Wojciech Moćko

Mechanical Properties of A359/SiCp Metal Matrix Composites at Wide Range of Strain Rates

The paper presents constitutive model of the aluminium metal matrix composite reinforced by a silicon carbide. Developed equation includes an empirically estimated term which takes into account softening effects of the composite due to reinforcement damages at a large strain. Experimental investigation of the aluminium based MMCs reinforced by silicon carbide of volume fraction equal to 0%, 10%, 20% and 30% were carried out. Tests were conducted at wide range of strain rates and large magnitudes of strains. Comparison between experimental and predicted data shows that the elaborated model may be applied for composite materials in computer simulations of large deformations.

Application of an extended Rusinek–Klepaczko constitutive model to predict the mechanical behavior of 6082-T6 aluminum under Taylor impact test conditions

The application of an extended Rusinek–Klepaczko constitutive equation to predict the mechanical response of 6082-T6 aluminum under the Taylor impact test conditions was presented in this article. The numerical results obtained in the study were verified through a comparison with the experimental data extracted from the Taylor anvil-on-rod impact experiments. It was concluded that the extended Rusinek–Klepaczko constitutive model predicts the behavior of the tested aluminum alloy under applied loading conditions with satisfactory accuracy. Moreover, it was found that the plastic wave phenomenon in this material is very limited and that there was no region of constant plastic wave velocity. Strain rates up to 1.6 × 104 s−1 were recorded during the Taylor impact experiments; therefore, this value may be set as the upper limit of the extended Rusinek–Klepaczko model for the alloy, which was validated with the anvil-on-rod experiment.

Effects of pre-fatigue on the strain localization during tensile tests of DP 500 steel at low and high strain rates

The analysis involved subjecting DP 500 steel to pre-fatigue loads, and then tension at high strain rates using Hopkinson bar. Digital image correlation method was used to investigate how the pre-fatigue loads change the strains’ distribution on the surface of the sample subjected to tension. The analysis involved both films recorded at low rates of deformation (1.0 × 10−2 s−1) using ARAMIS system and the images captured with a high-speed camera during dynamic deformations with a Hopkinson bar (6.0 × 102 s−1). It was noted, based on the micro-structural analysis, that pre-fatigue loads cause the formation of micro-damages in the examined material. Thus, macroscopically observed stress–strain characteristic as well as Huber–Mises substitute strains’ distribution determined locally by the image correlation method is also subject to changes. The observed effects include the following: reduction of deformation corresponding to the tensile strength, decrease in elongation at break, and increase in yield limit and tensile strength. The observed effects are intensified with an increased stress value and pre-fatigue cycles’ number. Furthermore, these phenomena are more intensive in the conditions of dynamic deformation.

Dynamic properties of aluminium alloys used in automotive industry

Around 20% of CO2 emitted as a result of human activity on Earth comes from transportation. One of the ideas, which lead to diminishing of the greenhouse gases emission, is reducing of fuel consumption. It may be achieved by introducing a new powertrain solutions as well as lowering overall vehicle weight. The reduction of vehicle weight may be obtained by both a new design of part and structures and application of a new materials i.e. aluminium alloys. It has to be emphasised that weight reduction is very important for combustion engine driven vehicles and electric driven vehicles as well. Mass lowering is especially desirable in the case of electric vehicles because they typically have a very narrow operational range usually lower than 200 km. Therefore even small weight reduction decreases energy consumption of EV and as a consequence increases its range, which is a crucial parameter for users. The results presented in this article were focused on high strength aluminium alloys of 6082-T6 and 7075-T6 types. Applying of those materials enables mass lowering of structures up to 50%, maintaining original functionality. Moreover, aluminium alloys may be also used in energy absorbing structures. The goal of tests was to obtain data required in FE analysis. In order to simulate dynamic phenomenon i.e. vehicle crash investigation of the stress-strain curves of alloys were carried out at wide range of strain rates using Hopkinson bar

Application of austenitic steels in energy absorbing structures

Occupants protection during vehicle crash is one of the major problems considered at vehicle designing stage. The increasing safety as well as comfort requirements results in the total vehicle weight increase. On the other hand, higher mass influence on the fuel economy of vehicle. Thus, it is very important to maintain or even reduce vehicle weight. One of the solutions of this problem is application of a new materials having good strength-to-weight ratio i.e. high strength steels (HSS). They are often used in protective and energy absorbing structures in automotive industry. One of HSS so-called austenitic steels are characterized by excellent mechanical properties such as: high strength, strong strain, and strain rate hardening. Those mechanical effects are consequence of microstructural evolution during strong deformations i.e. twinning and phase transformation. As a consequence those steels are very good material for the purposes of energy absorbing and protective structures. This article presents the results of mechanical properties tests of VP159 austenitic steel. On the basis of experimental data, the JC constitutive model has been calibrated. The obtained results were used for the computer simulations of energy absorbing of square profiles made of analysed material. The results were compared with other types of steel.

Numerical simulation of dynamic weld compression

The article presents some numerical results and experimental validation of Split Hopkinson pressure bar (SHPB) tests for welded S40NL steel. The goal of this research is to define material constants for modelling it in FEM. Steel was tested with Charpy impact test to determine properties of material. Next, the joint for welding was prepared. It was welded with electric arc welding method (MAG) with flux-cored wire. Hopkinson bar test is well-known experiment method used to determine material properties at high strain rates. The tests were performed in Institute of Fundamental Technological Research. Material properties for Johnson-Cook material model were obtained. Comparison between experimental results taken in quasi-static conditions and dynamic conditions proves that the behaviour of materials in those two states is quite different. Results from one type of loading condition cannot be used to create a realistic model of material when it is loaded dynamically. Numerical simulation of Hopkinson bars was performed on cylindrical model with known length and accelerated to high speed in direction of incident bar. For the purpose of the simulation, a Finite Element Code LS-DYNA was used. It allows simulation of dynamic response of SHPB system. The results show quite good agreement. The model can be used to simulate weld performance under high strain rate.

INFLUENCE OF ANISOTROPY ON THE VISCOPLASTIC PROPERTIES OF A HOT ROLLED Ti6Al4V TITANIUM ALLOY

In this work, the influence of strain rate on the anisotropy of the Ti6Al4V titanium alloy has been analyzed. Tensile tests of notched specimens were carried out in three loading orientations (0°, 45°, and 90°) with respect to the rolling direction, using the servo-hydraulic testing machine and Hopkinson bar. Investigation was supported by the digital image correlation analysis of strain distribution on the specimen surface and assessment of the fracture mechanism. The Ti6Al4V titanium alloy reveals a typical strain rate hardening behavior; however, strain rate sensitivity is independent of the loading orientation. Increases of the loading orientation results in material softening, observed as lowered yield stress, whereas plastic strain exponent and modulus remain unaffected. Fracture strain decreases with loading orientation at quasi-static and dynamic loading conditions.