F. Roudet

Interpretation of instrumented hardness measurements on stainless steel with different surface preparations

Abstract When measuring the indentation hardness of a material, the hardness number is generally not constant in the micro- and nano-hardness ranges but varies with the applied load. In the present work, we examine the role of surface modifications produced by different levels of polishing on this load dependence. The Vickers hardness of four stainless steel samples, three polished and one ground were measured in the micro- (0·1–10 N load) and macro-hardness (10–2500 N load) ranges. At loads exceeding ∼10 N, a constant, low bulk hardness that is the same for all polished samples, is found. We also find that each of the four methods of surface preparation leads to different rates of increase in microhardness with decreasing load. Although this pattern of hardness change is consistent with real variations of microhardness with depth, we examine various models that have been used to describe the indentation size effect to interpret and model the measurements. We note that most of the existing models can be fitted to the data but do not always allow for a reasonable physical interpretation. We propose a modification to the model of Abu Al-Rub and Voyiadjis (2004) to best capture the pattern of hardness variation. This model can be considered a hybrid of models based on dislocation or strain gradient plasticity theory.

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.

Hardness-load modelling applied to multilayer galvanised coatings

During the last past decades, galvanised steels have been extensively developed to improve corrosion resistance of steels due to their excellent chemical properties particularly in severe atmospheric conditions. The objective of this work is to predict the hardness-load variation in relation to the bath immersion time using both a multilayer coating hardness model and a layers growth modelling. The kinetic growth of each layer relates the thickness to the immersion time by a simple power law. The hardness of the intermetallic compounds is determined by applying a multilayer hardness model on classical Vickers microindentation data obtained at different indentation loads. Afterwards by combining the kinetic growth laws and the compounds hardness, it is possible to predict the surface hardness-load variation as a function of the immersion bath time.