Forward Creep Research Articles

MODELING AND INVESTIGATION OF THE INFLUENCE OF LOADINGMODE ON THE DEFORMATION PROCESS OF ASPHALT CONCRETEMATERIALS

The article considers a problem of calculating the deformations of forward and reversed creep tension of rheonomic materials. The hereditary theory of creep by Yu.N. Rabotnov is used to describe the nonlinear deformation process. Proposed a method for determining the necessary material characteristics from forward and reversed creep data. During the experiments, there were tested two batches of asphalt concrete samples. Also, was shown behavior of asphalt concrete at cyclic increasing and constant loading modes. At a temperature T = 24 °C were tested 10 samples at cyclic increasing loading mode. Cyclic stresses were equal to 0.041; 0.074; 0.111; 0.148; 0.183 MPa and duration period between loading and relax period were chosen to be 570 seconds. At cyclic constant loading were investigated individual samples for the parameters of forward and reversed creep at stresses of 0.041; 0.117. Then, investigated the affect of reloading two reversed creep process of asphalt concrete samples. From first batch of samples were tested 9 samples before destruction with a period 65 seconds: forward creep of samples at for the following 5 seconds; reversed creep at for the following 60 seconds. From the second batch of samples were tested 11 samples of asphalt concrete before destruction with period 70 seconds: forward creep for the following 10 seconds and reversed creep at for the following 60 seconds. Test results showed that the level of return of second batch increased than the first batch of samples. In the work were tested 5 samples of asphalt concrete according to the direct tensile scheme until a complete failure. Test temperature was T=22-24 °C. For each level of loading with constant rate: 0,6519; 0,4678; 0,0580; 0,0489; 0,0055 MPas-1.Using experimental data, were found parameters of the deformation at different constant rate of loading. As a result, all samples fractured brittle at small deformations.

Creep performance of carburized 316H stainless steel at 550 °C

The creep behaviour of two ex-service type 316H austenitic stainless steel casts with different Mn contents and austenite grain size, that were pre-carburized for either 8000 h at 550 °C or 3000 h at 600 °C in simulated CO2-rich gas environment of UK Advanced Gas-cooled Reactors, was evaluated by uniaxial creep testing at 550 °C in air. The low Mn (0.98 wt.%) steel grade with an average austenite grain size of 142 ± 110 µm presents a higher creep deformation rate and creep ductility compared to the high Mn (1.52 wt.%) steel cast with an austenite grain size of 81 ± 67 µm. Below an applied stress of 318 MPa, the presence of the sub-surface carburized layer reduces the minimum creep rate, likely due to the significantly modified material properties of the carburized layer and the presence of local compressive stresses. The stress exponent determined for the high Mn steel carburized at 600 °C (n = 9.6), and its reference non-carburized microstructure (n= 9.8), identifies the dominant creep mechanism to be dislocation climb & glide. However, crack evolution through M23C6-decorated grain boundaries within the carburized layer was observed in all the tested material's conditions at creep strains of 0.50%. The average crack length increases with creep strain and attains values close to the carburized layer thickness at creep strains of ε ≥ 1.00%. The prevalence and size of cracking increases with the local hardness carburized layer depth. At applied stresses σ ≥ 318 MPa, cracking on-loading prior to forward creep already occurs in the carburized layer, leading to a sharp increase in minimum creep rates of carburized specimens. These results demonstrate the impact a relatively thin layer of modified material can have on the materials creep performance; emphasising the requirement to consider the effects of the service environment in structural assessment procedures, particularly for high temperature applications including Generation IV and advanced modular reactor designs.

To travel hopefully… in search of the instant creep test

ABSTRACT Proverbially ‘To travel hopefully is better than to arrive.’ Many roads have been travelled in search of a short-term mechanical test that would reliably predict long-term component creep life. The motive is clear; whatever progress is made in inspection and monitoring, direct measurement of the relevant properties of the subject material – archival or ex-service – will always provide the most reliable indicator of its state and performance capability. The difficulties are equally clear: mechanism changes over the stress, temperature and time ranges involved; issues of specimen size and geometry – and their relation to stress state; adequate emulation of in-service variation in operating condition; and, for post-exposure tests, translating information between the sample position and the critical location. This paper focuses on: issues of mechanism change, both in simple extrapolation and across boundaries of stress state and test type identification of parameters that allow true comparison between forward creep, stress relaxation and constant strain rate testing Data obtained on low-alloy ferritic steels are examined and analysed, largely using the framework of continuum damage mechanics. Particular attention is given to the nature of the equilibria represented by tensile instability and the minimum creep rate. It is concluded that the ‘instant test’ is an unreachable ideal. However, the knowledge and understanding gained through the search have benefitted the larger objective and – as in every legendary quest – have made the journey worthwhile.

