Changes In Creep Behavior Research Articles

Microstructure Analysis and Creep Behaviour Modelling of Powder Metallurgy Superalloy

Microstructure analysis of Ni-based superalloy FGH96 under different ageing treatments were carried out in order to understand the microstructure-creep strength relationships of the alloy. It was found that the volume fraction of tertiary γ′ and the mean γ-channel width was significantly varied with different ageing treatments, leading to the changes in creep behavior. The dislocation/γ′ shearing mechanisms were also changed with ageing treatment. The volume fractions of both secondary and tertiary γ′ and the mean γ-channel width were quantitatively analyzed by electron microscopy. The quantified microstructures were used for a crystal plasticity-based constitutive model. It was observed that the crystal plasticity model can accurately simulate experimentally observed creep behavior of aged samples showing significant secondary creep stage.

Application of digital image correlation technique to evaluate creep degradation of sealing elastomers due to exposure to fluids

This work aims to evaluate degradation of viscoelasticity in terms of creep of four sealing elastomers (flouroelastomer/Viton A®, silicon rubber/vinyl-methyl silicone, chloroprene rubber and ethylene-propylene-diene monomer) upon exposure to engine lubricant and diesel by using digital image correlation technique in a creep experiment for determination of changes in creep behavior of elastomers. Short creep experiments (3 h) were conducted under a tensile load of 1.5N before and after the immersion in the fluids. Also, standard compatibility tests were simultaneously conducted to determine changes in mass, volume, tensile and tearing strengths, and hardness to extend the degradation study. Hence, the creep compliance behavior of the samples was successfully obtained and the creep degradation due to immersion tests was determined. It was found that this alternative method for evaluation of creep degradation of elastomers is very suitable and useful since very low complexity and strain measurement accuracy.

Identification, size classification and evolution of Laves phase precipitates in high chromium, fully ferritic steels

To fulfil the new challenges of the German “Energiewende” more efficient, sustainable, flexible and cost-effective energy technologies are strongly needed. For a reduction of consumed primary resources higher efficiency steam cycles with increased operating parameters, pressure and temperature, are mandatory. Therefore advanced materials are needed. The present study focuses on a new concept of high chromium, fully ferritic steels. These steels, originally designed for solid oxide fuel cell applications, provide favourable steam oxidation resistance, creep and thermomechanical fatigue behaviour in comparison to conventional ferritic-martensitic steels. The strength of this type of steel is achieved by a combination of solid-solution hardening and precipitation strengthening by intermetallic Laves phase particles. The effect of alloy composition on particle composition was measured by energy dispersive X-ray spectroscopy and partly verified by thermodynamic modelling results. Generally the Laves phase particles demonstrated high thermodynamic stability during long-term annealing up to 40,000h at 600°C. Variations in chemical alloy composition influence Laves phase particle formation and consequently lead to significant changes in creep behaviour. For this reason particle size distribution evolution was analysed in detail and associated with the creep performance of several trial alloys.

Changes in creep behavior and microstructure of model Mozzarella cheese during working

The effect of shear work input on the microstructure, fat particle size and creep behavior of model Mozzarella type cheeses was studied. Cheese samples were prepared in a twin screw cooker at 70 °C by mixing protein and fat phases together with different amounts of shear work input. Major changes in cheese structure were observed while working at 150 rpm and 250 rpm screw speeds. Confocal microstructures plus macroscopic observations showed systematic changes in structure with increased shear work inputs with unmixed buttery liquid observed at <5 kJ kg−1, typical Mozzarella type microstructures (elongated fat-serum channels) at 6–15 kJ kg−1 and homogeneously distributed, small size fat droplets at >58 kJ kg−1. At very high shear work inputs, > 75 kJ kg−1, striations or anisotropy in the microstructures had disappeared and small micro-cracks were evident. A 4-element Burger's model was found adequate for fitting the creep data of model cheese at 70 °C but a 6-element model was required at 20 °C. As shear work input increased retarded compliance decreased and zero shear viscosity increased indicating the more elastic behavior of the cheeses with higher shear work input. Changes in the protein matrix appear to be the main reason for increased elastic behavior.

Intact globe inflation testing of changes in scleral mechanics in myopia and recovery

The purpose of this study was to examine the effects of myopia-inducing and myopia recovery conditions on the scleral biomechanics of enucleated eyes of young chicks. Enucleated eyes from 5-day old chicks, with fiducial markers attached at 5 locations on the external sclera, were placed in a custom-built chamber filled with phosphate-buffered saline, and subjected to controlled increments in intraocular pressure (IOP). IOP was initially ramped from 15 to 100 mmHg and then maintained at 100 mmHg for one hour, with eyes photographed at a rate of 0.1 Hz over the same period. There were two experimental groups, one in which chicks were monocularly form deprived for four days to induce myopia, and the other in which chicks were allowed two days of recovery from myopia induced by two days of form deprivation. For all chicks, the contralateral (fellow) eyes served as controls. Myopic eyes showed less initial deformation relative to their fellows, while no difference was recorded between recovering eyes and their fellows over the same time frame. With exposure to sustained elevated pressure, eyes in all groups displayed time-dependent changes in creep behavior, which included a linear region of secondary, steady creep. The creep deformation of myopic eyes was significantly higher than that of their fellows, consistent with results of previous studies using uniaxial loading of scleral strips. When allowed only 2 days to recover from induced myopia, previously myopic eyes continued to show increased creep deformation. Compared to results reported in studies involving scleral strips, our whole globe testing yielded higher values for creep rate. Whole globe inflation testing provides a viable, less anatomically disruptive and readily adaptable method for investigating scleral biomechanics than uniaxial tensile strip testing. Furthermore, our results suggest that elastic stretching does not contribute to the increased axial elongation underlying myopia in young chick eyes. They also confirm the very limited involvement of the sclera in the early recovery from myopia, reflecting the well documented lag in scleral versus choroidal recovery responses.

