Stiffness Of Samples Research Articles

Fatigue crack initiation in lead-bismuth eutectic and its effect on the cyclic stress behaviour of austenitic stainless steel 316L

Active media such as liquid metals have an effect on the fatigue resistance of steels when compared to vacuum or inert environments. This effect might impact the initiation of fatigue cracks, their propagation, or both. This paper focusses on the initiation of fatigue cracks in an austenitic stainless steel when in contact with three environments: vacuum, air, and lead-bismuth eutectic (LBE), and the impact of such cracks on cyclic loading behaviour.Solid cylindrical samples of a 316L-type steel were fatigued under strain control in vacuum, air, and LBE in order to induce the initiation of cracks on their surface. The characteristics of fatigue damage in the form of surface microcracks were analysed with microscopy techniques. It was found that the nucleation of fatigue microcracks is enhanced by LBE, but the majority of these cracks do not propagate beyond the grain size of the steel (50 μm). An analysis with finite element methods showed that the large number of small (<10 μm) microcracks nucleated in LBE environments has a negligible impact on the sample's stiffness, as opposed to the fewer but deeper (100–500 μm) microcracks that initiate in air, which produce a loss of stiffness, observable on the mechanical stress response of the fatigue sample.A model based on the adsorption of LBE atoms along surface slip bands is proposed to explain the phenomenon of enhanced fatigue crack nucleation in LBE.

Dependence of the nanometer-scale structural heterogeneity of a bulk metallic glass on its fictive temperature

An interesting characteristic of metallic glasses is that their atomic-scale structure can be modified by processing to a degree that allows to tailor mechanical properties such as fracture toughness, Young's modulus, or hardness within a surprisingly large range without having to change its alloy composition. To be able to make full use of this ability, it is important to understand critical aspects of the glassy structure such as local ordering or density fluctuations, both of which manifest as structural heterogeneity. In this work, we reveal heterogeneity on the nanometer scale in samples made from a platinum-based bulk metallic glass that feature distinct atomic-scale arrangements. Toward this end, Pt57.5Cu14.7Ni5.3P22.5 was thermoplastically formed to feature virtually atomically flat surfaces followed by an annealing procedure that also sets the sample's fictive temperatures. Atomic force microscopy imaging was subsequently carried out at room temperature in both topography-mapping tapping mode and in a mode that allowed to locally track the sample's stiffness, modulations of which are interpreted as a reflection of structural inhomogeneity. The results show that samples featuring fictive temperatures close to the material's glass transition temperature Tg of 235 °C are the most homogeneous if compared to samples with fictive temperatures that are either lower or higher than Tg. This is qualitatively different from the trend for the absolute values of the average stiffness, which monotonically increases for lower fictive temperatures. These results are being discussed in the light of an uptick of local ordering when the sample's fictive temperature drops significantly below its glass transition temperature.

Dynamic stiffness of braided HMPE ropes under long-term cyclic loads: A full-scale experimental investigation

This paper presents a full-scale experimental investigation to study the nonlinear dynamic stiffness of high-modulus polyethylene (HMPE) fibre rope under long-term cyclic loading. Three kinds of ropes with different constructions and diameters were selected as test samples. The influences of mean load and load amplitude on axial stiffness and the corresponding evolution process were respectively studied. The experimental results showed that various braided ropes' dynamic stiffness was slightly different, which was mainly caused by the different twist angles. Furthermore, the non-dimensional stiffness of small-scale rope was lower than the large samples to some extent. This should be related to the small-scale ropes without an outer jacket were much softer and thus had more distinct deformation. The validations against two existing empirical expressions have also been performed. The results pointed out that these equations had some errors predicting the evolution procedure of the dynamic stiffness and the magnitude of small-scale sample's stiffness. On these accounts, a set of empirical expressions, taking into account braid constructions, diameter and input load conditions, has been newly derived. The comparison against the present experimental results proved its good reliability.

An apparatus to measure elastic dispersion and attenuation using hydrostatic- and axial-stress oscillations under undrained conditions.

An experimental apparatus is described for the investigation of frequency dispersion, and related attenuation, of fluid-saturated rocks under confining pressure and undrained boundary conditions. The forced-oscillation method is performed on cylindrical samples. The measurement of stress and strain under hydrostatic oscillations allows the dynamic bulk modulus to be inferred, while axial oscillations give access to dynamic Young's modulus and Poisson's ratio. We present calibration measurements for dispersion and attenuation on standard materials such as glass, plexiglass, and gypsum. Results show that for strain amplitudes below 10-5, robust measurements can be achieved up to 1 kHz and 1.3 Hz, respectively, for axial and hydrostatic oscillations. A new experimental design of the endplatens (sample holders) allows control of drained or undrained boundary conditions using microvalves. The microvalves were tested on a porous Vosgian sandstone. In addition, numerical modeling confirms that the resonances of the apparatus only affect frequencies above 1 kHz, with little sensitivity to the sample's stiffness.

Sensitivity of higher mode of rectangular atomic force microscope to surface stiffness in air environment

The sensitivity of the resonance frequencies and the amplitudes of the first four flexural vibration modes of atomic force microscopy with a rectangular cross-section cantilever in an air environment to variation of a sample's stiffness have been analysed. The cantilever has been modelled using Timoshenko beam theory, and the vertical and tangential forces between the tip and the sample in this simulation have been considered. The effect of tip position and the angle of the tip relative to the sample in changes of these sensitivities have been studied. Results indicate that for soft materials, the first mode compared with other modes is more sensitive to changes in the sample's elasticity, so the first mode is the best mode for supplying high-contrast images but by increasing the sample's stiffness, the higher mode will be sensitive, respectively. In addition, by increasing the angle between the cantilever and the sample's surface, the first mode is less sensitive and higher modes will be sensitive faster because of changes in material stiffness but in contrast, by increasing the distance between the tip's position and the free end of the cantilever, the sensitivity of the first mode increases and the higher mode will be sensitive for more stiff material.