Strain Rate Sensitivity Behavior Research Articles

Environmental embrittlement of two-phase Fe30Ni20Mn35Al15

It is shown that the ductility of lamellae-structured Fe30Ni20Mn35Al15 (in at. %), which consists of B2 and f.c.c. phases, is influenced by testing environment. Tensile tests performed in air at strain rates ranging from 3 × 10−6 to 3 × 10−1 s−1 showed that the elongation to fracture and ultimate tensile strength (UTS) increased with increasing strain rates below 3 × 10−3 s−1, and were independent of strain rate at ∼10.5% and 840 MPa for strain rates ≥ 3 × 10−3 s−1. In order to understand this strain-rate sensitive behavior, tensile tests were also performed in either dry oxygen or 4% hydrogen + nitrogen at different strain rates. The elongation and UTS in oxygen were insensitive to strain rate and close to those tested at 3 × 10−3 s−1 in air, whereas the elongation in hydrogen was 4% for strain rates ≤3 × 10−3 s−1 and increased to ∼10.8% at 3 × 10−1 s−1. The reduction of ductility in air and hydrogen-charged environment at low strain rate is attributed to hydrogen embrittlement.

A new micromechanics-based scale transition model for the strain-rate sensitive behavior of nanocrystalline materials

An original two-step “three phase” elastic–viscoplastic scale transition model is developed based on the combined self-consistent and Mori–Tanaka schemes. A coated inclusion is embedded within a matrix, wherein the inclusion represents grain interiors and the coating of the inclusion mimics the effects of grain boundaries and triple junctions. The predominant behavior within the grain interiors is captured through dislocation glide, whereas grain boundary (GB) dislocation emission and absorption, as well as thermally assisted GB sliding, describe the deformation processes within the coating describing the GB affected zone. Furthermore, an imperfect interface is assumed between the inclusion and the coating to account for viscoplastic grain boundary sliding along a stick-slip mechanism. Results and discussion focus on the competitive roles of GB sliding, GB dislocation emission/absorption, dislocation sweep in grain cores and collective dislocation plasticity, and the origins of the pronounced strain rate sensitivity of fcc NC materials.

Influence of Pre‐Straining and Bake Hardening on the Strain Rate Sensitivity of Automotive Sheet Steels

AbstractThe present investigation deals with the influence of pre‐straining with or without bake hardening on the strain rate sensitivity of automotive sheet steels in typical crash conditions. The strain rate sensitivity m has been determined by means of dynamic tensile tests in the strain rate range 0.005‐1000 s−1 and in the temperature range 233‐373K. A bake hardening heat treatment at 170 °C for 20 min without pre‐straining does not influence the m‐value in comparison to the base material condition. A small pre‐straining near plane strain condition, as commonly found in outer door panels, or a 10% uniaxial, plane strain and biaxial pre‐straining, as typically used in formed automotive crash components, without bake hardening does not affect the m‐value of sheet steels in comparison to the base material condition. Uniaxial 2% to 10% pre‐straining, longitudinal or transverse to rolling direction with subsequent bake hardening, does not clearly change the m‐value in comparison to the base material condition either. Small differences in the strain rate sensitivity behaviour are rather attributed to experimental scattering without real physical background.

Macro‐ and Nanomechanical Properties and Strain Rate Sensitivity of Accumulative Roll Bonded and Equal Channel Angular Pressed Ultrafine‐Grained Materials

AbstractSeveral processes of severe plastic deformation are suitable for the production of materials with ultrafine‐grained microstructures which are known to exhibit high strength and often good ductility as well as strain rate sensitive behavior. The most promising ones are equal channel angular pressing (ECAP) for bulk material and accumulative roll bonding (ARB) for the production of sheet material. In order to evaluate the influence of the process on these mechanical properties and the strain rate sensitivity, tensile tests, and nanoindentation tests were performed on material produced up to similar effective plastic strains of εARB = 6.4 and εECAP = 6.3. It could be shown that the macroscopic strength is slightly higher for ARB than for ECAP material and vice versa in nanoindentation. Independent of the testing method, the strain rate sensitivities and activation volumes are similar for both materials. Thus, both processes performed up to similar effective plastic strains lead to comparable improvements in the mechanical properties. Additionally it could be shown, that this comparison allows the identification of the dominant deformation mechanism which is responsible for the observed strain rate sensitivity.

