Influence of forming effects on the axial crush response of hydroformed aluminum alloy tubes

The impact behaviour of tubular hydroformed axial crush tubes is examined. The results of dynamic axial crush tests performed with both non-hydroformed and hydroformed AA5754 aluminum alloy tubes were compared to predictions from finite element models. Explicit dynamic finite element simulations of the hydroforming and crash events were carried out with particular attention to the transfer of forming history from the hydroforming simulations to the crash models. The values of tube thickness, work hardening, and residual stresses at the end of the hydroforming simulations were used as the initial state for the crash models. In general, simulations performed using the von Mises yield criterion with isotropic material behaviour gave reasonable predictions when compared to experimental data. It was found that it was important to account for the forming history of the hydroforming operation in the axial crush models. The results showed that work hardening resulting from hydroforming is beneficial to increasing the energy absorption during crash, whereas thickness reduction decreased the energy absorption. Residual stresses had little effect on the energy absorption characteristics. It was also shown that the energy absorption characteristics of tubes with the same mass could vary greatly by adjusting the geometry of the tube and the amount of work hardening experienced by the tube during hydroforming.     

Crashworthiness of helicopter subfloor structures

To improve a helicopter design concept, which meets structural and crashworthiness requirements, a research program is undertaken to study the energy absorption capability of a subfloor structure. In particular, crash tests are performed on the subfloor structure and on the intersection elements, which are components of the structure and can create high deceleration peak loads at the cabin floor level causing dangerous inputs to the occupants. Then the structures are analysed by a commercial explicit finite element code, PAM-CRASH, using detailed geometrical models, suitable materials models and the appropriate definition of contact forces and rivets. The analysis shows that the load-shortening diagrams present a good correlation to the experimental data and that the structural collapse predictions correspond closely to the observed behaviour during the experimental tests. Consequently the finite element analysis can be used to aid the designers in evaluating the crashworthiness of different structural concepts and can therefore be an important mean of reducing development costs.     

Stochastic simulations of square aluminium tubes subjected to axial loading

Stochastic simulations of square aluminium tubes of 6060 T6 aluminium alloy subjected to axial crushing have been performed and compared to an experimental program carried out previously at SIMLab. The main variables during testing were the extrusion length, the wall thickness of the extrusions and the impact velocity of the impactor. Three different buckling modes were observed: progressive buckling, transition from progressive to global buckling and global buckling. Progressive buckling was primarily observed in the short specimens, while increasing the length a transition mode took place. However, for the thick-walled tubes a direct global buckling mode was found. In the present study, it has been investigated if geometric imperfections modelled by assumed Gaussian random fields could explain the experimentally observed behaviour. Variation of the random field parameters by use of a factorial design resulted in variations in especially the buckling modes and consequently the average force, and the same buckling modes found in the experiments were obtained in the simulations. For the chosen model, the shape of the geometric imperfections seemed to be more important than the amplitude. Further, simulations of profiles with three different lengths subjected to impacts at two different velocities were performed. The estimated probability for progressive buckling to appear decreased as the length increased, while the change in velocity had minor effects for the chosen parameter range.     

内高压成形波节管承载特性分析

内高压成形波节管作为目前应用最广换热设备,其变形特点以及成形后承载特性均备受关注.本文通过数值模拟和实验研究的方法,首先分析波节管内高压成形壁厚分布规律、成形精度以及残余应力分布情况,然后分析成形后波节管在承载时,典型区域应力应变分布情况,得出波节管在承受不同载荷时的变形特点.研究结果表明:在内高压成形过程,当整形压力为290 MPa时,成形精度较好,根部过渡区域减薄率达21.63%,且此处残余应力最大.在承载过程,当波节管承受内压力自由胀形时,波节管等效应力的最大值出现在波节根部过渡区域,此处为承载的薄弱区域;当波节管承受轴向压缩和拉伸载荷时,波峰及其附近区域与之对应的产生轴向拉应变和轴向压应变,体现出波节管具备很好的轴向位移补偿能力.     

Effect of anisotropy,kinematic hardening,and strain-rate sensitivity on the predicted axial crush response of hydroformed aluminium alloy tubes

