Elastic Springback Research Articles

Holistic springback compensation procedure

The elastic springback during the manufacturing process of stamped car body components causes dimensional deviations. To compensate these deviations, the common approach is to modify the tool surfaces in the opposite direction of the deviations – this is called springback compensation. In the procedure of springback compensation various issues must be solved. To achieve the main goal of a dimensionally accurate part, it must be ensured that, on the one hand the distance normal to the sheet surface between the springback part and the target geometry is within a specified dimensional tolerance. On the other hand, it must be ensured that the surface areas and characteristic lengths of the springback part and that of the target geometry match as closely as possible. In the past, approaches/methods, such as the physical compensation method and the physical scaling approach, have been presented which can successfully counteract these problems. Furthermore, in a multi-stage process in subsequent operations a stable part position must be achieved and unwanted plastic deformations must be avoided during blankholder closing. Therefore, different compensation strategies have been presented, which can fulfil these requirements. However, in the publication of these methods, the problems were always considered individually. This paper shows how all the named requirements can be achieved by combining the individual methods in springback compensation.

On the Influence from Pressing Agent (PEG) on the Elastic and Plastic Mechanical Behavior of Hard Metal Powder Compacts

During unloading, pressed hard metal powder compacts expand (spring back), leading to unwarranted tensile stresses and, subsequently, crack initiation in the green body. Here, the elastic spring back and the green strength are analyzed for different types and amounts of the pressing agent PEG (polyethylene glycol). The results show that the plastic behavior, but not the elastic behavior, is influenced by a change of the pressing agent PEG. In this context, it should be stated that the risk for the initiation of cracks is influenced by both the elastic and plastic behavior after compaction. The spring back after compaction and the green strength defines to a large extent the risk for cracks. In addition, it is concluded that a standard three-point bending test is sufficient to analyze the risk for the initiation of cracks when comparing different spray-dried powders.

Optimization of the physical scaling approach through the adaption of the inverted membrane stresses

Dimensional deviations of stamped car body components caused by elastic springback still represent a significant problem for the development of the stamping tools. First, the springback causes deviations of the part in vertical direction to the part surface due to bending stresses. Second, the surface area of the part contracts (and also bends) due to elastic membrane stresses. The precise manifestation of the contraction depends both on the part stiffness and on the – usually heterogeneous – distribution of the membrane stresses. Usually, the resulting deformations cannot adequately be compensated by globally homogeneous scaling approaches. In order to carry out a locally correct compensation of the stamping tools, Birkert et al. have recently presented a physical scaling approach based on the inverted membrane stresses gained from the part in the closed die to compensate the active die surfaces. This approach delivered significantly better results but left potential for improvement. It is shown in the present paper how the existing approach and thus the scaling values can further be improved by adapting the inverted membrane stresses in an appropriate way. This is done by comparing the amount of strain changes during spring-back with those of the scaling process in a first step and adjusting the compensation stresses, in a second step, in such a way that the considered strain amounts are approximately identical.

Bridging the gap between design and manufacturing specifications for non-rigid parts using the influence coefficient method

The manufacturing process may lead non-rigid parts to endure large deformations which could be reduced during assembly. The manufacturing specifications of the single parts should refer to their free state or “as manufactured” state; the functional specifications should instead address the “as assembled” state. Therefore, a functional geometrical inspection requires dedicated fixtures to bring the parts in “as assembled” state. In this paper, through a linearized model that considers fixturing and elastic spring-back, we aim to correlate the functional specification to the manufacturing specifications. The model suggests a hybrid approach called “restricted skin model” that allows to reduce the degrees of freedom considering the form error when relevant. Firstly, the framework is verified in a mono-dimensional test case. Subsequently, it is verified including FEM simulation and actual measurement for two simple assemblies. The results show that the model can correctly interpret the theoretical assembly behaviour for actual applications. The use of the “restricted skin model” approach shows a negligible difference when compared to full FEM simulation with differences of 2.1 · 10−7 mm for traslations and 6.0 · 10−3 deg for rotations. The comparison with actual measurement values showed an error of ±0.2 mm at the assembly features. Furthermore, the linearized model allows a possible real-time application during production that enables to adjust manufacturing specification limits in case of process drifting.

