Wrinkling Limit Diagrams Research Articles

A theoretical model for predicting sidewall wrinkling in chain-die forming of channels with variable widths

As a kind of progressive sheet metal forming process, chain-die forming is suitable to form thin-walled high-strength channels. However, sidewall wrinkling is one of typical shape defects on the chain-die formed parts and hard to be predicted accurately. In this paper, the mechanism of sidewall wrinkling during the chain-die forming process is investigated numerically and experimentally. Considering the twist of transitional surface which is a characteristic feature of chain-die forming, a new theoretical model for predicting sidewall wrinkling is established by combining the energy method with finite element simulation. A typical channel with variable widths is designed as a benchmark case and the chain-die forming experiment is carried out to verify the accuracy of the proposed model. Moreover, based on the proposed model and response surface methodology, the influence of radial stress on sidewall wrinkling is analyzed and wrinkling limit diagrams in terms of geometrical parameters are established. The results show that the fold angle has the greatest influence on the sidewall wrinkling. Finally, a two-pass chain-die forming scheme is proposed and implemented to suppress the sidewall wrinkling on the channel. The present work provides a wrinkling prediction tool for the process design of thin-walled channels in the chain-die forming. • A new wrinkling prediction model for progressive forming is established. • Twisted transitional surface reduces the anti-wrinkle ability of sidewall. • Image recognition method is used to identify onset of sidewall wrinkling. • Effects of geometric parameters and stress state on sidewall wrinkling are studied.

The establishment of critical wrinkling judgment line for shear wrinkling instability of thin plate and its influencing factors

Wrinkle is a common and unpredictable forming defect in thin-walled components forming process. The complex stress loading paths caused by different forming conditions are the main reasons for the unpredictable wrinkling. It has important theoretical and practical significance to accurately predict wrinkling and instability of thin-walled components under different stress loading paths. In this paper, the shear wrinkling experiment was taken as the research object, and the Buckle-Explicit algorithm in ABAQUS was used to establish the numerical analysis model of shear wrinkling. The accuracy of the simulation algorithm was verified by comparative experiments; Based on the results of numerical analysis, the definition method of critical wrinkling time under shear wrinkling was clarified; By changing the size of test specimens, the influence of height and width on the critical wrinkling stress and strain of shear wrinkling was studied; By changing the shape of the specimen, different stress loading paths were obtained, and a unified wrinkle judgment diagram was established by using the stress–strain data under different stress loading paths; The prediction of that judgment diagram of specimens under different stress loading paths was verified; Taking the upper circular lower square specimens with different thickness as examples, the distribution law of the unified critical wrinkling judgment point was analyzed. It was found that the plate thickness had no obvious effect on the unified wrinkling limit diagram established in this paper. • A unified critical wrinkling judgment line suitable for different stress loading paths was established in sheet forming. • The unified critical wrinkling judgment line is suitable for sheet forming under some other working conditions. • An new experiment model and finite element analysis(FEM) model of shear wrinkling were designed.

Establishment of sheet metal forming wrinkling limit diagram (WLD) and research on the consistency of WLDs in different processes

Abstract With the actual demand for lightweight and high strength of metal parts, wrinkling has become one of the main problems restricting the overall forming of thin-wall members. Therefore, the efficient and accurate prediction and control of it have actual significance. In this work, a critical principal strain analytic model for plate and shell wrinkling was set up with the combination of energy method theory and plastic flow law of material. And a numerical procedure of establishing the wrinkling limit diagram (WLD) was developed based on the finite element method and the critical principal strain analytic model. With the sidewall wrinkling in deep drawing process of the conical cup and the wrinkling of square plate in the Yoshida bucking test (YBT) as the analysis object, the morphology and the characteristics of the WLD under different forming processes were studied, and the effect of loading conditions and stress loading history on the WLD was investigated, it is observed that the WLD is different under the same sheet material but different loading conditions, and indicated the WLD is dependent on the loading conditions.