There exists considerable motivation to reduce vehicle weight through the adoption of lightweight materials while maintaining energy absorption and component integrity under crash conditions. Aluminium and magnesium alloys, advanced high-strength steels, and composites are all proposed candidates for replacing mild steel in automotive structures. It was of particular interest to study the crash behaviour of lightweight tubular hydroformed structures. Thus, the current research has studied the dynamic crush response of hydroformed Al–Mg–Mn aluminium alloy tubes using both experimental and numerical methods. The research focused on axial crush structures that are designed to absorb crash energy by progressive axial folding. The main experimental parameter that was varied during the hydroforming process was the corner-fill radius of the tube. Numerical studies were carried out using explicit dynamic finite element models incorporating advanced constitutive material models to capture the measured forming and crash history. A constitutive model was implemented in the finite element models combining the Johnson–Cook strain-rate sensitivity model, a non-linear isotropic-kinematic hardening model, and the Yld2000-2d anisotropic model. Each effect was isolated, and it was shown that strain-rate sensitivity slightly increased the energy absorption capabilities while kinematic hardening and anisotropy effects decreased the energy absorption capabilities during axial crush. When including all three effects, the predicted energy absorption was less than the response predicted from simulations performed using the von Mises yield criterion and in reasonable agreement with measured data. It is recommended that a combined constitutive model be utilized for the study of materials that show sensitivity to the Bauschinger effect, strain-rate effects, and anisotropy.     

Crashworthiness design optimization using successive response surface approximations

 Finite Element (FE) method is among the most powerful tools for crash analysis and simulation. Crashworthiness design of structural members requires repetitive and iterative application of FE simulation. This paper presents a crashworthiness design optimization methodology based on efficient and effective integration of optimization methods, FE simulations, and approximation methods. Optimization methods, although effective in general in solving structural design problems, loose their power in crashworthiness design. Objective and constraint functions in crashworthiness optimization problems are often non-smooth and highly non-linear in terms of design variables and follow from a computationally costly (FE) simulation. In this paper, a sequential approximate optimization method is utilized to deal with both the high computational cost and the non-smooth character. Crashworthiness optimization problem is divided into a series of simpler sub-problems, which are generated using approximations of objective and constraint functions. Approximations are constructed by using statistical model building technique, Response Surface Methodology (RSM) and a Genetic algorithm. The approximate optimization method is applied to solve crashworthiness design problems. These include a cylinder, a simplified vehicle and New Jersey concrete barrier optimization. The results demonstrate that the method is efficient and effective in solving crashworthiness design optimization problems. Received: 30 January 2002 / Accepted: 12 July 2002 Sponsorship for this research by the Federal Highway Administration of US Department of Transportation is gratefully acknowledged. Dr. Nielen Stander at Livermore Software Technology Corporation is also gratefully acknowledged for providing subroutines to create D-optimal experimental designs and the simplified vehicle model.     

Microstructural and X-ray tomographic analysis of damage in extruded aluminium weld seams

In the present study, X-ray computer tomography is used to determine damage evolution in extrusion weld seams loaded close to failure for two comparable AlSiMg alloys with distinctively different grain structures. In the dispersoid free alloy, recrystallisation occurs upon extrusion and the weld seam has no effect on mechanical properties. No preferential void formation in the weld seam region was detected. In the non-recrystallising dispersoid rich alloy, the weld seam leads to a reduced ductility. There are no signs of internal void formation, and damage initiation was found to be close to the surface. It is argued that for the non-recrystallising alloy, the effect of the weld seam on the ductility is related to a sharp texture gradient across the weld seam.     

Thermal formation of corundum from aluminium hydroxides prepared from various aluminium salts

Aluminium hydroxides have been precipitated from various aluminium salts and the differences in their thermal behaviour have been investigated. Pseudoboehmite derived from the nitrate, sulfate and chloride all form γ-Al₂O₃ at ∼ 400°C but the formation of α-A1₂O₃ at 1200°C occurs more readily in the material derived from the sulfate. This contains a higher concentration of anionic impurities related to differences in the solubility of the original aluminium salts. The sulfate is retained in the gel to higher temperatures at which its eventual decomposition may lead to the formation of a reactive pore structure which facilitates the nucleation of α-A1₂O₃.     

Plasma ion assisted deposition of aluminium oxide and aluminium oxifluoride layers for applications in the ultraviolet spectral range

We present extended experimental material about optical and mechanical properties as well as the water content of aluminium oxide films, deposited by plasma ion assisted electron beam evaporation. A clear correlation between these experimental data is established and understood as being affected by the different degree of the porosity of the films. When adding fluorine as a reactive gas during deposition, aluminium oxifluoride layers can be obtained that appear nearly free of water, and combine UV-transparency with higher UV refractive indices than porous aluminium oxide layers.     

Crashworthiness characteristics investigation of silk/epoxy composite square tubes

This research concentrates on the evaluation of crashworthiness characteristics of natural silk/epoxy composite square tubes energy-absorbers. Composite laminate specimens were subjected to static axial compression load and experimental evaluation of the energy absorption capability of silk/epoxy composite. Specimens were in the form of square cross-sections with the dimension of 80 mm × 80 mm and a radius curvature of 5 mm. The variables in the experiment were the length of the tubes built 50 mm, 80 mm and 120 mm. Meanwhile, the thickness of the walls, consisting of laminates of silk/epoxy of 12, 24 and 30 plies, correspond to equivalent wall thickness of 1.7 mm, 3.4 mm and 4.2 mm, respectively. The parameters measured were the total absorbed energy (E_total), and the crash force efficiency (CFE). E_total is the measure of the amount of energy that the structure can withstand without failure and thus is a measure of its strength, while CFE gives a quantitative indication of the mode of failure of the composites. The mode of failure was observed using photography.     

