Change In Strain Amplitude Research Articles

Amplitude-initiated open hysteresis loop of <i>P</i>-wave attenuation in sandstone: experimental study

In the area of solid state physics and materials science, new knowledge has been attained in recent years about micro-nano-plasticity using high-precision measurements at low stresses and strain. Rock microplasticity is currently poorly understood, but in the future it may prove useful in resolving problems of a fundamental and applied nature. This study examines the effect of cyclically varying pulse amplitude and wave velocity on the attenuation parameters of longitudinal wave (P-wave) in sandstone. Laboratory measurements were performed on rock specimens using the reflected wave method in the frequency range of 0.5–1.4 MHz at five values of strain amplitude ~ (0.5-2.0)10 –6 . Trial simulations were performed, in order to establish the effect of amplitude-dependent wave velocity on the parameters of wave attenuation in the sandstone. Wave attenuation behavior under combined action of the amplitude-dependent factor and wave velocity deviation is complex. The change in strain amplitude shifts the attenuation peak 1/ Q p ( f ) in the attenuation-frequency coordinates. The maximum change in peak attenuation value due to the amplitude factor and wave velocity deviation reaches 3–4 %. An open wave attenuation hysteresis loop was identified as a consequence of the closed amplitude cycle A1(+) --- A1(+) --- A1(–), where A1(+) = A1(–). Open attenuation hysteresis occurs both in the cases of constant and variable wave velocities. The length of the open part of the attenuation hysteresis loop relative to the peak value of the attenuation is as follows: for constant wave speed, 62.63 %, in the mode of increasing wave speed, 91.58 %; and, in the mode of decreasing wave speed, 47.01 %. The effect of open hysteresis of wave attenuation in sandstone can be explained by the action of microplastic deformation detected in the experiments.

Dual-pulse phase-OTDR response to propagating longitudinal disturbance

A numerical model describing the effect of an external longitudinal disturbance acting on a sensitive fiber of a dual-pulse phase-OTDR is considered. In the proposed theoretical description it is assumed that the external disturbance forms a region of the elastic deformation in the optical fiber and the corresponding strain amplitude changes along this region in accordance with the shape of the Ricker wavelet. The region with the externally induced strain propagates along the fiber axis and causes the changes of the phases of the optical fields backscattered by this fiber. The resulting measured response in each spatial channel of the phase-OTDR consists of linear and nonlinear random contributions with respect to the external disturbance. The linear contribution is formed due to the change of the optical path within the gauge length—the distance between the beginnings of two scattering segments of a spatial channel. The nonlinear random contributions are produced due to the responses of the scattering segments themselves. These contributions degrade the phase-OTDR response fidelity. In particular, they lead to an error in the registration of the first arrival time of the propagating external disturbance.To verify the results of the numerical modeling, an experiment on registration of the external disturbance propagating along the fiber using a dual-pulse phase-OTDR has been carried out. In the experimental setup, the strained fiber regions with the same spatial shape of the strain propagating along the fiber were repeatedly excited. As a result, the random nature of the response measured at the phase-OTDR output has been confirmed, the statistical parameters of this response are in good agreement with the simulation results.

Self-heating and other reversible phenomena in cyclic testing of bituminous materials

This paper’s objective is to evaluate the reversible phenomena that take place when asphalt materials are subjected to cyclic loads, i.e., self-heating and thixotropy. A strain sweep test was adapted to capture the stiffness variation of binders with the change in strain amplitude. The evolution of the internal temperature of the binder during the test was measured. Results show that the temperature can increase very significantly during cyclic testing and can account for a great part of all stiffness reduction captured during the test at different strain amplitudes. These results led to the conclusion that internal heating should be very important in asphalt mixtures as well. For that reason two types of time sweep tests were performed on the same mixture, with the introduction of rest periods in one of them long enough to let the inside temperature of the material lower after cycling. The results showed that the specimen that was allowed to cool down did not experience any loss of stiffness, while the specimen submitted to the conventional time sweep test failed in a few cycles. These results show the importance of the sequencing of loading and discourage the application of the Miner’s law to estimate pavement life.

Fatigue Damage of Al/SiC Composites - Macroscopic and Microscopic Analysis

Abstract The results of comparative examinations of mechanical behaviour during fatigue loads and microstructure assessment before and after fatigue tests were presented. Composites of aluminium matrix and SiC reinforcement manufactured using the KoBo method were investigated. The combinations of two kinds of fatigue damage mechanisms were observed. The first one governed by cyclic plasticity and related to inelastic strain amplitude changes and the second one expressed in a form of ratcheting based on changes in mean inelastic strain. The higher SiC content the less influence of the fatigue damage mechanisms on material behaviour was observed. Attempts have been made to evaluate an appropriate fatigue damage parameter. However, it still needs further improvements.

