Creep Phenomena Research Articles

An approach of adaptive effective cycles to couple fretting wear and creep in finite-element modeling

Fretting wear and creep can occur simultaneously when two surfaces in contact vibrate against each other at relatively high temperatures. An ability to analyze combined creep and wear is important for applications involving engines, contacting surfaces within fluids of high-temperature heat exchangers, such as fuel rods in nuclear power stations, and contact with polymers close to their glass-transition temperatures. The two phenomena of creep and wear interact with each other through their roles in affecting the shape and stress fields at contacts. Wear is a phenomenon that is generally considered to have no inherent time scale associated with it, beyond the period of the fretting oscillations. Traditional computational models for wear do not require a specific reference to time, but creep models do. Currently, a reliable and efficient numerical algorithm to couple the relatively long time scales of creep to the, generally, short periods of vibrations is not available. In this paper, we present such an algorithm, using an approach of adaptive effective vibration cycles. We demonstrate how to ensure the efficiency and reliability of the modeling by bounding the magnitude of the stress redistribution along the interface during the effective cycle. This approach enables the simulation of problems involving coupled wear and creep. We show that this approach is robust, and that it can be used during full-slip and partial-slip fretting for geometries with smooth or discontinuous interfaces at the edge of the contact. In particular, we use these examples to illustrate some of the crucial roles of creep in wear, such as transitions between partial and full slip.

Monitoring Shear Strain in Shallow Subsurface Using Mini Pipe Strain Meter for Detecting Potential Threat of Slope Failure

Abstract The following study examines a new soil measurement method to achieve simple and reliable slope monitoring to ensure labor safety. The method measures the increase in shear strain in the shallow subsurface of the slope. The target that is measured in the soil using this method is different from that of the conventional methods, which use extensometers and inclinometers. Although the increase in shear strain in the shallow subsurface is considered negligible, this study discusses the potential application of the measurement for detecting the potential threat of slope failure. A mini pipe strain (MPS) meter was developed to measure shear strain in the shallow subsurface of slopes. A full-scale test model of slope failure was constructed via excavation to examine the detectability of the threat of slope failure using an MPS meter. Corresponding to the development of slip surfaces in deep positions, less than 0.5 % of shear strain in the shallow subsurface is clearly observable. Although the inverse of the shear strain rate (1/ν) showed a drastic decrease prior to failure, the slope did not fail immediately; in addition, creep phenomena was observed. An unstable slope may be considered a stable one because of the lag time prior to failure. Identifying the second or third creep could provide a few minutes for workers to escape. Accordingly, the threat of workers being injured by collapsing soil can be reduced by using the proposed method and sensor.

Analysis of Fatigue–Creep Crack Growth in the Superheater Header of a Power Plant Boilers and Estimation of Its Remaining Lifetime

As superheater header is exposed to high pressure and temperature in power plant boilers, it is one of the most sensitive parts in a power plant.Cracks may form where tubes and main steam outlet (nozzle) are connected to reservoir. This article examines a quarter-circle crack at the corner of a nozzle junction and its propagation steps under the influence of simultaneous interaction of creep and fatigue. Header loading in each cycle includes transient steps (increase and decrease in temperature and pressure) at the beginning and at the end of a cycle and intermediate steady state (fixed pressure and temperature) during operation. For crack growth calculations, stress distribution in a track-free part was calculated. Fatigue–creep crack growth was achieved using crack growth rules and the remaining lifetime was obtained. Research result shows that creep phenomenon is responsible for maximum crack growth.

Displacement Investigation of KNN‐Bitumen‐Based Piezoceramics in Asphalt Concrete

Piezoelectric material has excellent characteristics of electromechanical coupling so that it could be widely applied in structural health monitoring field. Nondestructive testing of piezoelectric technique becomes a research focus on piezoelectric field. Asphalt concrete produces cumulative damage under the multiple repeated vehicle load and natural situation, so it is suited material and structure for nondestructive application. In this study, a test system was established by driving power of piezoceramic, laser displacement sensor, computer, and piezo‐embedded asphalt concrete. Displacement, hysteresis, creeps, and dynamic behavior of KNN piezoceramic element embedded in asphalt concrete were tested. The results indicate that displacement output attained 0.4 μm to 0.7 μm when the loads were from 0 N to 150 N. The hysteresis was not obvious when the load was from 0 N to 100 N, aside from higher loads. The creep phenomenon can be divided into two parts: uptrend and balance. The more serious the asphalt binder ageing is, the larger the displacement is, when piezo‐asphalt concrete has already been in serious ageing.

