Term Creep Data Research Articles

Long term basic creep behavior of high performance concrete: data and modelling

This paper presents long term creep data from high performance concretes (HPC) sourced from different research programs or civil engineering projects carried out at IFSTTAR’s Paris laboratory, France. The authors are now making all this data publicly available through a data archive. The experimental procedures are described in detail, in particular that used to verify the constancy over time of the specimens’ compressive stress. It is shown that the majority of the concretes exhibit linear long-term behavior when plotted against log (time). Indeed, the paper describes modelling sourced from the literature, based on a logarithmic function of time which, despite its simplicity, matches the experimental data quite well. Using this modelling, we studied the behavior of HPC prepared from the same set of constituents. We have shown that the logarithmic change in creep is a quasi linear function of the water to cement ratio and aggregate concentration. Finally, the accuracy of the fib MC2010 model code has been tested on IFSTARR’s data bank.

Prediction of stress relaxation from creep data in terms of average creep rate

A new prediction model of stress relaxation from creep data in terms of average creep rate was established. An incremental calculation procedure has been further established to obtain stress relaxation data from creep data. To validate the effectiveness of the proposed conversion models, the predicted results were compared with those obtained by the previous continuum damage mechanics model as well as the stress relaxation experimental data using 1Cr10NiMoW2VNbN steel. Results showed that the creep–stress relaxation conversion model based on the average creep rate is better than that based on the continuum damage mechanics model because the former avoids the dispersion effect of fracture strain data, although the predicted results from the two models are both well in agreement with the experiment data.

Viscoelastic properties of kenaf reinforced unsaturated polyester composites

In order to quantify the effect of temperature on the mechanical and dynamic properties of kenaf fiber unsaturated polyester composites, formulations containing 10 wt.% to 40 wt.% kenaf fiber were produced and tested at two representative temperatures of 30°C and 50°C. Dynamic mechanical analysis was performed, to obtain the strain and creep compliance for kenaf composites at various styrene concentrations. It is possible to obtain creep curves at different temperature levels which can be shifted along the time axis to generate a single curve known as a master curve. This technique is known as the time–temperature superposition principle. Shift factors conformed to a William–Landel–Ferry (WLF) equation. However, more long term creep data was needed in order to further validate the applicability of time-temperature superposition principle (TTSP) to this material. The primary creep strain model was fitted to 60 min creep data. The resulting equation was then extrapolated to 5.5 days; the creep strain model of power-law was successfully used to predict the long-term creep behavior of natural fiber/thermoset composites.

Uncertainty in Damage Assessment and Remaining Life Prediction of Engineering Materials Used In Petrochemical Industry

In this paper creep damage assessment of about 11 years’ service exposed HP-40 grade of steel used in hydrogen reformer of a petrochemical industry has been carried out in terms of a discontinuous Markov process. Experimentally determined conventional creep data under identical testing condition were used in the present investigation. Scatter and damage accumulation due to creep deformation were evaluated through microstructural assessment using light optical microscope and scanning electron microscope. Quantification of creep damage was made from replicated creep data in terms of two damage parameters A and A*. Statistical analysis of void area fraction has been carried out extensively for the both top and bottom portions of the reformer tube at 870 o C in the stress range of 52-68 MPa. In addition, the proposed probabilistic model has been compared with the Kachanav’s Continuum Damage Mechanics (CDM) model. Both the approaches displayed quantitative experimental support. A residual life of A¢ÂŸÂ© 10 years is estimated at 870 o C/ operating stress. For 55 years’ service exposed Catalytic Cold Cracking (CCU) reactor vessel and Feed Processing Unit (FPU) distillation column materials of a petrochemical industry remnant life assessment studies were estimated by incorporating the uncertainty involved in calculation of LMP (Larson Miller Parameter) values and from extrapolation of stress vs. LMP plot. Variability of normalized creep damage for reactor and column materials is well approximated with the aid of Weibull distribution. As expected, it is observed that the distributions shift towards the higher range of damage with increase in service exposure time.

