Indiana University-Purdue University Indianapolis (IUPUI) / The passenger safety is one of the most important factors in the automotive industries. At the same time, in order to improve the overall efficiency of passenger cars, lightweight structures are preferred while designing the vehicle structures. Among various structural optimization techniques, topology optimization techniques are usually preferred to address the issue of crashworthiness. The hybrid cellular automaton (HCA) is a truly nonlinear explicit topology design method developed for obtaining conceptual designs of crashworthy vehicle components. In comparison to linear implicit methods, such as equivalent static loads, and partially nonlinear implicit methods, the HCA method fully captures all the relevant aspect of a fully nonlinear, transient dynamic crash simulation. Traditionally, the focus of the HCA method has been on designing load paths in the crash component that increase the uniform internal energy absorption ability; thus far, other relevant crashworthiness indicators such as peak crushing force and displacement have been less studied. The objective of this research is to extend the HCA method to synthesize load paths to obtain the different acceleration-displacement profiles, which allow reduced peak crushing force as well as reduced penetration during a crash event. To achieve this goal, this work introduces the concept of achieving uniform energy distribution at variable design simulation times. In the proposed work, the design time is used as a new design parameter in topology optimization. The desired volume fraction of the final design and the design time provided two dimensional design space for topology optimization, which is followed by the formulation of design of experiments (DOEs). The nonlinear analyses of the corresponding DOEs are performed using nonlinear explicit code LS-DYNA, which is followed by topology synthesis in HCA. The performance of the resulting structures showed that the short design times lead to design obtained by linear optimizers, while long simulation times lead to designs obtained by the traditional HCA method. To achieve the target crucial crash responses such as maximum acceleration and maximum displacement of the structure under the dynamic load, the geological predictor has been implemented. The concept of design time is further developed to improve structural performance of a vehicle component under the multiple loads using the method of multi-design time. Finally, the design time is implemented to generated merged designs by performing binary operations on topology-optimized designs. Numerical example of the simplified front frame is utilized to demonstrate the capabilities of the proposed approach. / 2019-11-21

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Crashworthiness design is one of the most critical areas of automotive design. It is really demanding for the structure and can therefore have a large influence on the final design. It is also difficult to model accurately and costly to simulate which has an important impact on the design process. Most car companies have now stopped addressing crashworthiness design with trial and error approaches, in favour of more advanced automated structural optimisation methods. While most relevant applications so far use size or shape optimisation, the ultimate way to achieve significant mass reduction is to use topology optimisation. However, topology optimisation methods for crashworthiness design are still a work in progress. Due to the high non-linearity of crash simulations, well-established classic topology optimisation methods cannot be applied directly to crashworthiness design. Alternative methods have been and keep being developed such as the Equivalent Static Loads method, the Ground Structure Approach or the Hybrid Cellular Automata (HCA). This thesis introduces an adapted version of Hybrid Cellular Automata using thin-walled ground structures. It combines the advantages of computing a real crash simulation while producing as an output a thin walled based topology needing minimal post-processing effort to be translated into a realistic design. In this method, the topology optimisation domain is filled up with a ground structure of thin walls which constitutes the elementary cells of the HCA method. These macro-elements replace the solid mesh elements used in the classic HCA approach. The details and implementation of the method are presented and discussed. Different application examples are detailed, including defining reinforcement patterns within extruded beams. Enriched space fillings patterns are studied and industrial application examples are presented. Eventually, recommendations for further studies and applications of the method are given.