Construction of a force method for estimating the longitudinal stability of the process of thin sheet rolling

We have analyzed theoretical features of rolls gripping capacity at simple rolling process under a steady mode. It is shown that depending on the conditions for deformation the ratio of the maximum gripping angle to a friction coefficient can be equal to, be less or more than 2. An experimental study has been performed involving the rolling of lead stepped samples measuring the forward creep at each step. Results of the experiment demonstrate that in the extreme case of deformation the forward creep is greater than zero, that is, there is a sufficient reserve of frictional forces so that reduction can be further increased, however, it is impossible. We have analyzed the balance of all horizontal forces at deformation site. It is shown that at each cross section the stretching horizontal contact forces are used not only to overcome the ejecting ones, but also to compensate for the longitudinal internal forces that occur as a result of plastic deformation of a metal. A force method for estimating the longitudinal stability of the sheet rolling process has been developed. An indicator of stability is a criterion that is determined from the diagrams of distribution of a normal contact stress and the stress of friction. It is shown that at a positive value of the criterion the rolling process proceeds under a steady mode; at a negative value, the stable process is impossible; and in the case it equals zero, the limit deformation occurs. A theoretical study has been conducted into determining the maximum gripping angle under different conditions of sheet rolling. It is shown that the ratio of the maximum gripping angle to a friction coefficient almost does not depend on a strip thickness, the diameter of rolls, as well as friction coefficient, and equals 1.43‒1.44. A decrease in the gripping capacity of rolls is explained by the effect, at a deformation site, not only of contact forces, but the internal forces as well.

Creep behaviour of AISI 316H stainless steel under stress-varying creep loading conditions: primary creep regeneration

ABSTRACTPrimary creep regeneration (PCR) is an important reported observation from creep under stress-varying conditions for several alloys. For a specimen deforming in the secondary creep regime, a stress reversal leads to an enhanced creep rate upon reloading due to reactivation of the primary creep regime (i.e. PCR). This paper focuses on an investigation of the PCR phenomenon during stress-varying creep loading for AISI 316H stainless steel at 650°C. The experimental observations clarify the influence of different parameters (e.g. forward creep stress level, reverse stress magnitude and forward and reverse accumulated inelastic strain) on the extent of PCR activation. In addition, a correlation between the extent of PCR activation and inelastic strain accumulation during the reverse loading period was found, which was employed to develop an empirical–phenomenological model for prediction of the creep behaviour of the alloy after stress transients (e.g. stress reversals).

The effect of creep strain rate on damage accumulation in Type 316H austenitic stainless steel

Many service components in power generation and aerospace industries operate at high temperatures and stresses that make them susceptible to creep deformation and damage. Their complex geometries and load multi-axiality are often treated only approximately in assessing their structural integrity via assessment codes that are based on standard creep tests. For example, the forward creep (defined here as constant load creep) test of round bars is not a true representation of the stress state that service components generally experience. The experiments conducted in this work used notched bar specimens to simulate the effect of stress triaxiality. The results from these experiments were then used to validate a well-established creep ductility exhaustion damage model. Although the damage model is largely based on uniaxial creep rupture tests, it has been previously adapted so that it can be applied to more complex states of stress. Rupture calculations were conducted prior to experimental testing to obtain an estimation of the duration of the experiments. The finite element simulation results, which utilised previously developed creep deformation and damage models, were then compared to the experimental data. It was shown that the model predicted the correct trend for the creep deformation and failure of the specimens and primary, secondary and tertiary creep behaviour of notched bars could be captured. The tests imply that the effective creep ductility was smaller at lower stresses, i.e., at slower strain rates creep strain was more damaging.

Influence of prior cyclic plasticity on creep deformation using crystal plasticity modelling

This paper proposes a simple, yet effective, modified crystal plasticity framework which is capable of modelling plasticity and creep deformation. In particular, the proposed model is sufficiently versatile to capture the effects of complex load histories on polycrystals, representative of those experienced by real materials in industrial plant. Specifically, the methodology was motivated by the need in the power generation industry to determine whether cyclic pre-straining influences the subsequent creep behaviour of type 316H austenitic stainless steel as compared to non-cyclically pre-strained material. Cyclic pre-straining occurs widely in plant and it is of paramount importance to accurately account for its impact on the subsequent deformation and integrity of relevant components.The framework proposed in this paper considers the effects of dislocation glide and climb in a relatively simple manner. It is calibrated using experimental tests on 316H stainless steel subjected solely to monotonic plasticity and forward creep. Predictions are then obtained for the creep response of the same material after it had been subjected to cycles of pre-strain. The predictions are compared to experimental results and good agreement was observed. The results show slower creep strain accumulation following prior cyclic loading attributed to hardening structures developed in the material during the cyclic pre-strain. The model also highlights the importance of accounting for directionality of hardening under reverse loading. This is hypothesized to affect the development of an internal stress state at an intragranular level which is likely to affect subsequent creep accumulation.

