汽车车身偏置碰撞抗撞性分析与结构改进

针对某乘用车在偏置碰撞形式下的安全性问题,对车身前部关键部件进行改进,实现汽车前舱刚度匹配,使整体结构合理变形。通过对仿真结果的分析并针对原车耐撞性能的不足,在车身纵梁前部设计导引槽式吸能结构,以改进前纵梁吸能变形模式;增加A柱加强件的厚度,以提高车架纵向刚度和乘员舱的强度。仿真结果表明:在整车质量仅增加2.24 kg的情况下,改进方案有效地提高了整车的耐撞性,整车的加速度峰值降低了6 g,乘员舱的变形量有所减小。     

组合薄壁直梁的耐撞性研究

针对汽车前部纵梁结构的耐撞性要求,设计了在矩形空心管内部填充成组圆管的组合梁结构,研究了组合直梁在轴向冲击载荷下的能量吸收与变形特性,并进一步研究了填充圆管直径和长度等参数变化对组合梁的性能影响。结果发现,矩形薄壁梁内部填充圆管以后,结构的碰撞吸能特性得到较大程度的提高;通过合理的改变填充圆管的数量和长度,可以较好地调整结构压溃过程中的碰撞力峰值载荷和均值载荷。     

Energy absorption of pressurized thin-walled circular tubes under axial crushing

By pressurizing cellular materials, honeycombs, or thin-walled structures, their energy absorption can be greatly enhanced, and this enhancement can be controlled by the applied pressure. This concept shines light on the possibility of achieving adaptive energy absorption. To investigate the effect of internal pressure on energy absorption of thin-walled structures, this paper presents a study of axial crushing of pressurized thin-walled circular tubes. In the experiments, three groups of circular tubes with radius/thickness ratio R/t=120-200 were axially compressed under different pressurizing conditions. The results show that with an increase of internal pressure, the deformation mode switches from diamond mode with sharp corners to that with round corners, and eventually to ring mode. In diamond mode, the mean force of the tubes increases linearly with internal pressure. The enhancement comes from two mechanisms: direct effect of pressure and indirect effect due to interaction between pressure and tube wall. After the deformation switches to ring mode, the enhancement resulting from the second mechanism becomes weaker. Based on experimental observations, the deformation mode, energy dissipation mechanisms as well as interaction between internal pressure and tube wall are analyzed theoretically and the theoretical results are in good agreement with the experimental ones.     

基于正面耐撞性仿真的轿车车身材料轻量化研究

以某轿车为研究对象,运用显式有限元理论,建立整车有限元模型,基于“汽车正面碰撞乘员保护设计规则(CMVDR294)”的耐撞安全性仿真,从满足整车正面耐撞安全性能的角度,分别采用高强钢和铝合金对车身主要覆盖件进行轻量化研究,使车身减质量分别达9.31 kg和53.10 kg,减质量效果达到11.30%和64.50%。对整车变形、整车与刚性墙的碰撞力、运动速度和加速度、主要零部件吸能等方面进行分析、评价,数值仿真验证了轻量化方案的可行性。     

Relation Between Friction and Plastic Deformation Examined by Using Periodic Asperity Arrays Fabricated on an AFM Multipurpose Cantilever

The relation between friction force and plastic deformation was determined by using FIB (focused ion beam)-processed multipurpose cantilevers for an AFM (atomic force microscope). First, a multipurpose cantilever consisting of a sliding block, parallel leaf spring, and hinge was made by using an FIB. Then, copper pyramids were rubbed against an unmodified area or periodic asperity arrays fabricated on the cantilever under loads of 23 to 960 nN. Under higher loads (900 nN), the friction coefficient measured for an asperity array was lower than that for an unmodified area and decreased with decreasing curvature radius of the asperity peak, and the specific volume change rate increased with decreasing curvature radius of the asperity peak.     