Creep and microstructure in powder metallurgy 15 vol.% SiCp–2009 Al composite

The creep behavior and microstructure of powder metallurgy (PM) 15 vol.% silicon particulate-reinforced 2009 aluminum alloy (SiCp–2009 Al composite) and its matrix PM 2009 Al were investigated over six orders of magnitude of strain rate and at temperatures in the range 618–678 K. The results show that the creep behavior of PM 15% SiCp–2009 Al composite resembles that of PM 2009 Al with regard to (a) the variations in both the apparent stress exponent and the apparent activation energy for creep due to applied stress, (b) the value of the true stress exponent, (c) the value of the true activation energy for creep, (d) the interpretation of creep in terms of a threshold stress, and (e) the temperature dependence of threshold stress. This resemblance implies that deformation in the matrix governs deformation in the composite. Analysis of the creep data in terms of creep rate against an effective stress shows that the creep behaviors of the composite and unreinforced alloy are consistent with the operation of viscous glide creep at low stresses. A comparison between the creep data of the composite and those of the unreinforced matrix revealed that the composite exhibited more creep-resistant characteristics than its matrix over the entire range of applied stresses.

High-Temperature Deformation and Ductility of a Modified 5083 Al Alloy

The high-temperature deformation of a 5.5% Mg and 0.6% Ca modified 5083 aluminum alloy was investigated in the temperature range from 573 to 723 K at strain rates in the range of 10−5-10−1 s−1. Ca was added to form an insoluble second phase in the range of temperatures tested to improve the high-temperature characteristics of this alloy. It was shown that the deformation behavior of the alloy could be divided into two regions with stress exponent, n of 3.5 and 13 at low and high strain rates, respectively. The apparent activation energy determined in both regions suggested that the deformation process is diffusion controlled in both regions. The slightly high value of n at the low-strain rate region (viscous glide) was attributed to the presence of threshold stress. The values of threshold stress showed an exponential increase with decreasing temperature and a dependence with an energy term Qo = 16.5 kJ mol−1. Analysis of creep data in terms of threshold stress and using diffusivity of Mg in normalizing the strain rates, revealed two types of deformation behavior. At high values of normalized strain rate $$ ({\ifmmode\expandafter\dot\else\expandafter\.\fi{\upvarepsilon }kT} \mathord{\left/ {\vphantom {{\ifmmode\expandafter\dot\else\expandafter\.\fi{\varepsilon }kT} {DGb}}} \right. \kern-\nulldelimiterspace} {DGb} > 10^{{ - 9}} ), $$ a high value of stress exponent of n = 10 is observed, and the exponential law creep takes place. At low normalized strain rates ≤10−9, the n value is 3 and the true activation energy, Q, is equal to 123 kJ mol−1 suggesting viscous glide of dislocations as rate-controlling mechanism. Enhanced ductility has been observed in the region of viscous-glide controlled deformation as a result of high strain-rate sensitivity.

Prediction of long term creep data for forged 1Cr–1Mo–0·25V steel

Although a transition from transgranular to intergranular fracture occurs and the tempered bainite microstructure evolves during creep exposure, new relationships allow rationalisation and extrapolation of the multibatch creep data sets reported for 1Cr–1Mo–0·25V steel. Specifically, analyses of results from tests lasting only up to 5000 h accurately predict the minimum creep rates, the times to various strains and the creep lives for stress–temperature conditions causing failure in 100 000 h and more. Validation of this new methodology would therefore offer the prospect of cost effective acquisition of long term creep design data.

New methodology for long term creep data generation for power plant components

AbstractUsing stress rupture results openly available for grade 91 steel, three features of a new methodology for creep data analysis are summarised. First, in contrast to traditional power law and parametric methods, 100 000 h creep strengths of power plant steels are predicted accurately by extended extrapolation of creep lives from tests lasting a maximum of 5000 h. Second, related approaches rationalise creep ductility measurements in a manner which provides information relevant to creep damage evolution in operational plant. Finally, reference is made to a set of new relationships developed to offer a physically meaningful foundation not only for rationalisation and extrapolation but also for interpretation of the creep and creep fracture properties of metals and alloys.

