Spring Back Research Articles

An extended homogenous yield function based anisotropic hardening model for description of anisotropic hardening behavior of materials

Except for the Bauschinger effect and permanent softening, work hardening stagnation and cross-effect are often observed deformation behaviors for sheet metals subjected to strain path changes. These complex deformation behaviors are assumed to have great effect on the sheet forming process and the following spring back. To constitutively, and more precisely model these plastic behaviors, extensive models have been successfully developed. Particularly, the combined isotropic-kinematic hardening model is powerful to capture the plastic behavior of sheet metals during forward and reverse loading. However, there is strong coupling between anisotropic yielding and anisotropic hardening with the kinematic hardening. Recently, Barlat et al. (2011) [61] proposed a homogenous yield function based anisotropic hardening model to describe the Bauschinger effect, which is not formulated with the kinematic hardening rule. In the present work, the original anisotropic hardening model is extended to capture the often observed cross-effect during multi-stage loading and work hardening stagnation during reverse loading. Although only four coefficients are newly introduced in the extended model, it can capture well the main trends of work hardening stagnation and cross-effect. The capability of this extended model is demonstrated with applications to two materials, namely, high strength steel and mild steel DC06.

Lessons from the death of a Tunisian salesman

The Arab Spring was triggered by the self-immolation of a Tunisian salesman leading to protests by ordinary people empathising with the salesman and demanding bread, jobs and a change in the lives of the youth. With the expansion of the movement to include internal and external political opposition, and in response to the hard-line tactics of beleaguered leaders, the focus of the uprising became a clarion call for leadership changes. However, two years after the start of these uprisings, Tunisia, Egypt and Libya are still in a state of crisis, even though new leaders have been elected into power. Thus, to contribute to the early detection of similar uprisings, their prevention and resolution, this article refocuses discourse and policy decisions about the Arab Spring back to the original demands of the protestors and to its root causes, namely the demand for improved lives. This will be done by analysing the life, death and memorial of the father of the Arab Spring: Mohamed Bouazizi.

Comparison of Material Models for Spring Back Prediction in an Automotive Panel Using Finite Element Method

Springback is a crucial factor in sheet metal forming process. An accurate prediction of springback is the premise for its control. An elasto-plastic constitutive model that can fully reflect anisotropic character of sheet metal has a crucial influence in the forming simulation. The forming process simulation and springback prediction of an automobile body panel is implemented by using JSTAMP/LS-DYNA with the Yoshida-Uemori, the 3-parameter Barlat and transversely anisotropic elasto-plastic model, respectively. Simulation predictions on spingback from the three constitutive models are compared with experiment measurements to demonstrate the effectiveness and accuracy of the Yoshida-Uemori model in characterizing the anisotropic material behavior of sheet metal during forming. With an accurate prediction of springback, it can provide design guideline for the practical application in mold design with springback compensation and to achieve an accurate forming.

Modeling and reducing edge effects in laser bending

Laser forming of sheet metal offers the advantages of requiring no hard tooling, no spring back and no external force, thus this technology reduces cost, increases flexibility and accuracy. Laser forming is a complicated and transient thermo-mechanical process involving elastic and plastic strain depended on time history. How to control and enhance the accuracy of the laser forming attracts much attention. This paper delivered a deep insight into the mechanism of the edge effects in laser forming, improved an existing analytical model to approximately predict the edge effects, and based on this model a new strategy was put forward to reduce the edge effects. Numerical simulations and experiments were carried out to validate the model and the presented strategy.

A Method of Die Surface Compensation after Spring Back Prediction in Sheet Metal Forming

Spring back in the process of sheet metal forming leads to deviations between design sizes and actual dimensions of stamping parts, to meet the precision requirements of deviations, the shape of die surface needs to be compensated in the opposite direction according to the predicted spring back by numerical simulation. In this article, the R S coordinate mapping method is used to revise coordinates of finite element nodes according to the predicted spring back, and the bi-cubic B spline surface interpolation method is applied to reconstruct die surface with CAD data format. An example shows that the algorithms are feasible and have some practical values.