Enhancing the Creep Resistance of Sn-9.0Zn-0.5Al Lead-Free Solder Alloy by Small Additions of Sb Element

The creep phenomenon is considered as one of the most important deformation mechanisms under working conditions. The present study has examined the microstructure and creep properties of Sn-9.0Zn-0.5Al solder alloy after adding a small amount of Antimony (Sb). Nominal compositions of Sb additions were chosen to be 0, 0.5, 1.0, and 1.5 wt.%. The minimum strain rate was reduced for the Sb containing solder alloy. The stress exponents, n, were found to be around 3.7 for all soldiers at 130°C. The stress exponent increases as the temperature drops from 100°C to 50°C, except for the 1.0% Sb alloy, where n 5.3 - 6.1 at all the temperature range (T = 50°C, 100°C and 130°C). The results reveal that the Sb-containing solder alloys have better creep resistance with greater ductility than the Sb-free alloy due to solid solution strengthening, and intermetallic compound SnSb particle hardening.

Creep analysis of the FG cylinders: Time-dependent non-axisymmetric behavior

In this paper history of stresses, strains, radial and circumferential displacements of a functionally graded thick-walled hollow cylinder due to creep phenomenon is investigated. The cylinder is subjected to an arbitrary non-axisymmetric two dimensional thermo-mechanical loading and uniform magnetic field along axial direction. Using equilibrium, strain-displacements and stress-strain relations, the governing differential equations of the problem containing creep strains are derived in terms of radial and circumferential displacements. Since the displacements are varying with time due to creep phenomenon, an analytical solution is not available for these equations. Thus, a semi-analytical procedure based on separation of variables and Fourier series together with a numerical procedure is employed. The numerical results indicate that the non-axisymmetric loading and the material grading index have significant effect on stress redistributions. Moreover, by proper selection of material for any combination of non-axisymmetric loading, one can arrive suitable response for the cylinder to achieve optimal design. With some simplifications, the results are validated with the existing literature.

Metallien virumismurron ja virumisväsymisen mallintaminen

Artikkelissa tarkastellaan metallien korkealämpötilan virumismurron ja virumisväsymisen mallintamista. Aluksi esitetaan lyhyt katsaus virumisen fysikaalisiin perusteisiin ja virumismalleihin. Kaytetyn termodynaamisesti konsistentin materiaalimallin konstitutiiviset yhtälöt esitetaan varsin perusteellisesti. Mallin materiaaliparametrit määritetään 7CrMoVTiB10-10 teräkselle lampötila-alueella 500 - 600 C. Malli on implementoitu ANSYS elementtimenetelmäohjelmiston USERMAT-aliohjelmaksi.

Mechanical properties of the oxide film on ELID electrolytic copper-based grinding wheel by nanoindentation

ABSTRACTThe mechanical properties of the oxide film on the copper based grinding wheel were studied by nanoindentation technique. Analysis of load displacement shows that the creep phenomenon occurs during the loading stage. Results show that the oxide film and the matrix have different characteristics, and the rigidity of the copper based grinding wheel is 0.6–1.3 mN/nm, which is weaker than that of the matrix; the hardness of oxide film is 2000–2300 MPa, harder than the matrix; and the elastic modulus of oxide film is 100–120 GPa, higher than the matrix.

