| Thin-walled tubes have been extensively employed as energy absorber.Conventionally,they have a polygonal section,such as triangle,square,hexagon and octagon.However,there are two defects of those conventional tubes.Firstly,the axially crushed tube usually renders an extremely high peak force,which may result in severe injury or damage to the people or structures being protected.Secondly,the collapse mode of conventional tubes is natural mode(i.e.,the folding waves form naturally)that limits the energy absorption capacity.Therefore,plenty of researchers devote themselves to improve the structural crashworthiness.For example,the tapered square or circular tubes and multi-cell tubes are proposed as energy absorbers.In addition,all kinds of dents,holes and patterns are introduced to the tube walls.In this paper,a novel origami-inspired tube known as the origami crash box is investigated.The idea that designing this tube is introducing a delicately designed origami pattern to conventional square tube.This strategy is used to reduce initial peak force and improve mean crushing force.Theoretical,experimental and numerical methods are employed to investigate the origami crash boxes when they are subjected to different load,including axial and oblique,quasi-static compression and low velocity impact.The theoretical prediction for mean crushing force of origami crash boxes under quasi-static compression has been carried out.The experiments have been designed to analyze and validate the developing mechanism of prediction errors in rare cases.In addition,the energy absorption characteristic of collapse modes has been analyzed.And the strategy for imperfection sensitivity analysis has been developed.It has been found that the complete diamond mode that is the most outstanding mode is not easy to be triggered due to that it is sensitive to imperfection.A bulkhead reinforced origami crash box is proposed as an imperfection insensitivity structure based on the above discovery.Moreover,a novel trapezoid origami crash box with trapezoid folded lobe is proposed.The energy absorption capacity of this tube is far superior to conventional square tube,octagonal tube and diamond origami crash box.The main works of the dissertation are as follows:1.The prediction formula of mean crushing force for complete diamond mode of origami crash boxes is deduced based on the super folding element theory.Quasi-static experiments on three types of origami crash boxes are designed to validate the numerical models.It is found from the comparison between numerical result and theoretical prediction that the theoretical formula could accurately predict the mean crushing force of origami crash boxes in most cases.The error analysis is performed based on the experimental and numerical methods aiming to find out the causes leading to the large errors between theoretical prediction and numerical results in rare cases.The result shows that about 10 percent of total energy is absorbed by two representative regions which are assumed as rigid plates in super folding element theory.This is the main cause resulting in the reduction of applicability of super folding element theory.In addition,the results obtained from quasi-static axial compression tests and numerical simulations suggest that the complete diamond mode is insensitive to geometric imperfection when the tube is subjected to quasi-static loading.And the tubes with longer modules performs better than those with short modules.(See Chapter 2)2.Thirty-three dynamic tests on thin-walled tubes including conventional square tubes and two types of origami crash boxes were conducted on a drop hammer rig.Complete diamond mode as well as two new collapse modes,which are local buckling mode and symmetric mode,were observed in tests.The comparison among those three collapse modes suggests that the complete diamond mode is the most efficient one and the symmetric mode is the most inefficient one in terms of energy absorption.The effect of local buckling on mean crushing force is presented to analyze the characteristics of those three collapse modes.It is found that the mean crushing force decreases with the increase of the number of buckling points.In addition,experimental results validate that origami crash boxes perform better than the conventional square tubes.And the mean crushing force of origami crash boxes with longer modules is larger than that with shorter ones.(See Chapter 3)3.Results from previous experiments reveal that unintentional imperfection could trigger the symmetric mode with low mean crushing force rather than a high-performance mode,known as complete diamond mode.Therefore,the imperfection-sensitivity of origami crash boxes is investigated in this paper.Appropriate geometric imperfection which is regarded as a substitution of the real defect is introduced into finite element models to trigger the symmetric mode.Numerical simulation shows that the specific energy absorption SEA declines with the increase in imperfection amplitude Ai.And a critical value of ratio Ai/t that is just able to trigger the symmetric mode is obtained.A detailed parametric analysis indicates that a suitable geometry is beneficial to improve the compliance of origami crash box,leading to stable collapse behavior with higher performance in terms of energy absorption.Moreover,a bulkhead reinforced origami crash box is proposed as a low imperfection-sensitivity energy absorption device.And an optimal wall thickness ratio t1/t is obtained through numerical analysis.(See Chapter 4)4.The energy absorption capacity of origami crash boxes subjected to oblique loading is investigated.A conventional square tube with identical weight is employed as benchmark.The comparative study reveals that the origami crash box is more desirable than the conventional square tube in most of the range of load angle.A parameter study is performed to assess the effect of geometry parameters on the energy absorption characteristics.The geometry parameters are tube length L,tube width b,module length I and width of folded lobe c.Considering that bamboo with large slenderness ratio could effectively resist wind load,a bulkhead reinforced origami crash box is proposed as a high-performance energy absorption device.And an optimum structure designed based on the parameter study is investigated.The result suggests that the proposed tube performs much better than the original design.(See Chapter 5)5.A thin-walled tube referred to as trapezoid origami crash box is proposed as an energy absorption device.The surface of this tube is prefolded in the light of a developable origami pattern that is delicately designed to introduce a special structure on the surface of a square tube.This special structure,known as trapezoid folded lobe,is employed as a type of geometric imperfection to lower the peak force and as a mode inducer to trigger the complete diamond mode.The quasi-static numerical simulations reveal that the complete diamond mode is successfully triggered.Moreover,geometric and compliance analysis suggest that three key parameters,the number of module M,dihedral angle ? and area ratio ?,could greatly affect the collapse behavior.Based on those analysis,an optimal trapezoid origami crash box is designed to compare with the conventional square and octagonal tubes of identical weight.Furthermore,a series of diamond origami crash boxes are analyzed to compare with the trapezoid origami crash boxes.The comparative results show that the trapezoid origami crash box is the most desirable in terms of energy absorption.(See Chapter 6)... |
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