Experimental investigation and modeling of non-monotonic creep behavior in polymers

The deformation behavior of two unfilled engineering thermoplastics, ultra high molecular weight polyethylene (UHMWPE) and polycarbonate (PC), has been investigated in creep test conditions. It has been found that a loading history (prior to the creep test) comprising of loading to a maximum stress or strain value followed by partial unloading to arrive at the target stress value can greatly modify the strain-time behavior. Under such a test protocol, while the expected increase in strain during creep (constant tensile load) is observed, at relatively low creep stresses specimens have also demonstrated a monotonic decrease in strain. In an intermediate stress range, specimens have demonstrated time dependent behavior comprising of a transition from decreasing to increasing strain during creep in tension. This paper presents experimental results to delineate these findings and explore the effect of prior strain rate on the qualitative and quantitative changes in the output (strain-time) behavior. Furthermore, modification of the viscoplasticity theory based on overstress (VBO) model into a double element configuration is introduced. These changes confer upon the model the ability to yield non-monotonic behavior in creep, and supporting simulation results have been included. These changes, therefore, allow the model to simulate strain rate sensitivity, creep, relaxation, and recovery behavior, but more importantly address the issue of non-monotonic changes in creep and relaxation when a loading history involves some degree of unloading.

Dynamic Characterization of Orthogneiss Rock Subjected to Intermediate and High Strain Rates in Tension

The dynamic characterization of rocks under intermediate and high strain rates is fundamental to understand the material behavior in case of heavy earthquakes and dynamic events. The implementation of material constitutive laws is of capital importance for the numerical simulation of the dynamic processes as those caused by earthquakes. These data are necessary and require experimental techniques able to induce on the rock materials state of loading reproducing the actual dynamic condition. The dynamic characterization has been carried out by means of two special apparatus: the split Hopkinson tension bar and the hydro-pneumatic machine. These equipments are briefly described with a discussion on the results of dynamic tension tests at three different strain rates (0.1, 10, 100 strain/s) on Onsernone Orthogneiss for loading directions 0°, 45° and 90° with respect to the schistosity. Results of the tests show a significant strain rate sensitive behavior, exhibiting dynamic tensile strength increasing with strain rate, up to about two times with respect to the quasi-static conditions in the case of 0° and 45° orientation and more than three times in the case of 90° at high strain rates. Dynamic increase factors versus strain rate curves for tensile strength were also evaluated and discussed.

Dynamic strain aging of a commercial Al–Mg–Si–Cu alloy during equal channel angular extrusion process

In this research, the occurrence of dynamic strain aging (DSA) phenomenon during equal channel angular extrusion (ECAE) of a commercial air cooled Al–Mg–Si–Cu alloy is investigated. In order to detect the negative strain rate sensitivity behaviour associated with DSA, samples were ECAE-ed at various temperatures and strain rates. Subsequently, the extruded billets were subjected to mechanical and physical tests and it was found that at a certain ECAE temperature range there is a critical strain rate above which DSA occurs. The implication of the results to strengthening of dilute solid solutions is discussed.

A cyclic viscoplastic and creep damage model for lead free solder alloys

Abstract The reliability of microelectronic components under cyclic thermomechanical loading is an important problem especially for new leadfree solder alloys. To investigate the low cycle fatigue strength of solder joints, material models are required, that can describe the constitutive inelastic deformation and damage behavior of solder materials. Such models form the basis for advanced numerical analyses by the finite element method. In the present contribution an appropriate material model that combines the viscoplastic constitutive model of Chaboche-type with the damage law of A.C.F. Cocks for porous creep will be introduced. The algorithm is reported for an implementation as a user defined material subroutine into the FEM-code ABAQUS®. The necessary parameters of the material model are identified using results of miniaturized double lap-shear experiments and tensile tests for a Sn96Ag3Cu1 solder alloy at various temperatures. The comparison of experimental and numerical results shows a good agreement with respect to strain rate sensitivity, relaxation and damage behavior of the investigated solder material. Finally, some numerical applications to surface mounted microelectronic devices are presented.

