Kinematic analysis of plastic instabilities occurring in microhardness tests

Plastic instabilities were observed to appear during dynamic ultramicrohardness testing of a solid solution Al–3.3 wt.% Mg alloy. The tests were carried out at room temperature with a Vickers hardness indenter in a computer-controlled dynamic ultramicrohardness testing machine. During the tests the applied load was increased from 0 to 2000 mN at constant loading rate. The instabilities appear as characteristic steps in the continuously recorded load-indentation depth curves. The physical basis for the occurrence of the instabilities is the interaction between moving dislocations and solute atoms, a phenomenon termed in the literature as serrated yielding, jerky flow, or Portevin-Le Châtelier effect. The instabilities start at a critical load, Fc, in the depth-load curve. Varying the loading rate, μ, by two orders of magnitude Fc was found to increase linearly with the loading rate.

In order to study the influence of sudden change of loading rate on crack propagation and failure mode of rock bridge of fractured rock mass, the specimens used in this paper were rock-like materials containing two prefabricated fractures. The mechanical properties and the failure mode of the specimens under different constant loading rates and sudden changes in loading rate were tested. The photographic monitoring and acoustic emission were carried out at the same time. The results show that: (1) In the process of sudden change loading, the specimen shows the characteristics of approximate elastic deformation, and the stress-strain curve after sudden loading is similar to the corresponding stress-strain curve at the corresponding constant loading rate. (2) Combined with acoustic emission detection, it is found that when the loading rate is not much different before the mutation, the loading rates is the same after the mutation and the stress-strain curve of the specimen is similar. In the low-speed loading stage, the acoustic emission count is generally low, while in the high-speed loading stage, the acoustic emission count is generally high. The sudden change from low-speed loading to high-speed loading easily induces stress drop, resulting in crack generation and increase of acoustic emission count. (3) The rate of the specimen during the crack development period plays a decisive role in the failure mode of the specimen. Before the sudden change of loading rate, the low-speed loading within a certain range has little effect on the specimen. When the high-speed loading is carried out when the stress is low in the early stage of the mutation, the acoustic emission count of the specimen is high, which will cause some damage inside the specimen. As a result, even if the rate is the same after the mutation, the final peak stress and failure mode may be different.

We use dimensional analysis to derive scaling relationships for self-similar indenters indenting solids that exhibit power-law creep. We identify the parameter that represents the indentation strain rate. The scaling relationships are applied to several types of indentation creep experiment with constant displacement rate, constant loading rate or constant ratio of loading rate over load. The predictions compare favourably with experimental observations reported in the literature. Finally, a connection is found between creep and 'indentation-size effect' (i.e. changing hardness with indentation depth or load).

The tensile strength measured by Brazilian test is termed the splitting tensile strength. The values of splitting tensile strength and corresponding strain may be affected by loading and/or strain rates used in the Brazilian test. In order to investigate the effects, several Brazilian tests with two-type loading modes were carried out. One is under a constant loading rate (BT-CLR), and the other is under a constant strain rate (BT-CSR). Lightly weathered sandstone and mudstone are selected as test materials. Experimental data indicate that, while the loading rate increases from 0.3 to 0.5 MPa/s, the splitting tensile strengths of sandstone and mudstone are increasing about 12.2% and 16.0%, respectively. With increasing the strain rate from 0.005 to 0.02 mm/s, the splitting tensile strengths of sandstone and mudstone increase about 9.3% and 12.8%, respectively. With increment of loading and/or strain rate, the critical strains of both sandstone and mudstone decrease. The effect degree, in percentage, on the mudstone is greater than one on the sandstone.

Material with good mechanical resistance in the high temperature operation is needed in the power generation industry. Due to the influence of stress, high temperatures and long operation of time, unpredictable failure often occurs resulting plant stopped working. The cause of failure, associated with some changes in mechanical properties of the material can be represented by microstructural conditions. Ferritic material SA-213 T22 is widely used in power plants. To get more information of those mechanical properties, the study of damage mechanisms has been conducted by analysis simulation. The Benefits of this study is to get proper operating condition to avoid unpredictable failures. Using finite element method, it is found that in constant temperature 580 °C with the load rate of 23.03MPa per second the strain is 55.25%. When the load rate is decreased to 3.84MPa per second the strain is 63.57%. In the constant temperature 300 °C with load rate 11,52MPa per second the strain is 27.55%. When the temperature increased to 560 °C the strain is 32.02%. In the constant temperature, the decreased of load rate give impact the increased of ductility. Otherwise in the constant load rate, the increased of temperature give impact only small amount to increase the ductility. To prevent the unpredictable failure the sudden increase of load rate should be avoided.

