Time-dependent Creep Deformation Research Articles

Prediction of Long – Term Creep Properties of Kenaf Fiber Unsaturated Polyester Composites

This research focuses on predicting long-term behavior of unsaturated polyester resin (UP) and kenaf unsaturated polyester composite. The objectives of these tests are to establish a relationship between stress, strain and time at constant loading and temperature. The results obtained from these tests are used in predicting the life and strength of the polymer material. Based on the 1,000 hours experimental data, curve fitting and Findley Power Law models are employed to predict long-term behavior of the material. The results showed that curve fitting model accurately predicted the non-linear time dependent creep deformation of these materials with acceptable accuracy.

Comparative investigation of creep behavior of ceramic fiber-reinforced alumina and silica aerogel

Ambient and high temperature creep experiments at constant stresses of 0.05MPa and 0.2MPa were conducted on a ceramic fiber-reinforced alumina aerogel. Experimental results show that at low temperature (below 300°C), there is no significant creep phenomenon at low stress level (0.05MPa), but time-dependent creep deformation is found at high stress level (0.2MPa) for the material. Scanning Electron Microscope (SEM) analysis was also conducted to understand the micro mechanism of the creep behavior. Crack initiation and propagation in matrix are the key factors that change the creep property. Comparative investigation of creep properties was also carried out between ceramic fiber-reinforced alumina and silica aerogel. Finally the application prospect of the two composites was discussed. The result shows that alumina aerogel has excellent creep resistance, thermal stability and heat insulation properties at high temperature (above 800°C), and it has a good application prospect in high temperature insulation fields, while the ceramic fiber-reinforced silica aerogel, limited by the sintering and poor creep resistance under high temperature, is more suitable for low temperature (below 600°C) insulation field.

Design of shaft linings to resist time dependent deformation in evaporite rocks

Potash, a fertiliser currently in demand worldwide, lies beneath beds of salt and other evaporites deep within the ground. The ground support challenge encountered when excavating through these strata is that of time dependent (creep) deformation control. The deformation is radial, causing the shaft excavation to close up over time, eventually loading the permanent lining with the full geostatic pressure and causing lining failure. In order to achieve a significant design life for an economical shaft lining in creeping rocksalt strata, a deformation zone filled with a compressible material is employed between the excavation and permanent lining. This helps resist the ground pressure, and reduce the magnitude and velocity of creep deformation. Optimum lining designs have been developed by predicting the creep deformation at a given time and by considering the behaviour of different compressible materials available to fill the deformation zone. Foamed concrete is the favoured material for an intermediate compressible lining.

Novel mathematical approaches for analyzing time dependent creep deformations in reinforced metals

The objective of this paper is to present some novel insights for solving a second stage creep problem in metal matrix composites. First, a new analytical approach is developed for obtaining some unknowns in second stage creep of short fiber composites under an applied axial load. The unknowns are the radial, circumferential, axial, shear and equivalent stresses, which are determined by approximation of creep constitutive equations and using proper assumed displacement rates. A nonlinear differential equation is solved employing suitable and correct approximate assumptions. Then, the difference of the stress components utilizing creep constitutive equations and assumed displacement rates is determined. Finally, the axial stress behavior in matrix is predicted by linear and nonlinear boundary value approaches, as well as displacement rates in matrix. For the purpose of the analysis, the steady state creep behavior of matrix material is described by an exponential law. As an important application, factor of safety n will be determined for fibers in order to have a good composite design. Based on the results, the aforementioned methods such as general boundary value approaches can be used to simply determine the approximate behavior of unknowns. These analytical results are then verified by the results of FEM simulation and other available research works. Interestingly, good compatibilities are found among the original mathematical approaches, numerical modeling and also previous available results.

