Occurrence of plastic instabilities in dynamic microhardness testing

Kinematic analysis of plastic instabilities occurring in microhardness tests

The plastic instability in the stress-strain curve and work hardening of a type AISI 316 austenitic stainless steel was investigated in the high temperature range of 150°C to 800°C for two strain rates. The results indicate the occurrence of plastic instability and serrations between 200°C and 650°C with peaks in both the tensile strength and work hardening rate. These plastic instability/serrations are associated with dynamic strain aging behavior. As a consequence, within the temperature interval of plastic instability, a negative value for the strain rate sensitivity was found. Based on the activation energy, it is proposed that a mechanism of substitutional solute atoms interaction with dislocations be responsible for the plastic instability.

Plastic instabilities were investigated by the depth-sensing microhardness test in binary high-purity Al–Mg alloys with different Mg contents. During the tests the applied load was increased from 0 to 2000 mN at constant loading rate. The instabilities appeared as characteristic steps in the load–depth curves during indentation. It was shown that the occurrence and development of the plastic instabilities depend strongly on the solute content. Furthermore, the plastic instabilities occurred only when the solute concentration was larger than a critical value, C0. From room-temperature tests on Al–Mg alloys, C0 was found to be 0.86 wt% Mg. The critical concentration, which is necessary to get plastic instabilities, was also interpreted theoretically.

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.

BackgroundSuture materials and techniques are frequently evaluated in ex vivo studies by comparing tensile strengths. However, the direct measurement techniques to obtain the tensile forces in canine skin are not available, and, therefore, the conditions suture lines undergo is unknown. A soft elastomeric capacitor is used to monitor deformation in the skin over time by sensing strain. This sensor was applied to a sample of canine skin to evaluate its capacity to sense strain in the sample while loaded in a dynamic material testing machine. The measured strain of the sensor was compared with the strain measured by the dynamic testing machine. The sample of skin was evaluated with and without the sensor adhered.ResultsIn this study, the soft elastomeric capacitor was able to measure strain and a correlation was made to stress using a modified Kelvin-Voigt model for the canine skin sample. The sensor significantly increases the stiffness of canine skin when applied which required the derivation of mechanical models for interpretation of the results.ConclusionsFlexible sensors can be applied to canine skin to investigate the inherent biomechanical properties. These sensors need to be lightweight and highly elastic to avoid interference with the stress across a suture line. The sensor studied here serves as a prototype for future sensor development and has demonstrated that a lightweight highly elastic sensor is needed to decrease the effect on the sensor/skin construct. Further studies are required for biomechanical characterization of canine skin.

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.

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

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.

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.

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.

On phase transition velocities of NiTi shape memory alloys

Characteristics of the dynamic strain aging (DSA) in the Portevin-Le Chatelier effect are experimentally investigated by dynamic indentation tests and numerically analyzed by using literature models. Experimental results obtained on Al–Mg alloys show that the occurrence and development of the plastic instabilities—serrated indentation—depend strongly on the solute content. During dynamic microindentation tests the amplitude of microhardness drops—similarly to the global hardness—and is changing as a power law function of Mg solute content with an exponent of 2/3. It has been shown that the term describing the effect of DSA in serrated flow is not proportional but rather a power expression of the local solute concentration, Cs, on the dislocation line with the exponent of 1/2. Together with this, the kinetics of solute segregation during DSA is controlled by the pipe diffusion.

The authors have suggested to use the actual restoring force of soil in an analysis of surface ground motion during earthquake. The analytical equipment consists of a microcomputer to calculate the vibration equation and dynamic triaxial testing machine to obtain the actual restoring force of soil.The micro-computer has two limits in the respect of a calculation velocity and a memory capacity. In order to remove the above limits, we develop new analytical equipment that uses a digital computer in a computer center. This computer is electrically connected to a dynamic triaxial testing machine by TSS (Time Shearing System). The dynamic behavior of very soft surface ground during the earthquake, such as a reclaimed ground, is analyzed.

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).