Strain Rate Jumps Research Articles

Thermomechanical treatments for a dual phase cast high entropy alloy

Thermomechanical treatments modify the cast microstructure of a dual phase hypoeutectic high entropy alloy. The as-cast microstructure consists of an FCC matrix and a BCC eutectic phase forming a semi-continuous network. The as-cast material is subjected to various thermomechanical treatments: I) hot compression; II) hot compression with strain rate jumps; III) multi-stage hot compression with intermediate stages of isothermal soaking; IV) hot compression followed by annealing. The material recovers rapidly after the strain rate jump and no dependence of the yield stress on the previous deformation step is observed. Much of the stress relaxation occurs within seconds during the hold for the multi-stage tests. A steady-state stress is reached for the first holding cycle, while progressively slower softening is observed for later holding cycles. The eutectic BCC phase fragments during hot deformation, while discontinuous dynamic recrystallisation associated with the formation of Σ3 occurs in the FCC phase. Precipitates are formed at high temperatures and pin the high-angle grain boundary motion. The rotation of the eutectic BCC phase and the precipitates during plastic deformation also promotes local lattice rotation of the FCC phase. As a result, a fine and intricate substructure is formed in the interdendritic spaces. Complete recrystallisation is not achieved during either hot deformation or annealing, suggesting that practical grain refinement of the investigated alloy would only be achieved at larger strains or a combination of larger deformations and intermediate annealing treatments.

Microstructure refinement of a cast high entropy alloy by thermomechanical treatments

This work investigates the hot compressive behaviour of a cast high entropy alloy. The alloy is subjected to four different thermomechanical treatments to refine the as-cast microstructure: hot compression tests at constant strain rates (0.001s−1 and 1s−1), hot compression tests with strain rate jumps (between 0.001s−1 and 1s−1), multi-stage hot deformation with holding intervals of 1 and 5 min, and hot compression at 0.1s−1 followed by annealing. The three first tests were carried out at 1100 °C and the deformation step of the last type was carried out at 1050 °C followed by annealing at 1150 °C. The deformation at 0.001s−1 promotes dynamic recrystallisation. Dynamic recrystallisation occurs when jumping from 1s−1 to 0.001s−1 and does not occur if the jump is from 0.001s−1 to 1s−1. Multi-stage interrupted tests show that 1 min holding between the stages promote dynamic recrystallisation, while static recrystallisation occurs more pronouncedly for 5 min inter-stage holding. Due to the large initial grain size, deformation does not occur homogeneously within the grains, and heterogeneous static recrystallisation occurs within the deformed specimen. Whether the alloy is solution heat treated or deformed in the as-cast condition does not affect the recrystallisation behaviour. Overall, refining the coarse cast microstructure of the investigated alloy is possible, although more deformation steps are needed to produce a homogeneous microstructure.

Size-induced twinning in InSb semiconductor during room temperature deformation

Room-temperature deformation mechanism of InSb micro-pillars has been investigated via a multi-scale experimental approach, where micro-pillars of 2 µm and 5 µm in diameter were first fabricated by focused ion beam (FIB) milling and in situ deformed in the FIB-SEM by micro-compression using a nano-indenter equipped with a flat tip. Strain rate jumps have been performed to determine the strain rate sensitivity coefficient and the related activation volume. The activation volume is found to be of the order of 3–5 b3, considering that plasticity is mediated by Shockley partial dislocations. Transmission electron microscopy (TEM) thin foils were extracted from deformed micro-pillars via the FIB lift-out technique: TEM analysis reveals the presence of nano-twins as major mechanism of plastic deformation, involving Shockley partial dislocations. The presence of twins was never reported in previous studies on the plasticity of bulk InSb: this deformation mechanism is discussed in the context of the plasticity of small-scale samples.

Development of LSTM networks for predicting viscoplasticity with effects of deformation, strain rate and temperature history

Abstract In this paper, long short-term memory (LSTM) networks are used in an original way to model the behavior of a viscoplastic material solicited under changing loading conditions. The material behavior is dependent on history effects of plasticity which can be visible during strain rate jumps or temperature changes. Due to their architecture and internal state (memory), the LSTM networks have the ability to remember past data to update their current state, unlike the traditional artificial neural networks (ANNs) which fail to capture history effects. Specific LSTM networks are designed and trained to reproduce the complex behavior of a viscoplastic solder alloy subjected to strain rate jumps, temperature changes or loading-unloading cycles. The training datasets are numerically generated using the constitutive viscoplastic law of Anand which is very popular for describing solder alloys. The Anand model serves also as a reference to evaluate the performances of the LSTM networks on new data. It is demonstrated that this class of networks is remarkably well suited for replicating the history plastic effects under all the tested loading conditions.

