Experimental and numerical investigation into impact bending collapse of rectangular hollow sections

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This thesis describes a program of research into the plastic moment - rotation (M-8) response of rectangular hollow section (RHS) steel tubes subjected to impact bending loads. The context for this research was the use of RHS in buses and vehicle rollover protection structures (ROPS) where the "plastic hinges" that form in the RHS members act as energy absorbing "crumple zones". In the design of such structures for crash worthiness, there is a need to understand the response of the plastic hinges to impact loading and this research has assessed the following methods used to determine the moment - rotation characteristics of plastic hinges under impact bending loads: * physical impact tests; * finite element modelling; and * the use of "dynamic magnifiers" to scale the results of quasi-static tests and analyses. In dynamic tests, the specimen's inertia complicates the measurement of plastic hinge properties. These complications were addressed by the design and development of a novel pendulum bend rig and instrumentation and the meticulous processing of test data. The test rig and procedures were successfully used to measure the bending collapse of 50x50x2 grade C350LO RHS specimens to a hinge rotation of 35°. Comparisons between the measured impact and quasi-static responses enabled the influence of the loading rate on plastic hinge response to be quantified. It is shown that the difference between the impact and static responses is essentially due to the RHS material's strain rate sensitivity. This influence of strain rate on the mechanical properties of the RHS material was characterised by a programme of uni-axial tensile tests at strain rates ranging from 10 -4 to 10 s -1. Detailed finite element analyses of the local buckling response were conducted using HKS ABAQUS-Standard. Excellent agreement between the predicted and measured impact M-8 responses of the local buckle was obtained using a "pseudo-dynamic" analysis procedure. This procedure correctly accounted for the effect of material strain rate sensitivity within a static analysis by controlling the loading rate. A critical assessment was made of the quasi-static scaling approach to predicting component impact response. The limitations of predicting impact moment and energy responses using a single scaling factor were demonstrated. It is shown that the most reliable results are obtained using a scaling factor derived experimentally. A theoretical approach proposed in the literature to predict RHS plastic hinge impact response is shown to overestimate the impact response of the RHS tested in this study. An alternative theoretical scaling factor is proposed that gave an improved prediction of RHS impact response.
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