Wrinkle Defect Research Articles

Research on Contact State and Its Effect on Forming Precision in Uniform-Contact Stretch Forming Based on Loading at Multi-Position

Uniform-contact stretch forming based on loading at multi-position (UC-SF) was designed to substitute for conventional stretch forming (C-SF) in the manufacturing of qualified three-dimensional surface parts of a specified shape. Since the integral rigid clamps are replaced by discrete clamps, the sheet metal can be bent in a transverse direction (perpendicular to the stretching direction), and the sheet metal can be automatically warped to the die surface during the loading process. In this paper, finite element numerical simulations were performed to research the contact state evolution and its effect on forming precision by two kinds of loading modes (UC-SF and C-SF). The evolutions of contact state for spherical and saddle-shaped parts were analyzed in different steps, and the results reflect that, in UC-SF, the contact region of curved surface parts is gradually extended in a long strip, and the effective formed regions of the final parts can be in contact with the die surface. However, in C-SF, it is difficult for the final parts to be completely in contact with the die surface, especially spherical parts of a large curvature. Moreover, it is found that the noncontact region of the saddle-shaped part is susceptible to wrinkling in C-SF. Conversely, in UC-SF, the sheet metal can be constrained by contact with a die surface, such that the noncontact region and wrinkle defect disappear and high-precision parts are formed. Finally, stretch forming experiments were carried out and the perfect curved surface part was formed; thus, the experimental results verify the feasibility and effectiveness of UC-SF.

Analysis and reduction of wrinkling defects for tube-hydroforming magnesium alloy components at elevated temperatures

Wrinkling defects commonly occur in tube hydroforming (THF) magnesium (Mg) alloy at elevated temperatures when the tube-end and axial-feeding regions of the workpiece are overheated. Most previously proposed methods for preventing such defects have been applied at room temperature and restricted by several limitations. Therefore, this paper presents a breakthrough in tool design through the appropriate control of temperature distribution of the Mg alloy AZ31B tubular material to minimise the wrinkling defects in THF at evaluated temperatures. The proposed cost-effective, simple and user-friendly collet-type device design was able to provide a non-isothermal condition for THF within an appropriate pre-heating time after die closing. An axisymmetric barrel-shaped component was taken as a prime example to demonstrate the methodology, in which various thermal potential differences between the axial-feeding and deformation regions were investigated using finite-element (FE) simulation so as to evaluate the wrinkling effects under various non-isothermal conditions. The results showed that the most satisfactory component could be obtained when the average temperatures of axial-feeding and deformation regions were around 240 and 330 °C, respectively. Subsequently, with the same approach, a wrinkle-free non-axisymmetric tubular bike-frame component was hydroformed successfully as a more realistic and practical application example.

Multiscale surrogate modelling of the elastic response of thick composite structures with embedded defects and features

This paper presents a multiscale modelling approach for thick composite structures containing internal defects and features. The proposed approach was developed using a surrogate model to represent the composite response on the meso-scale. A set of Representative Volume Element (RVE) models under periodic boundary conditions were used to sample the response at specified locations across the composite design space. As an example of its application, wrinkle defects of various severities were introduced to the RVE models to assess the defect contribution to the composite response. The homogenized responses from the meso-scale RVE models were then used as input to the surrogate model. To link the macro and meso scales, a set of 3D lamination parameters representing the composite layup were developed. A surrogate model using the 3D lamination parameters and the defect severity as input was built to link the macro-model to the meso-scale responses. The proposed multi-scale approach was verified against a set of high fidelity models with different levels of wrinkle defect severity. Good agreement was found between the new multi-scale approach and the more computationally expensive high-fidelity models.

A numerical study on the influence of composite wrinkle defect geometry on compressive strength

Out-of-plane winkling in continuous-fibre reinforced composites has a significant influence on compressive failure stress, which needs to be considered and evaluated during the design, manufacture and inspection stages, to achieve high-performance composite components. With the development of a three-dimensional characterisation based on non-destructive testing methods and finite-element modelling, it is possible to combine the two techniques to give a prediction of mechanical performance using directly measured geometry. This paper uses a new methodology developed for combining non-destructive characterisation and numerical analysis techniques to automatically create a series of models with controlled wrinkle geometry. It has been possible to determine the dependence of compressive strength on various wrinkle-severity and wrinkle-extent parameters. The outcome shows a dominant dependence on the maximum wrinkle angle in the load direction, with an additional dependence on the wrinkle wavelength for larger wrinkle angles. In terms of the extent of the wrinkled region, the strength reduces as the wrinkle extent in the load direction becomes concentrated locally or the wrinkled proportion of the cross-sectional area (perpendicular to the load) increases.

