High-speed Tensile Tests Research Articles

Mechanical behavior of DP780 dual phase steel at a wide range of strain rates

The mechanical behavior of DP780 cold-rolled dual phase steel was studied by means of high-speed tensile testing machine. The constitutive relation and strain hardening behavior was analyzed by modified Johnson-Cook (JC) model and modified Crussard-Jaoul (C-J) method, respectively. Results indicated that strain rates have significant effect on mechanical properties and plastic deformation of dual phase steel. Moreover, the modified JC model can well describe the experimental results. The plastic deformation of DP780 can be divided into two stages: the first low strain stage with stronger strain hardening capacity and the second high strain stage with weaker strain hardening capacity. Strain hardening behavior was influenced by strain rate, dislocations, plastic deformation of martensite and thermal softening; the strain hardening rate of DP780 steel was not sensitive to the strain rates. Micro-voids in the tensile samples were formed at ferrite/martensite interface or formed by the fracture of martensite.

Application of the Virtual Fields Method to determine dynamic properties at intermediate strain rates

Crash analysis simulation is now very important in automotive industry to assess automotive crashworthiness and safety. In order to acquire reliable crash simulation results, precise material behaviors at intermediate strain rates should be used as input data. To determine the stress-strain curves at various strain rates, the number of expensive and complicated experiments is large. The present study aims at determining the stress-strain curves of sheet metals at various strain rates from a single dynamic experiment. A new type of high speed tensile tester for sheet metal specimens was built and high speed tensile tests were carried out. Full-field heterogeneous strain fields were measured by a digital image correlation technique using a high-speed camera. The load data was acquired from strain gauges attached to the elastic deformation region on the specimen. Then, an inverse identification scheme with a rate dependent hardening law was applied to retrieve dynamic parameters. The stress-strain curves of advanced high strength steel at intermediate strain rates (100/s - 300/s) were successfully obtained from a single experiment.

Mechanical Performance of Resistance Spot Welded Steel Sheets

Mechanical performance of resistance spot welded steel sheets was evaluated under quasi-static and dynamic loading conditions. Numerical inverse calibration method was mainly utilized to characterize mechanical properties especially for flow curves and ductile fracture criteria of constituent zones of the welded joints. For the base sheets, standard tensile test and high-speed tensile test was conducted under quasi-static and dynamic loading conditions, respectively. Mechanical properties of the weld zones were characterized using the developed miniature tensile specimen. Implementing the mechanical properties and structure to the finite element model, finite element simulation was used to evaluate the mechanical performance of two distinct weld coupons.

高速引張試験の弾塑性解析に基づく ポリカーボネートの衝撃特性における時間-温度換算則の適用

高速引張試験の弾塑性解析に基づく ポリカーボネートの衝撃特性における時間-温度換算則の適用

Investigation on the influence of loading-rate on fracture toughness of AHSS grades

The automotive industry is striving for light body-in-white structures while maintaining or improving passenger safety. The aim of this paper is to investigate the influence of the loading rate on the fracture toughness of thin steel sheet metal of three advanced high strength steels. Although steel is a heavy material it plays a significant role for lightweight solutions in car bodies. Three different advanced high strength steel (AHSS) grades, namely dual-phase (DP), quench-partitioning (Q&P) and TRIP-assisted bainitic-ferritic (TBF), are investigated in the present paper. For crash relevant components it is of importance to know the material response under high loading velocities i.e. high strain rates. A standard tensile test system is used for low loading rates, a high-speed tensile testing setup is used to obtain high loading rates. The fracture toughness of the three AHSS grades is evaluated using the methodology of the Essential Work of Fracture (EWF). The tensile specimen used in the present work is the double edge notched tensile (DENT) geometry with a pre-developed crack. High-speed imaging is applied to verify the validity of the evaluation method Essential Work of Fracture at high rates of loading. Results from this work show that knowledge of fracture toughness would improve the understanding of fracture and crack propagation mechanisms for third generation high strength steels used for automotive components.