Research on the establishment and influence mechanism of critical wrinkling limit diagram (WLD) of plate and shell forming under mold constraints

With the rapid development of manufacturing technology in the fields such as aerospace, aviation, national defense, and automobiles, the demands for lightweight, toughening, and forming accuracy of products have increased. As one of the main defects in the plastic forming of thin-walled components, wrinkling failure has always been a key element in forming control. The sheet metal is restrained by the mold in actual forming process and the wrinkling is significantly affected by the boundary conditions. To study the effect of the mold constraints on the wrinkling failure in the actual forming process, a stretching wrinkling test of wedge plate with the boundary constraint conditions of mold was designed. By establishing the simulation model for wrinkling failure of sheet forming under mold constraints and its wrinkling limit diagram (WLD), the accurate prediction of sheet metal wrinkling failure form and the critical state in actual forming was realized. At the beginning of the research, the wedge plate tensile test was implemented for using as verification for simulation results, then the wrinkle simulation model of Buckle-Dynamic with the instability mode as the initial defect was established. By combining with the non-contact full-field strain measurement system DIC to obtain the strain data of the specimens in the test process, the accuracy of the simulation model was verified. On this basis, the critical wrinkling limit diagram (WLD) was established for the wrinkle partition on the wedge specimen, and the influence of stress state, stress loading path, principal compressive stress increment, principal stress ratio factors on the distribution and propagation direction of wrinkles, as well as the morphology and characteristics of WLD are discussed. The research content of this study is of guiding significance for the prediction of the actual wrinkling failure of sheet and shell forming.

Analytical and numerical investigation of wrinkling limit diagram in deep drawing of two-layer sheets with experimental verification

Wrinkling, which is primarily caused by insufficient blank holder force, is a significant issue that induces inconsistencies in forming parts, particularly in the deep drawing process. In this article, an investigation of the wrinkling in the deep drawing process of two-layer sheets is performed through an analytical approach, numerical method, and experimental tests. Increasing in the blank holder force, the process is under control by the proposed algorithm. Consequently, it aims to find the minimum required blank holder force to avoid wrinkling. The energy technique is utilized to predict the wrinkling in the analytical approach. Similarly, finite element simulations are implemented to investigate the effect of forming parameters on wrinkling. The experimental tests are performed to verify the analytical and numerical results. The impact of the material properties and stacking sequences (lay-up) on blank holder force and forming force are studied. Results show that the optimum blank holder force is dependent on the material properties, blank geometry, and layer stacking sequences. Also, a good agreement between analytical, numerical, and experimental results is achieved.

Determining factors affecting sheet metal plastic wrinkling in response to nonuniform tension using wrinkling limit diagrams

Abstract In thin plate plastic forming, the efficient, accurate prediction and control of wrinkling is a basic requirement. Because there is no clear means of determining the main strain space curve (similar to the fracture forming limit diagram) when establishing the critical wrinkling state, this work used diagonal tensile plate tests to verify a numerical simulation of plastic wrinkling in thin plates. This model included a buckling mode obtained by an eigenvalue buckling analysis as the initial shape defect in a dynamic explicit numerical simulation of the plastic deformation of the test pieces. This permitted calculation of the true instability morphology of the test piece and avoided the difficulty associated with simulating plastic deformation wrinkling morphologies using existing static implicit or dynamic explicit numerical simulation algorithms. The morphology and characteristics of the critical wrinkling limit diagram (WLD) associated with nonuniform tension were investigated. The effect of the specimen thickness and boundary conditions on the WLD in the main strain space were also examined and the effects of various processing parameters on crease resistance were established. Finally, the effectiveness of the WLD established in this work was verified based on using non-contact full-field strain measurements to examine the plastic wrinkling of thin plates. This research provides a means of establishing the critical wrinkling line for various conditions.

Erratum to: An analysis of wrinkling limit diagrams of aluminum alloy 5005 annealed at different temperatures

Erratum to: An analysis of wrinkling limit diagrams of aluminum alloy 5005 annealed at different temperatures

2111 Study on Effect of Blank Holder Gap on Cylindrical Cup Deep Drawing Process

This research is aimed to study on effects of Blank Holder Gap (BHG) on deep drawability, that has been investigated for a cylindrical cup deep drawing process from square blank shape. The BHG is an effective way to control material flow into the die cavity during forming process. The effects of BHG on deep drawing of circular cup are studied by using numerical simulation method with commercial code named LS-DYNA. The BHG is defined as a distance between die and blank holder surface (clearance). In this study the BHG is ranging from 1.05 to 1.60 mm when initial blank thickness is 1.00 mm. For failure (wrinkling and tearing) detection of deep drawing process Forming Limit Diagram (FLD) under Swift and Hill's criterion and Wrinkling Limit Diagram (WLD) are used. In order to find out an optimum BHG, and objective function of this research is defined as risk of tearing. From numerical simulation results, the BHG has a great effect on the forming quality of formed product. As ranging of BHG from 1.05 to 1.60 mm, a suitable BHG for deep drawing of circular cup for given tooling and material is tried to design. Moreover, a simple closed-loop algorithm had been developed in order to determine the variable BHF trajectory for constraint BHG during forming process.