Using discrete simulations of compaction and sintering to predict final part geometry

ABSTRACT A Discrete Element Method (DEM) model is used to simulate the compaction and sintering of ceramic oxides. The process kinematics is decomposed into loading, unloading and ejection of the pellet. Interactions between the particles are considered elastoplastic by implementing a model able to tackle large densities. A simplified approach is used in the sintering stage, which focuses on the final part geometry rather than kinetics. The results are in good agreement with experimental data and FEM simulations from the literature regarding density gradient, elastic spring-back and final geometry. The simulations show that the friction coefficient between the agglomerates and the die is the primary factor for the density gradient in the pellet. This density gradient induces non-homogeneous sintering, which results in a final geometry with a diabolo effect. It is the first time that DEM reproduces this effect with the advantage of considering explicitly the particulate nature of the powder.

A toolpath strategy for double-sided incremental forming of corrugated structures

This study focuses on developing a new toolpath strategy for manufacturing near-net shape corrugated structures using rectangular sheets whose two opposite edges are clamped based on the mechanics of Double-sided incremental forming (DSIF) process. The newly suggested strategy, Regional Plastic Incremental Bending (RPIB), is designed to capitalize on the effect of regional plastic bending in a way that a local geometric feature is incrementally formed without causing global elastic springback. The performance of the RPIB toolpath strategy is compared to the previously developed linear toolpath strategy that does not consider any anticipated springback errors in its design stage. It has been experimentally proven that the RPIB strategy results in significant improvements in the geometric accuracy of formed corrugated profiles. • A new toolpath strategy for manufacturing corrugated structures with Double-sided Incremental Forming (DSIF) technology. • Investigation of DSIF process characteristics under the condition of two opposing edges of the sheet are clamped. • Analysis of possible process failure modes in DSIF of corrugated structures.

On Dimensional Control in Rotary Stretch Bending of Aluminum Profiles: Loading-Path Effects

In this research, the influences of loading paths on dimensional accuracy in a newly developed flexible rotary stretch bending process were investigated by experimental and numerical approaches. A series of carefully controlled bending experiments were conducted using AA6082-T4 rectangular, hollow profiles. Several representative loading paths — including (i) kinematically-controlled simultaneous stretching and bending; (ii) pre-stretching prior to bending; (iii) post-stretching subsequent to bending; (iv) pre-stretching prior to and post-stretching subsequent to bending — were designed and tested for a given geometrical configuration of bent parts. The critical dimensional characteristics, such as global shape (elastic springback) and cross-sectional deformation, were analyzed and discussed for the products formed under the above-designed loading paths (i)-(iv). Furthermore, finite element analysis was conducted to gain additional insights into the deformation mechanisms that take place for various loading paths. Finally, a guideline for designing the loading paths was developed, thus providing improved process control for achieving formed products with high dimensional accuracy.

Multistage Deep-Drawing with Alternating Blank Draw-in for Springback Reduction in Transfer and Progressive Dies

Cold formed sheet metal parts made of advanced high-strength steels (AHSS) offer a high potential for a lightweight, durable and economic design. However, manufacturing dedicated, high-strength parts with cold forming technologies such as conventional deep-drawing often results in unacceptable shape deviations due to elastic springback after unloading the part from the forming tools. Therefore, various springback compensation methods have been established to ensure dimensional quality of such sheet metal parts. At the Institute for Metal Forming Technology (IFU Stuttgart), deep-drawing with alternating blank draw-in was developed in this context as a new approach to reduce springback and enhance cold forming of AHSS sheet metal parts. Presented work provides numerical sensitivity analysis as well as experimental studies about this new forming method. The asymmetric and alternating blank draw-in, which is changed within a multistage forming process sequence, results in an alternated bending over tool radii and leads to a beneficial stress superposition in the part wall area with reduced springback phenomena. Compared to conventional deep-drawn sheet metal components, springback of a benchmark part geometry could thus be reduced over 75 % by a three-stage forming process with an optimized blank draw-in kinematic.