Modelling the strain rate dependent hardening of constructional steel using semi-empirical models

The importance of modelling the strain rate dependent material behaviour increases since structural components more and more have to be designed against extreme events. With growing usage of numerical programmes, good models become necessary. The goal of the ongoing research project is to find reasonable models for simulations concerning steel constructions. This paper discusses various models to describe the strain rate dependent hardening of mild steel. In this context, high speed tensile tests were performed and evaluated. Four commonly used material models and a new model were introduced and calibrated to the test results: The Johnson-Cook model, a modified Johnson-Cook model by Huh and Kang, the Cowper-Symonds model and a combined Johnson-Cook/Cowper-Symonds model. These four models do not describe the observed behaviour of constructional steels satisfactorily. Unlike these models, a newly introduced model is able to describe both a yield plateau and the slope of the strain hardening, both as a function of the strain rate. The quality of the approximations is discussed and further investigated by performing notched bar tensile tests and simulations. Finally, recommendations are given on the basis that the modelling of a yield plateau and the negative strain rate sensitivity of the strain hardening is necessary for modelling structural components under extreme events.

Ductility and formability of three high-Mn TWIP steels in quasi-static and high-speed tensile and Erichsen tests

The ductility and formability properties of three high-Mn TWIP steels were investigated under quasi-static and high-speed deformation conditions. The ductility was evaluated from conventional and Hopkinson split-bar tensile tests at 1250 s−1 and the stretch formability was evaluated using Erichsen tests made with a special high-speed electro-hydraulic forming machine at about 1000 s−1. The data were related to microstructural features revealed using electron backscatter diffraction and X-ray diffraction. Furthermore, the stacking fault energy (SFE) was estimated using a thermodynamic approach. It was found that the 0.6C-22Mn and 0.2C-21Mn-0.23N steels (compositions in wt%) with SFEs of 23–24 mJ/m2 exhibited good elongation and a large Erichsen index at both low and high strain rates. These were attributed to intensive mechanical twinning though partly replaced by dislocation slip in deformation bands in the high-speed tests. However, it was noticed that the high-speed stretching failure of these TWIP steels occurred in the uniform elongation range without diffuse necking. In the austenitic - ferritic 21Mn-3Al-3Si steel strain-induced martensite was formed, but the ferrite phase seemed to impair formability.

Effect of strain rate on the dynamic tensile behaviour of UHMWPE fibre laminates

Ultra-high molecular weight polyethylene (UHMWPE) fibre has great potential for strengthening structures against impact or blast loads. A quantitative characterization of the mechanical properties of UHMWPE fibres at varying strain rates is necessary to achieve reliable structural design. Quasi-static and high-speed tensile tests were performed to investigate the unidirectional tensile properties of UHMWPE fibre laminates over a wide range of strain rates from 0.0013 to 163.78 s−1. Quasi-static tensile tests of UHMWPE fibre laminates were conducted at thicknesses ranging from 1.76 mm to 5.19 mm. Weibull analysis was conducted to investigate the scatter of the test data. The failure mechanism and modes of the UHMWPE fibre laminates observed during the test are discussed. The test results indicate that the mechanical properties of the UHMWPE fibre laminate are not sensitive to thickness, whereas the strength and the modulus of elasticity increase with strain rate. It is concluded that the distinct failure modes at low and high strain rates partially contribute to the tensile strength of the UHMWPE fibre laminates. A series of empirical formulae for the dynamic increase factor (DIF) of the material strength and modulus of elasticity are also derived for better representation of the effect of strain rate on the mechanical properties of UHMWPE fibre laminates.

