Advanced Materials Research Vols. 891-892

Abstract: Bone, a hard biological material, possesses a combination of high stiffness and toughness, even though the main basic building blocks of bone are simply mineral platelets and protein molecules. Bone has a very complex microstructure with at least seven hierachical levels. This unique material characteristic attracts great attention, but the deformation mechanisms in bone have not been well understood. Simulation at nanolength scale such as molecular dynamics (MD) is proven to be a powerful tool to investigate bone nanomechanics for developing new artificial biological materials. This study focuses on the ultra large and thin layer of extrafibrillar protein matrix (thickness = ~ 1 nm) located between mineralized collagen fibrils (MCF). Non-collagenous proteins such as osteopontin (OPN) can be found in this protein matrix, while MCF consists mainly of hydroxyapatite (HA) nanoplatelets (thickness = 1.5 4.5 nm). By using molecular dynamics method, an OPN peptide was pulled between two HA mineral platelets with water in presence. Periodic boundary condition (PBC) was applied. The results indicate that the mechanical response of OPN peptide greatly depends on the attractive electrostatics interaction between the acidic residues in OPN peptide and HA mineral surfaces. These bonds restrict the movement of OPN peptide, leading to a high energy dissipation under shear loading.

Abstract: A novel β-type, Ti-29Nb-13Ta-4.6Zr, referred to as TNTZ has been developed for biomedical applications. Its fatigue strength is one of the most important mechanical biocompatibilities of TNTZ because, in surgical applications, it will be used under cyclic loading conditions. The effect of the microstructural refinement by high-pressure torsion (HPT) on the fatigue behaviour of TNTZ is systematically investigated in this study. TNTZ subjected to HPT processing where the rotation number (N) is 20 (TNTZ_AHPT) after aging treatment (AT) shows a unique microstructure having ultrafine elongated grains (285 nm in length and 36 nm in width) with high-density dislocations, a large fraction of blurred and wavy boundaries consisting of non-uniform subgrains with high misorientation and nanostructured precipitated α phase. Remarkably, a good combination of high mechanical strength (1375 MPa) and low Young’s modulus (87 GPa), compared to that of Ti-6Al-4V (Ti64) ELI, is achieved for TNTZ_AHPT at N = 20. TNTZ_AHPT a great fatigue strength, which is comparable to those of (Ti64) ELI.

Abstract: The ocean environment necessitates the pile foundation supporting the offshore structures to be designed against cyclic load, moments and torques initiated by a combined action of waves, wind, tides, currents, etc. Such a complex loading condition induces progressive degradation in the pile-soil interactive performance introducing significant reduction in bearing capacity with increased settlement and displacements. The Author has carried out extensive experimental (laboratory model tests) and theoretical investigations (boundary element analysis) to study the salient features of this degradation and developed a design methodology for offshore pile foundation. The works conducted and the major conclusions drawn are highlighted in this paper.

Abstract: The pile foundations supporting offshore structures are required to be designed against cyclic load, moments and torques initiated by a combined action of waves, wind, tides, currents, etc. Such a complex loading condition produces progressive degradation in the pile-soil interactive performance which is likely to introduce significant reduction in bearing capacity with increased settlement and displacements. This paper is based on a numerical model developed by the Authors to study the response of pile foundation under lateral cyclic load in layered soil. The model is validated with a field test data and thereafter, parametric studies have been carried out. A brief description of the works conducted and the major conclusions drawn are highlighted in this paper.