Dynamic versus static artificial neural network model for masonry creep deformation

One of the inherent modelling problems in structural engineering is creep of quasi-brittle materials such as concrete and masonry. The creep strain represents the non-instantaneous strain that occurs with time when stress is sustained. It has long been known that creep in brickwork results in deformations that increase gradually over time, and dependable and accurate prediction models for the long-term, time-dependent creep deformation of brickwork structures are, therefore, required. Several models, with limited accuracy, have been developed over recent decades to predict creep in concrete and masonry structures. The stochastic nature of creep deformation and its reliance on a large number of uncontrolled parameters (e.g. relative humidity, time of load application, stress level) make the process of prediction difficult and the development of accurate mathematical models almost impossible. Artificial neural networks have been introduced as an efficient modelling technique for applications incorporating a large number of variables and have proven successful in many cases, especially in problems for which the characteristics of the process are difficult to describe using mathematical models. This study introduces a creep prediction model, based on non-linear autoregression with exogenous inputs (Narx), which is able to detect and consider time dependency (which is the major factor in creep deformation of brickwork structures) within its architecture. The performance of the proposed Narx model was verified with experimental creep data from brickwork assemblages collected over the last 15 years. The results show that the accuracy of the Narx model outperforms existing artificial neural network models and is able to achieve a prediction error of less than 12%, strongly suggesting that the Narx model is more suitable for modelling a time-varying process such as creep prediction.

Remarks on axisymmetric modelling of deep tunnels in argillaceous formations. II: Fissured argillites

In a companion paper, basic differences between the hydro-mechanical behaviour of plastic and stiff clay formations were highlighted and some issues, such as the influence of hydro-mechanical coupling and boundary conditions, were pointed out.This paper focuses on the effect of the scaly structure, frequently caused by tectonic shearing, typical of many relatively old and deep-sited argillaceous formations. Different engineering models for the prediction of short- and long-term response to tunnel excavation are compared. The proposed approaches consider some typical features of the hydro-mechanical behaviour of the plastic annulus around the tunnel, focusing on the influence of fissure opening, which may cause loss of saturation, in the short-term, and a general increase in permeability in the long-term. The case of the Raticosa tunnel (Italy), excavated through tectonised argillites, is chosen as an example case to apply the proposed approaches and investigate the aforementioned issues.Laboratory test results and in situ measurements are also discussed. The interpretation of convergence measurements, with the help of empirical correlations and the results of hydro-mechanical analyses, highlights the relevance of time-dependent creep deformation, which could therefore be tentatively distinguished from deformation due to consolidation.The potential of an “observational” approach in which early tunnel convergence measurements are used to predict the total final increment in convergence due to time-dependent deformation is also discussed.

Creep Predicting Model in Masonry Structure Utilizing Dynamic Neural Network

Problem statement: When the loads are applied to a brickwork structure, visco-elastic behavior upon their stress-strain relationships is exhibited, where the response can be classified into two separate parts: an instantaneous elastic strains and time-dependent creep strains. The creep strain represents the non- instantaneous strain that happens with time when the stress is sustained. Through the previous century, along with the alter in brickwork construction, A chain of creep tests on brickwork has shown that creep in brickwork be able to result in deformation that rise gradually with the way of time. Brickwork has considerable creep strain that is complicated to predict because of its reliance on several unrestrained parameters (e.g., relative humidity, time of load application, stress level). Dependable and precise prediction models for the long term, time-dependent creep deformation of brickwork structures are required. Artificial Neural Network (ANN) models have been determined useful and efficient especially in such problems for which the characteristics of the processes are difficult to describe using numerical models. Approach: This study introduces a creep prediction model based Focused Time-Delay Neural Network (FTDNN) which could detect and consider within its architecture the time dependency which is major factor in creep deformation in brickwork structure. Results: Performance of the proposed FTDNN model was examined with experimental creep data from brickwork assemblages collected over the last 15 years. Results showed that the FTDNN model has a relatively small prediction error compared to the other models with the error less than 15%. Conclusion: The results showed that the FTDNN model outperformed the existing ANN models and significantly enhance the accuracy of creep prediction.

