Results Of Stress Relaxation Tests Research Articles

Evaluation of time-dependent change in fiber stress profiles during long-term pull-out tests at constant loads using Raman spectroscopy

Constant-load pull-out tests were carried out on single-fiber model composite specimens for 500 to 1,000 hours in order to investigate the time-dependent change in fiber axial stress profiles resulting from matrix creep in unidirectional continuous fiber-reinforced composites. Three resins used as the matrix materials, in which single carbon fibers were embedded, were normal epoxy, a blend with a more flexible epoxy, and UV-curable acrylic. The time-dependent change in fiber stress profiles in the constant-load pull-out tests was measured using Raman spectroscopy, and creep and relaxation tests for the matrix resins themselves were performed. It was observed that the normal epoxy matrix composite exhibited only a negligible change in the fiber stress profile with time whereas the flexible epoxy and UV-curable acrylic matrices allowed, respectively, considerable and significant changes. These observations were shown to be consistent with the creep and stress relaxation test results of the matrix resins. It was also found that the time-dependent change in fiber stress was much slower in the experiment than in the prediction based on perfect bonding at the fiber/matrix interface. The interfacial slip that occurred in the composites tested could be responsible for the gradual variation in fiber stress profiles.

Calculation of viscoelastic properties of edible films: application of three models

The viscoelastic properties of edible films can provide information at the structural level of the biopolymers used. The objective of this work was to test three simple models of linear viscoelastic theory (Maxwell, Generalized Maxwell with two units in parallel, and Burgers) using the results of stress relaxation tests in edible films of myofibrillar proteins of Nile Tilapia. The films were elaborated according to a casting technique and pre-conditioned at 58% relative humidity and 22ºC for 4 days. The testing sample (15mm x 118mm) was submitted to tests of stress relaxation in an equipment of physical measurements, TA.XT2i. The deformation, imposed to the sample, was 1%, guaranteeing the permanency in the domain of the linear viscoelasticity. The models were fitted to experimental data (stress x time) by nonlinear regression. The Generalized Maxwell model with two units in parallel and the Burgers model represented the relaxation curves of stress satisfactorily. The viscoelastic properties varied in a way that they were less dependent on the thickness of the films.

Evaluation of Time-Dependent Change of Fiber Stress Profiles in Constant Load Single-Fiber Pull-Out Tests Using Raman Spectroscopy.

The time-dependent change of stress profiles resulting from matrix viscosity in unidirectional fibrous composites was studied using Raman spectroscopy, which enables us to determine fiber stress on the basis of the stress dependence of Raman band peak positions. Three kinds of model composites, in which single carbon fibers were embedded in three different resins, were tested by pulling the fibers at constant stresses for 500 to 1000 hours. The resins were normal epoxy, flexible epoxy and uv-curable acrylic. It was observed that a carbon fiber/normal epoxy composite exhibited negligible increase of stress transfer length with time whereas flexible epoxy and uv-curable acrylic allowed, respectively, considerable and significant changes of fiber stress profiles with time. These observations were shown to be consistent with the creep and stress relaxation test results of the matrix materials. It was however found that the time-dependent change of fiber stress profiles was much slower in the experiments than in the prediction based perfect bonding at fiber/matrix interface. It was then shown that the sliding at fiber/matrix interface, which was responsible for the slow change of fiber stress profiles, ocurred in the model composites tested.

The modelling of time-dependant deformation in wood using chemical kinetics

The development of rheological models to predict creep has led to the derivation of quite complex equations that can predict creep reasonably accurately. However, these models are conceptual and are not based on a fundamental under-standing of the actual deformation processes occurring within the material. The concept of modelling creep using a chemical kinetic approach is one that attempts to understand creep in wood at a molecular level and, from this, to develop models that more accurately predict creep deflections. This paper presents two models developed from chemical kinetic theory, that describe the time-dependant deformation of wood. The validity of applying these models to experimental data has been assessed by stress relaxation tests on thin samples of Sequoia sempervirens. Two stages of experimentation were carried out. In stage 1, both models were applied to the results of stress relaxation tests on 6 samples. Similar values of activation energy and activation volume were calculated by both models and a single energy barrier was found to dominate the deformation process. In stage 2, the effect of varying the initial applied stress on activation energy and activation volume was assessed by carrying out stress relaxation tests at stress levels of 25%, 30% and 35% of the short-term strength. Values of activation energy and activation were found to increase as the applied stress level decreased. Both models describe the time-dependent behaviour of wood well, however their ability to predict long-term creep deflections may be limited. Future work will develop these models further in order to improve long-term creep prediction and then apply them to the results of both creep and stress relaxation tests at a variety of stress levels and moisture contents in order to test their validity.