An analysis of wrinkling limit diagrams of aluminum alloy 5005 annealed at different temperatures

This paper deals with the wrinkling limit diagrams for Aluminum alloy 5005 sheets having thickness of 2.00 mm annealed at four different temperatures namely 200°C, 300°C, 400°C and 500°C. The study pertains to deep drawing into cylindrical cups through conical die using a flat bottom punch. It is proved by these researchers and others that the onset of wrinkling takes place when the ratio of strain increments (der / deθ) or the ratio of strain (er / eθ) reaches a critical value during the drawing process. These values which could be determined experimentally over which the wrinkling takes place, is shown in the form of wrinkling limit diagrams for the above grade at different annealing temperature. An attempt is also made to develop the wrinkling theory that predicts the wrinkling based on results obtained in the form of wrinkling limit diagrams established for the above grade at different annealed temperature. Further it has been observed from the stress ratio that there is a clear curve, that separates the region from safe and the wrinkling region. Further it was observed that the annealed sheets having high n-value, high R-value and high UTS /σy ratio improve the resistance against wrinkling.

Experimental evaluation of wrinkling limit diagrams for aluminium alloy 5052 sheets annealed at different temperatures

The sheet-metal-forming operations are principal processes in the automotive industry. Aluminium alloy 5052 sheet is a widely used material in the automotive industry. With the aim of finding the safe working strains for this metal under various forming processes, a set of wrinkling limit diagrams in terms of strain and strain increment ratios and a forming limit diagram with a wrinkling limit curve superimposed have been constructed for the same sheet metal with 2.0 mm thickness annealed at four different temperatures. It has been found that the onset of wrinkling takes place when the ratio (dε r/dε θ) of strain increments or the ratio (ε r/ε θ) of strain reaches a critical value during the drawing process. Further it was observed that, the higher the annealing temperature, the greater was the safe strain region, because of higher normal anisotropy and strain hardening index values.

Effect of annealing on combined forming, fracture and wrinkling limit diagram of Aluminium 5086 alloy sheets

In this work, a combined forming, fracture and wrinkling limit diagram have been experimentally evaluated for Aluminium 5086 alloy sheet annealed at three different temperatures namely 200, 250 and 300°C. With the help of the above diagrams, the safe range which avoids necking, fracture and wrinkles during forming of Aluminium 5086 alloy sheets were examined and the effect of annealing temperature on this range and other parameters namely microstructure, tensile properties and formability parameters were studied.

Effect of mechanical properties on wrinkling limit diagrams for Aluminum 5086 alloy annealed at different temperature

This article deals with the wrinkling limit diagrams for Aluminium 5086 alloy sheets having thickness of 2.00 mm annealed at three different temperatures namely 200, 250 and 300 °C. The study pertains to deep drawing into cylindrical cups through conical die using a flat bottom punch. When a conical die is employed, the need for a hold down or clamping ring is eliminated. However, this enhances the propensity of the blank to fail by wrinkling or buckling, particularly in the early stages of a drawing process in which thin sheet blanks are used. It is proved by these researchers and others that the onset of wrinkling takes place when the ratio of strain increments (der/deθ) or the ratio of strain (er/eθ) reaches a critical value during the drawing process. These values which could be determined experimentally over which the wrinkling takes place has been shown in the form of wrinkling limit diagrams for the above grade at different annealed temperatures. An attempt is also made to develop the wrinkling theory that predicts the wrinkling based on results obtained in the form of wrinkling limit diagrams established for the above grade at different annealed temperatures. Further it was observed that the annealed sheets having high n-value, high R-value and high UTS/σy ratio improve the resistance against wrinkling.