Nanoindentation tests in which oscillating loads were applied to specimens were carried out to study the work (Wf) required for a coating film to delaminate from its substrate. A sharp increase in the indentation depth occurred during coating film delamination. A theoretical model was developed for the present study to evaluate the difference between the work in the load-depth profiles obtained with and without delamination. Arranging that the maximum load was reached at the end of the loading process allowed us to obtain an approximately constant value for the indentation depth propagation rate during in one cycle. This allowed a determination of the number of oscillating cycles and, thus, the work required by assuming that there was no delamination during this delamination period. The depth propagation rates and the work required for coating film delamination were investigated by varying Pmax, Pmean, P0, and frequency (f). The depth propagation rates were slightly increased by increasing the Pmax value but were significantly lowered by increasing the frequency (f). The effects due to the changes in Pmean and P0 on the depth propagation rate were insignificant. The Wf value was slightly lowered by increasing the Pmax value but was significantly increased by increasing the oscillating load frequency (f). The effects of varying P0 and Pmean on the Wf value were found to be small. The Wf values showed an exponential drop when the depth propagation rate was increased; they were asymptotic to a constant value when the depth propagation rate was sufficiently high. Coating film delamination produced a sharp increase in the indentation depth even in the case of a very small load increase. The oscillating load-depth cycles appearing after delamination allowed us to establish the film-substrate contact behavior occurring during the loading/unloading process of one oscillating cycle.

Despite certain restrictions on its validity, the use of a triaxial apparatus is still a commonly used laboratory test method to determine soil behaviour. Solutions were obtained many years ago for evaluating mean excess pore pressure values in soils subject to continuous rates of loading. They have been used to predict mean excess pore pressures in consolidation tests with constant rates of loading. Such solutions may be suitable for predicting excess pore pressures in soil specimens, subject to continuous loading, in drained triaxial stress path tests. This note proposes the use of these solutions to select the rate of loading for continuously loaded, drained stress path triaxial tests. The analytical solutions of the equations in the known mathematical model may be used to predict the average excess pore pressure in a triaxial sample, subjected to time-dependent loading, at any time during a continuous triaxial stress path. This requires an estimate of the imposed excess pore pressure. A calculation method is given for this pressure, together with a proposed method for predicting deviation from the required effective stress path. As an illustrative example, a prediction of the effective stress path, during a typical drained triaxial stress path test, is presented.

When an engineering structure regarding a rock is affected by dynamic loads (due to the occurrence of natural hazards, such as earthquakes and landslides, or man-made hazards, such as explosions or impacts), correct prediction of changes in the strength behavior and deformability of the rock relative to its static state is necessary for reducing the damages and costs. On the other hand, rocks are always influenced by environmental conditions, such as temperature changes due to fire and weather during their lifetime, which should be considered when using them. In these cases, the mechanical behavior of the rock can usually be determined under different loading and environmental conditions using stress–strain curves. This study investigates rocks’ dynamic strength and deformability behavior at different loading rates and temperatures. For this purpose, 30 travertine rock samples from the Torshab mine, located in the Markazi province of Iran, were first heated up to 100°C, 200°C, 400°C, 800°C, and 1000°C (six temperatures), and then subjected under the impact pressure with different loading rates from (five) 11 m/s to 15 m/s using the split Hopkinson pressure bar test. Comparing the obtained dynamic stress–strain curve shows that at a constant loading rate, increasing the temperature, especially at higher temperatures, reduces the dynamic strength and increases the rock’s deformability. Moreover, in all cases, at a constant temperature, increasing the loading rate, especially at higher rates, increases the rock’s dynamic strength and deformability.

On phase transition velocities of NiTi shape memory alloys

Durability of polymer matrix composites: Viscoelastic effect on static and fatigue loading