环形梯度多胞结构的多工况耐撞性研究

针对传统多胞薄壁结构在受到冲击时峰值力过大以及在斜向碰撞时易产生全局弯曲而失稳的问题,提出了一种新颖的具有梯度材料分布特性的环形梯度多胞管。提出并推导了环形多胞结构的平均碰撞力的理论模型,并进一步采用实验方法验证了理论模型的可行性。采用数值分析的方法,对比研究环形梯度多胞结构和均匀多胞结构在多角度冲击工况下的吸能特性。研究结果显示：梯度多胞结构比均匀多胞结构具有更好的能量吸收能力和承载特性,尤其在大角度倾斜冲击工况下,梯度多胞结构具有更明显的耐撞性优势;并且,随着梯度指数的增大,梯度多胞结构的峰值力、能量吸收能力都会降低,并呈现逐渐收敛的趋势,梯度厚度区间也对梯度结构的耐撞性有重要影响。     

径向跳动对球面铣刀切削力的影响研究

研究了径向跳动对刀齿的实际切削半径、切屑形状以及切屑厚度的影响机理。研究了各刀齿沿刀刃螺旋线的切削微元实际切削半径的数学表示和变化规律,实际切削半径的变化改变了刀齿的切削路径,使各刀齿上切屑形状分布不均匀。建立了三维切削下切屑厚度的数学表示,提出了递延累加切屑厚度计算算法。实验验证表明,计算的切削力与测量结果能很好地吻合,瞬时切削力、切削力峰值、平均切削力的预测精度达到85%以上。     

Cutting surface quality analysis in micro ball end-milling of KDP crystal considering size effect and minimum undeformed chip thickness

Potassium dihydrogen phosphate (KH₂PO₄ or KDP) crystal is a typical soft-brittle optical crystal, and the size effect and brittle cutting mode are easy to appear in micro ball end-milling of KDP crystal. In this paper, micro-grooving experiments are conducted to study the size effect and brittle cutting in micro ball end-milling of KDP crystal with different feed rate and depth of cut. The cutting force, machined groove base quality and chip morphology are collected and analyzed carefully. The size effect is discovered by the phenomena of the existence of oscillations and relaxations in cutting force and hyper-proportional increase of specific cutting force, when the ratio of feed per tooth to cutting edge radius f_t/r_e is less than 1. While the brittle cutting mode is detected through the existence of sharp fluctuations in cutting force and cracks on the groove base when the ratio f_t/r_e is larger than 2. From the further comprehensive analysis of cutting force, specific cutting force, machined groove base quality and chip morphology, the cutting parameters with ratios of the maximum undeformed chip thickness in one cutting circle to cutting edge radius h_max/r_e around 0.14, 0.2 and 0.4 are regarded as size effect, optimal and brittle cutting points, respectively. The size effect, ductile cutting and brittle cutting zones are divided by the size effect and brittle cutting boundaries (points). Among the optimal points, the depth of cut of 2 μm with the ratio f_t/r_e of 1 is the optimal cutting parameter for micro ball end-milling of KDP crystal.     

Effects of defects on the in-plane dynamic crushing of metal honeycombs

The effects of defects and their distributions on the in-plane dynamic crushing of honeycomb panels were studied using explicit finite element modeling. The influence of defect locations and ratios is investigated on the deformation modes and the plateau stresses with respect to the impact velocity. Numerical results show that the dynamic performance of honeycomb displays a high sensitivity on the defect location, especially under low and moderate impact velocities. By introducing a defect correction factor β_m and using the one-dimensional shock wave theory, an empirical formula is given for the variation of honeycomb’s plateau stress with respect to the impact velocity and the defect ratio.     

The effect of tool edge radius on the chip formation behavior of tool-based micromachining

Chip formation behavior of micromachining is governed by the tool edge radius effect as reflected by the characteristic changes in plastic deformation at varying combinations of tool edge radius, r, and undeformed chip thickness, a. At high a/r above unity, concentrated plastic deformation takes place at the primary and secondary deformation zones akin to conventional macromachining. Decreasing a/r below unity promotes localized deformation ahead of the tool edge radius, with the expansion in fraction of the primary deformation zone and the simultaneous shrinkage in fraction of the secondary deformation zone following the reductions in total tool–chip contact length. Further decrease of a/r below a critical threshold brings forth a total suppression of secondary deformation zone and resulted in an ultimate localization of plastic deformation ahead of the tool edge radius. This is perceived as a transition in chip formation mechanism from concentrated shearing to a thrust-oriented behavior.     

