In-Situ Tensile Test on 316H Sent Using Digital Image Correlation

Abstract

At elevated temperature (550 °C) intergranular creep cracks are prone to develop in thermally and environmentally aged 316 stainless steel. To improve the understanding of mechanisms responsible of creep cracking, some micromechanical experiments have been conducted. Single Edge Notched Tensile specimens (SENT) made of 316H have been machined with a desired ratio of a/W = 0.15 with a the crack length and W the width specimen. After a fine polishing preparation, the samples are then thermally aged using an oxidizing treatment at 600 °C in a rich-carbon environment during 2000 h. It has been assumed that it induces carbide precipitation at grain boundaries (Cr₂₃C₆) leading to a loss of corrosion resistance and to a material embrittlement. In parallel, the oxide (Fe₃O₄) layer grows at the surface up to 50 μm. Its random structure is particularly convenient as a surface marker for the Digital Image Correlation (DIC). A tensile device placed in the SEM chamber is used to apply cycles of loading/unloading to the specimen combining this with a Scanning Electron Microscope (SEM) images acquisitions of the crack tip vicinity. Complementary finite element (FE) simulations of intergranular cracks in bicrystals have been performed and used as reference fields to develop an identification procedure of the crack tip position. It relies on kinematic measurements using a local approach and on projections using Linear Elastic Fracture Mechanic (LEFM) expressions. Experimental evidences have been obtained of plasticity developing ahead of the crack when a local load is applied. A quantification of biais due to the model errors when plasticity is taken into account is done as well as an assessment of the robustness of the procedure. We propose a comparison between FE modeling and full-fields measurements resulting from an in-situ tensile test in a pre-cracked sample.