Critical nutrient loads to prevent duckweed dominance loads in polder ditches were assessed using the eutrophication model PCDitch. In this article the ecological target was set at 50% duckweed coverage. This may be very high for ditches with a nature function, but is not unreasonable for ditches in agricultural areas, with upwelling nutrient rich groundwater, run-off and drainage. Since the change from a ditch with submersed vegetation to duckweed coverage is often a sudden shift, the choice of the amount of duckweed coverage does not influence the calculated loading very much. The main topic of this paper is to present a method to calculate critical loads of nutrients when ecological targets have been set. Sediment type, residence time and water depth influenced the critical loading rates. The calculated critical phosphorus load ranged from 1.8 to 10.2 g P m−2 year−1, while the calculated critical nitrogen load stretched from 12.1 to 43.8 g N m−2 year−1. The concentration ranges that were derived from the loading rate were 0.19–0.42 mg P l−1 and 1.3–3.3 mg N l−1. Since PCDitch does not distinguish between Lemna spp. and Azolla spp., no definite conclusions were drawn concerning the effects of nitrogen reduction. In a model situation a pristine ditch was loaded with phosphorus, which resulted into complete duckweed coverage during summer within a few years. When reducing the phosphorus load, it took 10 years before the original situation was reached again. Dredging would accelerate the process of recovery significantly, because the water depth would increase and the phosphorus release from the sediments in summer would decrease.

Estimation of physical properties of rock is essential to design rock structures and to evaluate the stability of rock slopes. It is well known that rock properties often vary from site to site, and even from specimen to specimen. So, in many cases, it requires many rock samples, much effort and cost to obtain the reliable data. In this study, an efficient testing method with small rock specimens was proposed and examined. At first, the preparation process for a cylindrical specimen, 10mm in diameter and 20mm in length, was developed. In the experimental work, the transparent triaxial vessel, developed by Okubo et al., was modified and used for the small specimen. Triaxial compression tests with constant loading rate were conducted with small specimens of Tage tuff, and it was found that cohesion, C, and angle of internal friction, φ, can be precisely obtained. By this test, the loading rate dependency of a stress-strain curve can be also obtained by just one small specimen with the loading rate alternately switching during a test. Small specimens are easy to carry and it is effective especially for geophysical researches in foreign countries. So, in this study, triaxial compression tests, noted above, were also conducted with rocks obtained around the Three Gorges Reservoir in China.

In the present paper, an experimental study was conducted to determine the factors affecting the point bearing capacity of pile foundations constructed in dry and saturated sandy soils. Model piles were installed as reinforced concrete bored piles cast-in-situ. Model pile foundations of various geometries resting at different depths in homogeneous sand of different saturation degrees (%0-100) were loaded statically to failure. The test results showed that the bearing capacity of piles did not significantly affect by the loading rate. At most 10% difference was observed in pile bearing capacity when the loading rate was between 0.7 and 2.5 mm/min. Subsequently, the load bearing capacities of the piles were determined at a specified constant loading rate. The point and total capacities of the piles were measured separately in the experiments, then test results were compared with theoretical values. Pile point capacities provided from pile load tests are smaller than the theoretical values. The differences between experimental and theoretical results have been attributed to the Nq values. The Nq values not only dependent on the internal friction angle of the soil but also the saturation degree of the soil, the pile diameter, and the effective stress. Nq values decrease since the pile length/pile diameter ratio increases.

Small amplitude oscillations are a commonly observed feature in prominences/filaments. These oscillations appear to be of local nature, are associated to the fine structure of prominence plasmas, and simultaneous flows and counterflows are also present. The existing observational evidence reveals that small amplitude oscillations, after excited, are damped in short spatial and temporal scales by some as yet not well determined physical mechanism(s). Commonly, these oscillations have been interpreted in terms of linear magnetohydrodynamic (MHD) waves, and this paper reviews the theoretical damping mechanisms that have been recently put forward in order to explain the observed attenuation scales. These mechanisms include thermal effects, through non-adiabatic processes, mass flows, resonant damping in non-uniform media, and partial ionization effects. The relevance of each mechanism is assessed by comparing the spatial and time scales produced by each of them with those obtained from observations. Also, the application of the latest theoretical results to perform prominence seismology is discussed, aiming to determine physical parameters in prominence plasmas that are difficult to measure by direct means.

Atomic force microscopy (AFM) is today an established tool in imaging and determination of mechanical properties of biomaterials. Due to their complex organization, those materials show intricate properties such as viscoelasticity. Therefore, one has to consider that the loading rate at which the sample is probed will lead to different mechanical response (properties). In this work, we studied the dependence of the mechanical properties of endothelial cells on the loading rate using AFM in force spectroscopy mode. We employed a sharp, four‐sided pyramidal indenter and loading rates ranging from 0.5 to 20 μm/s. In addition, by variation of the load (applied forces from 100 to 10,000 pN), the dependence of the cell properties on indentation depth could be determined. We then showed that the mechanical response of endothelial cells depends nonlinearly on the loading rate and follows a weak power‐law. In addition, regions of different viscous response at varying indentation depth could be determined. Based on the results we obtained, a general route map for AFM users for design of cell mechanics experiments was described.