Mehmet A. Guler

The effect of geometrical parameters on the energy absorption characteristics of thin-walled structures under axial impact loading

In this study, the crush behaviour of thin-walled straight and conical shell structures was systematically determined for various absorber designs and investigated comparatively under axial impact loading. The main parameters in the design of these structures are cross-section geometry, wall thickness and semi-apical angle. Several cross sections have been studied: circular, square and hexagonal. In the finite element simulations, these designs were fixed at one end and impacted by a rigid wall from the other with specified mass and velocity giving the required impact energy according to the European regulation ECE R29 by using explicit finite element code LS-Dyna. After crash simulations, energy absorption characteristics and crush forces were obtained for each crush element having different cross sections, wall thicknesses and semi-apical angles. Peak crush force, mean crush force, crush force efficiency and specific energy absorption (SEA) were calculated for a deformation length of 100 mm. In all cases it was found that tubes were crushed progressively. The results of the simulations showed that the square cross-sectioned energy absorber has the lowest crush force efficiency among three cross-section geometries. The crush force efficiency was found to be the highest for the circular absorber which has a semi-apical angle of 12.5° and a wall thickness of 2 mm. Finally, the peak crush forces were lowered by creating blanks and corrugations.

The influence of seat structure and passenger weight on the rollover crashworthiness of an intercity coach

Abstract A rollover event is one of the most crucial hazards for the safety of passengers and the crew riding in a bus. In past years it was observed that the deforming body structure seriously threatens the lives of the passengers after the accident, and thus the rollover strength has become an important issue for bus and coach manufacturers. Today the European regulation “ECE-R66” is in force to prevent catastrophic consequences of such rollover accidents, thereby ensuring the safety of bus and coach passengers. According to the said regulation the certification can be gained either by full-scale vehicle testing, or by analysis techniques based on advanced numerical methods (i.e., nonlinear explicit dynamic finite element analysis). The quantity of interest at the end is the bending deformation enabling engineers to investigate whether there is any intrusion in the passenger survival space along the entire vehicle. In this paper, explicit dynamic ECE-R66 rollover crash analyses of a stainless-steel bus under development were performed and the strength of the vehicle is assessed with respect to the requirements of the official regulation. Subsequently, starting from a baseline design, different considerations which are not currently mentioned in the regulation (i.e., passenger and luggage weight) and some worst case assumptions, such as the influence of the seat structure, were investigated. The nonlinear explicit dynamics code LS-DYNA as a solver and ANSA and LS-PREPOST softwares as FEA pre/post-processors were utilized throughout the bus rollover analysis project. The FEA model was generated by using PCs running on Linux Suse operating system, whereas the LS-DYNA solutions were performed on a multiple-processor workstation running on an AIX UNIX operating system. During the first stage, a verification of the analysis procedure following regulation ECE-R66 was performed. The verification of analysis is a compulsory requirement of the regulation, as it is the technical services responsibility (TÜV Süddeutschland in this case) to verify the assumptions used in the finite element analysis. The investigations indicated that the introduction of belted passengers increases the energy to be absorbed during rollover significantly (37% greater than the baseline), which severely impacts the rollover behavior of the pillars. When the vehicle is fully loaded (including passenger weight and luggage mass) the situation gets even worse. Even the tough center of gravity of the vehicle is lowered, and the total mass increases, which in turn gives the maximum intrusion. *This paper was originally presented at the 9th International LS-DYNA users conference, Detroit, MI, USA in June 2006, and published in the subsequent conference proceedings [14].