Abstract: A common opinion is that cast iron, especially grey cast iron, is not as notch sensitive as steel and has therefore not been treated by shot peening to suppress crack initiation. For a heterogeneous material that also is brittle, just like grey cast iron, the shot peening parameters needed to induce beneficial surface residual stresses can be problematic to identify. Fatigue testing under uniaxial loading with an R value of -1, on mechanically polished and shot peened specimens, has been performed to determine the fatigue strength at 10⁷ cycles as well as complete Wöhler-curves. Two different types of specimen geometries were tested, one smooth and one notched specimen having k_t equal to 1.05 resp. 1.33. With large shots and high peening intensity (heavy SP) the fatigue strength clearly decreased whereas small shots and low peening intensity (gentle SP) might have lowered the fatigue strength. A short annealing at 285° after gentle shot peening increased the fatigue strength. The results are discussed and explained based on x-ray diffraction (XRD) measurements, i.e. residual stress and full width at half maximum profiles, as well as microstructural investigations using scanning electron microscope (SEM).

Abstract: The efficient conservation, restoration and protection of stone historical buildings could not be dissociated from the researches of mechanism of stone decays. While various mechanisms of stone degradations are considered and studied in previous studies, in this paper is focused on the cumulative damage modeling of wall stones due to the fatigue induced from quotidian fluctuations of thermal and hydric conditions. The thermoporomechanics theory of partially saturated media is used to describe the behavior of stone wall and its interaction with climatic conditions. Further, the effective stress concept firstly introduced by Terzaghi is extended for partially saturated media providing a quite powerful tool for design and analysis [. The behavior of a typical stone wall from Chambord castle (Center region of France) is then simulated, taking into account the heterogeneity of the stone-mortar contacts. The climatic conditions are introduced in the model as boundary conditions. The records of temperature and humidity from meteorological stations close to castle are used to establish time-variation of condition with a time resolution of 6 hours. From performed numerical analyses, it is shown that variation of temperature and relative humidity leads to the variation and fluctuation of effective stress in the stone, more intensively on the outdoor. The contact of stone with mortar is a natural stress concentration center, but even there the stress is much lower than tensile strength of the white tuffeau stone. The fatigue of the stone due to the stress fluctuation induced by the variations of meteorological conditions seems to be a major factor of stone degradation. A model is used to assess the cumulative damage of the stone wall as a function of the time.

Abstract: In dry clutch systems, the clutch facing is an annular shaped continuous fibre composite with organic matrix (thermo set resins) which transmits the torque from the engine to the wheels. In use it is submitted to thermo-mechanical cycling. Due to the composite fibre organisation, the strain field under thermo-mechanical loading is not homogenous. Full field data is needed to describe the material behaviour. Digital Image Stereo-Correlation (DISC) was used to determine the coefficient of thermal expansion (CTE) of the material. To determine the effect of temperature and cyclic loading on the mechanical properties, the composite was subjected to different thermal cycles. The material properties are modified with increasing temperature and number of cycles. These results were confirmed by dynamic mechanical analysis which showed thermal ageing of the resin. The local information given by the strain fields revealed a non uniform evolution of the material properties with thermal cycling.

Abstract: Advanced areal (three-dimensional) characterisation of surface topography was applied to laboratory scale fatigue test specimens manufactured from the nickel based superalloy Alloy720Li. Finishing was deliberately manipulated to offer four distinct grades of topography. Subsequent low cycle fatigue performance was then correlated to a range of parameters selected to represent the surface topography. The aim of the ongoing study is to predict fatigue performance and aid to establish correlations between topographic parameters and fatigue life.

Abstract: The need to provide quality services, accurate diagnosis, timely rehabilitation and improvement in materials and operation of the turbines, have given way to scientific research crack initiation and crack propagation, the impending fracture and estimating life of the turbine components at normal conditions and resonance. The L-1 stage blades suffer high alternating stresses, for its size, exposure to high temperatures, and mechanical loads under repetitive strain of cyclic load. The lasts stages of blades are also subjected to severe centrifugal loads stress that, when combined with the alternating stress, is responsible for fatigue failures. In this work, the last stage L-1 blades of a steam turbine under cyclic load or fatigue were analyzed, the first instance to observe and measure the initiation crack and propagation crack under normal conditions operation and conditions on resonance; making the comparison to estimate the life time of the blade at both conditions.