Crashworthiness assessment considering the dynamic damage and failure of a dual phase automotive steel

Analyzing crash worthiness of the automotive parts has been posing a great challenge in the sheet metal and automotive industry since several decades. The present contribution will focus on one of the most urging challenges of the crash worthiness simulations, namely, an enhanced constitutive formulation to predict the failure and cracking of structural parts made from high strength steel sheets under impact. A hybrid extended Modified Bai Wierzbicki damage plasticity model is devised to this end. The material model calibrated using the experimental data covering high strain rate deformation, damage and failure successfully predicted the instability and subsequent response of the crash box under impact. Simulation results provide the deformation shape and deformation energy in order to predict and evaluate the vehicle crashworthiness. The simulations further helped in discovering the irrefutable impact of strain rate and stress state on the impact response of the auto-body structure. The strain rate is found to adequately affect the energy absorption capacity of the crash box structure both in terms of impact load and fold formation whereas the complex stress state has a direct association to the development of instability within the structure and early damage appearance within the folds

Influence of the geometry of hat-shaped specimens on the formation of adiabatic shear bands

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Experimental study of the high strain rate shear behaviour of Ti6Al4V

Three different high strain rate shear test techniques are applied on the titanium alloy Ti6Al4V. Two techniques for testing of bulk materials and one technique for sheet materials are used: torsion of thin-walled tubes, compression of hat-shaped specimens and tension of planar shear specimens. The tests are carried out on respectively torsion, compression and tensile split Hopkinson bar setups. Although shear stresses dominate the stress state in these three tests, the local stress state and its distribution and evolution are different. Therefore, the three techniques are considered to be rather complementary than equivalent tests. In this work, the value of the three test techniques for material characterization is evaluated. Where possible, digital image correlation (DIC) is used to clarify the test results. In addition, parameters difficult to assess experimentally are estimated through finite element simulations of the three tests

Numerical study of the influence of the specimen geometry on split Hopkinson bar tensile test results

Finite element simulations of high strain rate tensile experiments oil sheet materials using different specimen geometries are presented. The simulations component ail experimental study, using a split Hopkinson tensile bar set-up, Coupled with a. full-field deformation measurement, device. The simulations give detailed information on the stress state. Due to the small size of the specimens and the way they are connected to the test device, non-axial stresses develop during loading. These stress components, are commonly neglected, but, as will be shown, have a distinct influence on the specimen behaviour and the stress-strain curve extracted from the experiment. The validity; of the basic assumptions of Hopkinson experiments is investigated: the uniaxiality of the stress state, the homogencity of the strain and the negligibleness of the deformation of the transition zones. The influence, of deviations from these assumptions on the material behaviour from a Hopkinson experiment is discussed

Temperature development during sliding on different types of artificial turf for hockey

In the past, hockey players used to get burning and scraping injuries from making a sliding on artificial turf. In order to assess and compare this risk for different surfaces, a sliding tester has been developed, measuring temperature rise during a sliding. 3 surfaces have been tested: a sand-filled, a full synthetic hockey field and a third generation soccer field with sand and rubber infill. In dry conditions, the full synthetic field gave the highest temperature rise and the sand field the highest abrasion. In wet conditions, the temperature rise for all surfaces was much smaller

The use of 2D and 3D high-speed digital image correlation in full field strain measurements of composite materials subjected to high strain rates

The aim of this paper is to assess and compare the performance of both high speed 2D and 3D digital image correlation (DIC) configurations in the characterization of unidirectional carbon fiber reinforced epoxy composites in high strain rate tension in the transverse direction. The criteria for assessment were in terms of strain resolution and measuring the strain localization within the gauge section. Results showed the high-speed 3D DIC technique has lower strain resolution compared to the high-speed 2D DIC technique. In addition, the analysis of the full strain fields indicated that the 3D DIC technique could accurately locate and measure the concentrations of strains within the gauge section of the tested samples

High-speed sheet metal forming : numerical study of high speed Nakajima testing

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Inverse modelling with integrated digital image correlation and finite element method for estimation of static and dynamic fracture parameters

Classical material testing techniques experience limitations for material model parameter identification especially when heterogeneous strain fields occur. Indeed, when only global test parameters such as force and elongation are used to calculate material parameters, significant discrepancies are often observed between finite element simulations of the test and the actual test. These discrepancies can be attributed to inconsistencies in sample machining, material variations and testing conditions, however, also to the material parameter determination based on global data. Major improvements can be obtained when full field comparison of the test data with finite element analysis is considered to calculate the constitutive model parameters. In this study, a coupled numerical- experimental technique has been developed to estimate the fracture parameters of both ductile and brittle materials. The approach is applied to a set of smooth and notched specimens of a dual phase steel targeting at specific stress states and tested in tension at dynamic rates to extract the fracture parameters. Additionally, the method is utilized to find the model parameters from dynamic tensile tests on basalt epoxy composite. The parameter identification is based on an iterative finite element model update procedure in which the results obtained by the numerical simulation are compared with full field deformation measurements using a digital image correlation (DIC) technique. A least square cost function is used to assess the gap between the inhomogeneous displacement or strain fields obtained from measurements and the simulated fields. Minimization of the cost function is ensured by the Levenburg- Marquardt algorithm. Generally, the influence of a parameter on the displacement field is of the order of the image acquisition noise. The tight integration between mechanical model and digital image correlation enables direct identification of unknown parameters while regularizing the displacement field with a set of interpolation functions chosen to span local zones of interest thereby increasing the noise robustness. Through this method, the versatility of the finite element method is translated to the experimental realm, simplifying the existing experiments and creating new experimental possibilities. Moreover, to demonstrate the general applicability of the proposed method, the integrated approach is also coupled with contour plots so as to quantify the effect of radial inertia and end friction in static and dynamic compression tests

High strain rate testing of fibre-reinforced composites

With the increasing use of high performance composite materials in many applications, accurate constitutive material models are required to predict the high strain rate response of composite structures. The accuracy of these models is highly dependent on the quality of the experimental data used to validate them. The split Hopkinson tension bar is considered the most suitable experimental technique to study the high strain rate tensile behavior of composite materials. However, many challenges are associated with the high strain rate tensile testing of brittle composite materials, particularly the design of sample geometry and the optimization of the measurement techniques for accurate force and low deformation measurement. The aim of this work is to address these challenges when studying the high strain rate behavior of glass and basalt epoxy composites using the split Hopkinson tension bar. Several dog-bone sample geometries were assessed in terms of the establishment of the quasi-static stress equilibrium and the development of a homogeneous strain distribution across the gauge section, with the aid of FE models. Additionally, special semiconductor strain gauges were used on the bars to achieve a high force signal to noise ratio. Finally, the use of high speed digital image correlation technique to accurately measure the full strain fields locally in the gauge section of the sample was also discussed