Deformation behavior of a modified 5083 aluminum alloy

The deformation behavior of a 0.2% Zr and 1.6% Mn modified 5083 aluminum alloy was studied in the temperature range of 250–570 °C over seven orders of magnitude of strain rates. It was shown that the modified 5083 alloy exhibits threshold behavior in the temperature interval of 250–500 °C. There exists a temperature dependence of threshold stress with the energy term Q o = 18 kJ/mol. It was shown by transmission election microscopy that detachment of dislocations from dispersoids under extra force takes place. Threshold stress tends to disappear with increasing temperature at T > 500 °C despite the fact that evidence for interactions between mobile dislocations and dispersoids was found at these temperatures. Analysis of creep data in terms of threshold stress revealed three different types of deformation behavior. At high values of normalized strain rate, ε ˙ k T / ( D 1 G b ) > 4 × 10 − 8 , the exponential law creep takes place. At normalized strain rates, ε ˙ k T / ( D 1 G b ) , ranging from 4 × 10 −8 to 4 × 10 −14, the n value is ∼4 and the true activation energy, Q c, is equal to 139 ± 12 kJ/mol, suggesting high temperature dislocation climb. At low normalized strain rate, a transition to Newtonian viscous flow occurs.

Viscoplastic–plastic modelling of IN792

At high temperatures metallic materials behave in a viscous manner exemplified by strain rate dependence, stress relaxation and creep deformation. At low temperatures however, these effects are extremely small, and the behaviour is strain rate independent and shows no or very small relaxation effects. Finally there exists an intermediate region, in which the material behaviour is close to strain rate independent for high strain rates but at the same time shows time dependent inelastic effects, such as stress relaxation and creep. For IN792 this occurs at temperatures around 650 °C. The article describes the extension of a power-law viscoplastic model describing the behaviour of IN792 at 850 °C, also to describe the behaviour at 650 °C, by bounding the elastic–viscoplastic stress-space by a plastic yield surface. The model parameters have been estimated using data from creep test and tailored step relaxation tests, and the model fits well to both the step relaxation data aimed at resembling relevant component conditions and long term creep data.

High temperature creep of precipitation-strengthened Ni 3Al-based alloy

High temperature creep of a precipitation-strengthened Ni 3Al-based alloy with chemical composition Ni-20.2Al-8.2Cr-2.44Fe (at %) was investigated. The creep characteristics, including the stress exponent and activation energy for creep, are reported for the temperature range 1050–1150 K and applied approximately constant stresses ranging from 150 to 350 MPa. The steady-state creep rate is found to depend on the applied stress and temperature. The power law stress exponent for steady state creep is determined to be 4.1±0.1 and the apparent activation energy for creep is calculated to be 316±9 kJ mol −1. Analysis of the creep data in terms of an effective stress normalised with respect to the temperature dependence of the elastic modulus gives a true activation energy for creep of 301±9 kJ mol −1 and a stress exponent of 3. This activation energy for creep corresponds to the activation energy for lattice diffusion of Ni in Ni 3Al. The kinetics of the steady-state creep deformation within the studied temperature range is proposed to be governed by the diffusion controlled climb of dislocation loops along the α-Cr precipitates. During the tertiary creep stage an intensive formation of large slip bands is observed. The alloy fails by growth and coalescence of voids within the columnar grains and cavities at the grain boundaries.