Characterizing plasticity and fracture of sheet metal through a novel in-plane torsion experiment

In-plane shear specimens typically feature free gage section boundaries along which the state of stress deviates from that of pure or simple shear. As a consequence, the geometry of in-plane shear specimens needs to be carefully chosen to avoid any early fracture initiation from the free boundaries, before the actual failure strain for shear is reached at the specimen center. From this perspective, disc specimens for in-plane torsion experiments offer a significant advantage: they do not feature any free boundaries. However, detailed analysis suggests that circular groove need to be introduced (local thickness reduction) to ensure a strain localization away from the clamped specimen shoulders. In most existing in-plane torsion tests, the specimen is clamped on the inner diameter by applying out-of-plane compression to avoid any slipping. In such configurations, it is impossible to monitor the entire sheared circumference with cameras for digital image correlation. It is the goal of the present work to develop in-plane torsion test using grooved specimens with full optical access to the specimen for 2D or 3D DIC measurements. Furthermore, the experimental set-up will be designed for plasticity and fracture characterization at strain rates of up a few 100/s. Its main feature is a new clamping technique. After identifying a suitable specimen geometry through finite element simulations, experiments are performed on specimens extracted from aluminum alloys and steels sheets. The experimental campaign includes proportional loading, reversed loading and strain rate jumps. The full optical access to the sheared gage section area also enables the discussion of the effects of plastic anisotropy on the strain fields in in-plane torsion experiments. The results from the in-plane torsion experiments are also compared with the fracture strain measurements from in-plane shear experiments performed in a conventional uniaxial loading frame.

On the Tensile Tests of Polyurethane and Its Composites with Carbon Nanotubes

Neat polyurethane (PU) specimens and composites of polyurethane with variable amounts of multiwalled carbon nanotubes (MWCNTs) were subjected to tensile tests, stress relaxation tests, and strain rate jumps. Since the already published data about the effect of carbon nanotubes addition to polymer matrix are somewhat contradictory, great care was taken to understand the mechanical properties of neat PU specimens. The studies revealed that the tensile curves of neat PU are substantially influenced by several factors, such as strain rate, age, and thickness of the specimens. The addition of MWCNTs into the PU matrix had a negligible effect on the mechanical properties of composites at low strains (ε < 0.2). With increasing strain, the MWCNTs addition augmented the strength of the composites, and at high local deformations, as in the previously reported indentation experiments, the adhesion strength between functionalized carbon nanotubes and PU matrix substantially influences the mechanical behavior. The material response to the transient mechanical test showed an Arrhenius-type behavior and was analyzed accordingly. The magnitude of the activation volume scaled with the average length of the free segment of the polymer molecules.

Interactions of negative strain rate sensitivity, martensite transformation, and dynamic strain aging in 3rd generation advanced high-strength steels

Medium Mn transformation-induced plasticity (TRIP) steels, a subgroup of third generation advanced high-strength steels (AHSS), often present a combination of both dynamic strain aging (DSA) and TRIP. It has been previously shown that TRIP coincides with the passage of strain bands due to DSA, but their interdependence has not been well-established. This study seeks to explore the interactions between the two phenomena as well as the commonly-observed negative strain rate sensitivity. Digital image correlation (DIC) and in-situ magnetic measurements of the retained austenite fraction were employed to demonstrate the variations in mechanical behavior with increasing strain rate. Strain rate jump tests provided measurements of the strain rate sensitivity and were combined with either DIC to characterize PLC bands or with magnetic measurements to measure variations in the martensite fraction during the jumps. Finally, the micromechanics defining the peculiar bursts of martensite transformation in response to strain rate jumps are discussed in terms of a competition between dislocation mobility and austenite stability.

