High Speed Material Testing Machine Research Articles

Dynamic Hardening of AISI 304 Steel at a Wide Range of Strain Rates and Its Application to Shot Peening Simulation

The hardening behavior of AISI 304 steel is investigated at various strain rates, from the quasi-static state to ultra-high strain rates, because it is necessary for numerical simulation of high-speed deformation problems. This kind of testing at a wide range of strain rates has not been yet reported in the literature although it is indispensable for accurate numerical analyses where deformation takes place with a wide spectrum of strain rates. AISI 304 steel is a kind of austenitic stainless steel used in various engineering fields, which does not harden by heat treatment, but by cold working such as shot peening. In order to obtain hardening properties at each strain rate, tensile tests were carried out using a universal testing machine of the INSTRON 5583 for the quasi-static state, a high speed material testing machine of a servo-hydraulic type for intermediate strain rates, and a tensile split-Hopkinson bar for high strain rates with a digital image correlation method. The hardening properties at the ultra-high strain-rate region were obtained from Taylor impact test results by calibration with an experimental–numerical hybrid inverse optimization for reliable extrapolation results. Finally, the hardening flow stress curves were obtained at various strain rates from the quasi-static state to ultra-high strain rates by interpolating the data with the extended Lim–Huh model. The result shows that the yield stress of 759 MPa at the quasi-static state increased to 1429 MPa at a strain rate of 106 s−1, which is about 1.9 times of the quasi-static yield stress. As a demonstration example, the dynamic hardening properties obtained were applied to a shot peening simulation that required hardening curves at a wide range of strain rates from the static state to 106 s−1. The simulation result with the dynamic hardening properties is compared to that with the quasi-static properties. The comparison shows a notable difference in the maximum compressive residual stress by 32%, demonstrating that it is important to consider the dynamic hardening properties in such high strain rate simulation as shot peening for accurate simulation results.

Identification and formulation of stress-strain relationship for pure nickel and Inconel 718 in a wide strain rate range （10<sup>−3</sup>/ss~10<sup>4</sup>/ss）

Cold spraying is a surface coating technique in which a powder material is impacted on a base material at high speed without melting to form a dense coating layer. However, the stress-strain relationship in the strain rate range of 5 × 103 to 2 × 104/ss , which is required for Cold spraying, has not yet been sufficiently obtained. In this study, stress-strain relationships of pure nickel and Inconel 718 as target materials in a wide range of strain rates from quasi-static strain rate (10−3/ss) to ultra-high strain rate up to 104/ss were investigated by means of a Split Hopkinson bar method and a high-speed material testing machine (TS2000 Saginomiya co. Ltd.). From the experimental results, the stress-strain relationships of pure nickel and Inconel 718 in the wide strain rate range were clarified and a strain rate sensitive constitutive model was formulated by referring the Tanimura-Mimura (2009) model.

