Dynamic axial crushing of square tubes

Eighty-four dynamic tests on thin-walled square steel tubes having two different cross-sections with c/h = 30.25 and c/h = 32.18 and various lengths were crushed axially on a drop hammer rig. Approximate theoretical predictions were developed for the axial progressive crushing of square box columns using a kinematically admissible method of analysis. This theoretical study predicts four deformation modes which govern the behaviour for different ranges of the parameter c/h. New asymmetric deformation modes were predicted theoretically and confirmed in the experimental tests. These asymmetric modes cause an inclination of a column which could lead to collapse in the sense of Euler even for relatively short columns. The effective crushing distance is considered in the approximate theoretical analysis together with the influence of material strain rate sensitivity, which is important for steel even when the loadings are quasi-static. The simple equations presented herein for the design of axially crushed spuare box columns give reasonable agreement with the corresponding experimental results.     

Dynamic axial crushing of circular tubes

A series of axial crushing tests on steel circular cylindrical shells loaded either statically or dynamically is reported and compared with various theoretical predictions and empirical relations. A modified version of Alexander's theoretical analysis for axisymmetric, or concertina, deformations gives good agreement with the experimental results when the effective crushing distance is considered and provided that the influence of material strain rate sensitivity is retained in the dynamic crushing case.     

Energy absorption of a thin-walled cylinder with ribs subjected to axial impact

We performed experimental and theoretical analyses that show a thin-walled cylinder with stiff ribs can be used as a structural element to improve or adjust energy absorption characteristics. We conducted impact crushing tests using several different cylinders with ribs. The experimental results showed that the axisymmetric and non-axisymmetric crushing modes were dependent on not only the cross-section size but also on the distances between the ribs. A critical distance between the ribs was found to exist for generating axisymmetric and non-asxisymmetric crushing modes and it was more than double the wavelength of axisymmetric wrinkles regardless of cylinder size. The mean crushing forces of the axisymmetric modes were found to be roughly 1.3 times larger than those of the non-axisymmetric modes. The theoretical results based on plastic hinge behavior showed good agreement with the experimental results. The effects of material and cylinder size on the crushing behavior of a cylinder with ribs were expressed using approximate mathematical equations. The critical distance between ribs for generating axisymmetric or non-axisymmetric crushing mode was also expressed approximately. Stiff ribs appropriately spaced in a cylinder were found to be effective in absorbing a large amount of energy with a short crushing deformation.     

Dynamic progressive buckling of circular and square tubes

A series of over 120 axial crushing tests were conducted on circular and square steel tubes loaded either statically or dynamically. Approximate theoretical predictions for static and dynamic progressive buckling are developed. Fair agreement with the experimental results is achieved provided the effective crushing distance is taken into account and the infuence of material strain rate sensitivity is retained for dynamic loads.     

A finite element analysis of plane strain dynamic crack growth in materials displaying the Bauschinger effect

In this paper dynamic crack growth in an elastic-plastic material is analyzed under mode I plane strain small-scale yielding conditions using a finite element procedure. The main objective of this paper is to investigate the influence of anisotropic strain hardening on the material resistance to rapid crack growth. To this end, materials that obey an incremental plasticity theory with linear isotropic or kinematic hardening are considered. A detailed study of the near-tip stress and deformation fields is conducted for various crack speeds. The results demonstrate that kinematic hardening does not oppose the role of inertia in decreasing the plastic strains and stresses near the crack tip with increase in crack speed to the same extent as isotropic strain hardening. A ductile crack growth criterion based on the attainment of a critical crack opening displacement at a small micro-structural distance behind the tip is used to obtain the dependence of the theoretical dynamic fracture toughness with crack speed. It is found that for any given level of strain hardening, the dynamic fracture toughness displays a much more steep increase with crack speed over the quasi-static toughness for the kinematic hardening material as compared to the isotropic hardening case.     

Numerical modelling of dynamically loaded metal foam-filled square columns

The crushing behaviour of dynamically loaded metal foam-filled square columns has been investigated using an extended version of the existing self-similar pressure dependent constitutive model for metal foams. The model has been implemented in ABAQUS/Explicit and analyses have been conducted using different approaches to model the uniaxial and hydrostatic hardening behaviour of metal foams. A practical and reliable procedure to approximate the observed anisotropic behaviour within the computational framework of isotropic plasticity is introduced. The comparison between the available experimental and newly generated numerical results is presented in order to illustrate the accuracy and efficiency of the implemented model in predicting the crashworthiness of filled columns.     