Analysis and testing of ultrathin shell 2 m diameter reflector demonstrator

Ultra-thin shell foldable and self-deploying composite reflectors are being developed for space antennas to meet low cost, low mass and high surface accuracy. For Ku-band missions, a full-scale offset parabolic reflector antenna is designed by considering different concepts of monolithic and nonmonolithic structures, different stiffening schemes, manufacturing ease, and lower cost. The resulting reflector is based on stiffened spring back reflector concept but it provides much simpler design, significantly lower mass and lower cost. It is a simple monolithic structure and consists of a reflector surface and a flat skirt. The skirt is an integral part of the reflector and is used to stiffen the reflector. The full-size version is an offset parabolic reflector with a diameter of 6 m, a focal length of 4.8 m, and an offset of 0.3 m. In order to demonstrate the concept, 1/3-scaled version is designed, manufactured, and tested. Experiments of the demonstrator are conducted for quasi-static folding, dynamic deployment behavior, stiffness measurements, modal analysis, and surface accuracy in deployed configuration; the experimental results are compared with the results of finite elements solutions where applicable.

Controlling Edge Effects in Laser Bending

Laser forming of sheet metal offers the advantages of requiring no hard tooling, no spring back and no external force. Thus this technology reduces cost, increases flexibility and accuracy. Laser forming is a complicated and transient thermo-mechanical process involving elastic and plastic strain dependent on time history. How to control and enhance the accuracy of the laser forming attracts much attention. This paper put forward a new strategy to control and reduce the edge effects in laser forming. Numerical simulations and experiments were also carried out to validate the presented strategy.

Study and Analysis of Geometric Effect of Ball Burnishing Process of Different Materials and Evaluation of Forces and Strain for Ballizing Process

The process consists of forcing an oversized ball of a hard material through a premachines hole in softer material .The interference between the ball and the hole causes the hole to expand such that its deformation is partly plastic and partly elastic. The elastic deformation of the hole is recovered due to elastic spring back whereas the plastic deformation results in a slight permanent increase in the hole diameter after ballizing. Ball burnishing or Ballizing is a production process for improve the accuracy and surface finish of holes. This process is a mass production process for sizing and finishing holes. The sizing and finishing of holes depends upon the interference adopted for ballizing process. This paper is an attempt toward comparing surfaces effects ,Estimation of deflection, deformation, radial strain, stress and finished dimensions of Mild steel and Aluminium.

Creep-age forming of AA7475 aluminum panels for aircraft lower wing skin application

Creep-age forming (CAF) is an interesting process for the airframe industry, as it is able to form or shape panels into smooth, but complex, curvatures. In the CAF process, the ageing cycle of the alloy is used to relax external loads imposed to the part, through creep mechanisms. Those relaxed stresses impose a new curvature to the part. At the end of the process, significant spring back (sometimes about 70%) is observed and the success in achieving the desired form depends on how the spring back can be predicted in order to compensate it by tooling changes. Most of the applications relate to simple (non stiffened) panels. The present work deals with the CAF of aluminum panels for aircraft wing skin application. CAF was performed using vacuum-bagging autoclave technique in small scale complex shape stiffened panels, machined from an AA7475 alloy plate. An analytical reference model from the literature was employed estimate the spring back effect in such panel geometry. This model that deals with simple plates was adapted to stiffened panels using a geometric simplification, resulting in a semi-empirical model. The results demonstrate that CAF is a promising process to form stiffened panels, and the spring back can be roughly estimated through a simple model and few experiments.

Experimental analyses of the poly(vinyl chloride) foams' mechanical anisotropic behavior

AbstractAssessing a full set of mechanical properties is a rather complicate task in the case of foams, especially if material models must be calibrated with these results. Many issues, for example anisotropy and heterogeneity, influence the mechanical behavior. This article shows through experimental analyses how the microstructure affects different experimental setups and it also quantifies the degree of anisotropy of a poly(vinyl chloride) foam. Monotonic and cyclic experimental tests were carried out using standard compression specimens and non‐standard tensile specimens. Results are complemented and compared with the aid of a digital image correlation technique and scanning electron microscopy analyses. Mechanical properties (e.g., elastic and plastic Poisson's ratios) are evaluated for compression and tensile tests, for two different material directions (normal and in‐plane). The material is found to be transversely isotropic. Differences in the results of the mechanical properties can be as high as 100%, or even more depending on the technique used and the loading direction. Also, the experimental analyses show how the material's microstructure behavior, like the evolution of the herein identified “yield fronts” and a “spring back” phenomenon, can influence the phenomenological response and the failure mechanisms as well as the hardening curves. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Investigating spring back phenomena in double curved sheet metals forming