Some studies on wrinkling limit of commercially pure aluminium sheet metals of different grades when drawn through Conical and Tractrix dies

This paper deals with the wrinkling limit diagrams of annealed three different grades of commercially pure sheets namely ISS 19000, ISS 19600 and ISS 19660 having thickness of 2.00 mm when deep drawing into cylindrical cups through Conical and Tractrix dies using a flat bottomed punch. It is well established that, the use of either a Conical die or a Tractrix die can enhance the limiting draw ratio compared with that obtainable in a conventional drawing operation. When a Conical or a Tractrix die is employed the need for a hold down or clamping ring is eliminated. However, this enhances the propensity of the blank to fail by wrinkling or buckling, particularly in the early stages of a drawing process in which thin sheet blanks are used. It is proved by these researchers and others that the onset of wrinkling takes place when the ratio of strain increments (de r /de θ ) or the ratio of strain (e r /e θ ) reaches a critical value during the drawing process. These values which could be determined experimentally over which the wrinkling takes place has been shown in the form of wrinkling limit diagrams for the above three different grades. An attempt is also made to develop the wrinkling theory that predicts the wrinkling based on results obtained in the form of wrinkling limit diagrams established for the above grades.

Forming, fracture and wrinkling limit diagram for if steel sheets of different thickness

In this work, forming limit, fracture limit and wrinkling limit diagrams have been experimentally evaluated for various interstitial-free (IF) steel sheets of different thicknesses namely 0.6 mm, 0.9 mm, 1.2 mm and 1.6 mm, 0.85 mm noncoated and 0.85 mm coated. With the help of the above diagrams the suitability of the different steel sheets for press forming operations under different strain combinations were examined. The microstructural aspects, tensile properties and formability parameters were studied. Forming, fracture and wrinkling limit diagrams were evaluated for the above sheet metals and they were compared. The tensile properties and formability parameters were correlated with the forming, fracture and wrinkling limit diagrams.

Study on wrinkling limit of interstitial free steel sheets of different thickness when drawn through Conical and Tractrix dies

Abstract This paper deals with the wrinkling limit diagrams of cold rolled and close annealed of different grades of IF Steel sheets, which are used in Automotive Industries. The study pertains to deep drawing into cylindrical cups through Conical and Tractrix dies using a flat bottom punch. It is well established that, the use of either a Conical die or a Tractrix die can enhance the limiting draw ratio compared with that obtainable in a conventional drawing operation. When a Conical or a Tractrix die is employed the need for a hold down or clamping ring is eliminated. However, this enhances the propensity of the blank to fail by wrinkling or buckling, particularly in the early stages of a drawing process in which thin sheet blanks are used. It is proved by these researchers and others that the onset of wrinkling takes place when the ratio of strain increments (d e r /d e θ ) reaches a critical value during the drawing process. This value, which could be, determined experimentally over which the wrinkling takes place has been shown in the form of wrinkling limit diagrams for the above different grades of IF Steel sheet. An attempt is also made to rank the IF Steel sheet grade that appears to suppress the wrinkling based on results obtained in the form of wrinkling limit diagrams, established for the above different IF Steel sheet grades, in terms of the strain increments ratio and the stress ratio parameters. Further, it is noticed that as a thickness of IF Steel sheet increases the safe region also increases. This means that higher is the thickness better is the resistance against wrinkling. Drawing through the Tractrix die shows better resistance against wrinkling comparing with the Conical die.

A new method to accurately obtain wrinkling limit diagram in NC bending process of thin-walled tube with large diameter under different loading paths

To obtain the wrinkling limit of thin-walled tube by the NC bending process analytically, a new method has been proposed. Firstly, an FE model using ABAQUS/Explicit is employed to simulate the complex forming process. Thus, the plastic deforming behavior and wrinkling mechanism for thin-walled tube with large diameter under different loading paths (with or without boosting device) has been investigated and the reasonable assumptions for analytical analysis have been obtained. Secondly, according to the above simulation, an analytical wrinkling prediction model based on an energy criterion is established to calculate the wrinkling limit. Thirdly, an exponential function has been used to compensate for the effects of boundary conditions on onset of wrinkling by comparing the analytical results with corresponding numerical results to make the final results closer to reality. Thus the winkling limit diagrams with band distribution features are calculated. The method proposed is more suitable to the evaluation of formability for complex sheet forming process concerning wrinkling.

