Ultra-high creep resistant SiC ceramics prepared by rapid hot pressing

The high-temperature compression creep of additive-free β/α silicon carbide ceramics fabricated by rapid hot pressing (RHP) was investigated. The creep tests were accomplished in vacuum at temperature range 1500 °C–1750 °C and compressive loads of 200 MPa to 400 MPa. Under investigated condition the RHP ceramics possessed the lowest creep rate reported in the literature. The observed strain rates changed from 2.5 × 10^?9 s^?1 at 1500 °C and a lowest load of 275 MPa to 1.05 × 10^?7 s^?1 at 1750 °C and a highest load of 400 MPa. The average creep activation energy and the stress exponent remain essentially constant along the whole range of investigated parameters and were 315 ± 20 kJ?mol^?1, and 2.22 ± 0.17, respectively. The suggested creep mechanism involves GB sliding accommodated by GB diffusion and β?α SiC phase transformation.     

Investigation of three-stage compressive creep of a spinel forming refractory castable containing 8% MgO

A spinel forming castable with an initial MgO content of 8%, commonly applied in refractory linings in the steel industry, was characterised with regard to the creep behaviour. Three creep stages were observed for 1300, 1400, and 1500 °C for application relevant loads between 3 and 8 MPa for sintered samples. The parameters of the Norton–Bailey creep model were determined for all three creep stages. The added microsilica increased the creep strain rate in the temperature range above 1300 °C. The fitting of the results for only one temperature carries the risk of over-fitting. This was observed for the first creep stage at 1400 and 1500 °C. Nevertheless, for the investigated material, it was not possible to achieve a common fit for a larger temperature range. The evaluation of the creep parameters by different combinations of measurements allowed to obtain statistical information about the creep parameters.     

Tensile response of polyacrylonitrile fibers during air heating

Axial stresses generated by polyacrylonitrile filaments heated in air at constant length and length changes of filaments heated at constant load were measured. Fibers subjected to loads less than about 0.1 gpd shrank in the temperature range from about 40°C to 160°C. At about 160°C they began to stretch. Fibers that stretched out again to about their original lengths stiffened temporarily before undergoing a further elongation. At a temperature where the oxidation reaction begins to proceed with appreciable rate, elongation was retarded and finally reversed. Shrinkage was recorded during isothermal heating at 270°C, and a final length was approached when the oxygen content approached about 10 wt-%. The tension generated when the fibers were restrained from shrinking increased as temperature increased to 160°C but dropped in the temperature range of 160° to 250°C. Tension again built up during isothermal oxidation at 270°C. In the case of one of the samples, the tension generated below 160°C exceeded the ultimate tensile strength of the fibers above 200°C. This condition leads to tensile failures when the filaments are heated in a steep thermal gradient. The tensile behavior of the filaments is discussed in terms of the helical molecular model.     

Low stress creep response of PTFE sealants applied to PEM fuel cells

Viscoelastic properties of polytetrafluoroethylene (PTFE) play a crucial role in forecasting its long-term behavior in engineering applications. An attempt is made to explore the viscoelastic properties of PTFE sealants that are utilized in polymer electrolyte membrane fuel cell (PEMFC). It is to be noted that PTFE sealants are vulnerable to creep under constant loading at elevated temperatures. Moreover, the creep of sealants will lead to leakage of reactants from the cell, which affects the performance of PEMFC. PTFE is an excellent choice as a sealant material in low-temperature polymer electrolyte membrane fuel cell (LT-PEMFC), which operates in the temperature range of 60–80°C. PTFE can be prominently used as sealants in high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC), as it possesses no significant change in its physical properties within the temperature range of −150 to 300°C along with the working conditions of HT-PEMFC. In LT-PEMFC, the sealants will typically be subjected to low stresses in the range of 1–5 MPa. In this article, the creep response of PTFE sealant material is extensively studied at various temperatures of 25 (room temperature), 35, 45, 55, and 65°C and at three stress levels of 2, 3, and 4 MPa. The time–temperature superposition principle is utilized to develop master curve at a reference temperature of 25°C, to forecast long-term creep characteristics of PTFE sealants. Moreover, the master curve for creep compliance is developed for 4.5 h.     