Estudio De Caso: Comportamiento Mecanico Del Hierro Fundido Gris Perlitico Sometido Al Fenómeno De Termofluencia

The application of foundries has been extended in recent times due to the advancement in casting and cooling techniques. This leads to a better control of their mechanical properties by allowing them to replace steel in multiple applications. In this work, the results of an experimental study that seeks to obtain information on the combined effect of tension (1224 Kgf / cm2) and temperature (500, 550, 600 and 650 ° C) in the pearlitic gray cast iron was addressed. The methodology used is analytical, qualitative, quantitative, deductive, and experimental logic. The tests were carried out according to the ASTM E139 standard using flat specimens of reduced section sized according to ISO 6892: 1998 (E). From the specimens tested, specimens were extracted from the fractured area to perform metallographic and microhardness tests. The results showed that for the temperatures of 500, 550 and 600°C, no complete curves characteristic of the creep phenomenon were obtained, while at 650°C, the curve obtained was complete. Regarding the metallographic study, it did not present a considerable degradation at the different study temperatures despite reaching the fracture in all cases. From the study of the hardness, a similar behavior was obtained in the curves for all the specimens as measurements were made moving away from the fracture.

Underlying Physics of Conductive Polymer Composites and Force Sensing Resistors (FSRs). A Study on Creep Response and Dynamic Loading.

Force Sensing Resistors (FSRs) are manufactured by sandwiching a Conductive Polymer Composite (CPC) between metal electrodes. The piezoresistive property of FSRs has been exploited to perform stress and strain measurements, but the rheological property of polymers has undermined the repeatability of measurements causing creep in the electrical resistance of FSRs. With the aim of understanding the creep phenomenon, the drift response of thirty two specimens of FSRs was studied using a statistical approach. Similarly, a theoretical model for the creep response was developed by combining the Burger’s rheological model with the equations for the quantum tunneling conduction through thin insulating films. The proposed model and the experimental observations showed that the sourcing voltage has a strong influence on the creep response; this observation—and the corresponding model—is an important contribution that has not been previously accounted. The phenomenon of sensitivity degradation was also studied. It was found that sensitivity degradation is a voltage-related phenomenon that can be avoided by choosing an appropriate sourcing voltage in the driving circuit. The models and experimental observations from this study are key aspects to enhance the repeatability of measurements and the accuracy of FSRs.

Conjugate thermal creep flow in a thin microchannel

In the present work, we analyze asymptotically and numerically the conjugate heat transfer between a rarified gas flow and the lower wall of a thin horizontal microchannel. The laminar motion of the gas is originated only by the thermal creep or transpiration effect on the lower wall of the microchannel. It is well known the need to impose, in general, a variable temperature regime at the lower wall to induce the transpiration effect. Usually, it can be reached by setting a linear temperature profile as a boundary condition. However, in our case, we prefer to avoid this simplification taking into account that in practical applications, the temperature profile at the lower wall can be unknown. This case can occur, for instance, in a heat sink or a similar device with a well defined heat dissipation rate. Under this physical configuration, we can assume then that the bottom or external face of this heat sink with finite thermal conductivity is exposed to a uniform heat flux. On the other hand, the upper wall of the microchannel is subjected to a prescribed condition. The above conditions are sufficient to consider the simultaneous or conjugate heat transfer analysis of the heat conduction equation for the heat sink and the mass, momentum and energy equations for the gaseous-phase. Resulting governing equations are written in dimensionless form, assuming that the Reynolds number associated with the characteristic velocity of the thermal creep and the aspect ratio of the microchannel, are both very small. The velocity and temperature profiles for the gas phase and the temperature profiles for the solid wall are predicted as functions of the involved dimensionless parameters and the main results confirm that the phenomenon of conjugate thermal creep exists whenever the temperature of the lower wall varies linearly or nonlinearly.

Creep and recovery analysis of polymeric materials during indentation tests

Abstract The present study describes comparative results obtained experimentally on two polymeric surfaces during contacts with a rigid spherical indenter. An experimental setup specifically dedicated to transparent materials has been used to analyze the creep phenomenon over long holding time segments (10 5 s) and to study the recovery process once contact has been removed. As a function of imposed contact time, a normalized value of representative strain is defined to quantify creep and recovery steps. Experimental results indicate that the recovery of the deformed surface after contact is mainly dependent on the initial contact time. Finite Elements Modeling will be used to discuss further the time-depending behaviors of the materials.

