The paper presents a statistical study of nanoindentation results obtained in seven European laboratories that have joined a round robin exercise to assess methods for the evaluation of indentation size effects. The study focuses on the characterization of ferritic/martensitic steels T91 and Eurofer97, envisaged as structural materials for nuclear fission and fusion applications, respectively. Depth-controlled single cycle measurements at various final indentation depths, force-controlled single cycle and force-controlled progressive multi-cycle measurements using Berkovich indenters at room temperature have been combined to calculate the indentation hardness and the elastic modulus as a function of depth applying the Oliver and Pharr method. Intra- and inter-laboratory variabilities have been evaluated. Elastic modulus corrections have been applied to the hardness data to compensate for materials related systematic errors, like pile-up behaviour, which is not accounted for by the Oliver and Pharr theory, and other sources of instrumental or methodological bias. The correction modifies the statistical hardness profiles and allows determining more reliable indentation size effects.

A unified experimental-computational study on ductile fracture initiation and propagation during small punch testing is presented. Tests are carried out at room temperature with unnotched disks of different thicknesses where large-scale yielding prevails. In thinner specimens, the fracture occurs with severe necking under membrane tension, whereas for thicker ones a through thickness shearing mode prevails changing the crack orientation relative to the loading direction. Computational studies involve finite element simulations using a shear modified Gurson-Tvergaard-Needleman porous plasticity model with an integral-type nonlocal formulation. The predicted punch load-displacement curves and deformed profiles are in good agreement with the experimental results.

This work reports results from quasi-static nanoindentation measurements of iron, in the un-strained state and subjected to 15% tensile pre-straining at room temperature, 125 °C and 300 °C, in order to extract room temperature hardness and elastic modulus as a function of indentation depth. The material is found to exhibit increased disposition for pile-up formation due to the pre-straining, affecting the evaluation of the mechanical properties of the material. Nanoindentation data obtained with and without pre-straining are compared with bulk tensile properties derived from the tensile pre-straining tests at various temperatures. A significant mismatch between the hardness of the material and the tensile test results is observed and attributed to increased pile-up behaviour of the material after pre-straining, as evidenced by atomic force microscopy. The observations can be quantitatively reconciled by an elastic modulus correction applied to the nanoindentation data, and the remaining discrepancies explained by taking into account that strain hardening behaviour and nano-hardness results are closely affected by dynamic strain ageing caused by carbon interstitial impurities, which is clearly manifested at the intermediate temperature of 125 °C.

During their service in a nuclear power plant, structural materials are subject to high temperature, mechanical stress, corrosive coolant environments, and neutron irradiation. The combination of radiation, corrosion and thermo-mechanical loading represents a unique environment that materials must endure: radiation can significantly alter the material’s thermo-mechanical performance and the lifetime of a component. The conditions under which this happens are difficult to study and normally necessitate nuclear reactor experiments followed by post-irradiation examination in hot cells, which renders investigations complex, time consuming and expensive. While few attempts have been made to study the combined effects of radiation and corrosion with lead bismuth eutectic [1] or molten salts [2] none of these experimental set-ups were designed to study the combination of corrosion and irradiation together with mechanical loading. Here the development of a miniature test technique is presented where irradiation, corrosion and strain can synergistically be applied on a material in a controlled way. The experiment is based on scaling down the membrane bulge test [3], using liquid lead as a pressure medium at high temperature. Membrane type specimens are made from the structural material to study, with their back side pressurized thereby inducing corrosion and membrane stretching stress while irradiation damage is generated by proton irradiation at the front surface affecting the entire thickness to the back side of the sample. The paper presents the way how the engineering problems related to the combination of corrosion by liquid metal, stress application and irradiation of the sample have been addressed.

While force–displacement curves are often preferred in Small Punch (SP) tests due to the ease of the experimental set-up, they encompass significant uncertainties arising from frame compliance. In this work, a methodology is presented to predict yield stresses from the force vs. deflection curves. The present method relies on determining different force levels from the initial part of the force–deflection curve to reflect both the slope and the curvature instead of using a single force level only. The predicted yield stresses for different types of materials, that is, low- and high-strength alloys, are found to be in good agreement with the actual proof stresses with a maximum error of 16%.