Strong Strain Rate Sensitivity Research Articles

Quasi-Static and Dynamic Compressive Properties of AZ31 Magnesium Alloy Processed by Equal Channel Angular Pressing

Equal channel angular pressing (ECAP) was applied to control the microstructure of AZ31B magnesium alloy. Compression properties of both as-received and grain refined AZ31B alloy were investigated under quasi-static and dynamic loadings. The yield strength, ultimate strength, failure strain and flow stress under compression loading were studied. Both the as-received alloy and ECAPed alloy show strong strain rate sensitivity. Grain size, texture and strain rate jointly affect the deformation mechanisms. Through fractography analysis, the fracture mechanism was analyzed.

Orientation-dependent hardness and strain rate sensitivity in nanotwin copper

We observe a strong twin-orientation dependent hardness and strain rate sensitivity (m) in nanotwin copper. A highest m value of 0.059 ± 0.004 and an activation volume (V) of ∼10b3 are measured when deformation is predominately vertical to twin boundaries (TBs), whereas a much smaller m and larger V are observed for the direction parallel to TBs. Dislocation density is found to have a stronger impact on m and V in nanotwin materials, compared to that in coarse-grained materials.

Compression properties and work-hardening effect of the NiAl-matrix composite with TaB2 and TaB

The microstructure, compression property and work-hardening effect of the NiAl-matrix composite reinforced by 5 vol.% ceramic particulates (TaB and TaB2) were investigated. The compression properties and work-hardening capacity (Hc) under the different strain rates were also investigated. The NiAl-matrix composite has strong strain rate sensitivity. A high true ultimate strength of 1399 MPa, a true fracture strain of 18.7% and Hc = 1.84 were obtained under the strain rate of 1.0 × 10−4 s−1. The good compression properties may be attributed to the refinement of the grain size and the good combination between the ceramic particulates and the NiAl-matrix.

Effect of different strain rates on compression property and work-hardening behavior for the NiAl-matrix composite with 1.7wt.% NbB2 and NbxC

Abstract The microstructure, compression property and work-hardening effect of the NiAl-matrix composite with 1.7 wt.% NbB2 and NbxC (x = 1 or 2) fabricated by the combustion synthesis and hot pressing technique have been investigated in the study. The NiAl-matrix composite has strong strain rate sensitivity. The yield strength of the NbB2–NbxC/NiAl composite decreases while the fracture strain and the work-hardening capacity (Hc) increase with decreasing strain rate. A high true ultimate strength of 1472 MPa, a fracture strain of 20.1% and Hc = 1.41 were obtained under the strain rate of 2.0 × 10−5 s−1.

The effect of temperature and strain rate on the tensile properties of a Sn 99.3Cu 0.7(Ni) lead-free solder alloy

In this paper, the effect of temperature and strain rate of the lead-free solder alloy Sn 99.3Cu 0.7(Ni) was investigated. Tensile properties of this lead-free solder Sn 99.3Cu 0.7(Ni) at various temperatures and strain rates were determined and compared with those of the typical Pb-containing solder Sn 63Pb 37. During tensile tests of different temperatures and strain rates, the ultimate tensile strength (UTS) and 0.2% yield stress of the lead-free solder alloy Sn 99.3Cu 0.7(Ni) decreased with increasing test temperatures and decreasing strain rate. It was noted that mechanical properties of Sn99.3Cu0.3(Ni) showed strong temperature dependence and strain rate sensitivity in the tensile tests of various temperatures and strain rates. Temperature and strain rate impacted rigorously flowing behaviors of the lead-free solder Sn 99.3Cu 0.7(Ni) in the course of tensile deformation. The microstructure and fracture morphology of the lead-free solder Sn 99.3Cu 0.7(Ni) were analyzed by scanning electron microscope (SEM). In the SEM micro-graph of the fracture surface of this lead-free solder specimen tested, it was noted that increasing temperature affected the microstructure and morphology of fracture surface.

