Analysis of fracture behavior and stress–strain distribution of martensite/austenite multilayered metallic sheet
Highlights
► Multilayered metallic sheet is made by the lamination of brittle layer and ductile layer. ► Ductility of brittle layer increases by the suppression stress of ductile layer. ► The stress gradient between layers becomes similar in the fracture area. ► Void which is in the type of ductile fracture is observed in brittle layer. ► The fracture stain increased when each layer was thin.
Introduction
A multilayered metallic sheet (ML sheet) is fabricated for the improvement of light-weight transporters such as cars and aircrafts, for collision safety, and for the improvement of formability. Koseki et al. (2010) developed a ML sheet having high strength and an improved fracture strain achieved through the lamination of high strength steel and high ductility steel. This ML sheet was used in our study. Kawai (2001) reported the lightweight transport vehicle for environmentally conscious steel products and investigated the impact safety of sheet materials for this vehicle. Haug et al. (1994) emphasized the collision safety of a car through a research on transport vehicle crash, safety, and manufacturing simulation. Parsa and Yamaguchi (1998) investigated the increase in the formability of an aluminum-stainless laminated sheet. In the research conducted by Oya et al. (2010a), V-bending and hat-bending experiments were performed for the formability evaluation of an ML sheet. Park et al. (2004) investigated the flexural strength of a multilayer aluminum sheet. We want to obtain the desired properties of an ML sheet by combining all the good characteristics of a monolithic sheet. This ML sheet seems to be the promising material for the substitution of the high-strength steel sheets that are currently in use and for the realization of the high-strength, lightweight structural transporters with increased formability. Yanagimoto et al. (2010) recently demonstrated the enhancement of the bending formability of brittle sheet metal in multilayer metallic sheets in a study preceding the present study. However, as the study on formability and plastic forming is still insufficient, we investigate the stress–strain distribution or the fracture behavior. Through the study, we aim to devise a method for improving the formability of high-strength steel; further, we can find the defective part and use it for failure prediction and as data for a design.
By using an ML sheet that has alternately stacked ductile layers and brittle layers, we aim to develop a material that has excellent strength and ductility. The results of Nambu et al. (2009), who studied the effect of the interfacial bonding strength of a multilayered steel sheet, were referred to as a delamination data of this study. The method used in the research of Ohashi et al. (1992), which investigated the fracture behavior of a laminated composite in bend tests, was used for microstructure evaluation in the present study. Lee et al. (2007) studied the effect of annealing on clad-metal sheets. Oya et al. (2010b) have studied the characteristics and mechanical properties of an ML sheet through the experimental and analytical methods. As an ML sheet is fabricated using materials with different deformation behaviors, when tensile or bending deformation is applied to the sheet, mechanisms leading to a fracture can be considered to differ from those observed in the case of sheet with a normal monolithic layer. Therefore, in order to meet the design guidelines of the ML sheet or to clarify the forming limit, we need to investigate the mechanism of fracture. Inoue et al. (2008) investigated the fracture elongation of brittle/ductile multilayered steel composites, and Cao and Evans (1991) reported on the crack extension in ductile/brittle laminates. Coco et al. (2008) noted that a change in mechanical properties, strain localization, damage, and fracture mechanism occurred in compositionally graded materials because a change of flow stress brings about a change in strain hardening. We propose a new analysis method based on the stress change in the (x, y) direction with respect to the stress ratio of the two axes by using the research work of Kuwabara et al. (1998) and Geiger et al. (2005). Kuwabara et al. (1998) investigated work hardening in a steel sheet under biaxial tension, and Geiger et al. (2005) investigated the elliptical stress distribution graph. Chehab et al. (2010) investigated the characteristics of fracture strain as a function of the stress triaxiality of a lamellar structure and void nucleation by partial decohesion at the austenite–ferrite interface. Hannon and Tiernan (2008) also investigated the stress–strain distribution in the (x, y) direction. Elliptical plots were used to evaluate the change in stress and strain in order to investigate the changes in the stress gradient along the two axes of the ML sheet. The reason for the stretch increase with the improvement of ductility of the brittle layer is analyzed by the stress–strain distribution verification, and the necking and fracture that occur during the transformation process are evaluated by the micro-defects and the components of the consequently occurring stress–strain ratio. In this study, we conduct certain experiments on and an FEM analysis of an ML sheet, and evaluate the results.
Section snippets
Characteristics of ML sheet and outline of research
It is certain that the increase in the fracture strain of the brittle layer improves the formability of the ML sheet. It is necessary to verify the changes in the stress gradient and the plastic zone between the lamination layers for the quantification of the mechanical characteristics of the sheet. Changes in the loading-direction stress or effective stress between the two layers can be explained by studying the correlations of the changes in the vertically loaded stress. The increase in the
Experimental procedure
The ML sheet is laminated alternately with austenitic stainless steel in the soft layer and martensitic stainless steel in the hard layer. SS304 is used as the ductile layer, and WT780C and SS420J2 are used in the brittle layer as a high strength layer. The specimens are 1-mm-thick ML sheets with sufficient interface strength to prevent de-lamination. First, the soft layer is located at the outermost layer, and the hard layer is laminated inside of the soft layer; hot and cold rolling processes
Stress gradient and fracture
Through the verification of the stress change of the monolithic specimen and the laminated specimen, and the fracture characteristics according to lamination, the improvement of the mechanical properties of the ML sheet is estimated, and these data and the analysis system can be used as the data for predicting the formability of the ML sheet.
Fig. 2(a) shows the ML sheet fractured by the tensile stress; it shows that the voids near the fracture surface occur along the direction of maximum shear
Conclusions
The increase in the ductility, strength, and fracture strain of the ML sheet was investigated through the confirmation of the change in the stress gradient, formation of micro-defects, and layer interaction. The suppression stress in the direction of necking changed the stress gradient between the layers; consequently, a micro-defect was generated, and deformation over the fracture limit of the monolithic layer was possible in the brittle layer. At the necking point of the ML sheet, first,
Acknowledgement
The present research was conducted as part of the LISM (Layer-Integrated Steels and Metals) Project funded by the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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