DYNAMIC DUCTILE-BRITTLE TRANSITION AND FRACTURE TOUGHNESS MEASUREMENT OF METAL UNDER INTERMEDIATE-LOW LOADING VELOCITIES

The phenomenon of ductile-brittle transition and the measurement of dynamic fracture toughness of metallic materials under impact loading are important parts of the research on dynamic mechanical properties of metal materials. In view of the lack of understanding of ductile-brittle transition of metallic materials under impact loading and the difficulty in measuring the dynamic $J$-resistance curve of ductile materials at relative low loading rate, a method is proposed to measure the ductile-brittle transition process of 15MnTi and 11MnNiMo steels at different loading rates, and the effect of crack tip constraint on the rate change of dynamic ductile-brittle transition of the two materials, by using high-speed material testing machine and its corresponding special fixtures. The dynamic fracture toughness of 15MnTi steel under low loading rate was measured by adjusting the length of compression bar and changing the crack propagation by means of the brake of upper roller. The experimental results indicate that the CT specimen of 15MnTi steel is characterized as ductile fracture when loading velocity is lower than 0.025~m/s, and the fracture character of CT specimen of 15MnTi steel is ductile-brittle combination when loading velocity is between 0.1~m/s and 0.5~m/s, and brittle fracture of the CT specimen of 15MnTi steel starts from 0.5~m/s. The phenomenon of brittle fracture followed by ductile fracture for the CT specimen of 11MnNiMo steel occurs when the loading rate is greater than 1.5~m/s. The dynamic brittle fracture rate of 15MnTi and 11MnNiMo steels is significantly affected by crack tip constraint, and the dynamic brittle fracture rate of the material decreases obviously with the increase of in-plane constraint and out of plane constraint. It is also found that in the three-point bending tests, the fracture toughness of 15MnTi steel decreases slowly with the increase of loading rate, when the loading rate is lower than 8788~MPa$\cdot$mm/s.

Shear Stress Hardening Curves of AISI 4130 Steel at Ultra-high Strain Rates with Taylor Impact Tests

This paper is concerned with shear properties of AISI 4130 steel at various strain rates from the quasi-static state to ultra-high strain rates using a Taylor impact testing machine. Shear properties of metallic materials are known to be sensitive to the strain rate especially at high strain rates. In order to investigate hardening behavior under shear loading, in-plane shear tests are carried out at a wide range of strain rates from 10–3 s–1 to 103 s–1 with an INSTRON 5583 and a High Speed Material Testing Machine with a novel procedure to obtain hardening curves from shear tests. To induce shear deformation at ultra-high strain rates, a novel shear specimen applicable to Taylor impact tests is designed and then shear tests are conducted with a Taylor impact testing machine. Hardening curves are extrapolated by a dynamic hardening model and then calibrated by an experimental–numerical hybrid method with an inverse optimization scheme so that finite element analysis results match to shear tests results at ultra-high strain rates with the Taylor impact test. Shear fracture strains at ultra-high strain rates are also calibrated in the same manner. Calibrated flow stresses are higher than the extrapolated ones before calibration and calibrated shear fracture strains decrease more rapidly than the extrapolated ones before calibration at the ultra-high strain rate region.

Dynamic tensile response and failure mechanism of hi-lock bolt joint

The deformation mode and energy absorption property of aircraft sub-structure are of great significance for occupant protection during aircraft crash. The load transfer and failure mode of joint structures are one of important factors affecting aircraft structural deformation. This paper tries to study the dynamic failure behavior of aviation of hi-lock bolt joints under impact loads. Based on the shear resistance hi-lock bolt, the single-bolt single lap joints with two kinds of base metals (2024-T3 and 7050-T7451) were designed. The dynamic tensile tests of the joints were carried out by a high-speed material testing machine under four loading velocities, 0.01、0.10、1.00 and 3.00 m/s. The dynamic response, the ultimate load, the energy absorption and the failure mode of hi-lock bolt joints were measured and analyzed. The results show that the failure mode of the joints is greatly affected by the material strength of the base metal and the high lock bolt / nut, but less affected by the loading speed; as increasing the speed from 0.01 m/s to 3 m/s, the ultimate load and the energy absorption of 2024-T3 joints increase by 2.17% and 34.43% respectively, and the ultimate load and the energy absorption of 7050-T7451 joints increase by 5.53% and 6.58% respectively.

