Xavier Decoopman

Analysis of indentation size effect in copper and its alloys

Abstract For describing the indentation size effect (ISE), numerous models, which relate the load or hardness to the indent dimensions, have been proposed. Unfortunately, it is still difficult to associate the different parameters involved in such relationships with physical or mechanical properties of the material. This is an unsolved problem since the ISE can be associated with various causes such as workhardening, roughness, piling-up, sinking-in, indenter tip geometry, surface energy, varying composition and crystal anisotropy. For interpreting the change in hardness with indent size, an original approach is proposed on the basis of composite hardness modelling together with the use of a simple model, which allows the determination of the hardness–depth profile. Applied to copper and copper alloys, it is shown that it is possible to determine the maximum hardness value reached at the outer surface of the material and the distance over which both the ISE and the workhardening take place.

High Cycle Fatigue Damage Evaluation of Steel Pipelines Based on Microhardness Changes During Cyclic Loads

The aim of this work is to investigate changes in material surface microhardness during the different stages of fatigue life. Samples of API 5LX65 grade steel were submitted to high cycle fatigue tests in which different stages of microhardness changes are observed and then correlated to those observed by TEM images.

Role of plastic deformation on the efficiency of a nitriding treatment: modelling of the hardness-depth profile

To increase the hardness of ion-nitrided stainless steels, a plastic deformation is performed previously to the nitriding treatment in order to generate dislocations used afterwards as diffusion paths for atoms. In this work, influence of different plastic strain rates on the surface hardness improvement is studied. To obtain hardness values very close to the outer surface is not possible by classical hardness-depth profile, a simple model based on indentations performed perpendicularly to the surface is proposed. The predicted hardness-depth profile in different situations of work-hardening clearly demonstrates the efficiency of the prior plastic deformation on the global hardness improvement of the nitrided stainless steel. As a result, the hardened layer thickness increases from 20 µm to 35 µm when the plastic deformation rate increases from 0% to 80% of the maximum one. This is directly linked to the augmentation of the dislocation density by the work-hardening process.