Study on wrinkling limit of commercially pure aluminium sheet metals of different grades when drawn through conical and tractrix dies

Abstract This paper deals with the wrinkling limit diagrams of annealed three different grades of commercially pure sheets, namely ISS 19000, ISS 19600 and ISS 19660 having thickness of 2.00 mm when deep drawing into cylindrical cups through conical and tractrix dies using a flat bottomed punch. It is well established that, the use of either a conical die or a tractrix die can enhance the limiting draw ratio compared with that obtainable in a conventional drawing operation. When a conical or a tractrix die is employed the need for a hold-down or clamping ring is eliminated. However, this enhances the propensity of the blank to fail by wrinkling or buckling, particularly in the early stages of a drawing process in which thin sheet blanks are used. It is proved by these researchers and others that the onset of wrinkling takes place when the ratio of strain increments (dɛr/dɛθ) reaches a critical value during the drawing process. This value which could be determined experimentally over which the wrinkling takes place has been shown in the form of wrinkling limit diagrams for the above three different grades. An attempt is also made to rank the aluminium grade that appear to suppress the wrinkling based on results obtained in the form of wrinkling limit diagrams, established for the above three different grades, in terms of the strain increments ratio and the stress ratio parameters. Further, it is observed that the aluminium grade ISS 19660 having high strain hardening index value, low ratio of tensile to yield (σu/σy) and fairly good value of high normalized hardening rate shows better resistance against wrinkling. Drawing through the tractrix die shows better resistance against wrinkling comparing with the conical die.

Effect of mechanical properties on the wrinkling behaviour of three different commercially pure aluminium grades when drawn through conical and tractrix dies

Abstract Experiments have been carried out to evaluate the onset of wrinkling behaviour in three grades of annealed commercially pure aluminium sheets, namely ISS 19000, ISS 19600 and ISS 19660, having thickness 2.00 mm, while drawing them through conical and tractrix dies. The deep drawing of circular blanks of different diameters of the aforesaid sheet metals was carried out using flat bottom punch. The analysis of the experimental data has revealed that the onset of wrinkling takes place when the ratio of the plastic strain increments (dɛr/dɛθ) reached a critical value. It was further found that the measured strains followed non-linear paths in the wrinkling limit diagrams. Further it was observed that the aluminium grade having high strain hardening index value, low yield stress and high normalized hardening rate shows better resistance against wrinkling. Drawing with the tractrix die showed better resistance against wrinkling compared with drawing with the conical die.

Effect of annealing on the wrinkling behaviour of the commercial pure aluminium grades when drawn through a conical die

Abstract Experiments have been carried out to evaluate the onset of wrinkling behaviour in three grades of annealed commercial aluminium sheets namely ISS 19000, ISS 19600 and ISS 19660 having thickness 2.00 mm, while drawing them through a conical die. The deep drawing of circular blanks of different diameters of the aforesaid sheet metals was carried out using flat bottom punch. The analysis of the experimental data has revealed that the onset of wrinkling takes place when the ratio of the plastic strain increments (d e r /d e θ ) reached a critical value. It was further found that the measured strains followed non-linear paths in the wrinkling limit diagrams. Further, it was observed that the aluminium grade having high strain hardening index value, low yield stress and high normalized hardening rate shows better resistance against wrinkling. Furnace cooled aluminium grades showed better resistance against wrinkling compared with air cooled blanks.

Study on wrinkling limit diagram of anisotropic sheet metals

Abstract Surface distortions in the form of localized buckles and wrinkles are often observed in sheet metal forming process. In many cases the presence of wrinkles is unacceptable for the final product and the purposes of assembly. Because of the trend in recent years towards thinner gauges and higher strength, wrinkling is increasingly becoming a more common and troublesome mode of failure in sheet metal forming. In this study, a numerical analysis for evaluating a wrinkling limit diagram (WLD) for an anisotropic sheet subjected to biaxial plane-stress is presented. Here the scheme of plastic bifurcation theory for thin shells based on the Donnell–Mushtari–Vlasov shell theory is used. The effects of various material parameters — yield stress, strain-hardening coefficient and normal anisotropy — and geometrical parameters on the WLD are investigated numerically and compared with the results of the experiments of Kawai and Havranek.