Portevin-Le Chatelier effect triggered by complex loading paths in an Al–Cu aluminium alloy

ABSTRACTSerrated flow has been observed during non monotonic tensile tests of an Al–Cu aluminium alloy in the naturally aged state. The associated propagative localisation bands were observed by digital image correlation (DIC). In particular, the Portevin-Le Chatelier (PLC) effect and also Lüders bands were observed in interrupted tests during which the specimen was held for a length of time and also in tests with partial unloading followed by a holding time. Increasing strain rate jumps also triggered the PLC effect. These observations indicate the existence of the PLC effect in this material which was formerly considered insensitive to it at room temperature under monotonic loading conditions. There is no evidence of PLC serrations during constant strain rate tests. A strain ageing finite element model is used that captures the experimentally found PLC triggering effects.

Cвойства диаграмм нагружения и разгрузки, порождаемых нелинейным определяющим соотношением типа Максвелла для реономных материалов

В статье продолжены исследование и аттестация физически нелинейного определяющего соотношения типа Максвелла для нестареющих упруговязкопластичных материалов с двумя материальными функциями: комплекса моделируемых им реологических эффектов, области применимости, способов идентификации, настройки и дальнейшей модификации. Аналитически изучены общие качественные свойства семейства кривых нагружения и разгрузки с постоянными скоростями, порождаемых этим соотношением с произвольными материальными функциями в условиях одноосного квазистатического нагружения. Исследованы интервалы монотонности и выпуклости кривых и условия существования точек перегиба на участках нагрузки и разгрузки, величины максимальной деформации, скачков скорости деформации и пластической деформации за цикл нагрузки-разгрузки, их зависимость от максимального напряжения, скорости и длительности нагружения и разгрузки и от характеристик материальных функций. Обнаруженные свойства сопоставлены с типичными свойствами экспериментальных диаграмм нагружения и разгрузки классов реономных материалов с целью выявления феноменологических ограничений на материальные функции, арсенала возможностей и индикаторов (не)применимости определяющего соотношения.

Strain rate sensitivity of sintered silver nanoparticles using rate-jump indentation

Nanoindentation experiments were performed at room temperature on pressure-less sintered silver nanoparticles (AgNP) samples. Two representative die attach solder materials: sintered electrically conductive silver adhesive and conventional Sn-3.0Ag-0.5Cu solder were investigated. A novel technique of multiple strain-rate jumps is adopted accompanied by the continuous stiffness measurement, which can effectively determine the strain rate sensitivity (SRS) with good accuracy. Different strain rates and indentation depths are considered to obtain the nanomechanical properties such as hardness and Young's modulus. Compared with Young's modulus of sintered AgNP, the effect of applied strain rate is more influential to the hardness. In the loading stage, the SRS exponent decreases due to a smaller variation of hardness at a greater indentation depth. In the holding stage prior to the unloading of the applied indentation force, the creep displacement is relatively insensitive to the applied strain rate, however, the creep strain rate decays exponentially and the corresponding stress exponents are determined.

Effect of γ-radiation on the strain characteristics of a high-filled wood-plastic composite

The strain rate and the characteristics of the jumps at micro- and nanolevels were measured by the high-precision interferometric method for a wood-plastic composite irradiated to doses of 0–100 kGy. Radiation was shown to strengthen the material and change the characteristics of strain rate and value jumps. Strain jumps and mean-square deviations of the measured strain rate from its smoothened time dependence were determined for micro- and nanosized jumps. The change of these characteristics depending on the radiation dose of specimens was traced. A relation between the characteristics of micrometer jumps and the macroscopic strain was revealed.

Effect of activation on the porous structure and the strain and strength properties of beech wood biocarbon

The effect of activation on the size, specific volume, and surface area of pores in a monolithic biomorphic material obtained by carbonization of beech wood is studied. It is shown that under optimal activation mode with a steam heated to 970°C, the total pore volume and surface, determined by adsorption curves, increased by 20 and 18 times, respectively. With the use of high-precision interferometric procedure, strain curves are obtained under uniaxial compression with a stepwise loading, and the strain rate is measured with a step of moving of 325 nm for activated and nonactivated samples. Despite an increase in porosity, the strength and maximum deformation of the samples do not decrease. The behavior of the strain rate jumps is analyzed in the micro- and nanometer range. It is shown that the maximum size of the micrometer jumps (4 μm) correlates well with the average size of the possible strain area in the samples (the average distance between the pores of small size), and the minimum dimensions of the strain jumps are close to the size of mesopores. Assessment of the strain change and its rate upon activation indicates that the effect of activation on the strain and strength characteristics is defined by nanometer defects, the most likely of which are microand mesopores.