Asymmetric Hardening Behavior of AZ31B Magnesium Alloy Sheet with Large Strain at Various Strain Rates

This paper investigates asymmetric hardening behavior of the magnesium alloy sheet of AZ31B at different strain rates when deformation is large. Tensile tests are carried out at the quasi-static state of 0.001 s−1 and the intermediate strain rate of 100 s−1 utilizing the Instron 5583 and the high speed material testing machine (HSMTM), respectively. Compression tests are conducted at the strain rate of 0.001 s−1 and 100 s−1 using a jig fixture mounted on the testing machines to change the loading direction into the opposite direction. A compression testing method is developed to obtain compressive properties with large strain, which includes the attachment of a fork-type clamping device and the design of specimen dimensions to achieve large strain without buckling during the test. Experimental results reveal the asymmetric behavior showing that the tensile yield stress is much larger than the compressive one at both strain rates. The difference between the flow stress level in tension and compression becomes larger when the strain rate is higher. The compressive hardening behavior is very much different from the tensile hardening behavior showing that the compressive flow stress increases remarkably as the deformation proceeds while the tensile flow stress asymptotes to a certain value.

Characterization of hardening behaviors of 4130 Steel, OFHC Copper, Ti6Al4V alloy considering ultra-high strain rates and high temperatures

This paper is concerned with the characterization of dynamic hardening behaviors of metallic materials at ultra-high strain rates ranging from 104 s−1 to 106 s−1. As typical metallic materials, three kinds of materials are selected for body centered cubic (BCC), face centered cubic (FCC), and hexagonal close packed (HCP) structures: 4130 steel; OFHC copper; and Ti6Al4V alloy, respectively. In order to investigate the hardening behaviors for the three materials, uniaxial tensile tests are conducted at room temperature of 25 °C and a wide range of strain rates from 10−3 s−1 to 103 s−1 with the Instron 5583, a high speed material testing machine (HSMTM), and a tension split Hopkinson pressure bar (TSHB). To consider the thermal softening effect at different strain rates, tensile tests are additionally performed at an elevated temperature up to 200 °C and strain rates of 10−3 s−1, 10−1 s−1, and 102 s−1. Hardening curves obtained from several experiments are expressed with a constitutive model by multiplying a strain hardening term, a strain rate hardening term, and a thermal softening term to describe material behaviors at large plastic strain, high strain rate, and elevated temperature, respectively. A well fitted constitutive model is utilized to interpolate and extrapolate stress–strain relation from low to ultra-high strain rates. The experimental and extrapolated data are implemented into finite element simulations as the preliminary material properties before calibration. A hybrid experimental-numerical approach is applied to precisely calibrate the extrapolated hardening behaviors at ultra-high strain rates by utilizing the Taylor impact test results. During the Taylor impact tests, images of several sequentially deformed shapes of projectiles are acquired by a high speed camera to compare with numerical simulation results. The stress–strain curves after calibration give notable improvements in the accuracy of numerical estimation of the deforming shapes of projectiles at ultra-high strain rates.

Characterization of Hardening Behavior at Ultra-High Strain Rate, Large Strain, and High Temperature

This paper is concerned with the characterization of the OFHC copper flow stress at strain rates ranging from 10−3 s−1 to 106 s−1 considering the large strain and high temperature effects. Several uniaxial material tests with OFHC copper are performed at a wide range of strain rates from 10−3 s−1 to 103 s−1 by using a INSTRON 5583, a High Speed Material Testing Machine (HSMTM), and a tension split Hopkinson pressure bar. In order to consider the thermal softening effect, tensile tests at 25°C and 200°C are performed at strain rates of 10−3 s−1,101 s−1, and 102 s−1. A modified thermal softening model is considered for the accurate application of the thermal softening effect at high strain rates. The large strain behavior is challenged by using the swift power law model. The high strain rates behavior is fitted with the Lim–Huh model. The hardening curves are evaluated by comparing the final shape of the projectile from numerical simulation results with the Taylor impact tests.