Study of the temperature dependence of the uniaxial creep property of similar material of new soft rock

Using the experimental method, the experimental research of creep properties were conducted under different temperature ranging from 10°C to 60°C. The similar material of new soft rock consists of paraffin, which can obtain that the deformation contains the instantaneous elastic deformation and creep deformation through the uniaxial creep experimental results. And thus the increase of temperature has great influence on the creep characteristics of similar soft rock according to the creep curve of similar soft rock at 10°C to 60°C. With the increase of temperature, the slope of the stress-strain curve of similar soft rock is increasing, while the average of the creep modulus is decreasing, which means that the capacity of resist deformation is reduced. Therefore, the creeps law of high-temperature and short-time can be shown the creep phenomenon of low-temperature and long-time, and further shorten the creep experimental cycle.

A Viscoelastic Model for the Long‐Term Deflection of Segmental Prestressed Box Girders

AbstractMost of segmental prestressed concrete box girders exhibit excessive multidecade deflections unforeseeable by past and current design codes. To investigate such a behavior, mainly caused by creep and shrinkage phenomena, an effective finite element (FE) formulation is presented in this article. This formulation is developed by invoking the stationarity of an energetic principle for linear viscoelastic problems and relies on the Bazant creep constitutive law. A case study representative of segmental prestressed concrete box girders susceptible to creep is also analyzed in the article, that is, the Colle Isarco viaduct. Its FE model, based on the aforementioned energetic formulation, was successfully validated through the comparison with monitoring field data. As a result, the proposed 1D FE model can effectively reproduce the past behavior of the viaduct and predict its future behavior with a reasonable run time, which represents a decisive factor for the model implementation in a decision support system.

Peculiarities of stress field formation during cutting isotropic material by mining machine cutters

Peculiarities of the cutting process of isotropic material by mining machine cutters are considered. The objective of the studies is revealing regularities of the process of stress field formation in the pre-cutter zone of the breakable massif and assessment of the possibility of purposeful control of cutting process parameters. Taking into account the multifactorial nature and randomness of the process of elementary chippage formation, an experimental method of studies with the use of full-sized cutters was accepted as a determining principle. A stand for cutting an isotropic transparent material with an optical method of observing stress fields in the under-cutter zone, the procedure of conducting studies and results were presented in the paper. The use of quasi-isotropic acryl glass as an object for destruction allowed reducing the influence of multifactorial nature and randomness on the process of formation of the stress field and elementary chippage. The modes, excluding continuous chip formation, determined by, on the one hand, the phenomenon of material creep – at low cutting speeds, and, on the other hand – cutting speed modes by the terms of thermal conditions, were determined. Continuation of experimental studies of the cutting process of quasi-isotropic materials is aimed at revealing the most significant factors and determination of their influence on the change of phase parameters of elementary chippage and at revealing the very opportunity of formation of elementary chippage characteristics.

Physical-Mechanism Exploration of the Low-Cycle Unified Creep-Fatigue Formulation

Background—Creep-fatigue behavior is identified as the incorporated effects of fatigue and creep. One class of constitutive-based models attempts to evaluate creep and fatigue separately, but the interaction of fatigue and creep is neglected. Other models treat the damage as a single component, but the complex numerical structures that result are inconvenient for engineering application. The models derived through a curve-fitting method avoid these problems. However, the method of curving fitting cannot translate the numerical formulation to underlying physical mechanisms. Need—Therefore, there is a need to develop a new creep-fatigue formulation for metal that accommodates all relevant variables and where the relationships between them are consistent with physical mechanisms of fatigue and creep. Method—In the present work, the main dependencies and relationships for the unified creep-fatigue equation were presented through exploring what the literature says about the mechanisms. Outcomes—This shows that temperature, cyclic time and grain size have significant influences on creep-fatigue behavior, and the relationships between them (such as linear relation, logarithmical relation and power-law relation) are consistent with phenomena of diffusion creep and crack growth. Significantly, the numerical form of “1 − x” is presented to show the consumption of creep effect on fatigue capacity, and the introduction of the reference condition gives the threshold of creep effect. Originality—By this means, the unified creep-fatigue equation is linked to physical phenomena, where the influence of different dependencies on creep fatigue was explored and relationships shown in this equation were investigated in a microstructural level. Particularly, a physical explanation of the grain-size exponent via consideration of crack-growth planes was proposed.