Spring back is one of the main reasons for geometrical inaccuracy of stamping products. Many successful attempts have been done to calculate the amount of spring back of sheet metals, but most of them belong to the simple cases such as single curved U or V bending. In this paper an analytical formulation based on the moment–curvature relationships considering sheet thickness changes is presented for predicting spring-back in double curved sheet metal forming process. In this formulation displacement of neutral surface is also considered. The accuracy of the formulations is verified by comparisons with finite element and experimental results. The effects of some geometrical parameters such as thickness and curvatures radius on the spring back behavior of sheet metal have been investigated. Different sheet metals have been examined for modeling and experiments including SPCC steel sheets, commercial pure titanium sheets and cartridge brass sheets.

Influence of Punch Velocity on Spring Back in Micro Forming

In macro forming it is already known, that the punch velocity has an influence on the deep drawing process. This influence is considerably induced by the velocity dependent friction behavior between sample and tool. A further influence is the strain rate dependent forming behavior of the material. In micro range, the influence of punch velocity on the deep drawing process could, due to the size effects, be different from that in macro range, for example the spring back behavior. In this article the influence of punch velocity on the spring back behavior in micro deep drawing is investigated using strip drawing test with two different widths (1 mm and 2 mm). Experiments with aluminum strips with a thickness of 50 µm were performed with punch velocities ranged from 1 mm/s to 1000 mm/s. The strain behavior, which occurs with different punch velocities are investigated on the basis of microsections. The spring back of all samples was measured by an optical measurement system and compared with each other. From the reported work it can be concluded, that with increasing punch velocity the spring back of the complete system is increasing, while the spring back at cup wall stays constant. As reasons can be cited mass inertia effects due to the high velocities and the velocity dependent friction.

Mechanistic Model of Work-Piece Diametrical Error in Conventional and Ultrasonic Assisted Turning

The spring back is a major source of diametrical error occurring in turning of workpieces. In order to analyze diametrical error, an elastic deflection model of workpiece should be available. Many assumptions are usually taken into consideration when analyzing the elastic deformation of workpiece. Many studies have been done to find the analytical model of work-piece spring back and its suitable cutting force model. When compared with experimental data, developed models in these works are associated with considerable errors. In this paper, through using experimental results, an efficient elastic deformation model of workpiece has been obtained which can be compared with analytical model. Many experiments were performed, and some different functions were produced which predict the trend of experimental results. Finally, an appropriate model with the lowest error was suggested. The same process has been performed in ultrasonic vibrations assisted turning. The obtained functions will be suitable to compensate for the diametrical error occurring by machining forces. Finally this model has been used to obtain the amount of elastic spring back of tool and spindle.

Mechanistic Model of Work-Piece Diametrical Error in Conventional and Ultrasonic Assisted Turning

The spring back is a major source of diametrical error occurring in turning of workpieces. In order to analyze diametrical error, an elastic deflection model of workpiece should be available. Many assumptions are usually taken into consideration when analyzing the elastic deformation of workpiece. Many studies have been done to find the analytical model of work-piece spring back and its suitable cutting force model. When compared with experimental data, developed models in these works are associated with considerable errors. In this paper, through using experimental results, an efficient elastic deformation model of workpiece has been obtained which can be compared with analytical model. Many experiments were performed, and some different functions were produced which predict the trend of experimental results. Finally, an appropriate model with the lowest error was suggested. The same process has been performed in ultrasonic vibrations assisted turning. The obtained functions will be suitable to compensate for the diametrical error occurring by machining forces. Finally this model has been used to obtain the amount of elastic spring back of tool and spindle.

무금형 다점 펀치를 사용한 선체외판의 분할 성형 가공 정보 계산 시스템 개발

As a forming method for curved hull plates more efficient than the flame bending, mechanical bending using multi point press forming and die-less forming is discussed in this paper. the mechanical forming is a flexible manufacturing system for automatically forming of hull parts. It is especially suited to varied curved parts. This paper discusses a multiple point pressing machine composed of a pair of reconfigurable punches in order to achieve the rapid forming of curved hull plates using division forming and presents how forming information is obtained from the given design surface. Although the mechanical forming can be efficient in the metal forming, spring back after pressing is a phenomenon which must be carefully considered when quantifying the process variables. If the spring back is not accurately controlled, the fabricated shell plate cannot meet assembly tolerance. This paper describes the principles to calculate the proper stroke of each punch at the divided areas. the strokes are determined by an iterative process of sequential pressing and spring back compensation from an unfolded flat shape to its given design surface. FEA(finite element analysis) is used to simulate the spring back of the plate and the IDA(iterative displacement adjustment) method adjusts the offset of pressing punches from the deformation results and the design surface. The shape deviations of two surfaces due to spring back are compensated by integrated system using FEA and IDA method. For the practical application, It is aimed to develop an integrated system that can automatically perform the compensation process and calculate strokes of punches of the double sides' reconfigurable multiple-press machine and some experimental results obtained with mechanical bending are presented.