Evaluation of the effect of the strain rate on the tensile properties of carbon–epoxy composite laminates

This paper is concerned with evaluation and prediction of the tensile properties of carbon fiber-reinforced plastics laminates considering the strain rate effect at intermediate strain rates. Uniaxial tensile tests of carbon fiber-reinforced plastics laminates were conducted at various strain rates ranging from 0.001 s–1 to 100 s–1 using Instron 8801 and a high speed material testing machine to measure the variation of the elastic modulus and the ultimate tensile strength. Tensile test specimens were designed based on the ASTM standards and stacked unidirectionally such as [0°], [90°] and [45°] to predict the elastic modulus of carbon fiber-reinforced plastics laminates with various stacking sequences. The axial strain was measured by the digital image correlation method using a high speed camera and ARAMIS software to enhance the accuracy of the strain measurement. A prediction model of the elastic modulus of carbon fiber-reinforced plastics laminates is newly proposed in consideration of the laminate theory and the tensile properties of unidirectional carbon fiber-reinforced plastics laminates. The prediction model was utilized to predict the tensile properties of [0°/90°]s laminates, [±45°]s laminates, and [0°/±45/90°]T laminates for validation of the model. The elastic moduli predicted were compared with the static and dynamic tensile test results to confirm the accuracy of the prediction model.

Tension/compression hardening behaviors of auto-body steel sheets at intermediate strain rates

This paper is concerned with the effect of the strain rate on the tension/compression hardening behavior of auto-body steel sheets. Since the repeated tension and compression prevails in sheet metal forming of deep drawing, accurate material properties under tension/compression loading paths are important to predict deformed shapes of steel sheets with numerical simulation. The in-plane tension/compression tests have not yet been performed for the rate-dependent material properties which are indispensable for practical sheet metal forming simulation. An experimental technique is established to conduct tension/compression tests of DQ and DP590 steel sheets at various strain rates ranging from 0.001s−1 to 50s−1. A new clamping device for compression tests is designed to suppress a specimen from thickness-directional buckling with H-shaped clamping plates which are optimized for application of the DIC method. Novel devices to impose a designated tensile strain or compressive strain are also developed for tension/compression tests with the high speed material testing machine at intermediate strain rates. Tension/compression hardening curves are obtained with the variation of the strain rate. It is noted from the test results that the Bauschinger ratio is increased according to the increase of the strain rate. It represents the gradual decrease of the Bauschinger effect with the increase of the strain rate. In a transient period which occurs in loading reversal, the decrease of the tangent modulus appear slowly as the strain rate increases, so that the amount of the permanent softening becomes smaller as the strain rate becomes higher.

Characterization of flow stress at ultra-high strain rates by proper extrapolation with Taylor impact tests

This paper is to provide a novel systematic procedure to obtain the dynamic flow stress of a material at ultra-high strain rates ranging from 104 s−1 to 106 s−1 where hardening behaviors are difficult to acquire from conventional experiments. Uniaxial material tests with AISI 4340 steel are performed at a wide range of strain rates from 10−3 s−1 to 103 s−1 by using the INSTRON 5583, a high-speed material testing machine (HSMTM), and a tension split Hopkinson pressure bar (SHPB) testing machine. From the uniaxial tests above, stress–strain curves are obtained at the strain rates ranging from 10−3 s−1 to 103 s−1. However, stress–strain curves cannot be obtained at the strain rates higher than 104 s−1 due to the lack in experimental techniques. In order to characterize hardening behaviors at strain rates ranging from 104 s−1 to 106 s−1, Taylor impact tests are performed when the speed of a projectile is 200 m/s, 253 m/s, and 305 m/s, which entail ultra-high strain rates, high temperature, and large plastic deformation. Flow stresses at the ultra-high strain rates are characterized through an inverse optimization process by comparing the numerical simulation results with the experimental results of the sequentially deformed shapes of a projectile during the Taylor impact test. The thermal softening effect at different strain rates is also considered due to the elevated temperature caused by large plastic deformation. The flow stresses calibrated by the comparison are implemented to numerical simulation resulting in a good coincidence with the Taylor impact tests at different impact velocities. It is noted from the comparison that the yield stress and the comprehensive hardening curves proposed well describe the deformation behavior up to the strain rate of 106 s−1 beyond the strain rate range for conventional material testing.