Investigation of Wrinkling in Hydrodynamic Deep Drawing Assisted by Radial Pressure with Inward Flowing Liquid

The formation of wrinkling in deep drawing processes generally occurs in the two area of flange and cup wall. In this paper, a geometric method based on FEM simulation has been developed to investigate the wrinkling in the wall area of conical cups. Wrinkling wave variation was considered as the criterion of wrinkle defect. The forming process in this study is hydrodynamic deep drawing assisted by radial pressure with inward flowing liquid. The effects of radial pressure and cavity pressure on wrinkle phenomenon have been investigated. To verify the results of the simulation, experiments were performed on the steel sheets. Good agreement between the simulation and experimental results shows the reliability of this method in the wrinkling study. Results showed that radial pressure and cavity pressure have significant impacts on creation or prevention of wrinkling in the side wall.

Practicality and formability of integrally stiffened tube hydro-assisted forming

To achieve integrally stiffened tubular component, a new technology named as hydro-assisted forming was put forward, which successfully realized integral component forming. Experiment and finite element simulation were conducted to explore the practicality and formability of stiffened tube hydro-assisted forming. Based on plastic mechanics, the critical mechanics condition was analyzed and given. At last, the shape error and thickness distribution were given in detail. The results indicate that internal pressure induces not only tensile hoop stress but also “backward” bending moment, which are useful to eliminate wrinkle defect. But it is worthwhile to note that buckling defect would happen if the internal pressure was too high due to the effection of rib initial deflection. Shape accuracy is sensitive to internal pressure. Geometric error decreases as internal pressure, but increases after the internal pressure reaches a certain degree. Both welding residual stress and thermal deformation are successfully eliminated due to plastic deformation happens during hydro-assisted forming process. In view of hydro-assisted forming process, sound thickness distribution is an effortless advantage for bending deformation has less effection on thickness distribution. The maximum thinning ratio is only 5 % for hydro-assisted forming, which is far less than that of tube hydroforming. At last, an integrally stiffened tubal component with ellipticity of 2 was formed successfully by hydro-assisted forming technology, which rib height reaches 10 mm.

Tensile failure of laminates containing an embedded wrinkle; numerical and experimental study

The failure of a quasi-isotropic composite laminate containing an embedded out-of-plane fibre wrinkle defect was investigated under tension loading. Laboratory test specimens with controlled severity of fibre waviness were manufactured. Along with recording load–displacement data, high resolution camera images were taken at regular intervals which monitored the initiation and interaction of different damage mechanisms during test. Three-dimensional FE models were built following the geometry of actual test specimens. The information obtained from the tests was used to develop user material subroutines, implemented in Abaqus/Explicit as continuum damage and cohesive zone models for intra- and inter-ply failure respectively. The results of the simulations showed very good correlation with test observations in terms of failure load, location of damage initiation and interaction between different damage mechanisms for a range of waviness cases tested.

Out-of-plane ply wrinkling defects during consolidation over an external radius

If carbon fibre layers are prevented from slipping over one another as they consolidate onto a non-trivial geometry, they can be particularly susceptible to wrinkling/buckling instabilities. A one dimensional model for out-of-plane ply wrinkling during consolidation over an external radius is presented. Critical conditions for the appearance of wrinkles provide manufacturing strategies to eliminate such defects. Predicted wrinkle wavelengths and critical wrinkling conditions show good agreement with wrinkle defects observed in a spar demonstrator.

The Technology of Flexible Blank Holder Forming and the Study of its Numerical Simulation

Abstract. In this paper, we have discussed the principle of the flexible blank holder forming ( FBHF) technology, and established the finite element model of medal deep drawing process based on FBHF. Then we have discussed the FBHF technology by analyzing the system of finite element model. By simulating the forming of rectangular box, we can conclude that FBHF technology will have a greater advantage when the depth of stamping forming is much deeper. We have also compared the thickness distribution parameters, wrinkling parameters, and spring back parameters of rigid blank holder forming parts with FBHF parts. The result shows that the forming parts of FBHF technology has uniform stress and strain distribution, small spring back, the quality of inhibiting wrinkling, crack and other defects.

