PSI - Issue 68

E. Ezgi Aytimur et al. / Procedia Structural Integrity 68 (2025) 540–546 E. Ezgi Aytimur / Structural Integrity Procedia 00 (2024) 000–000

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vehicle manufacturers. One of the regulations is set by the National Highway Tra ffi c Safety Administration (NHTSA), a US agency of safety in transportation (NHTSA). On the other hand, crashworthiness tests are very expensive and time-consuming. Therefore, numerical simulations are used instead of real crash tests (Choudhari et al. (2019), Valayil and Issac (2013)). In the realm of crashworthiness simulations, the utilization of strain rate dependent material models holds paramount importance. These models o ff er a sophisticated means of understanding and predicting the behavior of materials subjected to high rates of deformation during collision (Sˇ krlec and Klemenc (2016)). Primarily, these models acknowledge the inherent complexity of material response under dynamic loading conditions, where the rate at which strain is applied significantly influences material behavior. One key facet of utilizing strain rate dependent material models lies in their ability to capture the intricate nuances of material behavior across a broad spectrum of loading conditions. Traditional material models, which assume a constant strain rate, often fall short in accurately representing the dynamic and nonlinear response exhibited by materials during high-speed impact events (Altair Radioss Manual (2021), LS-DYNA Keyword User’s Manual Volume II (2014)). In contrast, strain rate dependent models o ff er a nuanced approach that accounts for the varying rates of strain experienced by materials during a collision, thereby providing a more realistic portrayal of their mechanical properties. The incorporation of strain rate-dependent material models, such as the Johnson-Cook, and Cowper-Symonds models, profoundly enhances the understanding of crashworthiness in research and practical applications. These material models are indispensable in comprehending the dynamic behavior of materials subjected to high strain rates during crash events (Sˇ krlec and Klemenc (2016), Liu and Guedes Soares (2019)). Unlike static loading conditions, crash scenarios involve rapid and transient deformations, where the response of materials significantly deviates from their quasi-static behavior. While the Elastic model assumes instantaneous and reversible deformation under load, its inclusion in crashworthiness studies aids in establishing a foundational understanding of material behavior. By characterizing the initial sti ff ness and modulus of materials, the Elastic model serves as a reference point for evaluating the extent of plastic deformation and energy absorption during crash events (Altair Radioss Manual (2021)). The Johnson-Cook model introduces strain rate dependence and thermal softening e ff ects, making it particularly relevant for crashworthiness analyses involving high strain rate loading conditions. By incorporating empirical parameters that account for material properties such as strain hardening, and strain rate sensitivity, this model enables researchers to accurately predict material response under dynamic loading conditions. In the context of crashworthiness, the Johnson-Cook model facilitates the assessment of material behavior across a range of impact velocities and temperatures, o ff ering insights into energy dissipation mechanisms and failure modes. Similar to the Johnson-Cook model, the Cowper-Symonds model accounts for strain rate dependence, albeit with a simpler formulation. This model provides a computationally e ffi cient approach for characterizing material behavior under dynamic loading conditions, making it well-suited for crashworthiness simulations involving large-scale structural analyses. By capturing the essential features of strain rate-dependent behavior, the Cowper-Symonds model enhances the predictive capabilities of crash simulations, enabling engineers to optimize vehicle designs for improved occupant safety and structural integrity (Sˇ krlec and Klemenc (2016)). In this study, single impact simulations were conducted for three di ff erent material models of mild steel. The results were analyzed by comparing deformation, stress, deceleration, and restitution coe ffi cients. For the application of impact simulations, the studies of Valayil and Issac (2013) and Choudhari et al. (2019) have been taken as an example and carried out according to one of the regulations of NHTSA. The material is selected as mild steel and the strain rate e ff ect is observed according to the material card. Strain rate parameters are considered when the material is subjected to a large impact (Sˇ krlec and Klemenc (2016)). For this objective, Ansys LS-DYNA is preferred due to its ability to solve non-linear dynamic analysis and its large material library. Three di ff erent material models Johnson-Cook, Cowper-Symonds and elastic model (non-strain rate e ff ect) are investigated. The strain rate parameters of the two models Johnson-Cook and Cowper-Symonds are obtained from the study of Sˇkrlec and Klemenc (2016). The researchers studied mild steel E185 and present a general approach to estimating the material parameters at high strain rates. They evaluated strain rate parameters from experimental and numerical data. Thus, the material specifications are taken directly and applied to three di ff erent material cards. The material models are defined using 2. Materials & Methods