Optimization of functionally graded foam-filled conical tubes under axial impact loading

Metallic foams as a filler in thin-walled structures can improve their crashworthiness characteristics. In this article, nonlinear parametric finite element simulations of FGF foam-filled conical tube are developed and the effect of various design parameters such as density grading, number of grading layers and the total mass of FGF tube on resulting mode shapes, specific energy absorption and initial peak load is investigated. Multi design optimization (MDO) technique and the geometrical average method, both are based on FE model are applied to maximize the specific energy absorption and minimize the impact peak force by estimating the best wall thickness and gradient exponential parameter “m” that controls the variation of foam density. The results obtained from the optimizations indicated that functionally graded foam material, with graded density, is a suitable candidate for enhancing the crashworthiness characteristics of the structure compared to uniform density foam.     

Effects of concentrated rigid inclusions defects on the in-plane impact behavior of hexagonal honeycombs

Hexagonal honeycombs have exhibited significant advantages in energy absorption and they are increasingly used as absorbers under crush conditions. The in-plane crushing process of imperfect hexagonal honeycombs with concentrated rigid inclusions defects is simulated using finite element simulations. In each case, a constant velocity is applied to an impact plate which then crushed the honeycomb. Simulation results indicate that the defect location has a great influence on the deformation modes, especially at low and moderate velocity. After analyzing the apparent reflection about dynamic response at the impact end, the respective influences of local fraction of inclusions and foil thickness (relative density) on the crushing plateau stress on account of the crushing velocity are further discussed. Furthermore, the energy absorption capacity under constant velocity loading is studied. Due to the distribution of the concentrated rigid inclusions defects, the energy absorption can be controlled effectively.     

Hardening-softening behaviour of tubular cantilever beams

In the large plastic deformation of a tubular cantilever beam loaded by a force at its tip, the strain hardening of the material tends to increase the load-carrying capacity, while local buckling and cross-sectional ovalization (flattening) occurring in the neighbourhood of the root tends to reduce the moment-carrying capacity and results in structural softening. Experiments were carried out for seamless mild steel tubular cantilevers of radius/thickness ratio ranging from 9 to 20 to explore the development of local buckling and flattening and to examine its influence on the global load-carrying capacity of the beam. A simple theoretical model is proposed to predict the global hardening-softening behaviour of tubular cantilever beams in terms of the material properties and geometric parameters of the tube.     

二波谐波摩擦传动柔轮弹性变形径向力的研究

本文基于谐波摩擦传动二波模型,对柔轮的弹性变形径向力进行分析研究。通过建立空间的力学模型,推导出了自由边界条件下柔轮弹性变形径向力的表达式,然后又得出了可忽略固定边界影响的长度条件。最后,分析了柔轮弹性变形径向力与其长度,壁厚,半径等参数的关系。     

Quasi-static compression of electric resistance welded mild steel tubes with axial gradient-distributed microstructures

This paper presents the deformation behavior and crashworthiness of electric resistance welded mild steel tubes with axial gradient microstructures in quasi-static compression. Three sets of tubes were prepared, and regions of each tube were Induction heated and directly quenched (IH-DQ). The effect of the length to diameter (L/D) ratio, and length of the IH-DQ region on crushing characteristics was investigated, and compared with untreated tubes. The compression tests revealed that improved energy absorption can be obtained in IHDQ tubes if the collapse is controlled by the formation of a concertina buckling mode. However, there was a tendency to produce mixed or Euler buckling modes as the ratio of L/D increased. Meanwhile, the results of the crush experiments and the FEM models showed that the heat-treatment process should be precisely controlled to produce the correct type of microstructure, and circumferential uniformity of microstructure distribution.     

Dynamic crushing of a ductile cellular array

A range of deformation mechanisms occur during dynamic crushing of a tightly-packed array of thin-walled metal tubes. Within the array, both stable and unstable modes of deformation for individual tubes are observed; these depend on the packing arrangement, the deformation of surrounding tubes and the rate-of-crushing. Unstable deformation within narrow bands of cells accounts for most of the crushing in these systems. After unstable deformation is initiated, the average crushing force is insensitive to the rate-of-crushing; this feature makes cellular systems attractive for damage mitigation applications. Despite the highly localized nature of this deformation, the final extent of crushing is almost proportional to the ratio of the impact energy to an idealized energy dissipation capacity for a layer of cells.     