Strain rate jump induced negative strain rate sensitivity (NSRS) in aluminum alloy 2024: Experiments and constitutive modeling

Negative strain rate sensitivity (NSRS) leading to the strain localization and ultimately failure of material is one of the greatest challenges in the efficient usage of the Al alloys. NSRS in Al-Cu alloy (AA2024) is investigated with the help of uniaxial tension tests at constant strain rate and jump tests conducted with various strain rate jumps for different test temperatures. Digital image correlation (DIC) is adopted to locate the creation and movement of the heterogeneities. Strain controlled jump tests, in combination with DIC, prove to be an excellent tool to investigate NSRS in the alloy. Uniaxial tension tests performed at room temperature (RT) and elevated temperature (50°C) with constant strain rate ranging from 1×10−3 to 1×10−5/sec demonstrated strain rate insensitive deformation behaviour with the absence of serrations and plastic instability. In contrast, strain controlled jump tests conducted at −50°C, RT, and 50°C revealed the presence of NSRS during the strain rate jumps and plastic instabilities following the jump. Local strain mapping achieved from DIC confirmed the propagation of the localized shear bands around the strain rate jumps which is not observed during uniaxial tension tests under constant strain rate. For better understanding of the underlying mechanism kicking off the plastic instabilities, the strain controlled jump test is simulated using the existing phenomenological mesoscopic field dislocation mechanics (PMFDM) model where a dynamics strain aging (DSA) module is embedded in crystal plasticity framework. Numerical predictions of grain size effect suggest that size effect depends on trade off between contributions of polar dislocations (GNDs) to internal stress and plastic deformation, and can lead to an “inverse size effect” when plasticity is the major contribution. While explaining the experimental observations related to jump test, simulation results provided an opportunity to access the experimental aging time which is a crucial material parameter for alloys exhibiting NSRS.

Constitutive modelling of aluminium alloy sheet at warm forming temperatures

The formability of aluminium alloy sheet can be greatly improved by warm forming. However predicting constitutive behaviour under warm forming conditions is a challenge for aluminium alloys due to strong, coupled temperature- and rate-sensitivity. In this work, uniaxial tensile characterization of 0.5 mm thick fully annealed aluminium alloy brazing sheet, widely used in the fabrication of automotive heat exchanger components, is performed at various temperatures (25 to 250 °C) and strain rates (0.002 and 0.02 s-1). In order to capture the observed rate- and temperature-dependent work hardening behaviour, a phenomenological extended-Nadai model and the physically based (i) Bergstrom and (ii) Nes models are considered and compared. It is demonstrated that the Nes model is able to accurately describe the flow stress of AA3003 sheet at different temperatures, strain rates and instantaneous strain rate jumps.

Investigating the Plastic Behavior in Face-Centered Cubic Metals with Strain Rate Jumps

History of loading is the result of interaction of the material microstructure evolution and the loading conditions. The present work aims at an investigation of the behavior in face-centered cubic (FCC) metals under loading with a strain rate jump at different stages. In order to describe the behavior of FCC metals under loading we propose a mathematical model. This model is based on a system of ordinary differential equations (ODEs) and realized as software.

Global and local strain rate sensitivity of bimodal Al-laminates produced by accumulative roll bonding

Laminated metallic composites of 5 mm thickness, with a bimodal grain size distribution and alternating layers of commercial purity aluminum (Al99.5) and high purity aluminum (Al99.999) were successfully processed by accumulative roll bonding at room temperature up to 8 cycles. With increasing number of ARB cycles the layers of commercial purity aluminum become ultrafine-grained, whereas the grain size in the high purity aluminum remains clearly above 1 μm. The mechanical properties of the laminates were investigated by compression testing with strain rate jumps and by nanoindentation using continuous stiffness method and nanoindentation strain rate jump tests. Thereby the strain rate sensitivity was determined both globally, proving the bulk laminate material, and locally, proving the single layers of the laminate. It was found that the local strain rate sensitivity of the ultrafine-grained Al99.5-layers is increasing up to 4 ARB cycles, while the local strain rate sensitivity of the coarse grained Al99.999-layers remains rather constant independently of the number of cycles. In contrast to that, the global strain rate sensitivity of the bulk laminate increases up to 8 cycles without saturation. Thermally activated annihilation of dislocations at the grain boundaries and interfaces between the coarse grained and ultrafine-grained layers is proposed in order to interpret the findings.