The Effect of Negative Temperatures of Frozen Clay on Dynamic Strain Amplitude

Authors studied variation characteristics of dynamic strain amplitude of frozen clay with different temperatures by MTS-810 vibration tri-axial material testing machine. Results show that the variability is carried by instrument errors, operation errors, and so on. The average values are used to eliminate the variability of test results. With the increasing of the load levels, dynamic strain amplitude increases gradually. Negative temperature has a lager influence on the variation characteristics of dynamic strain amplitude, however, the influence is smaller when temperature above freeze points of water in the soil; when the temperature below it, dynamic strain amplitude changes with decreasing temperature; when temperature below-4°C, dynamic strain amplitude has no change with decreasing temperature. Dynamic strain amplitude grows with increasing dynamic stress amplitude.

A simple approach for the modeling of an ODS steel mechanical behavior in pilgering conditions

The optimization of the forming of ODS tubes is linked to the choice of an appropriated constitutive model for modeling the metal forming process. In the framework of a unified plastic constitutive theory, the strain-controlled cyclic characteristics of a ferritic ODS steel were analyzed and modeled with two different tests. The first test is a classical tension–compression test, and leads to cyclic softening at low to intermediate strain amplitudes. The second test consists in alternated uniaxial compressions along two perpendicular axes, and is selected based on the similarities with the loading path induced by the Fe–14Cr–1W–Ti ODS cladding tube pilgering process. This second test exhibits cyclic hardening at all tested strain amplitudes. Since variable strain amplitudes prevail in pilgering conditions, the parameters of the considered constitutive law were identified based on a loading sequence including strain amplitude changes. A proposed semi automated inverse analysis methodology is shown to efficiently provide optimal sets of parameters for the considered loading sequences. When compared to classical approaches, the model involves a reduced number of parameters, while keeping a good ability to capture stress changes induced by strain amplitude changes. Furthermore, the methodology only requires one test, which is an advantage when the amount of available material is limited. As two distinct sets of parameters were identified for the two considered tests, it is recommended to consider the loading path when modeling cold forming of the ODS steel.

Nonlinear amplitude–frequency characteristics of attenuation in rock under pressure

Laboratory experiments have been carried out to investigate the influence of change in strain amplitude on the frequency dependence of attenuation in samples of sandstone, smoky quartz and duralumin. The measurements were performed using the reflection method on pulse frequency of 1 MHz in the strain range ~(0.3–2.0) × 10−6 under a confining pressure of 20 MPa. The attenuation in rocks is nonlinearly dependent on frequency and strain amplitude. In sandstone for P-waves and in smoky quartz for P- and S-waves, the dependences Q−1p(f) and Q−1s(f) have the attenuation peak. With increasing amplitude, the peak frequency can shift towards both the lower and the higher frequencies. It depends on the location of the frequency of an incident (input) pulse with respect to the peak frequency on the frequency axis. For sandstone the peak frequency of P-waves shifts towards the higher frequencies. For smoky quartz the shift of peak frequency is absent in P-waves, and S-waves shift towards the lower frequencies. The attenuation at the incident frequency always monotonically decreases with amplitude, and the other frequency components have complex or monotonic characters depending on the location of the incident frequency in the relaxation spectrum. Q−1p(f) in duralumin has monotonic character, i.e. a relaxation peak in the measurement frequency band is absent. Attenuation strongly decreases with increasing frequency and weakly depends on strain amplitude. The curve Q−1s(f) has an attenuation peak, and its character essentially depends on strain amplitude. With increasing amplitude, the peak frequency shifts towards the lower frequencies. The unusual increase of peak frequency of the P-wave spectrum in the bottom reflection in comparison with peak frequency in spectrum of the initial reflection is detected. The unusual behaviour of attenuation is explained by features of the joint action of viscoelastic and microplastic mechanisms. These results can be used for improving methods of geological interpretation of seismic data.

