High Rate Tensile Tests Research Articles

Capability of New High Strength ADI‐Materials for Automotive Components under Crash Loading

The development of new high strength ADI (Austempered Ductile Iron) materials and their evaluation for light weight design purposes are parts of the German research project LEA. The Fraunhofer IWM is investigating the capabilities of ADI materials under crash loading situations by conducting high rate tensile tests which quantify the material's strain rate sensitivity. These investigations result in a reduction of the austenitic phase content of initially about 35%, decreasing to 5–10% after fracture (as characterized by X‐ray diffraction), indicating a phase transformation comparable to a TRIP effect. Several ADI 1000 and ADI 1200 samples show an increase in strength of about 100 MPa up to crash‐relevant strain rates of 100 s−1, and the elongation at fracture is constant or increasing, in some cases up to more than 20%. Thus, the newly developed high strength ADI materials exhibit good potential for crash loaded automotive components.

Straining of Metastable Austenite as a Way to Improve NiTi Alloy Functional Properties

The effect of metastable austenite straining on the basic functional properties of an NiTi alloy in quasi-equilibrium state was studied. Straining of austenite in a pre-martensitic state was shown to result in improvement of one-way and two-way shape memory effects (OWSME and TWSME). This improvement took place after straining in a narrow temperature range, and its manifestation at quasi-static and high-rate tensile tests was different: the increment of both characteristics was sufficiently large after quasi-static straining, but smaller after high-rate straining. Straining at temperatures that deviated significantly from the Ms temperature led to deterioration of OWSME and TWSME.

Investigating direct tensile behavior of high performance fiber reinforced cementitious composites at high strain rates

This research reported the process of measuring direct tensile stress versus strain response of high performance fiber reinforced cementitious composites (HPFRCCs) at high strain rates between 10s−1 and 40s−1. High rate tensile tests were performed using a strain energy frame impact machine (SEFIM) built by authors. The stress history of HPFRCC at high rates was estimated from two strain gauges attached on two sides of a transmitter bar while the strain history was obtained by analyzing the sequential images recorded using a high speed camera. HPFRCCs exhibited strong rate sensitivity, i.e., their tensile parameters, including post cracking strength, strain capacity, peak toughness, and number of cracks, were significantly enhanced as the strain rate increased although the enhancement was different according to the types of fiber. The source of the dynamic enhancements in the tensile parameters of HPFRCCs was discussed.

High rate tensile tests – Measuring equipment and evaluation

Polymers are widely used in protection systems, e.g. in cars. For dimensioning those parts, crash simulation is performed. However, crash simulation programs require reliable high rate mechanical properties to provide accurate results. As polymers are viscoelastic materials Young's modulus and yield stress are rate dependent quantities. In order to determine these properties, special experimental devices and special evaluation methods have to be used. In this paper, the modification of the clamps of a servo-hydraulic tensile testing machine to reach a global strain rate of 670/s is described. Furthermore, a new evaluation method is shown to determine Young's modulus and yield stress using a curve fitting procedure. The results show that both Young's modulus and yield stress depend logarithmically on the strain rate. It is also shown that the rate dependency of the Young's modulus can be used to get rid of the stress oscillations superimposed on the stress signal.

Stress wave effects in the Flying Wedge high strain rate tensile testing device

Stress wave effects in the high-rate tensile testing apparatus known as the ‘Flying Wedge’ have been investigated using both experimental and numerical approaches. The experiments were conducted on R2000 glass/epoxy composites and iron specimens at strain rates up to 103/s. Numerical simulations were carried out using the LS-DYNA hydro code. Both the numerical and the experimental results showed that the results for small and low-strength specimens were significantly affected by the stress waves that propagate through the parts of the testing machine following the impact. The time period of the resulting load oscillations was found to be similar to that calculated from one-dimensional stress wave theory for different load paths in the apparatus. Therefore, it is concluded that stress wave effects play a dominant role in the Flying Wedge for small and low-strength specimens and consequently this machine is not a suitable choice for dynamic testing of such specimens.

High Strain Rate Behaviour of Aluminium Alloy Sheet

This paper presents results from quasi-static and high rate tensile testing of three aluminum sheet alloys, AA5754, AA5182 and AA6111, all of which are candidates for replacing mild steel in automotive bodies. Tests were performed at quasi-static rates using an Instron apparatus and at strain rates of 600 to 1500 s-1 using a tensile split Hopkinson bar. Additionally, an in-depth investigation was performed to determine the levels of damage within the materials and its sensitivity to strain rate. The constitutive response of all of the aluminum alloys tested showed only mild strain rate sensitivity. Dramatic increases in the elongation to failure were observed with increases in strain rate as well as greater reduction in area. Additionally, the level of damage was seen to increase with strain rate.

Efficiency and identification procedures of damage models in dynamic

To accurately predict the damage and failure evolution occurring in crashworthiness and sheet metal-forming simulations, a realistic material description is required. The parameters needed to describe such damage models are not easy to obtain and there is no direct test to achieve these parameters. An inverse method to identify the Gurson and Lemaître damage model parameters is presented. Quasi-static and high-rate tensile tests have been carried out on thin notched specimen made of high-strength steel (THR800) from ARCELOR. The aim was to study the combined effects of strain rate and stress triaxiality on the damage material behaviour. Macroscopic measured responses are the global forces and the width at the bottom of the notch versus the elongation. An image-processing software has been developed to measure the evolution of the width at the bottom of the notch versus the elongation from the video tests. The damage parameters are identified by correlating experimental and numerical macroscopic measurements.

A tensile impact apparatus for characterization of fibrous composites at high strain rates

A high rate tensile testing apparatus called “flying wedge” for testing fibrous composites at strain rates up to 10 3 s −1 has been described. R2000 glass/epoxy composite has been tested at a very low strain rate of 10 −3 s −1 using the universal Instron tensile testing machine, and at high strain rates up to nearly 850 s −1 using flying wedge. The results show significant increase in failure stress and reduction in failure strain for dynamic tests with respect to quasi-static experiments. While the reduction in failure strain fluctuates from 60 to 75%, the increase in failure strength varies from 300 to even 500% for different ply angles. Although, the results may be affected by the stress waves, which propagate through the parts of the testing machine on the impact, it is found that the stress wave's effects do not play a determinant role in this regard. The results indicate that the rate of the increase of stress versus strain rate slows down as ply angle increases. Scanning electron microscopy of the fracture surface of the specimens did not reveal significant difference between the failure mechanisms at low and high strain rates.

On the influence of stress triaxiality and strain rate on the behaviour of a structural steel. Part II. Numerical study

Non-linear finite element analyses of quasi-static and high-rate tensile tests with smooth and notched axisymmetric specimens of the structural steel Weldox 460 E have been carried out. The constitutive relation and fracture criterion of Johnson and Cook, which were adopted in the simulations, have previously been determined for Weldox 460 E steel. First, a validation study was completed to assess the accuracy of the constitutive relation and fracture criterion. The numerical results were compared with experimental data from tensile tests under quasi-static and dynamic loading conditions. Secondly, the use of Bridgman's analysis in the identification of the fracture criterion was evaluated, and the influence of adiabatic heating and inertia on the stress triaxiality in the tensile specimens was investigated. The results were finally used to discuss the identification of fracture criteria based on tensile tests with smooth and notched axisymmetric specimens.

High-rate tensile tests of metallic sheet materials

High-rate tensile tests of metallic sheet materials