Minor cutting edge angles of sawing teeth: effect on cutting forces in wood

Optimising the cutting tool geometries can reduce waste while increasing timber yield. The industry is moving towards thin-kerf bandsawing of timber, and therefore, the geometry of the minor cutting edge and minor first flanks of cutting teeth become more important. Six cutting teeth with varying minor cutting edge angles (0^{circ }, 2^{circ }, 4^{circ }, 6^{circ }) and minor cutting edge clearance angles (2^{circ }, 4^{circ }, 6^{circ }) were tested by cutting into the heartwood and sapwood of frozen and non-frozen Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.). Single cutting tooth tests were conducted, and the cutting and feeding forces were measured. The cutting forces were higher for Scots pine due to its higher density. Frozen sapwood resulted in higher cutting forces. The feeding force of frozen sapwood stood out as it was highly negative (i.e. self-feeding) compared to frozen heartwood, non-frozen heartwood and non-frozen sapwood. An increase in the minor cutting edge angle and minor cutting edge clearance angles, and therefore more room for elastic spring-back, can lead to up to less friction and 40% lower cutting forces. Higher minor cutting edge angles and minor cutting edge clearance angles resulted in less-negative feeding forces (i.e. less self-feeding) for frozen sapwood but remained relatively unchanged for the other wood conditions.

Advanced part design method for springback minimization of stamped sheet metal car body components

The requirements for high-strength and low-weight car body structures prove challenging for the stamping process and cause growing problems with the elastic springback. Compensation approaches such as the geometrical adjustment of the stamping dies by the amount of and in opposite direction to the springback can be an effective solution in many cases. However, this approach can only lead to satisfactory results if the scatter of the springback is considered, controlled and in the best case reduced. The reduction of the scatter can be provoked by adaption of the part design. Therefore, different methods such as the topology optimization method, the so-called line-of-force method and shape optimization can be applied. In the present work an advanced topology optimization method is suggested. Based on the decomposition of the springback deformation into membrane and bending components, the most efficient optimization of the part design is found relying on unconventional topology optimization. For the proposed adapted topology optimization approach the excellent results are also confirmed when applied to the industry-relevant case of a truck rear panel.

The material removal and surface generation mechanism in ultra-precision grinding of silicon wafers

The ultra-precision grinding technology based on the workpiece self-rotational principle has been extensively employed to obtain a high surface accuracy and material removal rate during the wafer thinning process. However, no estimation methods and prediction models are currently available for the surface topography of ground wafers, especially considering the material removal characteristics between the grain and silicon. This paper proposed a novel three-dimensional topography modeling framework from a micro-scale perspective to reveal the material removal and surface generation mechanism underlying the wafer self-rotational grinding process. First, the wheel morphology was reconstructed via the random operation for grain position and size, while the combined effects of plastic side flow, elastic spring back and brittle fracture were quantized according to the elasticity theory principle. Then, a new method was developed to calculate the profile height of the wafer topography using kinematic transformation. The groove angle and surface roughness were used to characterize the wafer surface topography. The findings were consistent with the simulation results and validated the correctness of the model. The simulation model was used to examine the impact of the grain size and grinding parameters on the wafer surface topography, while the degree of influence was further evaluated according to the change rate of the groove density curve. This work provided new insights into the material removal behavior during wafer self-rotational grinding and optimized the grinding parameters to obtain a smoother surface.

Minor cutting edge force contribution in wood bandsawing

As the sawmill industry is moving towards thinner bandsaws for higher yields, it is important to study the cutting force in more detail. The cutting force can be split into two zones. Zone I concerns the force on the major cutting edge as well as the friction force on the major first flank. Zone II considers the forces on the minor cutting edges as well as the friction forces on the minor first flanks. Zone II cutting can significantly affect the cutting force and has not been studied in great detail. Frozen, non-frozen and dry heartwood of Norway spruce and Scots pine were cut using different tooth geometries and the cutting force was measured. The major cutting edge, clearance, band thickness, minor cutting edge angle and minor cutting edge clearance angle were investigated. The y-intercept of the cutting force–width graph was used as the Zone II force (at this point the Zone I forces are assumed to be zero). The Zone II force contribution to the cutting force was studied. The results show that frozen wood has less elastic spring-back and therefore less Zone II cutting. Dried wood showed a significantly higher degree of Zone II cutting (55−75% contribution to the cutting force). Changing the major cutting edge from 2.87 mm to 1.6 mm resulted in 10–15% higher Zone II force contributions.