Numerical modeling of tunnel excavation in weak sandstone using a time-dependent anisotropic degradation model

This paper presents a shear-induced anisotropic degradation model involving time-dependent behavior to simulate the deformational characteristics of weak sandstone. The stress–strain relationship of the proposed model was originated from the degradation of moduli K and G subjected to different loading conditions. An anisotropic factor β is introduced to indicate the tendency of shear-induced volumetric deformation. Furthermore, to incorporate time-dependent deformation behavior of sandstone, this anisotropic degradation model is further extended using a generalized Burger’s model. As a result, the proposed model is characterized by the following features: (1) being capable of describing shear-induced volumetric deformation, either compression or dilation, prior to the failure state; (2) being versatile in the time-dependent (creep) deformations; and (3) the anisotropic factor β serves as a convenient index regarding whether shear-induced volumetric deformation dilates or not. Afterward, the proposed model has been verified by comparing to experimental results. It is found that the proposed model is versatile in simulating short-term and long-term deformations of sandstone under different stress paths. Moreover, this model has been incorporated into finite element program and used to analyze a case of tunnel squeezing. Comparing with other existing models, it is found that the prediction of the proposed model is closer to reality and reveals a larger crown settlement, namely a squeezing condition, owing to larger extent of dilation zones. Overall, although the proposed model is a simple variable moduli model, it is capable of describing the key deformation behavior of weak sandstone reasonably-well, including time-dependent and shear-induced deformations.

Compression Stress–Strain Behavior of Sn-Ag-Cu Solders

New Pb-free alloys that are variations of the Sn-Ag-Cu (SAC) ternary system, having reduced Ag content, are being developed to address the poor shock load survivability of current SAC305, SAC396, and SAC405 compositions. However, the thermal mechanical fatigue properties must be determined for the new alloys in order to develop constitutive models for predicting solder joint fatigue. A long-term study was initiated to investigate the time-independent (stress–strain) and time-dependent (creep) deformation properties of the alloy 98.5Sn-1.0Ag-0.5Cu (wt.% SAC105). The compression stress–strain properties, which are reported herein, were obtained for the solder in as-cast and aged conditions. The test temperatures were −25°C, 25°C, 75°C, 125°C, and 160°C and the strain rates were 4.2 × 10−5 s−1 and 8.3 × 10−4 s−1. The SAC105 performance was compared with that of the 95.5Sn-3.9Ag-0.6Cu (SAC396) solder. Like the SAC396 solder, the SAC105 microstructure exhibited only small microstructural changes after deformation. The stress–strain curves showed work-hardening behavior that diminished with increased temperature to a degree that indicated dynamic recrystallization activity. The aging treatment had a small effect on the stress–strain curves, increasing the degree of work hardening. The yield stresses of SAC105 were significantly less than those of SAC396. The aging treatment caused a small drop in yield stress, as is observed with the SAC396 material. The static modulus values of SAC105 were lower than those of SAC396 and exhibited both temperature and aging treatment dependencies that differed from those of the SAC396 material. These trends clearly show that the stress–strain behavior of Sn-Ag-Cu solders is sensitive to the specific, individual composition.

Neural network modeling of time-dependent creep deformations in masonry structures

Stresses and deformations in concrete and masonry structures can be significantly altered by creep. Thus, neglecting creep could result in un-conservative design of new structures and/or underestimation of the level of its effect on stress redistribution in existing structures. Brickwork has substantial creep strain that is difficult to predict because of its dependence on many uncontrolled variables. Reliable and accurate prediction models for the long-term, time-dependent creep deformation of brickwork structures are needed. Artificial intelligence techniques are suitable for such applications. A model based on radial basis function neural networks (RBFNN) is proposed for predicting creep and is compared to a multi-layer perceptron neural network (MLPNN) model recently developed for the same purpose. Accurate prediction of creep was achieved due to the simple architecture and fast training procedure of RBFNN model especially when compared to MLPNN model. The RBFNN model shows good agreement with experimental creep data from brickwork assemblages collected over the last 15 years.