Investigating the quasi-static axial crushing behavior of polymeric foam-filled composite pultrusion square tubes

The capability of structures to absorb large amounts of energy is a crucial factor, particularly for structural components of vehicles, in reducing injury in case of collision. In this study, an experimental investigation was conducted to study the crashworthiness of polymeric foam-filled structures to the pultruded square cross-section E-Glass fiber-reinforced polyester composite tube profiles. Quasi-static compression was applied axially to composite tubes to determine the response of the quasi-static load displacement curve during progressive damage. Three pultruded composite tube wall thicknesses at different sizes were examined, and the effects of crushing behavior and failure modes were analyzed and discussed. Experimental results indicated that the foam-filled profile is superior to the non-filled foam composite tube profile in terms of the capacity to absorb specific energy.     

Threshold creep behaviour of discontinuous aluminium and aluminium alloy matrix composites: An overview

Several sets of creep data for aluminium and aluminium alloy matrix composites reinforced by silicon carbide particulates, silicon carbide whiskers or alumina short fibres are analysed. It is shown that for this class of discontinuous composites the threshold creep behaviour is inherent. Applying the concept of threshold stress, the true stress exponent of minimum creep strain rate of approximately 5 follows from the analysis even when the matrix solid solution alloy exhibits Alloy Class creep behaviour, for which the value of 3 for the true stress exponent is typical. The creep strain rate in the discontinuous aluminium and aluminium alloy matrix composites is shown to be matrix lattice diffusion controlled. The usually observed high values of the apparent stress exponent of creep strain rate and the high values of the apparent activation energy of creep are then rationalized in terms of the threshold creep behaviour. However, the origin of the threshold stress decreasing with increasing temperature but not proportional to the shear modulus in creep of discontinuous aluminium and aluminium alloy matrix composites is still awaiting identification. The creep-strengthening effect of silicon carbide particulates, silicon carbide whiskers and alumina short fibres is shown to be significant, although the particulates, whiskers and short fibres do not represent effective obstacles to dislocation motion.     

The effect of processing conditions on the energy absorption capability of composite tubes

Structures capable of absorbing large amounts of energy are of great interest, particularly in the automotive and aviation industries, in an effort to reduce the impact on passengers in the case of a collision. The energy absorption properties of composite materials can be tailored, thus making them an appealing option as a substitute of more traditional materials in applications where energy absorption is crucial. In this research, the effect of the processing conditions (with or without vacuum) on the specific energy absorption capacity of composite tubes was investigated. Tubes of circular and square cross sections were fabricated using orthophthalic polyester resin and plain weave E-glass fabric with fibers oriented at 0°/90°, with respect to the tube axis. Test specimens consisting of tube segments were prepared and tested under quasi-static compression load. Test results indicate that, among the conditions considered, tubes of circular cross section fabricated under applied vacuum display the highest level of specific energy absorbed. Ultimately, this investigation demonstrates the potential for tailoring the energy absorption properties of composite materials through controlled processing conditions.     

Electron stimulated desorption from aluminium alloy and aluminium coated stainless steel

The electron stimulated desorption (ESD) yield was measured at the internal surface of tubular samples made of aluminium alloy and aluminium coated stainless steel. The ESD yields were measured as a function of electron accumulated dose to a maximum of 3 × 10²² e⁻/m² at electron energy of 500 eV, then as a function of electron energy between 40 eV and 5 keV. The ESD yields as a function of electron energy were linear for all gasses except CH₄ and CO₂.     

Radio frequency plasma nitriding of aluminium at higher power levels

Nitriding of aluminium 2011 using a radio frequency plasma at higher power levels (500 and 700 W) and lower substrate temperature (500 °C) resulted in higher AlN/Al₂O₃ ratios than obtained at 100 W and 575 °C. AlN/Al₂O₃ ratios derived from X-ray photoelectron spectroscopic analysis (and corroborated by heavy ion elastic recoil time of flight spectrometry) for treatments preformed at 100 (575 °C), 500 (500 °C) and 700 W (500 °C) were 1.0, 1.5 and 3.3, respectively. Scanning electron microscopy revealed that plasma nitrided surfaces obtained at higher power levels exhibited much finer nodular morphology than obtained at 100 W.     