Gas-pressure Forming of an AlMg-Alloy Sheet at Elevated Temperatures

Forming of automotive leightweight parts using aluminium offers numerous advantages. Compared to other wrought aluminium alloys, in particular AlMg-alloys generally show a good formability which is favourable for the production of complex parts. However, forming of Mg-containing alloys at room temperature leads to yielding patterns preventing their implementation for class-A-surface applications. Furthermore, the formability of steel still exceeds that of AlMg-alloys at room temperature. Thus, in the present study, sheet metal forming is applied at a temperature range that is typical for warm forming. It is supposed to profit from the advantages of warm forming like high achievable strains and improved surface quality of the formed part, while not having the disadvantages of long production times and high energy consumption, which is correlated with superplastic forming. Applying fluid-based sheet metal forming in this paper, nitrogen is used as fluid working medium to satisfy the demand on high temperature resistance. Concerning the blank material used, formability of Mg-containing aluminium alloys shows strong strain rate sensitivity at elevated temperatures. To figure out the optimal strain rates for this particular process, a control system for forming processes is developed within the scope of this paper. Additionally, FE-simulations are carried out and adapted to the experiment, based on the generated process data. FE-investigations include forming of domes (bulging) as well as shape-defined forming, having the objective to increase formability in critical form elements by applying optimal strain rates. Here, a closed-loop process control for gas-pressure forming at elevated temperatures is to be developed in the next stages of the project.

The Mechanical Properties in the Vicinity of Grain Boundaries in Ultrafine-Grained and Polycrystalline Materials Studied by Nanoindentations

AbstractThe strength of structural materials strongly depends on the structure and properties of grain boundaries. Interfaces usually act as barriers to dislocation motion and therefore strengthen materials with decreasing grain size, quantitatively described by the well-known Hall-Petch relation. However, interfaces in nanocrystalline materials are often covered with impurities or second phases, which may influence the mechanical properties. With nanoindentation testing it is now possible to probe the strength of interfaces like grain boundaries directly on a nanometer scale. Therefore this method was used to investigate the properties in the vicinity of grain boundaries in polycrystalline materials with conventional grain size and in ultrafine-grained metals prepared by equal channel angular pressing (ECAP), where no impurities are introduced during processing. Measurements on an austenitic steel clearly show a decreasing hardness close to the interface opposite to the general expected behavior of strengthening. In this case segregation effects strongly influence the mechanical properties near the boundaries. The nanoindentation investigations on ultrafine-grained Al and Cu show a strong strain rate sensitivity. Inelastic effects are also found between unloading-loading segments during indentations.

The Mechanical Properties in the Vicinity of Grain Boundaries in Ultrafine-Grained and Polycrystalline Materials Studied by Nanoindentations

AbstractThe strength of structural materials strongly depends on the structure and properties of grain boundaries. Interfaces usually act as barriers to dislocation motion and therefore strengthen materials with decreasing grain size, quantitatively described by the well-known Hall-Petch relation. However, interfaces in nanocrystalline materials are often covered with impurities or second phases, which may influence the mechanical properties. With nanoindentation testing it is now possible to probe the strength of interfaces like grain boundaries directly on a nanometer scale. Therefore this method was used to investigate the properties in the vicinity of grain boundaries in polycrystalline materials with conventional grain size and in ultrafine-grained metals prepared by equal channel angular pressing (ECAP), where no impurities are introduced during processing. Measurements on an austenitic steel clearly show a decreasing hardness close to the interface opposite to the general expected behavior of strengthening. In this case segregation effects strongly influence the mechanical properties near the boundaries. The nanoindentation investigations on ultrafine-grained Al and Cu show a strong strain rate sensitivity. Inelastic effects are also found between unloading-loading segments during indentations.