Effect of Acrylonitrile-Butadiene-Styrene High-Rubber Powder and Strain Rate on the Morphology and Mechanical Properties of Acrylonitrile-Butadiene-Styrene/Poly (Methyl Methacrylate) Blends

Acrylonitrile-butadiene-styrene (ABS) high-rubber powder (HRP) toughened acrylonitrile-butadiene-styrene (ABS)/polymethyl methacrylate (PMMA) blends were prepared in a co-rotating twin screw extruder. The effect of ABS HRP on mechanical and morphology properties of ABS/PMMA blends was studied. It is shown that the toughness of ABS/PMMA blends was improved effectively with the incorporation of ABS HRP, while the elastic modulus and tensile strength decrease slightly. Tensile tests at different strain rates were carried out in order to investigate the influence of strain rate on mechanical properties of various blends. Finally, the strain rate sensitive behavior of different blends was explored by using the Eyring model.

Experimental and computational investigations on strain rate sensitivity and deformation behavior of bulk materials made of epoxy resin structural adhesive

The deformation behavior and strain rate sensitivity of an epoxy resin structural adhesive and a CTBN (carboxyl-terminated butadiene-acrylonitrile)-modified epoxy resin adhesive are experimentally investigated using an INSTRON-type material testing machine and a split Hopkinson pressure bar apparatus. The experimental results show some fundamental features of a typical compressive stress–strain behavior of amorphous glassy polymers with linear elastic and nonlinear inelastic deformation stages. In the inelastic deformation, a peak stress and a strain-softening stage after the peak can be observed in the entire range of strain rate from 10 - 4 to 10 3 s - 1 . In addition, it can be found that the relationship between the peak stress and strain rate on a semi-logarithmic plot is linear in the range of low strain rate. However, the slope of the curve changes at a high strain rate, and the nonlinear behavior of the peak stress can be obtained against the strain rate. In order to describe such a nonlinear peak stress–strain rate relationship and the deformation behavior of the structural adhesives in a wide range of strain rate on the basis of the experimental results, a plastic shear strain rate is formulated. Then, a three-dimensional constitutive model is derived based on a four-elements model with an elastic series element by considering the adhesive to be a glassy polymer. The plastic deformation rate tensor is expressed by the effective stress, which is the difference between the total stress and back stress, and the plastic shear strain rate proposed here. The stress tensor can be obtained by solving nonlinear simultaneous equation. The formulated constitutive model is implemented into the commercial FE code, ABAQUS/Explicit, and then a computational simulation is performed. As a result, the validity of the proposed model is shown by comparing the experimental result.

Split Hopkinson pressure bar multiple reloading and modeling of a 316 L stainless steel metallic hollow sphere structure

The high strain rate (600 s −1) compression deformation of a 316 L metallic hollow sphere (MHS) structure (density: 500 kg m −3; average outer hollow sphere diameter: 2 mm and wall thickness: 45 μm) was determined both numerically and experimentally. The experimental compressive stress–strain behavior at high strain rates until about large strains was obtained with multiple reloading tests using a large-diameter compression type aluminum Split Hopkinson Pressure Bar (SHPB) test apparatus. The multiple reloading of MHS samples in SHPB was analyzed with a 3D finite element model using the commercial explicit finite element code LS-DYNA. The tested MHS samples showed increased crushing stress values, when the strain rate increased from quasi-static (0.8 × 10 −4 s −1) to high strain rate (600 s −1). Experimentally and numerically deformed sections of MHS samples tested showed very similar crushing characteristics; plastic hinge formation, the indentation of the spheres at the contact regions and sphere wall buckling at intermediate strains. The extent of micro-inertial effects was further predicted with the strain rate insensitive cell wall material model and with the strain rate sensitive behavior of MHS structure similar to that of the cell wall material. Based on the predictions, the strain rate sensitivity of the studied 316 L MHS sample was attributed to the strain rate sensitivity of the cell wall material and the micro-inertia.