Morphology Change of Extrusion Molded Crystalline Polymer by Rolling Process

In this work, the spring back, dimensional change, internal structure and morphology of extruded polypropylene were investigated in order to consider the rolling characteristic. It was found that the internal structure, molecular orientation and crystallinity were changed a little by high spring back under low reduction in thickness. Crystal structure was extended along the rolling direction with high reduction in thickness due to the increase of plastic deformation. As a result, molecular orientation increased and crystallinity decreased at over 30% of reduction in thickness. Crystalline structure of surface was similar to the center part in cross-section at 70% reduction in thickness, and molecular orientation was constant.

Effects of time and temperature on the creep forming of 7075 aluminum alloy: Springback and mechanical properties

This paper deals with the experimental investigations of springback and mechanical properties of 7075 Al alloy in creep forming process. Results indicate that springback increased with decreasing time and temperature. Incorporating spring back and mechanical properties, it can be found that the appropriate forming cycle was 150 °C/24 h among all forming conditions.

The Design and Experiment Studies of Optical Adapter with Integrated Dust Protection and Shutter Design

High power lasers (> +21dBm) have gradually become the common solution for signal transmitting systems including regional cable television, Fiber To The Home (FTTH), and gigabit passive optical networks (G- PONs) due to their ability to generate signals that can be transmitted over long distances. However, if protective design is not implemented in the facility at the client side, users may be exposed to health hazards such as eye damage from these high-power lasers. High-power optical adapters with laser shutter use metal masks to prevent eye exposure to direct laser beams. They have progressively replaced conventional optical adapters and entered the market mainstream. Our study uses the Elastic-Plastic theory together with parametric design to investigate the effect of geometry on the initial spring-back angle of a laser shutter. Once the force stabilizes, the angles of the initial spring back are found to be the same as the simulated results for several attempts. In our study, it is observed that factors including the thickness of the metal masking plate, the initial design angle, the stiffness of the material and the boundary conditions have significant influence on the spring back angle. These can be used as references in design control.

보론강판의 열간 벤딩 공정에서 성형인자가 기계성질에 미치는 영향

In the hot press forming process (HPF), a martensitic structure is obtained by controlling the cooling rate when cooling a boron sheet that is heated up to over <TEX>$900^{\circ}C$</TEX>. The HPF process has various advantages such as the improvement in formability and material properties and minimal spring back of the deformed materials. The factors related to the cooling rate depend on the heat transfer characteristics between heated materials and dies. Therefore, in this study, the cooling rate is controlled by adjusting the heat transfer coefficient of the material at the pressing process. And, the mechanical properties and microstructure of the deformed material is demonstrated during the HPF process where cold dies are used to form the heated steel plate. This is achieved by varying the major forming conditions that control the cooling rate regarded as the most important process parameter.

Numerical Simulations of Superconducting Cables Bending and Winding

The assessment of the cold working, deformation, stress and strain distributions, and elastic spring back of superconducting cables conductors used for nuclear fusion experiments is a crucial issue for their manufacture process. The winding process of the ITER Toroidal Field (TF) conductors foresees their unbending from the drum they are delivered with and re-bending to their final shape. Nowadays the winding of the European Dipole (EDIPO) dummy coil is in progress and the final one will be challenging due to the conductors stiffness and the related spring back effect. In the light of this F4E ongoing activity and in view of the final design of the automatic winding process of the ITER TF coils a numerical tool capable to predict the plastic and elastic behavior of these conductors would be useful. To this end explicit Finite Element (FE) models have been implemented with the commercial code ANSYS/LS DYNA and simplified non linear calculations with CAST3M. Several numerical analyses have been performed by using different material properties, plasticity behaviors and loading conditions. The results of the analyses are presented and compared with experimental results and correlations with analytical solutions are investigated.