Response Characteristics and Adiabatic Heating during High Strain Rate for TRIP Steel and DP Steel

By using a static and high-speed material testing machine, tensile deformation behaviors of two kinds of Si-Mn TRIP (transformation induced plasticity) steels and DP (dual phase) steel were studied in a large range of strain rates (0.001 – 2000 s−1). Temperature variation during adiabatic heating and the amount of retained austenite at fracture were measured by an infrared thermometer and an X-ray stress analyser, respectively. The microstructure of steels was observed by optical microscopy (OM) and scanning electron microscopy (SEM) before and after tensile test. It was found from the experimental results that the tensile strength of these steels increased, and the fracture elongation firstly decreased and subsequently increased, as the strain rate increased in the range of 0.1 – 2000 s−1. The temperature raised during adiabatic heating of TRIP steel was in the range of 100 – 300 °C, while that of the DP steel was in the range of 100 – 220 °C. The temperature rise of these steels increased with increasing the strain rate, as well as the amount of the transformed retained austenite in TRIP steels. It was confirmed that austenite to martensite transformation is not suppressed by adiabatic heating.

Specimen and Grain Size Effects of Al1100 on Strain and Strain Rate Hardening at Various Strain Rates for Al1100

This paper is concerned with comparison of the tensile properties of Al1100 thin film in a micro-scale to that of Al1100 sheet in a macro-scale. The material properties of Al1100 film and sheet with a thickness of 96 μm and 1 mm respectively have been investigated at strain rates ranging from 0.001 to 100 s−1. The experiments were conducted with Static Micro-Material Testing Machine (SMMTM) and High Speed Micro-Material Testing Machine (HSMMTM) for micro-specimens and with Instron 5583 and high speed material testing machine (HSMTM) for macro-specimens. A reliable jig system for SMMTM and HSMMTM has been newly developed for easy installation of a specimen and accurate alignment between a specimen and the jig system to enhance the reproducibility of tests. The digital image correlation (DIC) method is employed to measure the axial strain of the specimens. In order to obtain a fine speckle pattern for the DIC method, a novel technique is employed to print the speckle pattern with fine particles by blowing sprayed particles before printing. The grain sizes of two Al1100 specimens have been compared and the number of grains in the gauge cross-section has been calculated to obtain the grain number which is related to the specimen size effect. Electron Back Scattered Diffraction (EBSD) images were obtained for both micro-specimens and macro-specimens and analyzed to measure the grain size. The Al1100 film with a smaller average grain size shows larger strain hardening than the Al1100 sheet with a larger average grain size.

Dynamic failure characterization of laser welds under combined loading conditions

This paper proposes a dynamic failure criterion for laser welds to describe the strain-rate dependent failure behavior of laser welds in the crash analysis of an auto-body. The failure criterion has been developed from experiments with nine different conditions of combined normal and shear loading including the normal loading and the pure shear loading. The experiments were carried out with a high speed material testing machine at intermediate strain rates ranging from quasi-static to 100 s−1. The dynamic failure tests were examined with four cases of the 25 mm stitch-type laser weld of CR340 1.2t, the 20 mm stitch-type laser weld of CR340 1.2t, the 25 mm stitch-type laser weld of SPCC 1.0t and the O-type laser weld with the 8 mm diameter of CR340 1.2t in order to evaluate the influence of the strain rate on the failure loads with respect to the base metal and the bead geometry. Based on the experimental data, a novel failure criterion was proposed for the description of the failure behavior of laser welds on the basis of the quasi-static failure criterion. The failure criteria are divided into two regions: the base metal failure criterion is expressed as a β-norm function; and the interfacial failure criterion is expressed as an exponential function. For the both failure criteria, the strain-rate sensitivity of the normal and the shear failure load are assigned as an exponential function of the logarithm of the strain rate. The change of the shear failure load with the increase of the strain rate is also considered to accurately construct the failure criteria. The failure mode and the onset of failure of the laser welds are determined when the maximum value reaches unity. The dynamic failure criterion proposed provides a fairly accurate description of the failure load obtained from the experiments under dynamic combined loading conditions.

