Jenn Terng Gau

A new model for springback prediction in which the Bauschinger effect is considered

Springback prediction is an important issue for the sheet metal forming industry. Most sheet metal elements undergo a complicated cyclical deformation history during the forming process. For an accurate prediction of springback, the Bauschinger effect must be considered to determine accurately the internal stress distribution within the sheet metal after deformation. Based on the foundations for isotropic hardening and kinematic hardening, Mroz multiple surface model, plane strain assumptions, and experimental observations, a new incremental method and hardening model is proposed in this paper. This new model compares well with the experimental results for aluminum sheet metal undergoing multiple-bending processes. As is well known, aluminum is one of the most difficult sheet metals to simulate. The new hardening model proposed in this paper is not only a generic model for springback prediction but also a hardening model for sheet metal forming process simulation.

An experimental investigation of the influence of the Bauschinger effect on springback predictions

Abstract An efficient and low cost multiple bending experiment has been designed to investigate the influence of the Bauschinger effect on springback in sheet metal forming. Most sheet metal elements undergo complicated deformations during forming process. In general, the influence of the Bauschinger effect must be considered for obtaining accurate springback predictions. In the work reported here, three different AISI steel sheet metals (high strength, bake hard, and AKDQ), and one type of aluminum sheet metal (AA6111-T4) were used as experimental materials. These materials are used widely in the automobile industry for car panels. From these experiments, it can be concluded that the influence of the Bauschinger effect on springback is more significant for aluminum AA6111-T4 than for steels. Both the deformation history and the Bauschinger effect must be considered to predict springback in AA6111-T4 stamping parts.

An experimental and analytical study on the limit drawing ratio of stainless steel 304 foils for microsheet forming

A series of micro-deep drawing experiments were conducted on stainless steel 304 foils with four thicknesses that were heat treated at four different temperatures. Due to heat treatments, a variety of different grain sizes and T/D ratios (the number of grains throughout thickness) were obtained. In this study, the limit drawing rations (LDR) of these foils were obtained; it has also been found that the factors that influence LRD of the foils include, but are not limited to, thickness, grain size, and T/D ratios. Tensile tests were conducted to obtain their mechanical properties that were used for two macroempirical equations to predict the maximum drawing load and LDR. It has been verified that the two equations can be applied to foils that are not thinner than 150 µm for reasonable predictions. However, the size effects are more noticeable and significant for the foils that are less than or equal to 100 µm so that the macroscale empirical equations cannot be applied to them.

A New Model for Springback Prediction for Aluminum Sheet Forming

A new model for springback, based on isotropic and kinematic hardening models, the Mroz multiple surfaces model, and observations from experimental data, is proposed in this paper. In this model, a material parameter (CM), which is significant after reverse yielding, is suggested to handle the Bauschinger effect. A simple, low-cost, multiple-bending experiment has been developed to determine CM for aluminum alloys AA6022-T4 and AA6111-T4. The new model fits available experimental results better than the isotropic and kinematic hardening models and the Mroz multiple surfaces model.

Size effect and forming-limit strain prediction for microscale sheet metal forming of stainless steel 304

Owing to the extensive applications of stainless steel 304 on a microscale, a series of microscale tensile and dome height tests were conducted to investigate its size effects on mechanical properties and formability. Based on the experimental results and observations and the Oh et al. fracture criterion, two new models were proposed in this paper for predicting the forming limit of stainless steel 304 foils in microscale sheet metal forming. For the mechanical properties study, foils of four thicknesses (150 μm, 100 μm, 50 μm, and 20 μm) heat treated at four temperatures (900 °C, 950 °C, 1000 °C, and 1050 °C) were used for the experiments. For the formability prediction, the first proposed model includes the effect of the strain path while the second proposed model considers the coupling effects of both the strain path and the size effects. The first model is superior to the Oh et al. criterion with respect to predicting the forming-limit strain of the foils, but it is not suitable for foils that are thinner than 100 μm. However, the second proposed model can be used for stainless steel 304 foils irrespective of their thicknesses and thickness-to-average-grain-size ratios T/D. It can also be concluded that the size effects must be considered in microformability when the foil thickness is smaller than 100 μm.

Tensile and micro stretch bending experiments for studying stainless steel 304 foil for micro sheet forming

Tensile test and a micro stretch bending test were conducted to study the behavior of the stainless steel 304 foils for micro sheet forming. In this study, 4 different thicknesses stainless steel 304 foils were used as specimens while 5 micro deep draw dies were used for micro stretch bending experiments. By observing the tensile test results, it can be found that the stainless steel 304 foils with T/D (thickness/average grain diameter: the numbers of grains throughout the metal thickness) 10. By observing the results of the micro stretch bending experiments, it can be concluded that the stainless steel foils with T/D>10 have less springback amount and smaller springback deviation in comparison with those with T/D 10 for micro sheet forming application.Copyright

Challenges in teaching modern manufacturing technologies

Teaching of manufacturing courses for undergraduate engineering students has become a challenge due to industrial globalisation coupled with influx of new innovations, technologies, customer-driven products. This paper discusses development of a modern manufacturing course taught concurrently in three institutions where students collaborate in executing various projects. Lectures are developed to contain materials featuring advanced manufacturing technologies, R&D trends in manufacturing. Pre- and post-surveys were conducted by an external evaluator to assess the impact of the course on increase in students knowledge of manufacturing; increase students’ preparedness and confidence in effective communication and; increase students’ interest in pursuing additional academic studies and/or a career path in manufacturing and high technology. The surveyed data indicate that the students perceived significant gains in manufacturing knowledge and preparedness in effective communication. The study also shows that implementation of a collaborative course within multiple institutions requires a robust and collective communication platform.

Formability evaluation by novel specimen designs in sheet metal forming with two-step strain paths

Limit dome height test is commonly used for evaluating the formability of sheet material. Through altering the geometry of specimens, different strain paths could be obtained to establish forming limit diagram of the material. By incorporating finite element analysis with ductile energy criteria, engineers can also predict the formability of material. The forming limit diagram or constants in ductile energy criteria are usually determined by experiments with linear strain path. However, the predictions may lose their accuracy when evaluating products with complex strain paths, which is commonly seen in sheet forming processes. Therefore, a better method for evaluating formability of material under complex strain path should be developed. In this study, novel specimen designs for limit dome height test are applied to generate different strain paths with two-step strain effect on the specimens. Different geometric parameters of the novel specimen design can alter the slope of strain paths and create different strain paths similar to the two-step deformation conditions occurred in actual sheet forming processes. Three different two-step strain path conditions are created experimentally, and the changes in strain path are verified with simulations. From the results, the predictions of forming limit based on linear strain path forming limit diagram could be overestimated or underestimated under two-step strain path conditions. Thus, the formability of material under two-step strain path conditions can be determined by corresponding experiments using novel specimen designs, instead of predictions made by linear strain path conditions.

An experimental study of the influence of size effect on springback of micro sheet forming

An efficient and low cost experiment was conducted to investigate the influence of grain size effects on springback of thin sheet metal. A three-point bending setup was used to test Aluminum 1100 O-Temper and Brass 26000 ½ Hard (H02). Thickness and average grain diameter were controlled to achieve a range of desired T/D (thickness/grain diameter) ratios because T/D ratio is a key factor that influences the springback behavior in micro sheet forming. The results from this experiment show that as T/D ratio becomes smaller (T/D>1 ∆ 1) the springback amount increases at a decreasing slope. And as T/D continues to become even smaller (T/D < 1) the springback amount begins to decrease. These behaviors demonstrate the influence of size effects on springback.