Investigation of dynamic flow and strength properties of Ti-6-22-22S at normal and elevated temperatures

This paper is concerned with the material behavior of Ti-6-22-22S titanium alloy as a function of strain rate and temperature. Results of measurements of the material behavior at compressive loading under uniaxial stress conditions over a strain rate range of 10 −4 s −1 to 10 3 s −1 and under uniaxial strain conditions at shock-wave loading with a strain rate of ∼10 5 s −1 are presented. The test temperature was varied from 20°C to 620°C. The uniaxial stress compression tests revealed a rather strong strain rate sensitivity and temperature dependence of failure and flow stresses. In plate impact tests, the dynamic yield stresses at plastic deformations of 0% and 0.2% and dynamic strength (spall strength) were measured as a function of temperature. The strain-rate and the temperature dependencies of the yield stress obtained from the uniaxial stress tests and from the shock-wave experiments are in good agreement and indicate that the thermal activation mechanism of plastic deformation of the alloy is maintained at strain rates up to 10 5 s −1 at least. The relative decrease in the yield strength in the plate impact tests much exceeds the spall strength decrease. That is considered as an evidence, that the spall strength magnitude is influenced more by the contribution of the nucleation of voids rather than by the growth of voids.

Plastic deformation of CVD textured tungsten—I. Constitutive response

The constitutive response of CVD textured tungsten in 〈001〉, 〈011〉 and random orientations under uniaxial compression subjected to a range of strain rates is investigated. Both 〈001〉 and 〈011〉 textured specimens revealed a strong strain rate sensitivity. The 〈001〉 specimens showed large ductility at all strain rates and significant strain softening during high strain rate deformation. The 〈011〉 texture undergoes homogenous deformation under quasi-static loading but failed prematurely due to extensive grain boundary cracking during high strain rate loading. The random orientation specimens exhibited brittle failure. The temperature rise during high strain rate deformation was measured using high speed infrared detectors. A physically based crystal plasticity model motivated by dislocation reactions in b.c.c. metals is outlined. The model predicts the dependence of constitutive response on the texture in tungsten polycrystals and suggests that the transverse tensile stresses which develop in 〈011〉 orientation due to the crystallographic asymmetry could be responsible for observed grain boundary cracking.

Stress-strain properties of structural steel at strain rates of up to 10 5 per second at sub-zero, room and high temperatures

A testing technique and method of processing the displacement-time data have been developed following which the stress-strain characteristics of structural steel at strain rates between 10 3 to 10 5 per second over a strain of about 50% and at different temperatures have been determined. The steel under present test condition within this strain rate range showed a strong strain rate sensitivity. The material inertia and temperature rise during high speed deformation were found to have mutually cancelling effect on the deduced flow stress. In determining the results, appropriate friction correction was also made and the results presented in this paper are all converted to those under frictionless condition. Finally, a constitutive equation has been proposed for the steel incorporating the effects of work-hardening and strain rate sensitivity of the material.

Experiments on dynamic plastic loading of frames

Tests are described on plane frames of mild steel and titanium (commercial purity) in which high intensity short duration pressure pulses were applied transversely to the beam member either uniformly over this member or concentrated at its center. The objective was to examine applications of two estimation techniques (upper bounds on deflections and the mode approximation technique) for major response features of pulse loaded structures at large deflections, taking account of strong plastic strain rate sensitivity. Loads over a range such as to cause final deflections up to about a third of the span were applied by detonating explosive sheet. Agreement between estimated and measured final deflections was often very good (generally conservative) but the intrinsic error of the mode technique was not observed as expected.

High Strain Rate Behaviour of Iron and Nickel

An investigation into the mechanical behaviour of iron and nickel at high strain rates is carried out, using a split Hopkinson bar method. Some special adaptations, a correction method for the effects arising from the adiabatic conditions of dynamic deformation and a simplified data processing procedure are described in detail. The test conditions covered a range of strain rates between 500 and about 10 000/s and temperatures from 20 to 500°C. For both metals, the results are presented by means of a family of true stress-true strain curves. The strong strain rate sensitivity at high strain rates indicates that the rate controlling mechanism differs from that operative at ‘static’ strain rates.