Mechanical characterization of a Low Density Sheet Molding Compound (LD-SMC): Multi-scale damage analysis and strain rate effect

Abstract This paper presents the results of an overall experimental characterization of the mechanical behavior of a Low Density Sheet Molding Compound (LD-SMC). LD-SMC is a polyester matrix containing mineral charge (CaCO3) reinforced by discontinuous bundles of glass fibers and Hollow Glass Microspheres (HGM). After a description of its specific microstructure using several experimental methods (notably a new ultrasonic method), the overall mechanical response of two microstructure configurations (Randomly Oriented (RO) and Highly oriented (HO)) is analyzed at both macroscopic and microscopic scales in the case of tensile and compression tests. HGMs are homogeneously distributed into the overall volume of the material. At the microscopic scale, in-situ tensile tests inside a SEM and fracture surfaces observations allows analyzing the specific damage mechanisms occurring during tensile and compression loading performed in the mold flow direction (HO-0°) and perpendicularly to it (HO-90°). A strong coupled influence of the presence of the HGM and fibers orientation has been emphasized. The results show that for HO-0° configuration fiber-matrix debonding appears to be the predominant damage mechanism, whereas for HO-90° configuration HGM-matrix debonding appears to be the predominant damage mechanism. High speed tensile tests are achieved using servo-hydraulic test equipment in order to study the strain rate effects (until 80 s−1) on mechanical macroscopic responses of HO-0°, RO and HO-90° samples. Strain rate has an obvious influence on the inelastic properties of LD-SMCs samples for all microstructures particularly on the damage threshold.

Research of the Load Bearing Capacity of Shape-Optimized Metal Inserts Embedded in CFRP under Different Types of Stresses

An efficient implementation of lightweight design is the use of continuous carbon fiber reinforced plastics (CFRP) due to their outstanding specific mechanical properties. Embedded metal elements, so-called inserts, can be used to join metal-based attachments to structural CFRP parts in the context of multi-material design. They differ from other mechanical fasteners and have distinctive benefits. In particular, drilling of the components to be joined can be avoided and, depending on the preforming, fiber continuity can be maintained using such elements. Thus, no local bearing stress is anticipated. Previous work published by the authors [1] dealt with a systematic research of the influence of different types of stresses on the load bearing capacity of welded inserts. This contribution aims at the investigation of the performance of shape-optimized inserts under the same types of loading to compare with the results of the welded inserts serving as a reference. For that purpose, the respective load bearing capacities were evaluated after preinduced damages from impact tests and thermal cycling. In addition, dynamic high-speed tensile tests (pull-out) were conducted under different loading velocities. It is shown that the load bearing capacities increased up to 19% for high velocities (250 mm/s) in comparison to quasi-static loading conditions (1.5 mm/min) showing an obvious strain rate dependency of the CFRP. Quasi-static residual strength measurements under tensile loading identified the influence of the respective preinduced damages of the insert. Influence of the thermal loading condition was evaluated by placing the specimens in a climate chamber and exposing it to various numbers of temperature cycles from-40 °C to +80 °C with a duration time of 1.5 hours each. Here, it turned out that already 10 temperature cycles decreased the quasi-static load bearing capacity up to 31%. According to DIN EN 6038 the specimens were loaded with different impact energies and the residual strength were measured carrying out pull-out tests. It could be shown that the damage tolerance is significantly lower for the shape-optimized insert due to failure-critical delamination. The optimized insert also endured lower impact energies and the influence on the performance was higher.

Dynamic Tensile Testing of High Strength Armor Steel Plates

In this paper was studied dynamic behavior of the armor steels Armox 500T and Secure 500 by testing specimens in quasi-static tensile test with strain rate 1∙10-3 s-1 and high-speed tensile test within range of intermediate strain rates from 100 s-1 to 400s-1 at the room temperature. Hardness test and quasi-static tensile test confirmed material properties specified by the manufacturer. Stress-strain diagrams showed very low strain-rate hardening effect at investigated strain rates. Total elongation at fracture was larger in case of Armox 500T for the whole strain rate range. Deformation energy density was calculated from the stress-strain curve and temperature rise due to adiabatic heating was estimated. Because of higher total elongation, Armox 500T was able to withstand higher deformation energy.

