Applicability of fracture toughness tests on ductile materials

SCK•CEN Mentor

Lescur Amke, alescur@sckcen.be, +32 (0)14 33 80 12

Expert group

Conditioning and Chemistry Programme

SCK•CEN Co-mentor

Stergar Erich , estergar@sckcen.be , +32 (0)14 33 31 80

Introduction

MYRRHA is short for ‘Multi-purpose hYbrid Research Reactor for High-tech Applications’ and is a first of its kind accelerator driven system currently developed at SCK•CEN. Being one of the largest research projects in Belgium, it will allow material developments for fusion reactors, material developments for GEN IV systems, fuel developments for innovative reactor systems and radioisotope production for medical and industrial applications. The selected primary coolant for the reactor part and at the same time spallation target is liquid lead-bismuth eutectic (LBE) which has significant safety benefits. Nevertheless, when building a new system it is essential to investigate its potential effects on structural and functional materials.  Among the challenges of using LBE as coolant is understanding its influence on mechanical properties of the structural materials.

 

Objective

The current candidate material for the structural parts of this new reactor is 316L(N) austenitic stainless steel, an extremely ductile material due to its high Ni content. In order to build MYRRHA, the materials should be characterized and the mechanical properties should be measured. One of the more important properties of a structural material is its fracture toughness, or its resistance to a crack growing. This is often done by tests where a tensile load is applied on fatigue pre-cracked CT specimen. For brittle materials with limited plasticity a KIC test is performed. JIC tests are performed when there should be accounted for more plasticity in the material. 

Analysis of the flow curve and CTOD data can be done with different techniques such as unloading compliance (UC), normalisation data reduction (NDR), energy normalization (EN), etc… For materials with limited plasticity, all of these techniques should result in same J-values and J-curves, within a reasonable statistical error. However, it has been observed that for highly ductile materials such as the 316L(N) under investigation for MYRRHA, the techniques give results differing in some cases by an order of magnitude.

During this internship, tests can be conducted on materials ranging from brittle to materials with small plasticity to extremely ductile. The goal is to investigate the practical limit of each of the different techniques and evaluate in what situations some technique is preferred over another. Additional information about the local plasticity of the material will be gained from the newly obtained DIC-system in the SMA group. This state-of-the-art technique allows us, by digital image correlation, to see the strain field on a flat surface, for example one side of a sample which is being tested.

The minimum diploma level of the candidate needs to be

Academic bachelor

The candidate needs to have a background in

Physics , Engineering