In-plane Crushing Analysis of Cellular Materials Using Vector Form Intrinsic Finite Element

The crushing of cellular materials is a highly nonlinear problem, for which geometrical, material, and contact/impact must be treated in one analysis. In order to develop a framework able to solve it efficiently and accurately, in this paper procedures for in-plane crushing analysis of cellular materials using vector form intrinsic finite element (VFIFE) is performed. A beam element of VFIFE is employed to handle large rotation and large deflection in the cell walls. An elastic-plastic material model with mixed hardening rule is adopted to account for material nonlinearity. In addition, an efficient contact/impact algorithm is designed to treat the complex contact/impact encountered in crushed cellular materials. Numerical results performed reveals that the procedures proposed in this paper are sound and reliable to simulate crushing of diverse cellular materials.     

焊接箱形截面不锈钢柱相关稳定性能分析

采用有限元法对焊接箱形截面不锈钢柱的局部与整体相关稳定性能进行分析,研究利用板件的屈曲后强度,以实现更为经济合理的构件截面设计。建立了可以准确模拟不锈钢非线性材料力学性能、截面焊接残余应力和构件的局部与整体几何初始缺陷等因素的精确有限元数值模型,并依据试验结果对模型的可靠性进行了验证。基于验证可靠的有限元模型,开展了系统参数分析,同时补充了大量的数值计算数据点。根据得到的试验和有限计算结果,对相关稳定承载力的理论计算方法进行了比较分析,表明现有的方法应用较为复杂,而且可能高估构件的相关稳定承载力。针对奥氏体型和双相型两类不锈钢提出了通用的计算修正系数,给出了新的直接强度法计算公式,经试验与有限元计算结果验证,表明其能够准确计算焊接箱形截面不锈钢柱的相关稳定承载力。     

Approximate elastic-plastic analysis of the static and impact behaviour of polymer composite sandwich beams

An elastic-plastic beam bending model has been developed to simulate the post-upper skin failure energy absorption behaviour of polymer composite sandwich beams under three-point bending. The beam skins consist of woven and chopped strand glass, while the core is a resin impregnated non-woven polyester material known as Coremat. A polyester resin was used for the construction. The theoretical model consists of a central hinge dominated by a crushing core and tensile elastic strains in the lower skin. Experimental measurements of the non-linear force-deflection characteristics for the beam are compared to the theoretical predictions from the model, and it is shown that the shear crushing of the core has an important effect on the behaviour of the beam. The model shows that the most important material properties are the lower skin tensile failure strain and the core crushing strength. Dynamic effects are included in the model in the form of a strain rate dependence of the core crushing stress and the strain rate dependence of the failure strain in the lower skin. The increase in material strength with strain rate gives rise to an improved energy absorption capacity for the beam under impact loading.     

反复水平荷载作用下偏心常轴压箱形钢柱抗震性能的数值分析

建立考虑材料非线性、几何非线性、几何初始缺陷、残余应力影响的有限元模型,对反复水平荷载作用下偏心常轴压箱形钢柱的受力性能进行模拟分析。通过与相关试验结果的比较,验证了有限元模型的正确性。在此基础上,对85个箱形构件进行数值分析,研究反复水平荷载作用下常偏压箱形钢柱的抗震性能。研究结果表明,腹板宽厚比是影响构件抗震性能的最主要因素,腹板宽厚比越大,构件的刚度、承载力退化越严重,延性越差。对于中等长细比的构件,当轴压比较小时,构件的抗震性能受整体失稳、局部屈曲和塑性变形的相互影响;但当轴压比较大时,整体失稳将起控制作用。柱顶弯矩使构件的骨架曲线发生平移,但柱顶弯矩对构件抗震性能的影响相对较小。根据有限元计算结果,回归出反复荷载作用下偏心常轴压箱形钢柱可承受的最大柱顶剪力和位移延性系数的拟合公式。     

瓦楞纸箱强度的静态仿真分析及试验研究

选用某B型单瓦楞纸箱,将其切割成3段,对各段分别进行抗压试验,以探讨各段对整个纸箱强度的贡献度。考虑瓦楞纸箱的材料非线性和几何非线性,采用ANSYS有限元分析软件对纸箱上段和中段以及整个纸箱进行抗压试验仿真分析,以得到纸箱各段和整个纸箱的压缩变形结果、压溃力和压溃位移。结果表明:仿真分析结果与抗压试验结果基本一致,从而验证了所建模型的可行性,且纸箱的强度基本上取决于横向皱褶。     

