An In-Situ Investigation of the Strain Partitioning and Failure Across the Layers in a Multi-Layered Steel

Background

Layered composites consisting of dissimilar materials have shown tremendous improvements in balancing strength with ductility. The details of strain partitioning across the layers, resulting in high ductility even in the brittle layer, are not well understood.

Objective

This study aims to quantify strain partitioning and understand the failure of rolled sheets of alternating austenite and martensite layers through in situ tensile experiments.

Methods

A novel high density speckle pattern with the sample surface as background is generated to resolve strain within and across the interface at the microscale. Simultaneous imaging of both the layered and top surfaces was performed to correlate strain and understand the localization leading to failure. Microstructural analysis and numerical simulations were performed to further understand the role of phase transformation and predict the stress–strain response, respectively.

Results

Both axial and transverse strain field heterogeneity was observed across the layers, with pronounced strain partitioning in the transverse direction and steep gradients near the interfaces. The restriction to the growth of micro-deformation sites in the thin austenitic layers led to a long neck region with local strain as high as 40% compared to the global fracture strain of 20%. During plastic deformation, the austenitic layers underwent phase transformation in the region of high Schmid factor, and the martensitic layers experienced texture evolution.

Conclusions

Small deformation bands within each layer grew and formed macroscopic shear bands leading to fracture. Finally, experimental results were compared with finite element simulations and the rule of mixtures, demonstrating a satisfactory agreement between the different approaches.