Solids Mechanic Laboratory

This paper presents a computational fatigue assessment method of mooring chains under tensile loading, it is composed of a mechanical analysis followed by a fatigue analysis. The mechanical analysis is performed in two steps: residual stress prediction and service loading. From this analysis, the shakedown cycle is extracted at the critical points, ie: the asymptotically stabilized stress-strain cycles. As the mooring chain under service loading is under elastic shakedown, the Dang Van fatigue criterion is applied for the fatigue analysis. The accuracy of the proposed fatigue assessment method is proved by comparing with the experimental results from full-scale fatigue testing of mooring chain in seawater. The numerical results match both the experimental observations with respect to the localization of the damage zone and the lifetime.

Electron Beam Melting (EBM) and Selective Laser Melting (SLM) are two of the most developed powder bed fusion based additive manufacturing processes. In this paper, fatigue properties of Ti-6Al-4V specimens built by EBM and SLM are compared. Several parameters are assessed: processes, manufacturing direction, surface roughness, HIP treatment. Fatigue properties are analyzed in relation with the crack initiation mechanisms identified on each specimen's post-mortem micrographs. Machined specimens show better fatigue properties as surface defects are removed. HIP treatment improves fatigue properties by decreasing the defects size. Small differences in lifetime are observed between horizontal and vertical building directions because of different defect shapes regarding the loading axis. Finally, EBM and SLM parts have approximately the same fatigue properties, which are equivalent to conventional casting processes.

We present here the study of Ba 2 Y 1+x Nb 1‐x O 6‐δ (x = 0, 0.05, 0.1, 0.15, 0.2) compounds with potential application as ion‐conducting materials in Solid Oxide Fuel Cells and/or Proton Conducting Fuel Cells. Yttrium is used here as an acceptor‐doping element in substitution to niobium to create oxygen vacancies by charge compensation. Nanopowders have been prepared by a freeze‐drying route and dense ceramics were obtained after shaping and treatment at high temperature. X‐ray diffraction analysis shows that single phase compounds are obtained for x values equal or below 0.2. Transport properties indicate that, in spite of a significant concentration of oxygen vacancies, the ion conductivity remains low in oxidative atmosphere. A higher conductivity is observed in humidified hydrogen showing that the material gets partially reduced in such conditions. High resolution STEM images and EDS chemical mapping show very well order of Y and Nb in Ba 2 YNbO 6 compound (x = 0), however when x = 0.1 a pronounced cationic exchange is observed between Y and Nb (Figure 1). Based on these results and Molecular Dynamics calculations, we noticed that the Y on Nb site induce an oxygen vacancies created by Yttrium doping. When the doping level is low, a long range diffusion would suppose that oxygen vacancies also locate close to Niobium atoms which does not seem to be possible in this material. Thus, the only way for oxygen vacancies to participate to long range ion diffusion is to percolate from a site close to a Y dopant to another site close to a Y dopant. These results were presented at an international conference EMRS Warsaw (2015) and gave rise to a publication that is being prepared.

Shape memory alloys (SMAs) are exploited in several innovative applications, experiencing up to millions of cycles, and thus requiring a fully understanding of material fatigue and fracture resistance. However, experimental and methodological descriptions of SMA cyclic response are still incomplete. Accordingly, the present paper aims to investigate the cyclic response of SMAs under macroscopic elastic shakedown and to propose a criterion for the high cycle fatigue of SMAs. A multiaxial criterion based on a multiscale analysis of the phase transformation between austenite and martensite and using the rigorous framework of standard generalized materials is proposed. The criterion is an extension of the Dang Van high cycle fatigue criterion to SMAs. The criterion is applied to uniaxial experimental data taken from the literature. It distinguishes run out from failure tests in the infinite lifetime regime. The analysis permits a novel insight into the development of a general multiaxial failure criterion for SMA materials and also suggests further experimental investigations to completely understand the fatigue behavior of SMAs under elastic shakedown.

Tetanized muscle myofibrils are often modeled as one-dimensional chains where springs represent half-sarcomeres (HS). The force–length relation for individual HSs (isometric tetanus) is known to have a ‘descending limb’, a segment with an apparently negative stiffness. Despite the potential mechanical instability on the descending limb, the isometric tetanus is usually interpreted as describing an affine deformation. At the same time, active stretching during tetanus around the descending limb is known to produce non-affine sarcomere patterns. In view of this paradox, the question whether the mechanical behavior of a myofibril can be interpreted as a response of a single contractile unit has been a subject of considerable controversy over the last 50 years. In this paper we question the claim that the isometric tetanus describes homogeneous configurations of the HS chain. To distinguish between the multitudes of non-affine equilibrium states available to this mechanical system, we propose to use the concept of a stored mechanical energy. While the notion of energy is natural from a mechanical point of view, physiologists have resisted it so far on the grounds that the contractile elements are active. We discuss how this objection can be overcome and show that the appropriately defined stored energy of a tetanized myofibril with N contractile units has exponentially many local minima. We then argue that the ruggedness of the ensuing energy landscape is responsible for the experimentally observed history dependence and hysteresis in the mechanical response of a tetanized muscle near the descending limb. A nonlocal extension of the chain model, accounting for surrounding tissues, shows that both the ground states and the marginally stable states are fine mixtures of short and long HSs. These mixtures are homogeneous at the macro-scale and inhomogeneous at the micro-scale and we show that the negative overall slope of the step-wise tetanus can coexist with a positive instantaneous stiffness. A salient feature of the nonlocal model is that the variation of the degree of non-uniformity with elongation follows a complete devil’s staircase.