Crushing analysis of polygonal columns and angle elements

Energy absorption characteristics of regular polygonal columns and angle elements under dynamic axial compression are investigated by using non-linear explicit finite element code LS-DYNA. The influence of central angle on deformation mode and mean crushing force of angle elements is studied. Based on two types of deformation mechanisms known by experiments, two types of initial indentation triggers are introduced. By assuming appropriate boundary condition on the edges, a simplified finite element model is adopted in the analysis of angle elements and validated by comparing with full polygonal column model. Numerical investigations are also carried out to study the influence of angle on angle elements with three and four panels by using the simplified model. Several useful conclusions are drawn about the axial crushing of polygonal columns and angle elements and can be used to guide the design of crashworthiness structures. In addition, a comparison is conducted between the numerical results and theoretical predictions of the mean forces of some special angle elements. Good agreement is obtained.     

Theoretical prediction and numerical simulation of honeycomb structures with various cell specifications under axial loading

The axial crushing of honeycomb structures with various cell specifications is studied analytically and numerically. Based on the Super Folding Element theory, a new method for predicting the mean crushing stress of honeycomb structures with various cell specifications under axial loading is developed. In this new theoretical method, two types of simplified folding modes named SFM1 and SFM2 are proposed. The mean crushing stress and the folding wavelength for honeycomb structures with various cell specifications are then determined by a minimum principle. The effective crushing distance and the loading rate effect are both considered. In order to illustrate the effectiveness of the proposed approach, numerical simulations are carried out by employing the explicit finite element code LS-DYNA. The bond of the honeycomb panels is simulated by using a tie-break contact. It can be seen that the analytical solutions are in agreement with the numerical results as well as the Wizerbicki’s solutions.     

Effect of natural stitched composites on the crashworthiness of box structures

In the present paper the effects of stitching on the energy absorption and crashworthy behaviour of composite box structures will be studied. The combination of unidirectional carbon fibre-reinforced polymer (CFRP) and glass fibre-reinforced polymer (GFRP) composite materials are used to laminate the composite boxes. Delamination study in Mode-I with the same lay-up was carried out to investigate the effect of stitching on delamination crack growth on energy absorption of stitched and non-stitched composite box structures. The double cantilever beam (DCB) standard test method was chosen for delamination studies. For non-stitched and stitched composite boxes the lamina bending and brittle fracture crushing modes were observed. It was found that the stitched composite boxes which show higher fracture toughness in Mode-I delamination tests, are not necessarily able to absorb more crushing energy in comparison with non-stitched composite boxes. It was also observed that the position of stitched area can affect the crushing mode and consequently energy absorption capability of composite box structures. The main reason can be related to other mechanisms such as bending, friction and bundle fracture which significantly contribute to energy absorption. The analytical model based on energy balance approach is proposed to estimate the mean crushing force, F_m, in axial crushing of square composite box.     

薄壁开孔圆管在轴向荷载作用下的理论研究

研究薄壁开孔圆管的轴向耐撞性有助于其在缓冲、吸能领域的广泛应用。通过分别考虑开孔区域和未开孔区域的能量吸收特征并引入材料的应变强化效应,根据塑性铰理论建立了轴向荷载下开孔圆管轴对称压溃模式的理论模型,得到了弯曲应变能、拉伸应变能、平均压溃力、比吸能的解析表达式。分析结果表明:该理论模型的预测结果与数值和实验结果相吻合;正则化平均压溃力会随半皱褶长细比的降低而显著增加;单层孔数对正则化平均压溃力的影响会随管壁厚度的增加或孔半径的减小而降低;比吸能可通过减少单层孔数或减小孔半径提高。     

合肥新桥机场航站楼大型箱形钢柱的拟静力试验

通过8根箱形钢柱在偏心常轴压、柱顶反复水平荷载作用下的拟静力试验,研究地震作用下合肥新桥机场航站楼大型箱形钢柱的壁板稳定性及其抗震性能。试验结果表明：在小震作用下,合肥新桥机场航站楼箱形钢柱处于弹性受力状态,且壁板稳定;在大震作用下,箱形钢柱壁板基本处于弹塑性受力状态,出现塑性变形,但壁板仍然满足稳定性要求。腹板宽厚比是影响构件抗震性能的重要因素,腹板宽厚比越大,滞回曲线越不饱满,试件的承载力及刚度退化越严重。最后,提出大跨度空间结构箱形钢构件的加工制作构造措施及抗震设计建议。     