Time-dependent creep deformation of the coating-based system under in-plane bending moment

The aim of this paper was to investigate the creep deformation of coating-based system under in-plane bending moment. An analytical model was proposed to predict the creep strain and stress within the coating-based system through extending the classic beam theory. Once the material properties and thicknesses of the coating and substrate were identified, the implementation of the model was fairly straightforward. A basic understanding of the time-dependent behavior of the coating-based systems was achieved by using this model, which can be used to guide their design and fabrication. Specific results were calculated for the time-dependent curvatures of the NiCrAlY coating/IN718 substrate systems, and the stresses as well as strains in them. The effects of the exposure time, coating thickness, and creep-law parameter of the coating on the stress and curvature of the coating-based systems were discussed.

Experimental investigation of some parameters on isothermal cyclic fatigue creep behavior of AISI 3137 steel at low homologous regimes

Experimental investigation of some parameters on isothermal cyclic fatigue creep behavior of AISI 3137 steel at low homologous regimes

Thermal cycling analysis of flip-chip solder joint reliability

The reliability concern in flip-chip-on-board (FCOB) technology is the high thermal mismatch deformation between the silicon die and the printed circuit board that results in large solder joint stresses and strains causing fatigue failure. Accelerated thermal cycling (ATC) test is one of the reliability tests performed to evaluate the fatigue strength of the solder interconnects. Finite element analysis (FEA) was employed to simulate thermal cycling loading for solder joint reliability in electronic assemblies. This study investigates different methods of implementing thermal cycling analysis, namely using the "dwell creep" and "full creep" methods based on a phenomenological approach to modeling time independent plastic and time dependent creep deformations. There are significant differences between the "dwell creep" and "full creep" analysis results for the flip chip solder joint strain responses and the predicted fatigue life. Comparison was made with a rate dependent viscoplastic analysis approach. Investigations on thermal cycling analysis of the temperature range, (/spl Delta/T) effects on the predicted fatigue lives of solder joints are reported.

The Influence of Biofilm on the Mechanical Behavior of Sand

Abstract The growth of microbes on the surface of an object results in the formation of a biofilm, which in sand can lead to a reduction of hydraulic conductivity and can be used for groundwater containment and remediation. Sample preparation and testing techniques for triaxial and oedometer experiments were devised for the purpose of examining the influence of biofilm on the stress-strain-time properties of Ottawa sand. Results have shown that biofilm has a negligible influence on the strength and stiffness of the sand but increases time-dependent creep deformation. An elasto-viscoplastic constitutive model was calibrated for biofilm and non-biofilm sand and implemented into a finite-element code for the purpose of predicting creep deformation of a slope and a foundation resting on level ground. Numerical simulations showed that additional creep behavior of the biofilm sand had a negligible influence on the behavior of these geotechnical engineering facilities.

Mechanisms of high-temperature fatigue failure in alloy 800H

The damage mechanisms influencing the axial strain-controlled Low-Cycle Fatigue (LCF) behavior of alloy 800H at 850 C have been evaluated under conditions of equal tension/compression ramp rates (Fast-Fast (F-F): 4 X 10(sup -3)/s and Slow-Slow (S-S): 4 X 10(sup -5)/s) and asymmetrical ramp rates (Fast-Slow (F-S): 4 x 10(sup -3)/s / 4 X 10(sup -5/s and Slow-Fast (S-F): 4 X 10(sup -5) / 4 X 10(sup -3)/s) in tension and compression. The fatigue life, cyclic stress response, and fracture modes were significantly influenced by the waveform shape. The fatigue lives displayed by different loading conditions were in the following order: F-F greater than S-S greater than F-S greater than S-F. The fracture mode was dictated by the ramp rate adopted in the tensile direction. The fast ramp rate in the tensile direction led to the occurrence of transgranular crack initiation and propagation, whereas the slow ramp rate caused intergranular initiation and propagation. The time-dependent processes and their synergistic interactions, which were at the basis of observed changes in cyclic stress response and fatigue life, were identified. Oxidation, creep damage, dynamic strain aging, massive carbide precipitation, time-dependent creep deformation, and deformation ratcheting were among the several factors influencing cyclic life. Irrespective of the loading condition, the largest effect on life was exerted by oxidation processes. Deformation ratcheting had its greatest influence on life under asymmetrical loading conditions. Creep damage accumulated the greatest amount during the slow tensile ramp under S-F conditions.

