Portevin‐Le Chatelier effect in AlMg3% studied using elevated temperature nanoindentation

The Portevin-Le Chatelier (PLC) is a plastic instability observed in different alloys, and particularly in aluminum alloys, which is characterized by a serrated flow during plastic deformation. The PLC effect originates from the competition between gliding of mobile dislocations and pinning of these dislocations by diffusing solute atoms. This dynamic strain hardening leads to a negative strain rate sensitivity which is often used to characterize or quantify the PLC effect. The PLC effect has been widely investigated in the case of stress-strain curves obtained in macroscopic uni-axial tests. However, in the case of the aluminum matrix composites Al/AlCuFe, it has been observed that copper atoms diffuse during the material synthesis form the reinforcement particles to the aluminum matrix. The aluminum matrix thus presents a heterogeneous concentration of copper atoms leading to local PLC effect. Nanoindentation test is the best way to characterize locally this mechanical effect. However, strain rate is not a convenient parameter for nanoindentation tests since the complex strain field below the indent, as well as the increase of the contact area during the test, makes difficult the definition a single strain value. Another way to investigate a local PLC effect would be thus the perform nanoindentation tests at different temperatures rather than different strain rates. This poster will present experimental results from elevated temperature nanoindentation studies on an AlMg3% alloy, used as a model material for easy comparison with uniaxial tests, in the temperature range from 25-300°C. The experiments were performed in displacement controlled mode in a recently developed vacuum high temperature nanoindenter based on active surface referencing and non-contact tip and sample heating. In this configuration, the PLC effect appears as successive load drops on the loading curves. The temperatures of the tip and the sample surface were calibrated and matched in order to minimize thermal drift. With increasing temperature, the magnitude of load drop decreased whereas its occurrence frequency increased. The load drop magnitude and its occurrence frequency were statistically analyzed for different temperatures of testing. The results will be discussed in terms of an expanding plastic volume beneath the indenter interacting with the solute atoms in the complex stress field of the indenter

An in‐situ indentation system for high dynamic nanomechanical measurements

Nanoindentation is typically confined to quasi-static strain rates of testing. This poster presents the development of an in-situ indenter designed to measure the response of materials at high strain rates and high oscillation frequencies at the nanoscale. This builds up on the previous work that was the first to report on in-situ nanoindentation in a SEM in 2004 which eventually resulted in the founding of the company Alemnis AG, today one of the key players in in-situ high temperature and high dynamic nanoindentation. The motivation for variable strain rate studies is that this allows analysis of activation parameters of the physical deformation processes. Once the activation parameters are known, the deformation mechanism(s) can be determined and materials science approaches to improve materials performance can be developed. Ultra-high frequency nanoindentation enables high strain rate studies and high cycle fatigue tests that can be performed within reasonably short timespan. Compared to other actuation principles, piezo actuators offers very fast response time and high force density and are compatible with vacuum environments. At the technological heart of this innovation is a transducer called “SmarTip” consisting of a diamond tip mounted on miniaturized and embedded three-axis piezo-actuators and sensors. The SmarTip allows a full range displacement of 1μm along the three axes and to measure forces up to 1N. The theoretical bandwidths are up to 10kHz and 40kHz for lateral and axial displacements respectively. We aim to reach strain rates as high as 105s-1 meaning that the speed of displacement must reach 60mm/s for a displacement of 600nm. With such high ambitions, several parameters have to be taken into consideration such as resonant frequencies of the indenter, self-heating and cabling inducing spurious capacitance. This poster will report on these design aspects, instrumentation and technique development in addition to presenting initial data on high strain rate and high cycle fatigue tests at the micron scale. It is hoped that the multi-axis capabilities of the SmarTip will result in additional breakthroughs for applications on nano-tribology, fretting and more generally on the translation of dynamic mechanical analysis (DMA) to the micro/nanoscale. Acknowledgments Research work partially co-funded by the Commission for Technology and Innovation (CTI), the State Secretariat for Education, Research and the Innovation Eurostars program and project UHV

Continuum damage interactions between tension and compression in osteonal bone

Skeletal diseases such as osteoporosis impose a severe socio-economic burden to ageing societies. Decreasing mechanical competence causes a rise in bone fracture incidence and mortality especially after the age of 65 y. The mechanisms of how bone damage is accumulated under different loading modes and its impact on bone strength are unclear. We hypothesise that damage accumulated in one loading mode increases the fracture risk in another. This study aimed at identifying continuum damage interactions between tensile and compressive loading modes. We propose and identify the material constants of a novel piecewise 1D constitutive model capable of describing the mechanical response of bone in combined tensile and compressive loading histories. We performed several sets of loading–reloading experiments to compute stiffness, plastic strains, and stress-strain curves. For tensile overloading, a stiffness reduction (damage) of 60% at 0.65% accumulated plastic strain was detectable as stiffness reduction of 20% under compression. For compressive overloading, 60% damage at 0.75% plastic strain was detectable as a stiffness reduction of 50% in tension. Plastic strain at ultimate stress was the same in tension and compression. Compression showed softening and tension exponential hardening in the post-yield regime. The hardening behaviour in compression is unaffected by a previous overload in tension but the hardening behaviour in tension is affected by a previous overload in compression as tensile reloading strength is significantly reduced. This paper demonstrates how damage accumulated under one loading mode affects the mechanical behaviour in another loading mode. To explain this and to illustrate a possible implementation we proposed a theoretical model. Including such loading mode dependent damage and plasticity behaviour in finite element models will help to improve fracture risk analysis of whole bones and bone implant structures