Further analysis of the Monkman–Grant relationship for 2·25Cr–1Mo steel using creep data from the National Research Institute for Metals

The National Research Institute for Metals in Japan regularly publishes long term creep data which is not as readily available in the UK. Parametric extrapolation techniques have proved unsuccessful on such data sets and so interest has recently turned to the Monkman–Grant relationship. A number of shortcomings in past estimates of this relationship make them unsuitable for extrapolation. These include failure to make use of censored data and to identify the distribution for rupture lives, and neglect of the known dependency of the Monkman–Grant relationship on test conditions. This paper addresses these issues to derive an empirical relationship capable of extrapolation. Incorporating unfailed specimens into the analysis substantially altered the predictions obtained from the Monkman–Grant relationship. Uncertainty over the shape of the failure time distribution also increased the confidence limits associated with these predictions. The intercept of the Monkman–Grant relationship was shown to vary with absolute temperature and this was modelled to allow the relationship to be extrapolated to operating conditions for boiler tubes. A weak linear dependency of variability in rupture life on the minimum creep rate was also identified.

Modelling of time dependence in ultra-high-modulus polyethylene based on Raman microscopy

Previous research dealt with the modelling of time dependence in ultra-high-modulus polyethylene in terms of mechanical systems. Although such attempts were highly successful in describing time-related phenomena in melt-spun polyethylene, they were less able to account for the behaviour of gel-spun material. Moreover, in the early experiments, only the macroscopic deformation could be determined. Recent advances in Raman microscopy have enabled a study of the changes in internal strain associated with crrep and recovery. This short paper considers some preliminary creep data in terms of mechanical models which, despite their simplicity, are nevertheless able to provide a phenomenological basis for a number of the observed features: most notably the attainment of a strain maximum in one of the differently stressed structural components.

Indentation Creep Studies of Cross-linked Glassy Polymer Films

ABSTRACTIndentation creep testing was done on cross-linked glassy polymers based on polystyrene, specifically poly(styrene-co-divinylbenzene) (PS-DVB) and poly(divinylbenzene) (PDVB). The continuous depth-sensing capabilities of the Nano-Indenter II were used to measure the time-dependent response to indentation at constant applied load. The raw creep data in terms of indentation depth vs time showed that PDVB was about 20% more resistant to penetration than PS-DVB. A data analysis program was developed that converted the raw data to stress σ and plastic strain rate ε and generated the plastic flow curve that was observed to follow the power law . The stress exponent a for PS-DVB increased with applied loads between 1 and 27 mN and was generally larger in magnitude than the exponent for PDVB. When compared at equivalent stress and indentation depth, the plastic strain rate for PDVB was observed to be about 100 times slower than for PS-DVB. This was attributed to the higher degree of cross-linking increasing the characteristic relaxation time governing creep flow in these polymers.

The influence of test frequency, temperature, and environment on the fatigue resistance of an Ni3Al–B/Cr/Zr intermetallic alloy

Stress-controlled fatigue tests have been carried out on an Ni3Al–B/Cr/Zr alloy, at 600 °C and 800 °C in air and in vacuum at various test frequencies. Decreasing the test frequency and/or increasing the temperature leads to a decrease in the number of cycles to failure and a gradual disappearance of a fatigue fracture zone. This trend is shown to be related to a true creep component. Environment has a weak interacting effect on fatigue life but strongly influences the fracture path in the fatigue zones, with fracture becoming partly or entirely intergranular when the environment is changed from vacuum to air. It is suggested that most of the fatigue life is spent in initiating a crack. Comparisons are made with some creep data in terms of fracture paths and time to rupture. Fatigue life at 800 °C is shown to be entirely controlled by creep damage at the lowest test frequency of 0.2 Hz.

A METHOD OF CREEP DAMAGE SUMMATION BASED ON ACCUMULATED STRAIN FOR THE ASSESSMENT OF CREEP‐FATIGUE ENDURANCE

Abstract— A method of combining long term creep data with relatively short term mechanical behaviour to provide an estimate of creep‐fatigue endurance is presented. It is proposed that the creep‐fatigue effect in high temperature cyclic deformation is governed by a difference in strain rate around the cycle and the associated variation in ductility with strain rate.