A method for determining the shor- and long-term behaviour of slender reinforced concrete columns in sway frames

A rigorous model of the short- and long-term behaviour of a slender reinforced column and beam sub-assemblage in a sway frame is developed. The analysis includes cracking at service load levels, creep and shrinkage deformations and the second-order slenderness effects. The analysis shows that second-order moments and dejections may be substantial at service loads. These moments and dejections are accentuated by cracking and the time-dependent creep and shrinkage deformations in the concrete, and are calculated in this Paper without recourse to the contentious effective length factor. This behaviour is illustrated by computer.

Modeling Creep in Thermoplastic Composites

A creep model for unidirectionally reinforced fiber composites was devel oped based on a one-parameter potential function. The same potential function was used to describe plastic flow and time-dependent creep deformation. The material constants in the model were determined by using the uniaxial tension test of off-axis coupon specimens. In terms of effective stress and effective creep strain, the anisotropic creep behavior can be given by a stress-dependent master creep curve irrespective of loading direction relative to fiber orientation. DuPont's AS4/PEKK thermoplastic composites were used in the creep experiment. Results for both unidirectional and laminated composites were presented.

Some mechanical aspects of pingo growth and failure, western Arctic coast, Canada

Many closed-system pingos are underlain by sub-pingo water lenses, and the same is probably true of numerous open-system pingos. In the early growth stage the bending of the frozen overburden of a pingo by a sub-pingo water lens can be compared to the bending of a thin elastic plate. Although the assumptions of elastic plate theory do not apply fully to a growing pingo, because time-dependent plastic and creep deformation are involved, the application of elastic plate theory nevertheless helps to explain the peripheral normal faulting and spring flow of pingos, summit failure, the ease with which elongated pingos appear to collapse, and the changing roles played by the radius and overburden thickness of pingos from early growth to the cessation of growth.

クリープ条件下の疲労き裂伝ぱ下限界挙動とき裂開閉口

The fatigue crack growth behavior of SUS 304 stainless steel at 650°C was investigated under low stress ratios R on the basis of the non-linear fracture mechanics. Emphasis was placed upon the near-threshold fatigue crack growth behavior and crack closure under creep conditions. It was found that in the very low da/dN region and the high da/dN region the relation between the cycle-dependent crack growth rate da/dN and stress intensity factor range ΔK obtained for -1≤R<0.2 deviated from the same correlation obtained under 0.5≤R. The values of fatigue threshold ΔKth for low R were much greater than those obtained for high R. Such a strong R dependence of the crack growth behavior found in the low R region was eliminated by the use of J-integral range ΔJf, eff, in which the effect of crack closure as well as that of material non-linearity were taken into account. The near-threshold crack growth behavior was found to be very sensitive to the loading history including frequency. This was rationalized in terms of the effect of oxide-induced crack closure enhanced by fretting as well as that of time-dependent creep deformation.

Elevated temperature creep crack growth: State-of-the-art review and recommendations

Presented in this paper is a detailed literature review of the emerging technology of creep crack growth. The application of this technology is directed at components which must withstand thermo-mechanical loads in high temperature range. Examples are components of breeder reactor systems and turbine disk alloys for aerospace applications. Component failures due to creep crack growth have been reported to occur in heat-affected zones and in defective welds. However, current design codes (such as ASME) do not permit crack-like defects in creep range. The essential feature of creep is that it is a thermally activated process involving significant time-dependent plastic and creep deformations. Much effort in the past decade has been directed toward experimentally identifying the parameters that would govern crack growth under creep conditions. However, no completely successful single parameter has emerged. The review is divided into a discussion of the basic creep deformation mechanisms, experimental crack growth correlations, analytical predictions, and numerical computations. Greater emphasis is given to describing the developments in the past five years. Finally, a combined numerical and experimental program is recommended for further research in this area. An alternate strategy to select pertinent experimental data from the published literature and to use them to conduct numerical/analytical work is also suggested, which would be much more economical to pursue.