UGS in giant offshore salt caverns to substitute the actual Brazilian NG storage in LNG vessels

This paper evaluates the importance of underground natural gas storage in Brazil and the selection of an area for such, considering logistical and geomechanical aspects. Currently, the power plants are fueled by natural gas. This gas consumption is seasonal, volatile and associated with the level of rainfall in the hydroelectric power plants' basins. Therefore, Brazil has been using floating stocks of Liquefied Natural Gas (LNG) in LNG carriers. This contingency storage is inefficient and costly. This article demonstrates the technical feasibility of replacing LNG floating stocks by underground caverns created by dissolving rock salt domes offshore. Rock salt has a negligible permeability to most fluids and gasses even under high pressure. The rock salt skeleton also develops the phenomenon of creep, through which can absorb high levels of strain and can self-heal cracks and faults over time. This article also presents the design of the caverns by computer simulation. The computer codes were developed by (Costa, 1978) and (Costa, 1984), since than theses codes have been used in several different types of geomechanical engineering projects.

Thin Thermally Efficient ICECool Defense Semiconductor Power Amplifiers

Abstract Traditional power electronics for military and fast computing applications are bulky and heavy. The “mechanical design” of electronic structure and “materials” of construction of the components have limitations in performance under very high temperature conditions. The major concern here is “thermal management.” To be more specific, this refers to removal of high-concentration hotspot heat flux >5 kW/cm2, background heat flux >1 kW/cm2, and “miniaturization” of device within a substrate thickness of <100 μm. We report on the novel applications of contact-based thermoelectric cooling (TEC) to successful implementations of high-conductivity materials - diamond substrate grown on gallium nitride (GaN)/AlGaN transistors to keep the hotspot temperature rise of device below 5 K. The requirement for smarter and faster functionality along with a compact design is considered here. These efforts have focused on the removal of higher levels of heat flux, heat transfer across interface of junction and substrate, advanced packaging and manufacturing concepts, and integration of TEC of GaN devices to nanoscale. The “structural reliability” is a concern and we have reported the same in terms of mean time to failure (cycles) of SAC305 (96.5% tin, 3% silver, 0.5% cu) solder joint by application of Engelmaier's failure model and evaluation of stresses in the structure. The mathematical equation of failure model incorporates the failure phenomena of fatigue and creep in addition to the dwell time, average solder temperature, and plastic strain accumulation. The approach to this problem is a nonlinear finite element analysis technique, which incorporates thermal, mechanical, and thermoelectric boundary conditions.

Contemporary overview of soil creep phenomenon

AbstractSoil creep deformation refers to phenomena which take place in many areas and research in this field of science is rich and constantly developing. The article presents an analysis of the literature on soil creep phenomena. In light of the complexity of the issues involved and the wide variety of perspectives taken, this attempt at systematization seeks to provide a reliable review of current theories and practical approaches concerning creep deformation. The paper deals with subjects such as definition of creep, creep genesis, basic description of soil creep dynamics deformation, estimation of creep capabilities, various fields of creep occurrence, and an introduction to creep modeling. Furthermore, based on this analysis, a new direction for research is proposed.

Effect of creep phenomena on room-temperature tensile properties of cast & forged P91 steel

Abstract In present research work, the as-received cast & forged (C&F) P91 steel was subjected to the creep test at temperature of 620 °C and 650 °C for applied stress of 120 MPa. The room temperature tensile test was conducted after normalizing and tempering (N&T) treatment of the ruptured creep specimen. The standard tensile-test specimen was prepared from the gauge section of creep ruptured specimen. The N&T treatment was performed to restore the microstructure and mechanical properties of virgin P91 steel (N&T P91 steel). The microstructure of creep fractured specimen in ruptured state and N&T condition were characterized by using field-emission scanning electron microscope (FE-SEM) with energy dispersive X-ray spectroscopy (EDS). The fracture surface morphology of crept specimen and the tensile tested specimen was also studied. The effect of prior creep deformation on the mechanical strength was more significant in the sample with longer creep rupture life.