Crash response of advanced high-strength steel tubes: Experiment and model

The performance of non-hydroformed and hydroformed structural steel tubes in component-level crash testing was investigated using both experimental and analytical techniques. In particular, the focus was on high-strength steels that may have potential to enhance crashworthiness of automobiles. Monolithic tubes made from multiple materials and wall thicknesses were considered in this study. The following materials were used: conventional drawing quality (DDQ) steels; high-strength low alloy (HSLA-350) steels; and advanced high-strength steel (AHSS) materials comprising the dual phase alloys DP600 and DP780. The goal of this research was to study the interaction between the forming and crash response of these materials in order to evaluate their potential for use in vehicle design for crashworthiness. The tubes were hydroformed using two methods known as low- and high-pressure processes. Material characterization of all materials was carried out through quasi-static and high strain rate tensile tests in the range of 0.00333–1500 s⁻¹, and rate sensitive constitutive models for all materials were developed. The nonlinear explicit dynamic finite element code LS-DYNA, in conjunction with the validated constitutive models, was used to simulate both the hydroforming processes and the crash tests performed on the tubes. The energy absorption characteristics of the different tubes were calculated and the results from the numerical analyses were compared against the experimental data. This comparison was performed in order to determine whether the interactions between forming and crush could be adequately predicted using finite element analysis. The effects of thickness changes, work hardening, and component geometry, which resulted from hydroforming, on the crash response were also investigated. A study of the significance of strain rate and the importance of performing detailed material characterization on the accuracy of the numerical analysis was performed. Also, a parametric study on the effect of transferring forming history data between simulations on the accuracy of the numerical analysis was performed, and the importance of carrying forward the histories between multiple forming simulations was demonstrated.     

Effect of aluminium phosphate as admixture on oxychloride cement

The effect of admixing of aluminium phosphate on oxychloride cement in the matrix has been investigated. It is shown that aluminium phosphate retards the setting process of the cement and improves water-tightness.     

Plasma focus assisted carburizing of aluminium

Carburizing of aluminium using the energetic carbon ions emitted from a low energy (1.45 kJ) Mather type plasma focus device operated with methane is reported. The aluminium samples are placed in front of a hollow tapered aluminium anode at a fix distance and are exposed to different carbon ion dose. Raman spectroscopy and X-ray diffraction analyses confirm the presence of Al₄C₃ phase in the irradiated samples. Raman spectroscopic analysis verifies the existence of sp² and sp³ hybridized carbon content also in the treated aluminium surface. Energy dispersive X-ray spectroscopy analyses show a gradual increase of carbon concentration in the treated samples as a function of ion dose. Scanning electron microscope results reveal that the surface smoothness improves initially with ion beam exposure, but deteriorates with increasing ion dose above an optimum value. Vickers micro-indentation technique reveals maximum hardness of about 1990 ± 100 MPa, approximately 7 times as compared to that of aluminium bulk, for the sample treated with 30 plasma focus discharges.     

Forming and fracture limits of aluminium alloy

Abstract

Forming and fracture limits of an AA 3104-H19 aluminium alloy sheet were studied by hydraulic bulging and Marciniak type deep drawing and tensile tests. The alloy appeared to be highly anisotropic, exhibiting distinctly different fracture patterns in the rolling and transverse directions. The preferred fracture direction was transverse to the rolling direction. In the tensile test, samples loaded in the rolling direction failed transverse to the rolling direction, but in the transverse direction, the fracture was inclined at ～55° to the tensile axis. In some cases, two such competing fractures in the characteristic directions could be observed. Scanning electron microscopy studies revealed a typical ductile fracture pattern. The fracture occurred by shearing in the through thickness direction, and typical alternating shear lips in a direction inclined at ～45° to the through thickness direction could be observed. Forming limit diagrams for both rolling and transverse directions were determined from the experiments. The measured limit strains in uniaxial tension were predicted well by the modified Rice–Tracey theory, but in equibiaxial tension, the theory overestimated the fracture limit strains.     

Scheduling continuous aluminium casting lines

This study considers the problem of scheduling casting lines of an aluminium casting and processing plant. In aluminium processing plants, continuous casting lines are the bottleneck resources, i.e. factory throughput is limited by the amount of aluminium that can be cast. The throughput of a casting line might be increased by minimizing total setup time between jobs. The objective is to minimize setup time on production lines for a given time period while balancing workload between production lines to accommodate potential new orders. A mathematical formulation for scheduling jobs to minimize the total setup time while achieving workload balance between the production lines is presented. Since the casting scheduling problem is an NP-hard problem, even with only one casting line, a four-step algorithm to find good solutions in a reasonable amount of time is proposed. In this process, a set of asymmetric travelling salesman problems is followed by a pairwise exchange heuristic. The proposed procedure is applied to a case study using real casting data.