Dynamic energy absorption characteristics of hollow microlattice structures

Hollow microlattice structures are promising candidates for advanced energy absorption and their characteristics under dynamic crushing are explored. The energy absorption can be significantly enhanced by inertial stabilization, shock wave effect and strain rate hardening effect. In this paper we combine theoretical analysis and comprehensive finite element method simulation to decouple the three effects, and then obtain a simple model to predict the overall dynamic effects of hollow microlattice structures. Inertial stabilization originates from the suppression of sudden crushing of the microlattice and its contribution scales with the crushing speed, v. Shock wave effect comes from the discontinuity across the plastic shock wave front during dynamic loading and its contribution scales with v². The strain rate effect increases the effective yield strength upon dynamic deformation and increases the energy absorption density. A mechanism map is established that illustrates the dominance of these three dynamic effects at a range of crushing speeds. Compared with quasi-static loading, the energy absorption capacity at dynamic loading of 250 m/s can be enhanced by an order of magnitude. The study may shed useful insight on designing and optimizing the energy absorption performance of hollow microlattice structures under various dynamic loads.     

Some observations on the dynamic elastic-plastic buckling of a structural model

The use of a simple imperfection-sensitive elastic-plastic model for studying the non-linear buckling of short columns and cylndrical shells under dynamic axial compression is discussed in this paper. The axial impact loading of the model by a mass with an initial velocity is considered as a particular example. The critical impact velocities are determined for wholly elastic and elastic-plastic materials with linear strain hardening characteristics. The results show that the maximum initial kinetic energy at the transition between stable and unstable behaviour after impact is sensitive to the magnitude of the material strain hardening parameter. It is also evident from the results that impact due to larger masses leads to larger lateral displacements at buckling and that the instability of a column is more sensitive to the presence of initial imperfections at higher impact velocities. Some guidance is offered on the choice of the various parameters in the idealized model for the analysis of practical engineering structures.     

Experimental and theoretical study of the lateral compression process on the empty and foam-filled hexagonal columns

In this article, a new analytical model of plastic deformation during the flattening process on hexagonal metal columns under the lateral loading in the quasi-static condition is presented. Based on the introduced model, some theoretical relations are derived to forecast the average and instantaneous lateral load of the hexagonal column in two different conditions: empty and polyurethane foam-filled. Then, some lateral compression tests were carried out on the empty and foam-filled metal columns and the experimental results were compared with the theoretical predictions by the present formulas that showed an admissible correlation. The theoretical relations estimate variations of the lateral load in terms of lateral displacement. The theoretical analysis shows that the lateral load on the hexagonal column during the flattening process is dependent on the column wall thickness, column length, and material properties of the column and polyurethane foam-filler. Finally, the effects of geometrical characteristics of the hexagonal columns and material properties of the columns and foams on the lateral load are investigated, experimentally.     

Development and application of the material constitutive model in springback prediction of cold-bending

The accuracy for cold-bending springback prediction is determined by the sensitivity and accuracy of the material constitutive model. Thus, the material constitutive model is developed and improved by many researchers, and the improved models are applied in the springback calculation with various materials in finite element simulation or theoretical analysis. To provide a reference for the researchers studying cold-bending springback problems, a review of the development and application of the material constitutive models is presented in this paper, which conducts from the elastic behavior, the anisotropy, and the work-hardening. It can be summarized as: (1) Springback prediction result is higher and more accurate when the variable elastic modulus and the nonlinear recovery are considered. (2) The isotropic hardening leads to an overestimation of the springback, which can be avoided by a hardening model describing the Bauschinger effect. (3) The hardening model has greater impact on springback than the yield criterion. (4) Good accuracy of the springback prediction can be achieved when the variable elastic modulus effect, the material anisotropy and the nonlinear hardening are considered together. It is also found the theory development and practical application of the material constitutive models are out of line, due to lacking further experiment, or that the stress loading–reloading history within a bending part may be not so complex as that “ratchetting behavior” discussed.