Relationship between strain rate concentration factor and stress concentration factor

In this study, the strain rate concentration is considered for high speed tensile test, which is now being recognized as a standard testing method. To evaluate the impact strength of engineering materials under high impact speed, Izod and Charpy tests are unsuitable since they cannot control the impact speeds and therefore do not coincide with the real failure of real products. For smooth specimens, the strain rate can be determined from the tensile speed u/t and specimen length l as ε̇smooth=u/tl. For notched specimens, however, the strain rate at the notch root ε̇notch should be analyzed accurately. In this study, therefore, the strain rate concentration factor defined as Ktε̇=ε̇notch/ε̇smooth is studied with varying the notch geometry and specimen length. It is found that the strain concentration factor Ktε̇ can be estimated from stress concentration factor Kt∗.

Dependency of tensile strength of ductile cast iron on strain rate and temperature

The dependency of the tensile strength and the notch strength on strain rate and temperature were investigated for conventional ferrite-pearlite type ductile cast iron (JIS-FCD500) to make clear the applicability of ductile cast iron to components for welded steel structures. High speed tensile tests were conducted on notched and smooth specimens with varying strain rate and temperature. Charpy absorbed energy was also evaluated on notched specimen with varying temperature. It is found that the tensile strength is in a good relationship with strain rate-temperature parameter R for the wide range of strain rate and temperature. With decreasing R parameter, both and increase even when Charpy absorbed energy starts decreasing. It should be noted that the notch strength is always larger than the tensile strength at room temperature in the range of R parameter required for the welded structures. Therefore, the tensile strength is confirmed to be useful for the structural design.

Erratum to: New tension mechanism for high-speed tensile testing machine

Erratum to: New tension mechanism for high-speed tensile testing machine

Characterization of dynamic hardening behavior using acceleration information

Abstract Crash analysis simulation is very important in automotive industry to assess automotive crashworthiness and safety. In the FE simulation, accurate dynamic hardening behavior should be used as input data to provide reliable results. But, it is difficult to obtain precise hardening properties at intermediate or high strain rates due to inaccurate measurement of load caused by the inertial effect. In this study, a new methodology was applied to retrieve dynamic strain hardening properties of sheet metal specimens. The virtual fields method (VFM) was adopted as an inverse method to identify hardening parameters without load information. As an initial study, Swift model for a rate independent hardening law was selected for an elasto-plastic constitutive model. In order to validate the proposed methodology in the experiments, a new type of high speed tensile tester for sheet metal specimens was built and high speed tensile tests were performed. Digital image correlation technique using a high-speed camera was utilized to measure strain and acceleration fields so that the identification is carried out from the measured quantities. The validation of the proposed VFM identification procedure using the acceleration will be performed by comparing with the conventional procedure using a load-cell.

Study on effect of strain rate on elongation in advanced high strength steel

Abstract Automotive parts made of Advanced High Strength Steel (AHSS) deform at various strain rates during the press forming process, car crash, etc. It is known that conventional steels are affected by strain rate. AHSS is increasingly being used for reducing the weight of automobiles. Since such high tensile steels are also used more and more in other industries, it is important to investigate the relationship between stress and strain with changing strain rate for AHSS in order to improve the accuracy of deformation simulation. In this paper, a high speed tensile test method is proposed and results of investigations on the stress-strain curves with changing strain rate for AHSS are reported. High speed tensile tests were conducted using a servo-hydraulic controlled testing machine. Quasi-static tests were carried out by conventional testing apparatus. The test material was dual phase cold-rolled steel sheet with tensile strength of 980MPa. By using specimens with newly designed shapes, they could be broken at the center stably at various tensile velocities. Experimental results showed that the ductility of AHSS presented nonlinear behaviors dependent on the strain rate. Elongation was 23% at strain rate 0.1 s-1 and 29% at 1000 s-1. Uniform and necking elongations were measured at various strain rates. As a result, it was found that the necking elongation was larger than the uniform elongation with respect to strain rate. In order to investigate the reasons for this, the temperature was measured during the tests. The results revealed that at high strain rates, the temperature exceeded 520K, confirming that the tensile stress of AHSS increases due to the strain rate and ductility is changed by heat generated during plastic deformation.

