Assessment of Damage Models in Sheet Metal Forming for Industrial Applications

Abstract

Due to increasing demands to reduce C02-emission and to augment occupant’s safety new modern materials are developed ongoing. Because of relatively low production costs, high strength and simultaneously good formability the advanced high strength steels (AHSS) are applied among others for the lightweight design of body-in-white components in the automotive industry. Their already mentioned properties follow from the presence of mixed mild and hard ferrous phases. Due to this multiphase microstructure of the most AHSS steels, a complex material and damage behavior is observed during forming. The damage grows in a ductile manner during plastic flow and the cracks appear without necking. They are often characterized as the so called shear cracks. The damage predictions with standard methods like the forming limit curve (FLC) lack accuracy and reliability. These methods are based on the measurement of linear strain paths. On the other hand ductile damage models are generally used in the bulk forming and crash analysis. The goal is to prove if these models can be applied for the damage prediction in sheet metal forming and which troubles have to be overcome. This paper demonstrates the capability of the Gurson-Tvergaard-Needleman (GTN) model within commercial codes to treat industrial applications. The GTN damage model describes the existence of voids and they evolution (nucleation, growth and coalescence). After a short introduction of the model the finite element aspects of the simulative damage prediction have been investigated. Finally, the determination of the damage model parameters is discussed for a test part.

A Review of Sheet Metal Forming Evaluation of Advanced High-Strength Steels (AHSS)

“…The modified void volume fraction at macroscopic failure is represented by f * u = f * ( f f ). The calibration process for the GTN model requires determination of a total of eight parameters, where a few parameters can be found in the existing literature (q 1 , q 2 , and S N ) [230]. Moreover, f f is frequently assumed to be 10% [262].…”

“…Nevertheless, the influence of stress triaxiality on the fracture behavior has to be taken into account due to the void formation mechanism, which might be described by modifying model parameters [261]. Doig and Roll [230] stated that the GTN model is appropriate to treat real industrial cases. In their study, the GTN model accurately predicted the localization of cracks in a cross-die part.…”