Artificial neural network modelling and optimization of elastic and an-elastic spring back in polymer parts produced through ISF

Artificial neural network modelling and optimization of elastic and an-elastic spring back in polymer parts produced through ISF

A dissipative model for deep-drawing simulations: Elastic springback prediction and incremental forming strategies

In industrial applications, with geometries of fabric and textile composites becoming more and more complex, new deep drawing strategies are required. In this context, modern incremental forming strategies appear to be very advantageous. Moreover, recent approaches have emerged making it possible to reach decent conclusions about the incremental forming strategy and the ability to predict the elastic spring back as well as the residual stresses in a draped woven fabric. The aims of this paper were thus several. The first part presents a discussion of the differences between classical models (often hyperelastic) and new ones (dissipative, hysteretical) under large strains. Subsequently, these models were compared in application to different deep-drawing geometries. It was found that more sophisticated models were interesting for complex forming, while common hyperleastic ones were efficient enough for simple geometries. The last part introduces some results concerning spring-back and new capabilities obtained by such dissipative models. Finally, two incremental forming tool geometries were also presented and discussed.

Adapted part design methods for springback minimization of stamped sheet metal car body components

In sheet metal forming, both the dimensional accuracy of the manufactured parts and the robustness of the forming process are significantly compromised by elastic springback effects. In contrast to traditional compensation methods mostly relying on modifications of the tool geometry, a part geometry-based approach reduces the amount of springback by appropriately manipulating the part stiffness through slight modifications of the part geometry. To date, part-geometric measures are mostly based on practical experience and trial-and-error processes resulting in a need for simple and effective simulation-based approaches. In the present work three novel simulation-based approaches for a suitable modification of structural component design are suggested relying on unconventional topology optimization, the so-called line-of-force method and shape optimization. First tests with a classical hat profile reveal that the optimized geometries, compared to the nominal one, lead to significant reductions of the springback after stamping and simultaneously to a notable decrease of the process variation when parameters of the forming process are varied slightly. For the adapted topology optimization approach the excellent results are also confirmed when applied to the industry-relevant case of a truck rear panel.

Tool wear and spring back analysis in orthogonal machining unidirectional CFRP with respect to tool geometry and fibre orientation

Machining abrasive carbon fibre reinforced polymers (CFRP) is characterised by extensive mechanical wear. In consequence, the cutting edge micro-geometry and thus the tool/material contact situation are continuously changing, which affects process forces and machining quality. As a conclusion, a fundamental understanding of the tool wear behaviour and its influencing factors is crucial in order to improve performance and lifetime of cutting tools. This paper focuses on a fundamental tool wear analysis of uncoated tungsten carbide cutting inserts with different combinations of fibre cutting angles and tool geometries. For this purpose, orthogonal machining experiments with unidirectional CFRP material are conducted, where the wear progression of the micro-geometry is investigated by means of five wear parameters lα, lγ, γ*, α*, and bc. For detecting the actual contact zone of the cutting edge and to measure the elastic spring back of the material, the flank face is marked via short pulsed laser processing. Furthermore, the process forces and the wear rate are measured. It is shown that the material loss due to wear clearly varies along the tool’s contact region and is highly dependent on the clearance angle and the fibre cutting angle Φ, while the influence of the tested rake angles is mostly negligible. Especially in machining Φ=30° and Φ=60°, a strong elastic spring back is identified, which is more intense for smaller clearance angles. For all tested configurations, the material’s elastic spring back increases in intensity as wear progresses which, in combination with the decreasing clearance angle, is the main reason for high thrust forces.

Effect of Compaction Pressure and Moisture Content on Post-Agglomeration Elastic Springback of Pellets.