On functionally-graded crashworthy shape of conical structures for multiple load cases

Many studies on energy absorbers have been focused on tapered tubes because they have significant advantages in crashworthiness and provide a desired constant load-deflection response. However, few studies have been reported on tapered tubes with nonlinearlyvariable diameters along the longitudinal direction. This paper presents thin-walled Functionally graded tapered tube (FGTT) with a diameter varying nonlinearly subject to axial (0°) and oblique (10°, 20°, 30°) impacts. To explore the advantages of FGTT, conventional Straight/Conical circular tube (SCT/CCT) with the same mass are compared; and FGTTs with a gradient exponent n > 1 are found to be preferable to others in terms of energy absorption capacity under small impact angles. Then, crashworthiness analyses of different crushing distances are conducted and it is found that under a large impact angle (e.g. 20°, 30°), FGTT with a short crushing distance (e.g. 40 mm) have a higher mean crashing force than long crushing distance (e.g. 120 mm), especially for n > 1. In addition, the effect of geometric parameters, such as the gradient exponent n and diameter range ΔD between top (incident) and bottom (distal) diameters of FGTTs, are also studied and it is found that the FGTT with ΔD = 40 mm and n > 1 exhibits better crashworthiness than the others under small impact angles (0°, 10°). This paper demonstrates that such FGTT structures have a certain potential to be an energy absorber.

     

Load-Bearing Capacity in Quasi-Static Compression and Bearing Toughness of Silicon Nitride Balls

Results of ball-on-ball crushing tests on bearing-grade silicon nitride balls and a new analysis of the dependence of the peak load on the ball radius are presented. The variation of the peak load with the ball radius is consistent with a fracture-mechanics analysis, which suggests that the peak load corresponds to a transition from stable to unstable growth of cone cracks in the balls, and the critical cone crack size scales with the ball size. It is recommended that a ball-size independent bearing toughness be calculated and reported based on the crushing test results rather than a crushing strength, which decreases with increasing ball size.     

Machining with tool-chip contact on the tool secondary rake face—Part I: a new slip-line model

Given the growing number of applications of groove-type chip breaker tools in modern machining, it is becoming increasingly important to study the tool-chip contact on the tool secondary rake face. This type of tool-chip contact significantly changes not only the state of stresses in the plastic deformation region, but also changes the distribution of forces and temperatures over the tool rake face. A new slip-line model accounting for the tool-chip contact on the tool secondary rake face is proposed in this paper. The model also takes into account chip curl and incorporates seven slip-line models developed for machining during the last six decades as special cases. Dewhurst and Collins's matrix technique for numerically solving slip-line problems and Powell's algorithm of nonlinear optimization are employed in the mathematical formulation of the model. The inputs of the model include (a) the tool primary rake angle γ₁, (b) the tool secondary rake angle γ₂, (c) the tool land length h, (d) the undeformed chip thickness t₁, (e) the ratio of hydrostatic pressure P_A to the material shear flow stress k, (f) the ratio of frictional shear stress τ₁ on the tool primary rake face to the material shear flow stress k, and (g) the ratio of frictional shear stress τ₂ on the tool secondary rake face to the material shear flow stress k. The outputs of the model include (a) the cutting force F_c/kt₁w and the thrust force F_t/kt₁w, (b) the chip up-curl radius R_u, (c) the chip thickness t₂, and (d) the natural tool-chip contact length l_n.     

In-plane vibration analysis of cantilevered circular arc beams undergoing rotational motion

Equations of motion of cantilevered circular arc beams undergoing rotational motion are derived based on a dynamic modeling method developed in this paper. Kane’s method is employed to derive the equations of motion. Different from the classical linear modeling method which employs two cylindrical deformation variables, the present modeling method employs a non-cylindrical variable along with a cylindrical variable to describe the elastic deformation. The derived equations (governing the stretching and the bending motions) are coupled but linear, so they can be directly used for vibration analysis. The coupling effect between the stretching and the bending motions, which could not be considered in the conventional modeling method, is considered in this modeling method. The effects of rotational speed, arc angle, and hub radius ratio on the natural frequencies of the rotating circular arc beam are investigated through numerical analysis.