Rate-dependent hardening model for polymer-bonded explosives with an HTPB polymer matrix considering a wide range of strain rates

This article is concerned with the effect of the strain rate on the strain hardening behavior of polymer-bonded explosives at a wide range of strain rates ranging from 0.0001 s–1 to 3870 s−1. Inert polymer-bonded explosive simulants are prepared as specialized particulate composites to acquire analogous mechanical characteristics to polymer-bonded explosives for safety reasons. Uniaxial compressive tests were conducted from quasi-static states to intermediate strain rates ranging from 0.0001 s−1 to 100 s−1 with cylindrical specimens using a dynamic material testing machine (INSTRON 8801) and a high-speed material testing machine. An experimental method was developed for uniaxial compressive tests at intermediate strain rates ranging from 10 s−1 to 100 s−1. Split Hopkinson pressure bar tests were performed at high strain rates ranging from 1250 s−1 to 3870 s−1. Deformation behavior was investigated using captured images from a high-speed camera. The strain hardening behavior of polymer-bonded explosive simulants was formulated as a function of the strain rate with the proposed rate-dependent hardening model based on the DSGZ model. The model is capable of representing the complicated strain rate effects on the strain hardening behavior for rate-sensitive materials with a second-order exponentially-increasing function of the strain rate sensitivity. The rate-dependent hardening model of polymer-bonded explosives can be readily applied to prediction of deformation modes of polymer-bonded explosives in a warhead that undergoes severe dynamic loads.

동적하중을 받는 기능성 고폭화약조성 시뮬런트 재료물성 연구

This paper is concerned with the material properties of functional high explosive(FHX) simulant at various strain rates ranging from <TEX>$10^{-4}/sec$</TEX> to <TEX>$10^1/sec$</TEX>. Material properties of FHX at high strain rates are important in prediction of deformation modes of FHX in a warhead which undergoes dynamic loading. Inert FHX stimulant which has analogous mechanical properties with FHX was utilized for material tests due to safety issues. Uniaxial tensile tests at quasi-static strain rates ranging from <TEX>$10^{-4}/sec$</TEX> to <TEX>$10^{-2}/sec$</TEX> and intermediate strain rates ranging from <TEX>$10^{-1}/sec$</TEX> to <TEX>$10^1/sec$</TEX> were conducted with JANNAF specimen using a tensile testing machine, INTRON 5583, and developed high speed material testing machine, respectively. Uniaxial compressive tests at quasi-static strain rates and intermediate strain rates were conducted with cylindrical specimen using a dynamic materials testing machine, INSTRON 8801. And cyclic compressive loading tests were performed with various strain rates and strains. Deformation behaviors were investigated using captured images obtained from a high-speed camera.

EFFECT OF STRAIN RATE ON DYNAMIC DEFORMATION BEHAVIOR OF DP780 STEEL

Tensile properties and deformation behavior of DP780 steel were studied using servo-hydraulic high-speed material testing machine,SEM and TEM.The effects of strain rate and the mechanism were investigated.The results showed that the strength and ductility of DP780 steel remained almost unchanged as the strain rate increased at strain rates lower than 10°s~(-1). When the strain rate was over 10~1 s~(_1),the strength and the strain-hardening coefficient increased remarkably.Ductility of DP780 steel increased significantly at the strain rates ranging from 3×10~1 to 5×10~2 s~(_1).The deformation resistance increased with increasing the strain rate due to the stronger short range resistance induced by the acceleration of dislocation motion in the ferrite matrix. Increasing strain rate up to 3×10~1 s~(_1) resulted in a considerable increase of the amount of mobile dislocation,which was the main reason for the increasing uniform elongation and fracture elongation of DP780 steel at the strain rate ranging from 3×10~1 to 5×10~2 s~(_1).Interface of ferrite-martensite in DP780 steel was the main location for pile-up of dislocation,crack initiation and propagation. The ability of inhomogeneous plastic deformation of DP780 steel increased due to the decreasing plastic strain energy difference between the ferrite matrix and ferrite-martensite interface and the consequent delaying initiation and propagation of microvoids at ferrite-martensite interface induced by the increasing work hardening degree of ferrite matrix with the increasing strain rate.