The Influence on their Resilience of a Partial Removal of the Deforming Stress on Compressed Elastomers

If there is a relatively low reduction in the original deforming stress applied to an elastomer seal which is subjected to a constant compression during its practical use, this may result in a considerable drop in its resilience and the sealing capacity associated with this. The degree of the stress relaxation caused by a specific reduction in the initial deforming stress in addition to the viscoelastic and ageing processes in the elastomer was determined for six vulcanisates and a thermoplastic elastomer during long-term storage at 23, 70 and 1000°C. The stress relaxation values determined in this way were compared to the results of stress relaxation tests performed according to DIN 53 537 (method B). In parallel to the relaxation measurements, a determination was made of the compression set which is obtained if the heated and deformed test pieces are cooled to room temperature in the clamping device before the stress is removed.

The Influence on their Resilience of a Partial Removal of the Deforming Stress on Compressed Elastomers

If there is a relatively low reduction in the original deforming stress applied to an elastomer seal which is subjected to a constant compression during its practical use, this may result in a considerable drop in its resilience and the sealing capacity associated with this. The degree of the stress relaxation caused by a specific reduction in the initial deforming stress in addition to the viscoelastic and ageing processes in the elastomer was determined for six vulcanisates and a thermoplastic elastomer during long-term storage at 23, 70 and 1000°C. The stress relaxation values determined in this way were compared to the results of stress relaxation tests performed according to DIN 53 537 (method B). In parallel to the relaxation measurements, a determination was made of the compression set which is obtained if the heated and deformed test pieces are cooled to room temperature in the clamping device before the stress is removed.

Stress relaxation at room temperature for sheet single-crystal molybdenum

1. Results of stress relaxation tests by two different procedures (compensation and static) for rolled sheet single-crystal molybdenum at room temperature and with initial stresses σ0 < 0.4σ0.2 are close in value. Stress relaxation curves calculated taking account of the change in distribution of bending stresses over the specimen cross section in this region of initial stresses almost coincide with experimental curves. 2. Values of stress relaxation limit on reaching residual strain of 0.001 and 0.005% on a base of 500 h, and also the stress relaxation limit σ nom o = 70 ± 10 MPa below which relaxation does not occur, have been determined in specimens of rolled sheet single-crystal molybdenum. 3. In specimens of rolled single-crystal molybdenum tested with initial stresses of σ0 0.4σ0.2 for 500 h, 90% of the residual strain accumulates in the first 25 h.

Rhesus monkey intervertebral disk viscoelastic response to shear stress.

A viscoelastic model of the shear response of the Rhesus monkey intervertebral disk, represented by a series chain of four Kelvin units is presented. Two types of investigations are carried out to assess the validity of the model: 1) determination of material properties by comparison with experimental strain creep behavior of the disk; and 2) validation of this viscoelastic model by accurately predicting the experimental results of stress relaxation tests. The use of the series Kelvin units approach provides the first analytical mechanical model capable of predicting the creep and relaxation functions for the intervertebral disk in shear.

Time Dependent Behavior of Normally Consolidated Clays Under Shearing

In order to analyze the time-dependent behavior of normally consolidated clays, a nonlinear theory with a single integral representation has been developed. The equilibrium properties, as well as the time-dependent properties, are taken into consideration in the theory. It is shown that the proposed theory can be reduced to a simple form by making use of the results of stress relaxation tests on a normally consolidated clay. After experimentally confirming that the relationship between the excess pore water pressure and the maximum principal strain is only slightly influenced by the rate of strain, it was shown that the proposed theory successfully ex-plains the rheological properties of the clay under conditions of undrained steady shearing. Furthermore, the effect of time on the undrained compressive strength in the case of a plastic clay which has been consolidated under no lateral strain is also discussed. The results suggest that the proposed theory is possibly applicable for analyzing the time-dependent behavior of Ko-consolidated samples of normally consolidated clays.

Comparison of the results of stress relaxation tests on rings and bars

1. The conditions of relaxation (and of any other process of plastic strain) of a bent ring differ from the conditions of relaxation of a stretched bar. The systems of ideal elastic or ideal plastic strain may in many cases lead to substantial errors. Numerical results were found by Chizhik [7, 8] using an example of relaxation of screw springs. 2. On comparing the results of relaxation of rings and bars, a number of contradictory factors, which can sometimes compensate each other and produce the impression that the calculations for the relaxation of a ring using the system of linear stress curves is applicable, should be taken into consideration. Without taking into account all factors described in teh paper, it is impossible to speak convincingly about the intrinsic comparison of the results of tests on rings. 3. On discussing and comparing data on relaxation, a strict distinction should be made between the loading to a given strain and that to a given stress. In the latter case, the specimen has an increased relaxation resistance. 4. The problem of comparing the results of relaxation tests on rings and bars and the methods of calculation call for broader theoretical and experimental studies. The existing methods of calculation [1] should be substantially corrected.