Fatigue failure of sandwich beams with face sheet wrinkle defects

Abstract This paper presents experimental fatigue results for GFRP face sheet/balsa core sandwich beams with face sheet wrinkle defects, subjected to fully reversed in-plane fatigue loading. An estimate of the fatigue design limit is presented, based on static test results, finite element analyses and application of the Northwestern University failure criteria. The presence of a wrinkle defect reduced the fatigue life by approximately 66%, compared to that of an unnotched reference laminate. Furthermore, the results from the fatigue tests revealed that the design limit was initially overestimated, as the specimens loaded close to the predicted design limit typically failed before reaching the target life, or reached test run-out with visible face sheet damage indicating imminent final failure in the worst case. It was found that specimens would reach target life with no visible or otherwise detectable damage by lowering the fatigue load amplitude below 80% of the predicted design limit. By extrapolating the test results it appears that the undamaged specimens would reach a fatigue life of 107–108 load cycles and would thus be safe for design of wind turbine blades.

Optimum of Drawbead Design and Restraining Forces by Sensitivity Analysis Method Combined with Inverse Approach in Sheet Metal Forming Process

A improved algorithm to optimize the restraining force of equivalent drawbead was proposed base on BGFS(Broyden-Fletcher-Goldfarb-Shanno) algorithm combined with a simplified finite element method called inverse approach(IA). The forming limit curve (FLC) and the wrinkle limit curve (WLC) were considered as the objective function to reflect the influence of Fracture and wrinkle defect in sheet metal forming process. The optimized result was more accurate than those procedures only consider the variation of thickness distribution. The optimized process was also very efficient due to the simplified assumptions of the IA. Two stamping parts were presented to validate the accuracy of this optimum algorithm.

Investigation of failure mechanisms in GFRP sandwich structures with face sheet wrinkle defects used for wind turbine blades

Wrinkle defects can be formed during the production of wind turbine blades consisting of composite monolithic and sandwich laminates. Earlier studies have shown that the in-plane compressive strength of a sandwich panel with wrinkle defects may decrease dramatically. This study focuses on the failure modes of sandwich specimens consisting of thick GFRP face sheets with a wrinkle defect and a balsa wood core subjected to in-plane compression loading. Three distinct modes of failure were found, and the strain distributions leading up to these failures were established by use of digital image correlation (DIC). Finite element analyses were subsequently conducted to model the response of the test specimens prior to failure, and generally a very good agreement was found with the DIC measurements, although slight differences between the predicted and measured strain fields were observed in the local strain values around the wrinkle defect. The Northwestern University (NU) failure criterion was applied to predict failure initiation, and a good correlation with the experimental observations was achieved.

Surface wrinkle defect of carbon steel in the hot bar rolling process

It is well known that surface defect is a common problem encountered in the multi-stage hot bar rolling process of carbon steel. In this study, the phenomenon was investigated by simulating the process by the finite element technique to identify the location where the surface defect might occur and checking the surface qualities obtained from the compression tests at various temperatures and strain rates to clarify the important parameter governing the possible surface defect formation. Also, the surface temperature was measured by employing pyrometer to support the experimental observation. The levels of temperature and specific deformation energy obtained from finite element simulations depending on the roll groove geometry were compared with the experimental observation to better understand the formation of the surface defect in the hot rolled bar. Based on this study, the surface defect might be formed by dissipating the excessive deformation energy accumulated by generating the new surface at the lower level of temperature where recrystallization cannot occur. According to this work, the comparison of the specific deformation energy level for determining the instability of the hot working process might be interesting for further investigation.

Instability map based on specific plastic work criterion for hot deformation

Estimating workability is very important to complete the hot or cold working process successfully without making any crack. Instability map was developed by utilizing conventional plastic work criterion in the current study. Hot compression tests were conducted using Gleeble 3800 simulator to obtain flow stress curves as a function of temperature and strain rate. Based on the compression results, new instability map based on the specific plastic work was developed for estimating the instability during the hot deformation process. The critical specific plastic work value was determined by investigating the surface irregularity and shear band formation in the compressed specimens. Then, the instability map determined was applied for predicting surface wrinkle defect during the multi-pass hot bar rolling in combination with the finite element analysis. The predicted results using new instability map suggested matched well with the observations obtained from the industry. The developed approach is easy to apply to the available simulation tools to determine necessary processing parameters.