Numerical modeling of failure was performed for magnesium alloys with circular and square cross-sections under axial compression. The failure criterion was employed using material model 124, where failure was simulated using the element deletion method. LS-DYNA material model 124 (MAT_124) was calibrated using stress-strain curves in compression and tension. This approach, combined with MAT_124, captures the material asymmetry. Comparisons with experiments showed that the failure criterion accurately predicted the stress-strain behavior during axial compression tests of the round tubes of magnesium alloy, AZ31. A parametric study was also performed to investigate the effects of various phenomena on simulated results. Numerical modeling of square magnesium tubes during bending was also simulated for extruded magnesium alloys AZ31, AM30 and AM60. The failure criterion, based on element erosion, was used in these models to simulate fracture for all three alloys. Comparisons with experiments, for all three alloys, showed that the proposed numerical model accurately predicted the force-displacement curves during bending. Engineering strain at failure was found from the tensile test curves for the three magnesium alloys (AZ31, AM30 and AM60). Simulations were done to predict local strain at the necking region at this engineering strain. The necking strain was incorporated in the failure criterion, which considerably improved results for the bending simulations. Numerical modeling of slow and fast axial compression tests were also performed for AM30, AM60 and AZ31 magnesium tubes with square cross-section. Comparisons with experiments, for all three alloys, showed that the proposed numerical model accurately predicted the force-displacement curves during quasi-static and high-speed crush tests. Furthermore, the predicted fracture locations and patterns were in good agreement with experimental observations. Finally, new failure criteria was employed to improve the crashworthiness behavior of magnesium alloys by several tube design variations. Magnesium tubes cladded with aluminum and magnesium tubes with alternating strips of aluminum were simulated. Magnesium tubes with thinned sections and spirals were also simulated. Results showed that most of the design modifications increased the crashworthiness of magnesium alloys tubes.

No description available.

No description available.

Numerical modeling of failure was performed for magnesium alloys with circular and square cross-sections under axial compression. The failure criterion was employed using material model 124, where failure was simulated using the element deletion method. LS-DYNA material model 124 (MAT_124) was calibrated using stress-strain curves in compression and tension. This approach, combined with MAT_124, captures the material asymmetry. Comparisons with experiments showed that the failure criterion accurately predicted the stress-strain behavior during axial compression tests of the round tubes of magnesium alloy, AZ31. A parametric study was also performed to investigate the effects of various phenomena on simulated results. Numerical modeling of square magnesium tubes during bending was also simulated for extruded magnesium alloys AZ31, AM30 and AM60. The failure criterion, based on element erosion, was used in these models to simulate fracture for all three alloys. Comparisons with experiments, for all three alloys, showed that the proposed numerical model accurately predicted the force-displacement curves during bending. Engineering strain at failure was found from the tensile test curves for the three magnesium alloys (AZ31, AM30 and AM60). Simulations were done to predict local strain at the necking region at this engineering strain. The necking strain was incorporated in the failure criterion, which considerably improved results for the bending simulations. Numerical modeling of slow and fast axial compression tests were also performed for AM30, AM60 and AZ31 magnesium tubes with square cross-section. Comparisons with experiments, for all three alloys, showed that the proposed numerical model accurately predicted the force-displacement curves during quasi-static and high-speed crush tests. Furthermore, the predicted fracture locations and patterns were in good agreement with experimental observations. Finally, new failure criteria was employed to improve the crashworthiness behavior of magnesium alloys by several tube design variations. Magnesium tubes cladded with aluminum and magnesium tubes with alternating strips of aluminum were simulated. Magnesium tubes with thinned sections and spirals were also simulated. Results showed that most of the design modifications increased the crashworthiness of magnesium alloys tubes.

No description available.

Thin-walled metallic components have been widely used as energy absorbers. One key drawback is the high initial crippling load, which typically results in passenger injuries. It is the objective of this study to introduce taper angle to thin-walled prisms, and to examine the crushing response of thin-walled frusta. Nonlinear finite element models of thin-walled frusta of different cross-sectional geometries were developed. Experimental investigations were conducted to validate these models. The effects of key design parameters on the energy absorption characteristics of frusta were explored. Comparison between thin-walled prisms and frusta show that taper angle helps to reduce the initial crippling load and increase the resistance to global buckling. To take advantage of the interaction effects, a novel multi-frusta configuration was developed and it was shown that the energy absorption efficiency is significantly increased. The results of this work are valuable for enhancing the crashworthiness performance of thin-walled metallic energy absorber.

Thin-walled metallic components have been widely used as energy absorbers. One key drawback is the high initial crippling load, which typically results in passenger injuries. It is the objective of this study to introduce taper angle to thin-walled prisms, and to examine the crushing response of thin-walled frusta. Nonlinear finite element models of thin-walled frusta of different cross-sectional geometries were developed. Experimental investigations were conducted to validate these models. The effects of key design parameters on the energy absorption characteristics of frusta were explored. Comparison between thin-walled prisms and frusta show that taper angle helps to reduce the initial crippling load and increase the resistance to global buckling. To take advantage of the interaction effects, a novel multi-frusta configuration was developed and it was shown that the energy absorption efficiency is significantly increased. The results of this work are valuable for enhancing the crashworthiness performance of thin-walled metallic energy absorber.