Deformation of nonfired refractories based on phosphate binders. 5. Creep specifics in cement mixtures

The results of experimental studies of aluminosilicate cement are described identifying two temperature intervals of deformation, which differ in their creep regularities. The first range is typical of composites with an unstable low-temperature structure (heat treatment up to 800°C), and the second interval is typical of relatively stable high-temperature structures (> 1000°C). Two ranges of deformation are identified based on stress. In the first range the deformation process is determined by sintering shrinkage under a load which is lower than the surface tension; the second deformation range is typical of loads exceeding the surface tension. __________ Translated from Novye Ogneupory, No. 5, pp. 47–51, May, 2007. Continuation. See beginning in Vol. 48, Nos. 1–2, 2007.     

Some aspects of the mechanical response of BMI 5250‐4 neat resin at 191°C: Experiment and modeling

The inelastic deformation behavior of BMI‐5250‐4 neat resin, a high‐temperature polymer, was investigated at 191°C. The effects of loading rate on monotonic stress–strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. Effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods during both loading and unloading. The strain‐time behavior was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 191°C. The model was employed to predict the response of the material under monotonic loading/unloading and multi‐step load histories. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Creep of aromatic polyamide fibres

Creep of Kevlar 29, Kevlar 49 and PRD 49-III fibres was investigated. The fibres exhibited transient creep and the strain-time relationship was represented by a logarithmic time law. The creep strain recovered with time when the load was removed. Upon reloading to the same creep stress the strain-time relationship was again logarithmic but the creep rate was reduced. Modulus measurements were made during the creep test and these showed that the modulus increased with time. This result indicated a crystallite rotation mechanism which could account for the experimentally observed creep strain. Creep in PRD 49-III fibres exhibited a small temperature dependence over the temperature range 20°C to 150°C. The apparent creep activation energy was consistent with the range of values reported for hydrogen bonding. This suggests one possible creep mechanism in which the combined action of stress and thermal activation causes rearrangement of intercrystalline bonds in the crystallite boundaries resulting in boundary creep. Boundary creep allows crystallite rotation which produces the macroscopic creep strain. Boundary creep is discussed in terms of the fibre morphology and a model of delayed elasticity.     

Compression creep behaviour of precursor-derived SiCN Ceramics

The creep behaviour of SiCN materials derived from polyvinylsilazane (PVS) and polyhydridomethylsilazane (PHMS) precursors was investigated in the temperature range between 1200 and 1550°C at compressive stresses between 30 and 250 MPa in air. Both materials show very similar creep behaviour. Decreasing strain rates with time were observed. Even after 4×10⁶ s creep deformation, stationary creep was not observed. Temperature dependence of the creep behaviour of such materials is very low. Dense passivating oxide layers were found on the surface of creep specimens tested in the temperature range up to 1500°C. At 1550°C active oxidation was observed.     

Durability study of a polymeric composite material for structural applications

An experimental study was carried out to investigate the mechanical behavior of a structural carbon based composite for infrastructure industry. Creep and load relaxation experiments were conducted to investigate the rate sensitivity behavior for a temperature range of 20–60°C. The results show that the mechanical properties were degraded under elevated temperature conditions. A threshold stress was measured, at any given temperature, below which no observable strain rate is detected. Results show that load relaxation experiment can be used as an effective tool to study durability and long‐time creep behavior. The load relaxation test methodology for the prediction of model parameters was found to be more time and cost efficient than traditional long‐time creep tests.     

Statistical description of the uniaxial creep behavior of polypropylene foam

The basis of a statistical method for the analysis of creep data is described. The method consists of response surface fitting to a Taylor series expansion of a function about a point. The method is capable of treating multiaxial stress data and includes other variables, such as temperature, without undue mathematical complications. In addition, the statistical approach can account for such things as experimental error and sample variation. The uniaxial compressive creep-recovery behavior of a newly developed polypropylene foam was measured under loads of 140–705 g./cm.² and temperatures of 23–74°C. The foam has a nominal density of 0.07 g./cc. and a mean molecular weight between crosslinks of 10,000. The creep behavior is described by a Taylor series expansion through the second order of a function of applied load, test temperature, foam density, and log time.     

Tensile Creep Behavior of a Gas-Pressure-Sintered Silicon Nitride Containing Silicon Carbide

The tensile creep behavior of a gas-pressure-sintered silicon nitride containing silicon carbide was characterized at temperatures between 1375° and 1450°C with applied stresses between 50 and 250 MPa. Individual specimens were tested at fixed temperatures and applied loads. Each specimen was pin-loaded within the hot zone of a split-tube furnace through silicon carbide rods connected outside the furnace to a pneumatic cylinder. The gauge length was measured by laser extensometry, using gauge markers attached to the specimen. Secondary creep rates ranged from 0.54 to 270 Gs⁻¹, and the creep tests lasted from 6.7 to 1005 h. Exponential functions of stress and temperature were fitted to represent the secondary creep rate and the creep lifetime. This material was found to be more creep resistant than two other silicon nitride ceramics that had been characterized earlier by the same method of measurement as viable candidates for high-temperature service.     