Modelling and analysis of creep data for plastics at elevated temperatures

Absract Procedures are considered for determining the long-term creep behaviour of plastics at ambient temperature from experimental data obtained from short-term tests over a range of elevated temperature. Errors in long-term properties derived from the application of time-temperature superposition can arise unless proper account is taken of changes to creep behaviour caused by physical ageing. Short-term changes in age state and, hence, creep behaviour occur when the temperature of a specimen is raised. These changes are caused by the erasing of physical ageing that has occurred at ambient temperature prior to the temperature increase. Subsequent reactivation of physical ageing leads to further changes in creep behaviour with time at the elevated temperature. The results of creep tests at different temperatures have been analysed by theoretical expressions that separate the contributions to creep behaviour from structure and temperature changes.

Low-cycle thermal-mechanical fatigue as an accelerated creep test

An accelerated creep test (ACT) has been developed for ferritic/martensitic creep resistant steels and weld metals, aiming to speed-up the microstructure transformation to the same extent like it occurs during real multi-year creep. Using low-cycle thermal-mechanical fatigue on Gleeble physical simulator, the compositions of precipitated phases, mainly carbides, could reach in less than 30 hours the state near to the thermodynamic equilibrium, as predicted by Thermocalc. Accordingly the equivalent depletion of alloying elements occurred in the steel’s / weld metal’s matrix, resulting in decrease of creep strength prior to failure by elevated temperature fracture. Based on TEM observations discussed are precipitation processes related to changes in creep behaviour as well as dislocations interactions with the precipitates resulting in the acceleration of creep. A particular role in the acceleration of the carbide precipitation and the steel’s matrix transformation was ascribed to meta-stable mobile edge dislocations of a〈001〉 Burgers vector, which were found to effectively transport interstitial elements. The thermalmechanical fatigue procedure of the ACT consists of compression-tension cycles containing steps of generating these edge dislocations as reaction products between intersecting families of a/2〈111〉 screw dislocations saturated with interstitials and dragging the interstitials dissolved in cores of the a〈001〉 edge dislocations to finally dispose the interstitials at grain boundaries or phase boundaries while annihilating these edge dislocations. The developed ACT procedure obeys the following principles: •The basic temperature and applied strains prevent odd transformations like secondary dissolution of carbides or intensive formation of non-equilibrium phases.•The final deformation at fracture is like at real creep — just a few pct in total.•The depletion of weld metal or steel matrix in alloying elements is achieved similar to that of crept steels and the carbide phases at onset of cracks are not different. This ACT procedure was already effectively applied in 2001-04 within 5thFP EU R&D project “SmartWeld” (G1RD-CT-200100490) to development of new generation weld metals for power generation applications, and most recently was implemented in studies on new materials for super-critical power-generation components and on determining of remnant lifetime of power plants, in the course of EU collaboration actions COST-536 and COST-538. Actually the ACT is included into research/technical services provided to interested customers by the manufacturers of the Gleeble series physical simulators, i.e. the DSI company from Poestenkill, NY, USA.

Creep Behavior of High-Strength Concrete with Polypropylene Fibers at Elevated Temperatures

This paper will explain how experimental studies have been conducted to examine the creep behavior of high-strength concrete (HSC) with polypropylene (PP) fibers of 2 kg/m3 (0.125 lb/ft3) when exposed to elevated temperature. Various types of heating and loading schemes are considered, including transient temperature and stepwise loading paths. Tests are carried out at temperatures up to 700°C (1292°F) and a stress ratio up to 0.6. HSC with PP fibers performs better than normalstrength concrete (NSC) at temperatures below 500°C (932°F) and shows an obvious increase in creep behavior at 700°C (1292°F). At the same stress ratio and duration of 120 minutes, the creep strain developed at 700°C (1292°F) was approximately 10 times more than that developed at 500°C (932°F). There was no abrupt change in creep behavior at or approximately 2200°C (428°F) when the PP fibers evaporated. It is likely that the markedly different response at 7000°C (1292°F) was related to the change in the microstructure of HSC at high temperatures. Increasing the stress ratio at constant temperature will lead to an abrupt increase in creep strain rate. HSC with PP fibers showed a large transient creep strain, and this phenomenon becomes obvious at higher temperatures. Finally, fitting equations are proposed to represent the creep behavior of HSC with PP fibers.

Physical aging in polystyrene: Comparison of the changes in creep behavior with the enthalpy relaxation

AbstractPhysical aging in polystyrene was studied by annealing samples isothermally (at 22°, 57°, and 93°C) for various lengths of time and measuring the changes in the enthalpy and the flexural creep property. The enthalpy decrease reached a constant value after about 10 hours of aging at 93°C, indicating an apparent attainment of equilibrium, whereas the creep behavior continued to change with prolonged aging even after 10 hours. At room temperature it took about a year of aging to induce a measurable change in enthalpy, while the creep behavior showed changes clearly only after hours of aging. These and other items of evidence indicate that the effect of aging on the creep behavior cannot be interpreted solely in terms of the free volume concept. Thus the hope of being able to predict the mechanical properties of aged polymers from the measurement of specific volume or enthalpy alone is not likely to be realized.