Advanced High-Strength Steels: Science, Technology, and Applications examines grades, types, microstructures, thermal processing, deformation mechanisms, properties, performance, and applications of advanced high-strength steels (AHSS). The book introduces the drivers and solutions for building lighter, safer, and more efficient automobiles and how using AHSS accomplishes weight reduction, fuel economy, and crash safety. The complex interrelationship between AHSS composition, processing, microstructure, and mechanical properties is reviewed. Design guidelines, manufacturing hurdles, advanced forming, sustainability, and evolving grades of AHSS are also discussed in the book. For information on the print version, ISBN: 978-1-62708-005-7, follow this link.

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Plastic deformation in sheet metal consists of four distinct phases, namely, uniform deformation, diffuse necking, localized necking, and final rupture. The last three phases are commonly known as nonuniform deformation. A proper forming limit diagram (FLD) should include all three phases of the nonuniform deformation. This paper presents the development of a unified approach to the prediction of FLD to include all three phases of nonuniform deformation. The conventional method for predicting FLD is based on localized necking and adopts two fundamentally different approaches. Under biaxial loading, the Hill’s plasticity method is often chosen when α(=ε2/ε1) <0. On the other hand, the M-K method is typically used for the prediction of localized necking when α > 0 or when the biaxial stretching of sheet metal is significant. The M-K method, however, suffers from the arbitrary selection of the imperfection size, thus resulting in inconsistent predictions. The unified approach takes into account the effects of micro-cracks/voids on the FLD. All real-life materials contain varying sizes and degrees of micro-cracks/voids which can be characterized by the theory of damage mechanics. The theory is extended to include orthotropic damage, which is often observed in extensive plastic deformation during sheet metal forming. The orthotropic FLD model is based on an anisotropic damage model proposed recently by Chow and Wang (1993). Coupling the incremental theory of plasticity with damage, the new model can be used to predict not only the forming limit diagram but also the fracture limit diagram under proportional or nonproportional loading. In view of the two distinct physical phenomena governing the cases when α(=ε2/ε1) < or α > 0, a set of instability criteria is proposed to characterize all three phases of nonuniform deformation. The orthotropic damage model has been employed to predict the FLD of VDIF steel (Chow et al, 1996) and excellent agreement between the predicted and measured results has been achieved as shown in Fig. 1. The damage model is extended in this paper to examine its applicability and validity for another important engineering material, namely aluminum alloy 6111-T4.

Varying the blank holder force during forming can lead to higher formability and accuracy, and better part consistency. Process control, using on-line adjustment of the blank holder force to follow a reference process variable (e.g., the punch force, the draw-in, etc.) trajectory has been applied to sheet metal forming. However, process controller design has not been thoroughly addressed. In this paper, the essential part for systematic process controller design, i.e., modeling a sheet metal forming process, will be addressed in terms of control terminology (e.g., the process model, the model uncertainty and the disturbance). A process model for u-channel forming, i.e., a mathematical relationship between the blank holder force and the punch force, is presented and experimentally validated. Characterization of the model uncertainty mainly due to small variations in blank size, sheet thickness, material properties and tooling shape due to die wear and the disturbance which is mainly due to friction is developed. [S1087-1357(00)01304-6]

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