Viscoelastic behavior of flexible slabstock polyurethane foam as a function of temperature and relative humidity. II. Compressive creep behavior

The compression creep behavior was monitored at constant temperature and/or relative humidity for two slabstock foams with different hard-segment content. The tests were performed by applying a constant load (free falling weight) and then monitoring the strain as a function of time over a 3-h time period. A near linear relationship is obtained for linear strain versus log time after a short induction period for both foams and at most conditions studied (except at temperatures near and above 125°C). The slope of this relationship or the initial creep rate is dependent on the initial strain level, espcially in the range of 10–60% deformation. This dependence is believed to be related to the cellular structs buckling within this range of strain. At deformations greater than 60% and less than 10%, the solid portion of the foam is thought to control the compressive creep behavior in contrast to the cellular texture. Increasing relative humidity does cause a greater amount of creep to occur and is believed to be a result of water acting as a plasticizer. For low humidities increasing the temperature from 30 to 85°C, a decrease in the rate of creep is observed at a 65% initial deformation. At 125°C, an increase in the creep rate is seen and is believed to be related to chemical as well as additional structural changes taking place in the solid portion of the foams. The creep rate is higher for the higher hard-segment foam (34 wt %) than that of the lower (21 wt %) at all of the conditions studied and for the same initial deformation level. This difference is principally attributed to the greater amount of hydrogen bonds available for disruption in the higher hard-segment foam. © 1994 John Wiley & Sons, Inc.     

Creep-rupture behaviour of notched oxide/oxide ceramic matrix composite in combustion environment

ABSTRACT

Creep-rupture tests were performed in the combustion environment on double-edge notch and centre hole oxide/oxide ceramic matrix composite specimens. The specimens were exposed to the maximum temperature of 1250?±?50°C in the notch region where the combustion flame directly impinged. Specimens were loaded to the desired creep load levels and the loads were sustained till either the specimens ruptured or a run-out time of 25?h was achieved. Optical and scanning electron microscopes were used to characterise specimen damage. The test results were compared to its counterparts in 1200°C (isothermal) laboratory air environment. At a given creep life, the applied creep stress for both the notch types was generally lower in the combustion environment than the laboratory air environment. Finite element simulations attributed lower applied creep stress in the combustion environment to the presence of thermal gradient stresses, which were not present in the isothermal laboratory air environment.     

DETERMINATION OF CROSS-GRAIN PROPERTIES OF CLEARWOOD SAMPLES UNDER KILN-DRYING CONDITIONS AT TEMPERATURE UP TO 140° C

ABSTRACT

Small specimens of Pinus radiata have been tested to determine the creep strain that occurs during the kiln drying of boards. The samples have been tested over a range of temperatures from 20°C to 140°C. The samples, measuring 150 × 50 × 5 mm, were conditioned at various relative humidities in a pilot-plant kiln, in which the experiments at constant moisture content (MC) in the range of 5-20% MC were undertaken to eliminate mechano-sorptive strains. To determine the creep strain, the samples were brought to their equilibrium moisture content (EMC), then mechanically loaded under tension in the direction perpendicular to the grain. The strain was measured using small linear position sensors (LPS) which detect any elongation or shrinkage in the sample. The instantaneous compliance was measured within 60 sec of the application of the load (stress). The subsequent creep was monitored by the continued logging of strain data from the LPS units.

The results of these experiments are consistent with previous studies of Wu and Milota (1995) on Douglas-fir ( Pseudotsuga menziesii ). An increase in temperature or moisture content causes a rise in the creep straw while the sample is under tension. Values for the instantaneous compliance range from 1.7 × 10^?3 to 1.28 × 10^?7 MPa^?1 at temperatures between 20°C and 140°C and moisture content in the range of 5-20%. The rates of change of the creep strains are in the Order of magnitude 10^?7to10^?8s^?1 for these temperatures and moisture contents. The experimental data have been fitted to the constitutive equations of Wu and Miloia (1996) for Douglas-fir to give material parameters for the instantaneous and Creep strain components for Pinus radiata.     

