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Title: Sandwich steels for crash energy absorption applications
Author: Sharma, Sanjeev
ISNI:       0000 0004 5371 1359
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2014
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This thesis has examined the applicability of steel–polypropylene–steel sandwich materials for the role of axial energy absorbers, an application previously undescribed in the literature. The results show that energy absorption performance of steel–polypropylene–steel sandwich materials can be predicted to within –2% and +8%, as well as highlighting the potential for their use in automotive applications. The work has demonstrated that the deformation modes in the steel–polypropylene–steel sandwich mimic the monolithic metal crash structure, however, with smaller fold radii, hypothesised to be due to shear in the polypropylene core. It was observed that increasing the core thickness increased the radius of the folds in the structure when undergoing collapse. Though due to the variability in the folding patterns of sandwich material in the crash structures seen in this work, it could not be stated with certainty. From the physical testing, the effect of core thickness for a fixed skin thickness is also defined. The physical tests showed a linear relationship between increasing core thickness and mean crush force. Further, the effectiveness of increasing the core thickness on the specific energy absorption was identified. The testing also showed an unprecedented >60% increase in energy absorption from quasi–static to dynamic for all three thicknesses of Steelite sandwich material, a level not seen in monolithic metal crash structures. Hence, suggesting an increased strain rate sensitivity of steel in MPM sandwich materials over the monolithic steel, a property which has been suggested in the literature for tensile tests but unknown in axial crash deformation. The testing demonstrated the potential for the crushing mode to change from a desirable progressive crushing mode to an undesirable and difficult to predict progressive failure. This occurred with a 7:1 core to skin thickness ratio, though failure of the steel skin is seen at all ratios. A 70%:30% ratio of thickness for the polypropylene core to steel skin is shown to minimise steel skin failure, i.e. the individual steel skin thickness should be no less than 15% of the total sandwich thickness. Finite element analysis presented in this thesis shows a single shell element model with laminated shell theory invoked can be used in LS–DYNA to predict the performance of the steel–polypropylene–steel sandwich materials. However, there is a potential thickness limit for which the model is applicable for the single hat and backplate crash structure considered; further research would be required to increase the confidence in the model. The single shell element model was accurate to within +8% of the physical test results. An analytical solution fitted the LS–DYNA single shell element model well and showed increasing the core thickness is more effective at increasing the specific energy absorption than increasing the skin thickness. The analytical solution also shows the potential for a steel–polypropylene–steel sandwich with a core to skin ratio of 70%:30% ratio by thickness to equal the performance of high strength aluminium alloys.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council ; University of Warwick
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TL Motor vehicles. Aeronautics. Astronautics