Effect of polypropylene fiber processing conditions on fiber mechanical behavior

An investigation into the mechanical behavior of melt‐spun isotactic polypropylene (iPP) fibers is reported. Two different iPP formulations, PH835 and Exxon3854, synthesized using Ziegler–Natta and metallocene catalysts, respectively, and spun at take‐up velocities ranging from 1000 to 3000 m min−1 were subjected to uniaxial tensile loading, cyclic loading and creep tests. The strain rate sensitivity was determined by performing strain rate jumps. Injection molded specimens from the same iPP formulations were tested under the same conditions. The fiber birefringence increases slightly with increasing take‐up velocity, while the crystallinity is approximately insensitive to this process parameter in this range of velocities. Fibers from the two iPP samples behave differently at large plastic strains despite having the same birefringence and crystallinity. Differences are also seen in creep. The behavior of fibers is significantly different from that of the injection molded samples of the same iPP and same crystallinity. These have lower strain hardening rate, smaller failure strains, close to zero strain rate sensitivity and exhibit a yield point phenomenon. The difference is associated with the different nature and spatial organization of the crystals and inter‐crystalline amorphous and mesomorphic phases. © 2014 Society of Chemical Industry

Experimental Study of the Mechanical Behavior of Materials under Transient Regimes of Superplastic Deforming

Transient regimes of deforming are always present in any technological process and can be taken into account and used more widely if properly studied. The behavior of the materials under such regimes is becoming even more interesting if initial microstructure is coarse grained and undergoes transformation in the process of deforming. One of the transient processes which happen in any Superplastic deformation is the initial stage of loading, before steady superplastic flow starts. Initial parts of stress-strain curves during superplastic deformation are not frequently studied experimentally but provide very important information about mechanical properties of material. They are also necessary for development and verification of the constitutive equations. The results of experimental analysis of the behaviour of titanium alloys under superplastic conditions at the initial stages of loading and also under unloading are presented here. Another type of transient regimes of deforming is represented by the strain rate jumps. In such kind of experiments if the amplitudes of the jumps are big enough, the shifts of the corresponding parts of the stress-strain curves about the basic ones (hardening or softening) can be observed depending on the amplitude of the jump and microstructure of the material. Some experimental results related to this effect are discussed in this paper. The applicability of some constitutive equations for description of the observed results is discussed. The necessity of involving visco-elastic properties of material for proper description of its behavior in some regimes of deforming is also mentioned.

On the criterion for compensation to avoid elastic–plastic transients during strain rate change tests

Determination of the apparent activation volume and the instant strain rate sensitivity from strain rate change tests is a non-trivial task due to the short elastic–plastic transient. The flow stress can be back-extrapolated towards the incipient value, or the transient can be eliminated by a compensation technique consisting of an instant displacement introduced exactly at the moment of strain rate change. The criterion for determination of the correct amount of compensation is derived analytically. It was found that the established “experimental” criterion for the level of compensation gives an error of up to 20% in activation volume for soft tensile test rigs and affects the Haasen plot. It is shown that back-extrapolation of the stress gives the correct compensation. However, undesired changes in dislocation density may occur and modify the transients. A combination of slightly under-compensated strain rate jumps and back-extrapolation is recommended.

High temperature deformation of a Cu 40Zr 44Ag 8Al 8 bulk metallic glass

The deformation behavior of a Cu 40Zr 44Ag 8Al 8 bulk metallic glass in its supercooled liquid region was investigated in compression. Strain rate jumps were carried out at various temperatures ranging from about T g to T g + 20 K. The results show that the alloy exhibits quasi-Newtonian behavior at the highest temperature with a transition to non Newtonian flow when temperature decreases and strain rate increases. The mechanical behavior has been analyzed in the framework of the free volume model. The material has been characterized after deformation at T g + 20 K by XRD and comparison has been made with the same material simply held at high temperature for the same duration. It was observed that deformation slightly accelerates crystallization which results in a very important hardening on the stress–strain curve. However, even if there is no hardening at high temperature which suggests that no crystallization has occurred, some hardening is observed at room temperature even for short holding times at high temperature.