Investigation and prediction of springback in rotary-draw tube bending process using finite element method

Rotary-draw tube bending operation is one of the most universal methods used for the tube forming processes. Similar to the other forming methods, some problems such as wall thinning, cross-section distortion, wrinkling, and springback can also be seen on the tubes formed by rotary-draw bending operations. Springback is a very common problem and its prediction plays a crucial role in increasing the efficiency of the tube bending operations and also to overcome the difficulties in the assembly processes. Tube diameter, wall thickness, bend radius, bend angle, and coefficient of friction can be considered as the most effective parameters that cause the variation of springback magnitude. In this study, not a simple one-at-a-time sensitivity analysis, but a thorough investigation of the springback phenomena involving interactions between the geometrical and mechanical parameters is done and surrogate models are developed via the data obtained from finite element analysis using a multi-purpose explicit and implicit finite element software LS-DYNA to analyze the non-linear response of structures. The constructed surrogate models can be utilized to perform fast prediction of springback for a given combination of parameters. Three different surrogate modeling techniques are exploited and it is found that the linear polynomial response surface approximations can provide acceptable accuracy. Finally, experiments are conducted to validate the accuracy of surrogate models. It is observed that the cross-validation error predictions are close to the errors observed in the experiments.

Investigation of combined effects of cross section, taper angle and cell structure on crashworthiness of multi-cell thin-walled tubes

ABSTRACT Crash box design has a substantial importance to reduce the fatalities in a frontal crash. In this study, four different types of multi-cell tubes, namely straight-circular, straight-square, tapered-circular and tapered-square geometries, are considered as energy absorbing components. For each type, seven different cell structures are designed, and the crashworthiness of these designs is assessed based on two different metrics: crush force efficiency (CFE) and specific energy absorption (SEA). When the thickness and the taper angle are fixed, the multi-cell design having the best performance is found to have 165% larger CFE and 237% larger SEA compared to the single-cell design having the worst performance. By varying the thickness, the CFE and SEA performances of the best design can be further increased by 5% and 7%, respectively. Similarly, by varying the taper angle, the SEA performances of the best design with varied thickness can further be increased by 4%. Highlights Impact behaviour of several multi-cell straight and tapered tubes are investigated All multi-cell models have larger CFE and SEA values than the single-cell models Tapered-circular tube has the best, straight-square has the worst crush performance CFE of the best multi-cell design is 177% larger than the worst single-cell design SEA of the best multi-cell design is 275% larger than the worst single-cell design

Closed-Form Solution of the Frictional Sliding Contact Problem for an Orthotropic Elastic Half-Plane Indented by a Wedge-Shaped Punch

In this paper, the frictional contact problem of a homogeneous orthotropic material in contact with a wedge-shaped punch is considered. Materials can behave anisotropically depending on the nature of the processing techniques; hence it is necessary to develop an efficient method to solve the contact problems for orthotropic materials. The aim of this work is to develop a solution method for the contact mechanics problems arising from a rigid wedge-shaped punch sliding over a homogeneous orthotropic half-plane. In the formulation of the plane contact problem, it is assumed that the principal axes of orthotropy are parallel and perpendicular to the contact. Four independent engineering constants , , , are replaced by a stiffness parameter, , a stiffness ratio, a shear parameter, , and an effective Poisson’s ratio, . The corresponding mixed boundary problem is reduced to a singular integral equation using Fourier transform and solved analytically. In the parametric analysis, the effects of the material orthotropy parameters and the coefficient of friction on the contact stress distributions are investigated.

Analyzing batch-to-batch and part-to-part springback variation of DP600 steels using a double-loop Monte Carlo simulation

Many automobile companies are actively exploring the use of high-strength dual-phase steels as an alternative to aluminum and magnesium alloys owing to their light weight, low cost and durability. However, dual-phase steels have a tendency to springback more than other structural steels in a forming operation owing to their high tensile strength. In addition, variations in manufacturing process parameters and material properties cause springback variation over different manufactured parts. Therefore, it is an important task to reduce the magnitude of springback, as well as its variation within, to produce robust and cost-effective parts. This article investigates minimization of the magnitude and variation of springback of DP600 steels in U-channel forming within a robust optimization framework. The computational cost was reduced by utilizing metamodels for prediction of the springback and its variation during optimization. Three different allowable sheet thinning levels were considered in solving the robust optimization problem, and it was found that, as the allowable thinning increased, the die radius decreased, thereby the magnitude and variation of springback reduced. A simple sensitivity analysis was performed and the yield stress was found to be the most important random variable. Finally, a double-loop Monte Carlo simulation method was proposed to calculate part-to-part and batch-to-batch springback variations. It was found that, as the batch-to-batch variation of yield stress increased, the batch-to-batch springback variation increased, while the part-to-part springback variation remained unchanged.