Effect of strain rate on deformation behavior of TRIP steels

Tensile deformation behavior of Si–Mn TRIP (TRansformation Induced Plasticity) steel with vanadium and without vanadium and the DP (Dual Phase) steel of the same composition were studied in a large range of strain rate (0.001–2000s−1) by routine material testing machine, rotation disk bar–bar tensile impact apparatus and high-speed material testing machine of servo-hydraulic type. In situ measurement of the transformation of retained austenite was performed by means of X-ray stress apparatus in order to have detailed knowledge about the transformation of retained austenite at quasi-static tensile. Microstructure of steels before and after tensile were observed by means of optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). It is shown that there is no yield plateau observed on the stress–strain curve at quasi-static condition for TRIP steel containing vanadium because the vanadium carbide suppress the formation of Cottrell atmosphere in matrix. Retained austenite of Si–Mn TRIP steel containing vanadium transforms to martensite at loading stress of 502MPa (its yielding strength is 486MPa), while the transformation of retained austenite in matrix of Si–Mn TRIP steel without vanadium happens when its yielding process is finished at quasi-static tensile. It is confirmed that phase transformation of retained austenite in TRIP steel is strain induced phase transformation. It is noted that tensile elongation of TRIP steel at dynamic tensile is always lower than that at quasi-static tensile. That is because gradually strain induced phase transformation of retained austenite in TRIP steel is suppressed by deformation localization at dynamic tensile.

Determination of true stress–true strain curves of polymers at various strain rates using force equilibrium grid method

This article is concerned with the tensile characteristics of polymers at various strain rates. Tensile characteristics at the high strain rate are important in the prediction of deformation modes of polymeric components which undergo high-speed deformation during car crashes. Uniaxial tensile tests at strain rates ranging from 0.001 to 100 s−1 were conducted using a tensile testing machine (INSTRON 5583) and developed high-speed material testing machine. Since the deformation of polymers is accompanied by localized deformation and a conventional extensometry method is no longer valid for strain measurement, a force equilibrium grid method (FEGM) is newly proposed to acquire accurate true stress–true strain curves of polymers. FEGM is utilized with images of the non-uniform distribution of deformation at each elongation stage captured by a high-speed camera in combination with a grid method. The relation between true stress and true strain at the desired strain rate and increment was determined by collecting the load data from the equilibrium and deformation images.

Investigation of Elongation at Fracture in a High Speed Sheet Metal Forming Process

This paper investigates the dynamic elongation at fracture of conventional steels, advanced high strength steels and nonferrous metals, such as aluminium and magnesium alloys. Dynamic tensile tests were carried out using a high speed material testing machine at various strain rates ranging from 0.001/s to 200/s. The results show that the elongation at fracture of sheet metals does not simply decrease with the increase of the strain rate. The elongation of SPCC, SPRC450R, TRIP600 and AZ31 decreases when the tests are carried out under the quasi-static state at the strain rate of 0.1/s, but increases again when the tests are carried out at the strain rate of 0.1/s up to the strain rate of 200/s. Furthermore, DP600 and AA7003-T7 show the tendency that the tensile elongation increases as the strain rate increases. This tendency is related to the microstructure and forming history of the sheet metal. It is concluded that localized strain rate hardening in the necking region induces the enlargement of the necking region and thus the increased elongation. This phenomenon is worth being considered to predict the fracture of sheet metal products in high speed sheet metal forming.