Dynamic viscoelasticity of hybrid kevlar and glass fiber reinforced LLDPE in the molten state

AbstractKevlar and glass fibers were used to reinforce linear low density polyethylene (LLDPE), and composite sheets of 0.8, 1.5 and 2.5 mm thicknesses were obtained by using a compression molding technique. Dynamic viscoelastic properties of non‐hybrid and hybrid composites of various compositions at 200°C are evaluated. Storage modulus (G′) and loss modulus (G″) increase with angular frequency (ω) and reinforcement. Replacement of glass fiber by Kevlar at constant loading of fibers in LLDPE increases the value of G′, G″ and η′. The fractured surface of composite shows the gradient orientation of fibers particularly in 2.5 mm thick sheet. Top and bottom layers show relatively two‐dimensional orientation as compared to the middle layer, which shows random orientation. The orientation of fibers decreases G′ and η′ of Kevlar fiber and hybrid fiber hybrid fiber reinforced LLDPE composites. The effect of change in distance between parallel plate of rheometer (change in strain amplitude) on dynamic rheological properties is studied and reported here.

Dynamic Viscoelastic Properties of Organic / Inorganic Fibres Reinforced lldpe Composites in Molten State

Abstract Dynamic rheological parameters such as storage modulus, G’, loss modulus, G”, and dynamic viscosity, h’, at 200°C were studied for Kevlar fibres, glass fibres and their hybrids reinforced linear low density polyethylene (LLDPE). Parallel plate rheometer was employed for these tests. G’, G” and h’ increased with the increased reinforcement and angular frequency, w. Two sets of reinforcement, 10 and 20 vol.% of fibres are used in LLDPE. The composition of fibres in hybrid composites was varied. The replacement of glass fibres with Kevlar increases the values of G’, G” and h’. The values of these rheological parameters also increased with the thickness of the composite. This increase was associated with the decreased average orientation of fibres present in the composite. The effects of the change in strain amplitude on G’ and G” is also studied and reported here.

Effects of Amplitude-History and Temperature-History on Multiaxial Cyclic Behavior of Type 316 Stainless Steel

The effects of the history in strain-amplitude and temperature variation on the multiaxial cyclic behavior of type 316 stainless steel were discussed by performing a series of total-strain controlled cyclic tests under uniaxial tension-compression and circular strain paths. Constant strain rate of 0.2 percent/min was specified throughout the tests. The effects of strain amplitude history were examined by changing the strain amplitude between 0.2 percent and 0.4 percent (step-up and step-down tests) at room temperature, 400°C and 600°C. For temperature history dependence tests, the temperature was changed between 200°C and 600°C, 400°C and 600°C, 500°C and 600°C, by specifying a constant strain amplitude of 0.3 percent. It was observed that for the step-up change in strain amplitude the prior cycle showed apparently no influence on the subsequent cyclic accommodation for the uniaxial and the multiaxial cycles at room temperature, 400°C and 600°C. For the decrease in strain amplitude, however, the definite effect of the prior cycle was observed at 400°C, while at higher temperature it disappeared. The effect of the temperature history, on the other hand, appeared only in the case of the temperature-decrease during the uniaxial cycle.

On the cyclic stress-strain curve evaluation in low cycle fatigue

Constant strain amplitude tests, strain amplitude changes and short cut procedures for cyclic stress-strain curve determination in low carbon steel were analysed in order to understand the differences or similarities they show in the cyclic stress-strain response. These observations were supplemented by electron microscope observations of the resulting dislocation structure after constant amplitude loading and strain amplitude changes. It was found that the previous cyclic history considerably influences the cyclic stress-strain response. Constant amplitude cycling after an amplitude change has a tendency to bring both the stress-strain response and internal dislocation structure into a steady state identical with that reached by cycling the virgin specimen at the same (final) strain amplitude. The actual stress-strain response during variable amplitude loading depends on the cyclic history. The lower bound of this response is the cyclic stress-strain curve obtained from tests on companion specimens and the upper bound is the stabilised hysteresis loop at the highest strain level. Between these extremes lies the cyclic stress-strain curve obtained from the incremental step test. This cyclic stress-strain curve approximates reasonably well the stabilised cyclic stress-strain response in variable amplitude loading.

The effect of the strain amplitude changes on the stress amplitude and resistivity of torsionally fatigued copper

Polycrystalline copper wires were cyclically strained in torsion in liquid nitrogen and the effect of the strain amplitude changes on the stress amplitude and resistivity was followed. It was found that both stress amplitude and resistivity are able to increase or decrease in dependence on the applied plastic strain amplitude. When the strain amplitude was decreased, the reversibility was not complete. By means of intermediate annealing it was found that both the dislocation density and the point-defect concentration follow the changes of the plastic strain amplitude.