Prediction of long-term ABS relaxation behavior

The applicability of time–temperature superposition to tensile stress relaxation of ABS plastics has been verified at strains from 0.5 to 5% for temperatures in the range of 10–50°C. Master curves have been compiled to predict the long-term stress relaxation at 23°C. and a stress–strain–reduced time surface has been constructed. A comparison of relaxation times and activation energies has confirmed that a strain increase facilitates stress relaxation up to yield. The decay of relaxation modulus at linear viscoelastic strains was shown to be equivalent to that of tensile creep modulus. By normalizing the master curves to originate at yield stress and then converting them into multiaxial from the strain which gives the best data fit with long-term hydrostatic pipe-burst strength was shown to be at yield or beyond. The ABS yield-strain master curves at 23°C. were shown to match satisfactorily the long-term pipe-rupture data. Activation energies for ABS relaxation have been compared below and above the rigid matrix T_g, to assess the degree of stiffening of the polymer in the solid state.     

Nonlinear behavior of linear low-density polyethylene

To predict the response of polyethylene thin films subjected to stress for a long time, it is necessary to understand the influence of stress on either the relaxation modulus or creep compliance. Extensive testing has been conducted on 20-micron-thick samples of a particular linear low-density polyethylene film at temperatures from 23°C to −50°C. When reduced to creep compliance and compared with results from dynamic mechanical analysis (DMA), the influence of nonlinearities in the response function is apparent. However, the use of a two-step loading procedure has produced sufficient data to discriminate between the effect of stress on amplitude and time on the creep compliance. It has been found that a master curve of compliance generated by DMA equipment may be used in conjunction with certain nonlinear functions to accurately predict the response of the polyethylene. Perhaps of more importance is the observation that the principles of simple time-temperature superposition, commonly used with linear viscoelastic characterization, are insufficient for use with polyethylene films at most stress levels of interest.     

Influence of multi-walled carbon nanotubes on creep behavior of adhesively bonded joints subjected to elevated temperatures

Polymeric materials are prone to creep loading. This paper is aimed to study the effect of multi-walled carbon nanotubes (MWCNTs) on creep behavior of adhesively bonded joints. Neat and MWCNTs-reinforced adhesively bonded joints were manufactured and tested under creep loading at elevated temperatures. Two MWCNT weight percentages of 0.1 and 0.3 were used for reinforcing the single lap joints (SLJs) and the joints were tested at different temperature and load levels. The results showed that 0.1 wt% of MWCNTs resulted maximum improvements in creep behavior of adhesive joints. Adding 0.1 wt% of MWCNTs into the adhesive layer caused maximum reductions of 57%, 60% and 47% in the steady-state creep rates of the joints tested at 30, 40 and 50°C, respectively. Furthermore, 0.1 wt% of MWCNTs resulted maximum reductions of 29%, 33% and 37% in the creep strains corresponding to a specific creep loading time and maximum reductions of 23%, 45% and 49% in the elastic strains corresponding to the time at which creep loading started.     

Densification mechanisms and microstructural evolution during spark plasma sintering of boron carbide powders

Micron-sized boron carbide (B₄C) powders were subjected to spark plasma sintering (SPS) under temperature ranging from 1700 °C to 2100 °C for a soaking time of 5, 10 and 20 min and their densification kinetics was determined using a creep deformation model. The densification mechanism was interpreted on the basis of the stress exponent n and the apparent activation energy Q_d from Harrenius plots. Results showed that within the temperature range 1700–2000 °C, creep deformation which was controlled by grain-boundary sliding or by interface reaction contributed to the densification mechanism at low effective stress regime (n = 2,Q_d = 459.36 kJ/mol). While at temperature higher than 2000 °C or at high stress regime, the dominant mechanism appears to be the dislocation climb (n = 6.11).     

High temperature compressive creep behavior of BaCe0.65Zr0.2Y0.15O3−δ in air and 4% H2/Ar

The proton conductive material BaCe_0.65Zr_0.2Y_0.15O_3−δ has great potential for the separation and purification of hydrogen. However, due to the demanding application conditions regarding both temperature and atmosphere, the elevated temperature structural stability needs to be characterized and warranted. Hence, in this research work, the elevated temperature compressive creep behavior of BaCe_0.65Zr_0.2Y_0.15O_3−δ in the temperature regime of 850°C to 1200°C was studied in both air and 4% H₂/Ar as a function of the applied stress. The results indicate different creep mechanisms depending on atmosphere and temperature range. While dislocation creep was observed in 4% H₂/Ar over the full range, a dislocation creep mechanism was observed in air at temperatures ≤1050°C and a diffusional creep mechanism at temperature ≥1100°C. A detailed microstructural analysis of the post-creep test specimens revealed that the exposure to oxygen leads to localized stoichiometric changes and a decomposition at the surface.