construction. Using the CRM by the dry process method is not commonly used, although it has great advantages such as it is less fuel consuming and it does not require storage container like the wet process method. This research evaluates the mechanical properties of dense graded asphalt rubber mixtures manufactured using the dry process. The results obtained from this mixture compared with similar asphalt mixture without CRM. The mechanical properties of all mixtures evaluated using a set of tests such Marshall Stability and flow test, moisture susceptibility test, indirect tensile strength test, dynamic modulus and flow number test. The research results showed that using CRM with 0.75% of aggregate’s weight increased the mixture’s stability, flow and enhanced its cracking and permanent deformation resistance.

Selective laser melting of dual phase AlCrCuFeNix high entropy alloys: Formability, heterogeneous microstructures and deformation mechanisms

Preparing dual-phase high-entropy alloys (DP-HEAs) by selective laser melting (SLM) has never been achieved owing to high crack susceptibility induced by rapid solidification. Here we design and fabricate new face-centered cubic (FCC) and body-centered cubic (BCC) DP-HEAs based on BCC AlCrCuFeNi HEA using SLM. Results show that the addition of Ni facilitates the columnar-to-near-equiaxed transition and improves the formability of the as-built AlCrCuFeNix (2.0 ≤ x ≤ 3.0) HEAs. Especially, the as-built AlCrCuFeNi3.0 HEA exhibits modulated nano-sized lamellar or cellular dual-phase structures and possesses the best combination of ultimate tensile strength (∼ 957 MPa) and ductility (∼ 14.3%). Post-deformation research reveals that the FCC phase is deformed through planar dislocation slip with {111} <110> slip systems, and stacking faults (SFs). In the ordered BCC (B2) phases, high densities of Cr-rich nano-precipitates make B2 phase severely distort during tension, thus triggering the formation of deformation nano-twins and SFs on {112} planes. Strain-activated B2-to-FCC phase transition occurs in the B2 phase. Moreover, serrated tensile flow is first discovered in DP-HEAs due to the continual initiation and propagation of twins in the B2 phase. The uncovered synergy of various deformation modes and the underlying back stress strengthening induced by heterogeneous microstructures contribute to the high ultimate tensile strength and good ductility of the as-built AlCrCuFeNi3.0 HEA.

On the tensile flow stress response of 304 HCu stainless steel employing a dislocation density based model and electron backscatter diffraction measurements

ABSTRACTTensile flow stress response of 304 HCu stainless steel is investigated using a dislocation density based model and electron backscatter diffraction (EBSD) studies. The model considers two types of dislocations explicitly, i.e. mobile and forest, to model the flow behaviour in the temperature range 300–973 K, at strain rates 3×10 −3 and 3×10 −5 s-1. The flow behaviour indicates the predominance of thermal recovery at higher temperatures, except at 923 K/3 × 10−5 s−1. The dislocation model predicts a rapid increase in both mobile and forest dislocation densities at the early stages of deformation and thereafter reach saturation/steady state. Higher strain rate leads to an increase in dislocation densities with concomitant increase in peak flow stress, indicating significant dislocation multiplication. The dislocation densities decreased with an increasing temperature and is attributed to thermally activated recovery by glide (slip and cross-slip) and climb of dislocations. However, an offset to the thermal recovery is observed at 873–923 K at 3 × 10−5 s−1 wherein the magnitudes of peak flow stress, boundary and forest dislocation densities are of similar magnitude at both the temperatures, thereby signifying the occurrence of matrix hardening by DSA and precipitation. Boundary dislocation densities estimated from EBSD data have shown affinity to that of predicted forest dislocation densities.

Predicting the Tensile Behavior of Ti-6.6Al-3.3Mo-1.8Zr-0.29Si Alloy via the Temperature-Dependent Crystal Plasticity Method.

