A study of localisation in dual-phase high-strength steels under dynamic loading using digital image correlation and FE analysis

Tensile tests were conducted on dual-phase high-strength steel in a Split-Hopkinson Tension Bar at a strain-rate in the range of 150-600/s and in a servo-hydraulic testing machine at a strain-rate between 10-3 and 100/s. A novel specimen design was utilized for the Hopkinson bar tests of this sheet material. Digital image correlation was used together with high-speed photography to study strain localisation in the tensile specimens at high rates of strain. By using digital image correlation, it is possible to obtain in-plane displacement and strain fields during non-uniform deformation of the gauge section, and accordingly the strains associated with diffuse and localised necking may be determined. The full-field measurements in high strain-rate tests reveal that strain localisation started even before the maximum load was attained in the specimen. An elasto-viscoplastic constitutive model is used to predict the observed stress-strain behaviour and strain localisation for the dual-phase steel. Numerical simulations of dynamic tensile tests were performed using the non-linear explicit FE code LS-DYNA. Simulations were done with shell (plane stress) and brick elements. Good correlation between experiments and numerical predictions was achieved, in terms of engineering stress-strain behaviour, deformed geometry and strain fields. However, mesh density plays a role in the localisation of deformation in numerical simulations, particularly for the shell element analysis

Effect of pre-straining on localisation and fracture of dual-phase steel at elevated rates of strain

The present paper is aimed at studying the effect of pre-straining on the localisation and fracture in dual-phase steel under dynamic loading conditions. This study was conducted by using tensile tests at elevated rates of strain.A dual-phase steel DP800 sheet of thickness 1.5 mm was pre-deformed to three different levels of effective strain 1%, 4% and 8%. Thereafter, sub-size tensile specimens were machined from the pre-deformed sheets in the pre-strained direction. Tensile tests at elevated rates of strain in the range of 10−3 s−1 to 103 s−1 were conducted by using a conventional servo-hydraulic machine and a split-Hopkinson tension bar. A high-speed video camera was used to record the sequence of deformation during testing. A full-field measurement technique, namely digital image correlation, was utilized to measure the local strains up to fracture from the recorded digital images. It was found out that the pre-straining and dynamic loading have a marked effect on the strain localisation and subsequent fracture

A hydro-pneumatic machine for intermediate strain-rates: Set-up, tests and numerical simulations

A hydro-pneumatic machine for performing material tests at intermediate strain-rates is described, and the principle of operation is given. The behaviour and operation of the hydro-pneumatic machine are further evaluated by an analytical model and non-linear finite element analysis. Several tests were conducted on Weldox 460E steel and aluminium alloy 7075 at strain rates from 1 to 150 s−1. A modified Johnson-Cook model was used to represent the material behaviour at high strain rates. Finally, the accuracy of the measurement signals and stress wave reflections in the hydro-pneumatic machine are discussed by comparing the experimental data with results from non-linear finite element analysis

Comparative Numerical and Experimental Modal Analysis of Aluminum and Composite Plates

The paper presents the comparative analysis of the dynamic behavior of two rectangular plates of different material, aluminum and composite. While their global geometric dimensions (length, width and thickness) are similar, their inner structures are quite different. Whereas aluminum plate can be considered isotropic, the composite plate is a unidirectional carbon-epoxy laminate. Modal characteristics of the two plates were determined both numerically and experimentally and a comparative analysis of the obtained results was performed. Responses of the plates were documented by an optical, contactless 3D digital image correlation (DIC) system that contains a set of high-speed cameras capable of recording the movement of the white-and-black stochastic pattern applied to the upper surfaces of the plates. Numerical simulations were performed by the finite element method (FEM) in the commercial software package ANSYS. The plates were excited by a modal hammer and allowed to freely oscillate. In order to determine the natural frequencies of the plates the recorded time-domain responses were post-processed, i.e. converted to the frequency domain by fast Fourier transform (FFT). The first three natural modes were successfully experimentally established and compared to the corresponding numerical values. Since the differences between the two sets of results are less than 5%, the applied experimental technique can be considered valid and suitable for a wide range of engineering problems involving vibrations