Warm forming simulation of Al–Mg sheet

The accuracy of warm forming simulations depends to a large extend on the description of the yield surface with temperature and strain-rate dependent hardening and on the modeling of friction. In this paper, the anisotropic behavior of the sheet is described by using the Vegter yield locus, which is purely based on experimental measurements. For work hardening, the dislocation based Nes model is used, in which the evolution of microstructure is defined by three internal state variables. The model incorporates the influence of the temperature and strain rate effect on the flow stress by means of the storage and dynamic recovery of dislocations. It is demonstrated that the Nes model is able to describe the flow stress of Al–Mg alloys up to 250 ° C at different strain rates. It also represents the negative strain rate sensitivity behavior of Al–Mg alloys at temperatures below 125 ° C. The simulation of uniaxial tensile tests shows that the model is able to predict the strain localization. Cylindrical cup deep drawing simulations are presented using shell elements. Data from experimental deep drawing tests is used to validate the modeling approach, where the model parameters are determined from tensile tests.

Preparation and Properties of PP/PLA /Multiwall Carbon Nanotube Composites Filaments Obtained by Melt Compounding

The present paper studied the thermal and mechanical properties of atmospheric pressure plasma jet (APPJ) treated multiwall carbon nanotubes/polypropylene/polylactic acid nanocomposite filaments. The experiments included tensile tests, differential scanning calorimeter (DSC), Scanning electron microscopy (SEM) experiments. DSC studies showed that there were a distinct shift in Tg and a relatively moderate change in Tm for different systems. The activation volumes of CNTs/PP/PLA nanocomposite filaments have been calculated to describe strain rate sensitive behavior of CNTs/PP/PLA nanocomposite filaments by following Eyring’s equation based on the tensile test results.

Dynamic behavior of a Mediterranean natural stone under tensile loading

Historical buildings are important structures commonly occurring in Mediterranean cities. The behavior of their constituent materials under high dynamic loads is fundamental to investigate the vulnerability of such structures under extreme dynamic events. The main aim of our investigation was to study the effect of high dynamic loading conditions on a classical porous natural stone from the Naples area, namely yellow tuff, used in hundreds of historical buildings and monuments in Naples and other Mediterranean cities. Hence, dynamic characterization was performed through high strain-rate failure tensile tests. A wide range of strain-rates was investigated, from 10 −5 s −1 to 50 s −1. The obtained data were processed to obtain stress–strain relationships at different strain-rate levels. The results reveal that Neapolitan yellow tuff presents a significantly strain-rate sensitive behavior, exhibiting dynamic tensile strength increasing with strain-rate, up to about three times that from quasi-static conditions in the case of very high strain-rates. Dynamic increase factors (DIFs) vs. strain-rate curves for tensile failure stress were also evaluated and discussed.

Effect of strain rate on tensile behavior of polypropylene and carbon nanofiber filled polypropylene

In the present paper, vapor-grown carbon nanofibers were mechanically dry-mixed with polypropylene powder and extruded into filaments with a single screw extruder. Then tensile tests were performed on the single filament at a strain rate range from 0.02 min −1 to 2 min −1. Results indicate that both neat and nanophased polypropylene were strain rate-strengthening material. The tensile modulus and yield strength both increased with increasing strain rate. Results also show that infusing polypropylene with nanofibers increases tensile modulus and yield strength, but decreases ductility. Finally, based on the tensile test results, a nonlinear constitutive equation was developed to describe strain rate-sensitive behavior of neat and nanophased polypropylene.