Uniaxial tensile flow properties of a duplex Ti-6.6Al-3.3Mo-1.8Zr-0.29Si alloy in a temperature range from 213 K to 573 K are investigated through crystal plasticity modelling. Experimental results indicate that the initial yield stress of the alloy decreases as the temperature increases, while its work-hardening behavior displays temperature insensitivity. Considering such properties of the alloy, the dependence of the initial critical resolved shear stress (CRSS) on temperature is taken into account in the polycrystal plasticity modelling. Good coincidence is obtained between modelling and the experimental results. The determined values of CRSS for slip systems are comparable to the published data. The proposed polycrystalline model provides an alternative method for better understanding the microstructure–property relationship of α + β titanium alloys at different temperatures in the future.

The Role of Microstructure on the Tensile Plastic Behaviour of Ductile Iron GJS 400 Produced through Different Cooling Rates—Part II: Tensile Modelling

Tensile testing on ductile iron GJS 400 with different microstructures produced through four different cooling rates was performed in order to investigate the relevance of the microstructure’s parameters on its plastic behaviour. Tensile flow curve modelling was carried out with the Follansbee and Estrin-Kocks-Mecking approach that allowed for an explicit correlation between plastic behaviour and some microstructure parameters. In the model, the ferritic grain size and volume fraction of pearlite and ferrite gathered in the first part of this investigation were used as inputs, while other parameters, like nodule count and interlamellar spacing in pearlite, were neglected. The model matched very well with the experimental flow curves at high strains, while some mismatch was found only at small strains, which was ascribed to the decohesion between the graphite nodules and the ferritic matrix that occurred just after yielding. It can be concluded that the plastic behaviour of GJS 400 depends mainly on the ferritic grain size and pearlitic volume fraction, and other microstructure parameters can be neglected, primarily because of their high nodularity and few defects.

Validation of a New Quality Assessment Procedure for Ductile Irons Production Based on Strain Hardening Analysis

A mathematical procedure based on the analysis of tensile flow curves has been proposed to assess the microstructure quality of several ductile irons (DIs). The procedure consists of a first diagram for the assessment of the ideal microstructure of DIs, that is, the matrix where mobile dislocations move, and a second diagram for the assessment of the casting integrity because of potential metallurgical discontinuities and defects in DIs. Both diagrams are based on the dislocation-density-related constitutive Voce equation that is used for modeling the tensile plastic behavior of DIs. The procedure stands on the fundamental assumption that the strain hardening behavior of DIs is not affected by the nature and the density of the potential metallurgical discontinuities and defects, which are expected to affect only the elongations to fracture. However, this fundamental assumption is not obvious, and so its validity was evaluated through tensile testing Isothermed Ductile Irons (IDIs) 800, showing a wide scatter of elongations to rupture. The analysis of the strain hardening behaviors supported by strain energy density calculations of IDIs tensile tests proved that the fundamental assumption was valid and the quality assessment procedure could be applied to IDIs. A modified Voce equation was also introduced to improve the fitting of the experimental tensile flow curves and the strain energy density calculations.

A novel two-step intercritical annealing process to improve mechanical properties of medium Mn steel

In this paper, we present a novel two-step intercritical annealing process, i.e. a-few-minute interruption during heating followed by a short intercritical annealing, to improve the mechanical properties of medium Mn steel without prolonging the intercritical annealing (IA) period. Both experimental investigations and numerical simulations were performed to investigate the microstructural inheritance between the two steps and its influence on the tensile properties. The increase of interruption temperature led to a gradual change of tensile flow curve from the discontinuous yielding to the continuous yielding at lower yield strength due to more martensite and less austenite formed after the IA at 670 °C. It is concluded that the two-step IA process can improve the mechanical properties of medium Mn steel when it is optimized, compared to the non-interrupted IA process. The strategy for optimizing this novel process is to carefully tailor cementite precipitates before annealing as the nuclei of austenite for promoting relatively coarse austenite grains formed with Mn gradients inside. This kind of austenitization on the deliberately introduced nuclei finally led to a considerable fraction of austenite grains retained adjacent to martensite, causing strength and ductility improved simultaneously. Moreover, we demonstrate that such a deliberate design of two-step IA process can be guided by the quantitative modeling of austenitization kinetics on a cementite nucleus.