Papers by Keyword: Wrinkle

Abstract: Ultrasonic measurement is used to evaluate wrinkles during sheet metal forming. The authors developed a new apparatus for investigating the relationship between the wrinkles in press forming and the ultrasonic reflection characteristic of an angle beam. The new evaluation apparatus was composed of probe-fixing parts, an upper die, a middle die, and a lower die. A specimen was sandwiched by a pair of dies in the evaluation apparatus. Angle beam probes for transmission and reception were set on the upper die. Specimens in the form of plates having periodic trapezoidal wrinkles were fabricated by electro-discharge machining. In the new evaluation apparatus, the specimen is placed in contact with the die. Wrinkling was found to change the ultrasonic reflection characteristic of an angle beam. The new apparatus can thus be used to evaluate wrinkles using the angle beam technique.

Abstract: This study suggests a novel method to detect wrinkles using the frequency characteristic of reflected ultrasonic wave. The experimental apparatus was manufactured to simulate the press forming. A plate specimen possessing periodic wrinkles in a trapezoidal shape was also prepared to simulate actual wrinkles during the press forming process. These wrinkles affected the reflected wave of ultrasonic wave. The power spectrum of reflected wave at a frequency of 0.8 MHz was changed by the occurrence of wrinkles. The comparison of the frequency characteristics provided obvious difference between wrinkled and flat specimens. As a result, the present work confirmed that the frequency characteristic of reflected wave would detect the formation of wrinkles.

Abstract: In this paper, the sheet hydroforming process of 2A12 aluminum alloy with uniform die cavity pressure on to the blank is proposed and investigated both primarily through the finite element method (FEM) and experiments. The influence of the die cavity pressure curve on the quality of the products was explored and the measures to promote the sheet formability were discussed. The results from the studied case indicate that the profile of the cavity pressure was one of the fundamental parameters directly related to the product's quality and precision. Excessive or insufficient initial pressure is not conducive for the reduction of wall thickness thinning and guarantee of wall thickness uniformity. And the wall thickness thinning is reduced and the thickness evenness is improved by increasing the maximum cavity pressure within a proper range. Moreover, an optimum cavity pressure curve generated by the numerical and experimental methods was properly applied in forming the aluminum alloy part without rupture and with slight wrinkle in the flange area. The study demonstrates that the results of simulations based on the identified parameters were in reasonable agreement with those from experiments.

Abstract: In this study, the authors employ a multi-point die-support structure to hold the upper die for deep drawings in order to adjust the distribution of the blank holding force (BHF) so as to eliminate wrinkles. The developed multi-point support structure has 12 support cells (support units) between the upper die and the outer slide of a double-action press; the cells are metal cylinders working as springs. The support unit has a ball contact at the interface with the upper die, and the interface freely rotates and slides horizontally. The support unit has strain gauges on the side surface, and the bearing load at each unit can be determined, as well as the elastic deformation. The bearing load distribution is observed through a trial blow, and then the support units are manually relocated to better distribute the supporting points to create the appropriate BHF distribution. To demonstrate the efficiency of the suggested structure, the authors perform deep drawing with off-centered setting of a blank to create wrinkles intentionally. They then employ the multi-point die-support system, relocate the support units, and eliminate wrinkles.

Abstract: In this work, the effect of wrinkled microstructures on surface wettbility of ginkgo leaf was investigated. The upper and lower layers of the ginkgo leaf were comparatively measured by Scanning Electron Microscopy (SEM) for contrast of morphological characters. Measurement results show that the upper epidermis is mainly composed of regularly long-rangle stripes of cellular structures with gently smooth cell wall. On the other hand, the lower one is made up of chaotically isolated cellular structures with wrinkled cell wall. The comparative measurement of contact angle (CA) and roll-off angle in the upper and lower epidermis with deionized water was conducted. Wetting behaviors indicate that the lower has stronger hydrophobicity and weaker adhesion than the upper. Through theoretical analysis, the difference in wettability and adhesion was the result of isolated micro-bumps made by hierarchical wrinkles on the lower epidermis, leading to form solid-air-liquid interface between water and wrinkled surface. This research explains the difference of ginkgo leaf in wettability, and has important reference value in the design and preparation of the surface for the biomimetic materials.

Abstract: Wrinkle evolution under temperature and compression was experimentally explored in a metal film deposited on a compliant polymer. After wrinkle formation, the samples undergo thermal annealing in different temperature circumstances. In the range of 75-150°C, the wavelength and amplitude of wrinkles became both smaller as the temperature increased, indicating that annealing effectively releases residual stress and promote material modification. In contrast, compression only changed wrinkle rearrangement, and did not alter the wavelength and amplitude. These results suggest that external stimulation can transform wrinkled surfaces. This work provides the guidance for service conditions of surface wrinkling.

Abstract: The thin metal plastic forming is an indispensable metal forming technology. Wrinkling is one of the common defects in plastic forming. If this kind of defects can be used properly, the formability of metal will be better. In this paper, the recent researches on the methods of identifying wrinkles, distinction of the useful wrinkles and harmful wrinkles, control and application of the wrinkles are summarized. The useful wrinkles are expected to improve the forming property.

Abstract: Subject to a compressive membrane force, a film bonded to a compliant substrate often forms wrinkles. In the past, the studies of such wrinkles are mostly based on the assumption of semi-infinite substrate. However, such an assumption cannot be satisfied in real applications. This paper focuses on wrinkling of a rigid film on a compliant thin-substrate. The finite element method used to analyze wrinkling and investigate the relationship between the critical wavelength, material parameters (such as film Youngs modulus, substrate Poisson's ratio) and geometric parameters (such as substrate thickness). Meanwhile, the finite element results are compared with analytical results.

Abstract: The ultimate goal of the metal forming industry is to form components made of a specific material into a required shape without experiencing wrinkling, bifurcation and spring-back. This study is devoted to develop the whole process by making rules of precision for specific part geometry. For inner clutch shell a computer simulation system and the general procedure of dynamic simulation in sheet metal forming are presented. Finally compute all simulation results with reference to the original part property and recommend the appropriate process flow to produce the part, the comparison with tryout results shows that the formality simulation of thick panel with precise die face is reliable and accurate.

Abstract: In the present work, a fast pyrolysis process is developed for YBa₂Cu₃O_7-δ (YBCO) films preparation with trifluroacetates metalorganic deposition technique (TFA-MOD). The decomposition of TFA salts as well as the surface morphology of pyrolyzed films is investigated with respect to the pyrolysis temperature and its heating rate. Two typical surface morphologies for the pyrolyzed films, being smooth or buckled, are revealed subjecting to the heating rate and film thickness, which related to the subsequent YBCO films property. Smooth and wrinkle-free pyrolyzed films are achieved by using this fast pyrolysis process with a heating rate as high as 5°C/min. The final crystallized YBCO films exhibit good performances with the superconducting transition temperature of 91 K and critical current density of 2.0 MA/cm² at 77 K, suggesting a rapid, promising MOD route available for scale-up preparation of YBCO coated conductors.