Simulation of the strain rate sensitive flow behavior of SiC-particulate reinforced aluminum metal matrix composites

Strain rate dependent compression mechanical behavior of an SiC-particulate reinforced Al (2024-O) metal matrix composite (MMC) with different particle volume fractions was numerically investigated at various strain rates. Calculations were performed using axisymmetric finite element unit cell model, in which an elastic SiC particle was embedded inside a strain rate sensitive viscoplastic Al matrix. Stress–strain curves of Al matrix material were derived from Split Hopkinson Pressure Bar experiments at various strain rates and used as inputs in the FEM model. Numerically computed stress–strain curves and strain rate sensitivity were compared with those of experiments for a 15% SiC-particulate reinforced MMC. Computed strain rate sensitivity of the MMC was found to be higher than that of the matrix alloy and increased with increasing strain contrary to the strain independent matrix strain rate sensitivity. The strain rate sensitivity of the MMC was also found to increase with increasing particle volume fraction at the same particle size. Finally, several possible reasons including assumptions used in the model, adiabatic heating, microstructural variations between the composite matrix and matrix alloy, particle shape and distribution and damage accumulation for the small discrepancy found between computed and experimental stress–strain curves and strain rate sensitivity of the composite were discussed.

Negative strain rate sensitivity in ultrahigh-strength nanocrystalline tantalum

Nanocrystalline tantalum prepared through direct current magnetron sputtering exhibits a negative strain rate sensitivity behavior under nanoindentation; i.e., the hardness value of nanocrystalline tantalum decreases with increasing indentation loading rates. High resolution transmission electron microscopy reveals a beta- to alpha-phase transformation underneath the indents, suggesting that the pseudohexagonal to body-centered cubic structural transformation in nanocrystalline tantalum is likely pressure induced. Evidence of shear banding was documented along the edge of indents, consistent with the negative strain rate sensitivity observed.

Effect of SiO 2 nanoparticle on thermal and tensile behavior of nylon-6

The present study investigates the effect of SiO 2 nanoparticle on the thermal and mechanical properties of nylon-6. Firstly, nanoparticles with 0, 1, and 2 wt.% loading were dry-mixed with nylon-6 by mechanical means and extruded into filaments using a single screw extruder. Then, tensile tests were performed on the single filament at the strain rate range of 0.02–2 min −1. Experimental results show that both neat and nanophased nylon-6 were strain rate strengthening materials. The tensile modulus, yield strength, and ultimate tensile strength (UTS) all increased with increasing strain rate. Experimental results also show that infusing nanoparticles into nylon-6 can increase tensile modulus, yield strength, hardening modulus, and UTS. Forty-five percent enhancement in tensile modulus and 26% enhancement in UTS were observed in the 2 wt.% system as compared with neat nylon-6. At the same time, thermal properties of neat and nanophased nylon-6 were characterized by TGA and DSC. TGA thermograms have shown that the nanoparticle infused systems are more thermally stable. DSC studies also have indicated that there is a moderate increase in T g without a distinct shift in the melting endotherm for the nanophased systems. At last, based on the tensile test results, a nonlinear constitutive equation was developed to describe strain rate sensitive behavior of neat and nanophased nylon-6.

Some recent developments in the dynamic inelastic behaviour of structures

This article contains a brief review of some recent developments on the response of structures subjected to large dynamic loads, which produce large inelastic strains. It is focussed on topics which are of interest in the design of naval architecture and ocean engineering structures. In particular, comments are offered on the strain rate sensitive behaviour of materials, including various high strength steels, dynamic rupture strains, and the dynamic inelastic failure of structures. Other topics discussed briefly include the dynamic energy absorbing characteristics of structures, scaling, and the response of fibre metal laminates.

Strain rate sensitive behavior of wollastonite-reinforced ethylene–propylene copolymer composites

Abstract A comparative study of the effect of strain rate on plastic deformation behavior of neat ethylene–propylene copolymer (EP) and wollastonite-reinforced ethylene–propylene copolymer composite (EP-W) is made in terms of strain rate sensitivity index. EP and EP-W exhibit low- and high-strain rate sensitive regimes characterized by distinct deformation features. EP is characterized by wedging that leads to predominantly fibrillated mode of fracture in association with ductile ploughing, while EP-W is characterized by wedge and ridge micromechanism of deformation. The final fracture in EP-W occurs by a combination of fibrillated and brittle mode of deformation.