Mechanical Modeling of Thin Films Bonded to Functionally Graded Materials

In this study the contact problems of thin films bonded to Functionally Graded Materials (FGM) are considered. In these problems the loading consists of any one or combination of stresses caused by uniform temperature changes and temperature excursions, far field mechanical loading, and residual stresses resulting from film processing or in the manufacturing process of the graded coating. The primary interest in this study is in examining stress concentrations or singularities near the film ends for the purpose of addressing the question of crack initiation and propagation in the substrate or along the interface. The underlying contact mechanics problem is formulated by assuming that the film is a “membrane” and the FGM an elastic continuum, and is solved analytically by reducing it to an integral equation. The calculated results are the interfacial shear stress between the film and the graded substrate, mode II stress intensity factor at the end of the film and the axial normal stress in the film.

The Frictional Contact Problem of Sliding Rigid Parabolic Stamps on Graded Materials

This study presents an analytical procedure to determine the contact stress distribution at the surface of an FGM coating perfectly bonded to a homogeneous substrate. The coating is assumed to be loaded by a rigid stamp of a semi‐circular or circular profile. Using Fourier transformations, the contact mechanics problems are reduced to a singular integral equation of the second kind. Singular behavior of the unknown contact stress distribution at the end points is determined by following a function theoretic method. The singular integral equation is solved numerically using an expansion‐collocation technique. Main results of the study are the normal and lateral contact stress components and required contact forces as functions of material parameters. It is shown that a substrate that is softer than the FGM coating could be useful in decreasing the magnitude of the positive lateral stress at the trailing end of the contact.

Design optimization of an automobile torque arm using global and successive surrogate modeling approaches

The design of lightweight automotive structures has become a prevalent practice in the automotive industry. This study focuses on design optimization of an automobile torque arm subjected to cyclic loading. Starting from an available initial design, the shape of the torque arm is optimized for minimum weight such that the fatigue life of the torque arm does not fall below that of the initial design and the maximum von Mises stress developed in the torque arm does not exceed that of the initial design. The stresses are computed using ANSYS finite element software, and the fatigue life is calculated using the Smith–Watson–Topper model. Surrogate-based optimization approach is used to reduce the computational cost. Optimization results based on global surrogate modeling and successive surrogate modeling approaches are compared. It is found that the successive surrogate modeling approach results in 28.7% weight reduction for the torque arm, whereas the global surrogate modeling approach results in 25.7% weight saving for the torque arm.

Mixed-Mode Stress Intensity Factors in a Homogeneous Orthotropic Medium Loaded by a Frictional Sliding Rigid Flat Stamp

In this study, the crack problem for a homogeneous orthotropic medium loaded by a sliding rigid flat punch is considered. The homogeneous orthotropic medium is assumed to be a half-plane and is subjected to both normal and tangential forces through the sliding action of the punch. The crack on the homogeneous orthotropic medium is supposed to a depth of and is parallel to the direction of the normal force. The effect of the geometrical parameters and coefficient of friction on the mixed-mode stress intensity factors (mode I and mode II) is investigated using a computational approach using the finite element method. Augmented Lagrange method is used for the contact algorithm between the rigid flat punch and homogeneous orthotropic half-plane. This study may provide insight to the engineers in understanding the crack mechanisms in orthotropic materials in a comprehensive way and to identify early crack propagations under frictional loadings accurately.