Renewable energy sources (RES) represent an increasing share of global energy production. Biomass has the highest potential of all RES. Biomass is used to produce solid biofuels, liquid biofuels, and gaseous biofuels. One of the main directions of research on solid biofuels is to optimize the agglomeration process. The main factors determining the characteristics of the final product in the production of pellets are process and material parameters. Process parameters include compaction pressure, temperature, and geometry of the matrix channel. The parameters of the material are the type of biomass, moisture content, degree of fragmentation, and method of preparation of the material (e.g., drying). The process of pressure compaction is always associated with the negative phenomenon of elastic springback. The aim of this work was to check the influence of compaction pressure and material moisture content on the springback value. The research was conducted on three materials (giant miscanthus, cup plant and Virginia mallow), using four different pressures (131, 196, 262, and 327 MPa) and three different moisture levels (8, 11, and 14%). For all material springback values, the range was 9–16%. Statistical analysis showed that for all plants tested, the effects of compaction pressure and moisture content significantly affected the elastic springback value. Areas of high value springback in the pattern of process parameters were determined.

Single and multiple springback technique for construction and control of thick prismatic mesh layers

Abstract We suggest an algorithm for construction of semi-structured thick prismatic mesh layers which guarantees an absence of inverted prismatic cells in resulting layer and allows one to control near-surface mesh orthogonality. Initial mesh is modelled as a thin layer of highly compressed prisms made of hyperelastic material glued to the triangulated surface. In order to compute robust normals at the vertices of the surface mesh we use quadratic programming algorithm based on the nearest ball concept. This pre-stressed material expands, possibly with self-penetration and extrusion to exterior of computational domain until target layer thickness is attained. Special preconditioned relaxation procedure is proposed based on the solution of stationary springback problem. It is shown that preconditioner can handle very stiff problems. Once an offset prismatic mesh is constructed, self-intersections are eliminated using iterative prism cutting procedure.Next, variational advancing front procedure is applied for refinement and precise orthogonalization of prismatic layer near boundaries. We demonstrate that resulting mesh layer is ‘almost mesh-independent’ in a sense that the dependence of thickness and shape of the layer on mesh resolution and triangle quality is weak. It is possible to apply elastic springback technique sequentially layer by layer. We compare single springback technique with multiple springback technique in terms of mesh quality, stiffness of local variational problems and mesh orthogonality or/and layer thickness balance.

МОДЕЛИРОВАНИЕ ПРОЦЕССА ГИБКИ ПЕРСОНИФИЦИРОВАННЫХ ИМПЛАНТАТОВ, ИСПОЛЬЗУЕМЫХ ПРИ РЕКОНСТРУКЦИИ ДЕФЕКТОВ НИЖНЕЙ ЧЕЛЮСТИ

In this paper, using the finite element method, a comparative study on bending schemes of standard titanium perforated reconstruction plates used in the reconstruction of mandibular defects is presented. Currently, there is a need for equipment for high-precision bending of perforated plates, with the purpose of сreating a specific geometry that follows the natural curvature of the patient's jaw. The personalization of the implant allows to restore the natural symmetry of the patient's face and, therefore, accelerate the process of his social reintegration. The article compares six bending schemes corresponding to different bending directions, angles and levers. The results obtained allow to understand the specificities of the process, evaluate its strain characteristics and elastic springback after bending, which are necessary for the development of equipment for bending personalized implants.

A Python and Java software approach for 2.5 axes, self-adaptive stretch forming process and IoT solution

Stretch forming is a metal forming process that implies bending and stretching a sheet of metal over a die, with the sheet being plastically deformed into the desired shape. The purpose is to obtain large parts. One of the main aspects is that the metal sheet is locked into position by gripping jaws. A hydraulic ram is raised into the metal sheet, therefore increasing the tensile forces. This process is used to draw intro shape materials like aluminium, magnesium, titanium alloys, stainless steel, Inconel. These materials have in common the fact that they have poor formability or the elastic spring-back has unacceptable values. Taking this into consideration, the nature of the materials along with the fact that the metal sheet can crack, due to excessive strain, we propose a self-adaptive stretch forming process. Underlining this process is the stress-strain curve, that has three inputs: force, sample section area and strain. For each of these factors, our method uses a particular approach, as in a Python-based software and Android-based IoT solution, that uses stress and strain data. Furthermore, it controls, in real-time the hydraulic press to the point at which the material is stretch close to its ultimate yield strength.