Jieshi Chen

A method for evaluating the degradation of edge stretchability incorporating punching damage based on Marciniak and Kuczynski model

Edge stretchability refers to a sheet metal’s capability to resist edge cracking during edge forming and flanging. In this article, a quantitative method has been proposed to evaluate the effect of punching or trimming of sheet metals on the degradation of their edge stretchability. This method adopts the Marciniak and Kuczynski concept to quantify edge damages due to punching or trimming. A novel index, the effective failure strain ratio, is introduced. Effective failure strain ratio is strain-based, and it is defined as the ratio of the actual edge failure strain to the theoretical edge failure strain. The algorithm to calculate effective failure strain ratio based on hole-expansion simulations is detailed. The magnitude of effective failure strain ratio depends on the damage value, which corresponds to the edge damage caused by preprocessing such as punching. Numerical studies are conducted to demonstrate the applicability of this method. The results show that the degradation of edge stretchability of materials with higher hardening exponent (n-value in power hardening law) is more sensitive to edge damage. Hole-expansion experiments using two grades of dual-phase steels are conducted to validate the conclusions deduced from the simulations. The comparison between the experimental and numerical results shows that the proposed method is able to predict phenomena appearing in the experiments. The quantitative relationship between damage value and the punching clearance has not been established in this work yet, which requires extensive experimental investigation. However, a qualitative link has been clearly demonstrated, and this method provides a new perspective to express the pre-damage and its effect straightforwardly.

Forming limit prediction for two-stage aluminum alloy sheet forming process considering the effect of normal stress

Purpose – The purpose of this paper is to analyze theoretically the influence of normal stress on the formability of aluminum alloy sheets in non-linear strain paths. Design/methodology/approach – Four loading modes of non-linear strain paths are investigated in detail to consider the effect of normal stress on formability of aluminum alloy sheets. Findings – Results show that the influence of normal stress in the first stage can be ignored. However, the normal stress in the second stage enhances the formability of aluminum alloy sheets obviously. Besides, the normal stress in the second stage is found to have larger effect on forming limit stress than that in the first stage. Research limitations/implications – Maybe more experiment data should be obtained to support the theoretical findings. Originality/value – This current study provides a better understanding of normal stress effect on the formability of aluminum alloy sheets in non-linear strain paths. Since the reacting stage of normal stress play i...

Forming limits of anisotropic sheets with non-power-law hardening

For aluminum alloys and some advanced high-strength steels, the tensile flow curve exhibits a tendency to saturate. The suitability of constitutive equations was analyzed for aluminum 6111-T4 and a general non-power-law hardening model was adopted in the derivation of forming limits incorporated material anisotropy with varying R-values. A bifurcation analysis was conducted for the left-hand-side FLD under the assumption of proportional loading and zero-extension necking orientation. Analytical results showed good correlation with Nakajima forming limit test data for aluminum sheets following Voce hardening law.

Accurate Die Design for Automotive Panel Stamping Considering the Compensation Related with Die Deflection and Blank Thinning

In order to improve assembly accuracy, automotive body panels have to be fabricated with higher dimensional and surface quality requirements, therefore the die faces should be designed more accurately to consider more relevant factors. In the presented study, we proposed algorithms to realize the following functions: through forming process simulation, the thinning distribution on the deformed blank was extracted as first kind of compensation; through die structural CAE analysis which automatically mapped the boundary contact forces onto the die surfaces from process simulation results, the die deflection was calculated as second kind of compensation. These two quantitative contributions were added together to compensate the die face.The proposed methodologies were programmed and integrated with LS‐Dyna and HyperWorks, and also integrated with Autoform and CATIA linear CAE functionalities separately. In addition, a software toolkit to calculate the contacting ratio was also developed to evaluate the effec...