쇄빙선 강재의 내충격 특성에 관한 실험적 연구: 제1부 강재 특성

This paper presents a study on the crashworthiness of the scaled-down stiffened panels used on a Korean icebreaker. In order to validate the crashworthiness of the panels, this paper provides various mechanical properties such as the results of a CVN test, quasi-static tensile test, and high-speed tensile test at arctic temperatures. Two types of steels (EH32 and FH32) were chosen for the material tests. CVN tests revealed that the two steels were equivalent up to −60℃ in terms of their impact energy absorption capacity. However, the toughness of FH32 was significantly superior to that of EH32. EH32 showed slightly higher flow stresses at all temperature levels compared to FH32. The improvement ratios of the yield strengths, tensile strengths, plastic hardening exponents, etc. for FH32, which were obtained from quasi-static tensile tests, showed an apparent ascending tendency with a decrease in temperature. Dynamic tensile test results were obtained for the two temperatures levels of 20℃ and −60℃ with two plastic strain rate levels of 1 s^−1 and 100 s^−1. A closed form empirical formula proposed by Choung et al. (2011;2013) was shown to be effective at predicting the flow stress increase due to a strain rate increase.

New tension mechanism for high-speed tensile testing machine

As concerns of dynamic tensile measuring, a new tension mechanism driven by liner motor in high-speed tensile testing machine is presented. It satisfies the requirements for testing the dynamic tensile properties of plastic materials with strain rates from 1 to 100 s-1, which is relatively difficult to test for currently available machines. For this purpose the idle stroke principle is used to realize the dynamic load of plastic material with static grip. Computer aided engineering is used: i) to analyze and optimize the tension mechanism thus allowing reducing the system shock caused by collisions. ii) to avoid the influence of plastic materials specimen stress and strain uniformity on the measuring system. The analysis results agreed well with the established empirical relationships based on experimental evidence. Two sets of experiments for different strain rate tensile were conducted thus allowing proving the reliability and credibility of the designed tension mechanism.

Static and dynamic material properties of CFRP/epoxy laminates

Carbon fiber reinforced polymer (CFRP) has been extensively used to strengthen structures owing to its outstanding mechanical properties. With an increasing threat from terrorist bombing attacks and accidental explosions, the application of CFRP has been extended to mitigate the effect of blast loading on structures. A better understanding of the dynamic material properties of CFRP/epoxy laminates at high strain rates is therefore needed for more reliable analysis and design of CFRP strengthened structures under dynamic loadings. In this study, the unidirectional tensile properties of CFRP (SikaWrap®-230C) and epoxy resin (Sikadur®-330) laminates is investigated experimentally over a wide range of strain rates. Quasi-static and low-speed tensile tests are conducted at strain rates varying from 7×10−5s−1 to 0.07s−1. Then, high-speed tensile tests are performed using a high-speed servo-hydraulic testing machine at strain rate from about 10s−1 to 240s−1. The testing results show that both the tensile strength and the stiffness of the CFRP/epoxy laminates are insensitive to loading speed when the strain rate is less than 50s−1. However, when strain rate is over 50s−1, both the tensile strength and the coupon stiffness increase with the increase of strain rate. High-speed camera images are used to assist inspecting the failure modes of CFRP/epoxy laminates. It is found that under high-strain rate tension CFRP/epoxy laminates fail differently from that at low-strain rate. The different failure mode is believed to contribute to the increment of laminate strength. The testing data are analyzed together with available testing results on CFRP/epoxy laminates at various strain rates. Empirical formulas of dynamic increase factor for CFRP material are derived for better prediction of material strength at various strain rates.

高速引張試験によるカーボンおよびガラス繊維織物を用いたFRTPとFRPの45°方向の特性

高速引張試験によるカーボンおよびガラス繊維織物を用いたFRTPとFRPの45°方向の特性