Effect of prior deformation on the subsequent creep and anelastic recovery behaviour of an advanced martensitic steel

The creep and anelastic recovery characteristics of a 10%Cr steel have been systematically investigated at 600°C after subjecting the test material to various prior deformation histories. Constant-load forward creep tests on specimens, either with a tensile or compressive preloading history, indicated that over- and reverse-preloading respectively decreases and increases the early primary creep rate of the steel. The extent of decrease (or increase) in early primary creep rate is also found to be directly proportional to the magnitude of stress during prior loading while such a correlation is not clearly evident for material deformation in the secondary and tertiary stages. Specifically, the creep rate in the secondary and tertiary stages is lower for specimens with a compressive prior loading while the rupture time is notably shorter for tensile pre-loaded specimens. The observed effect of prior loading on the early primary creep behaviour can be explained by considering micro-backstress development (as a consequence of dislocation pile-up formation during the prior loading phase) that subsequently introduces a kinematic hardening effect to the material's viscoplastic response. The second set of experiments involve monitoring the anelastic recovery behaviour immediately after accumulation of a similar amount of time-dependent strain either under forward creep (load control mode) or stress relaxation (strain control mode) condition in completely unloaded 10%Cr steel specimens at 600°C. Experimental observations indicate that the higher the stress magnitude during the prior loading phase, the greater and faster the anelastic recovery at zero stress. Further findings show the mode of prior deformation (creep or relaxation) to also not noticeably influence the subsequent anelastic recovery behaviour. The observed anelastic recovery characteristic can be mechanistically interpreted by consideration of the time-dependent material back-flow due to the relaxation of dislocation bows and pile-ups generated during the prior deformation.

Effect of elastic follow-up and ageing on the creep of an austenitic stainless steel

Elastic follow-up is a mechanical boundary condition lying between constant load and constant strain control. It exists in many engineering components operating at high temperature and can result in dramatically different creep stress relaxation and strain accumulation rates in a localized region of a component. We have performed creep tests under constant load, constant strain and elastic follow-up control on an aged (additional 22,000 h) 316H austenitic stainless steel after service in a nuclear power station for 65,000 h. Primary and secondary forward creep models with parameters derived from the constant-load data were able to describe constant-load creep adequately, but not able to predict stress relaxation and elastic follow-up. We show that this is because ageing has increased the constant-load creep strain rate significantly but has no effect on stress relaxation creep strain rate. Ageing promotes the formation of ferrite/chi phase at grain boundaries which are preferential sites for creep cavitation under load control. However, creep cavitation is less likely under constant strain and elastic follow-up control because a high creep strain rate, large creep strain and stress cannot coexist under these boundary conditions.

Predicted life of P91 steel for cyclic high temperature service

The P91 steel is widely used in high temperature components of power plants, and it is a candidate material for Gen-IV reactors. The P91 steel has relatively attractive mechanical and physical properties combined with resistance to stress corrosion cracking in water–steam environments. This study aimed to explore the combined cyclic, creep and relaxation behaviour of P91 material. Uniaxial specimens were subjected to cyclic loadings with periodic forward creep or relaxation at peak stress. The results indicate that prior creep or intermediate relaxation periods up to 72 h will influence the subsequent cyclic softening of P91, but do not significantly reduce the cyclic life. In contrast, prior cycling has a detrimental effect on the subsequent creep life. A simplified creep-fatigue model is shown to predict life better than usual code-based approaches. Improved verification of all models would benefit from the availability of more extensive long-term data on P91 steel.

Creep deformation and stress relaxation of a martensitic P92 steel at 650 °C

This paper develops methods to predict creep stress relaxation in the presence of combined boundary conditions and explores the influence of primary-secondary stress dependent creep properties on predictions for a martensitic P92 steel at temperature of 650°C. A series of forward creep and elastic follow-up experiments have been conducted. A summary is provided of empirical creep equations for forward creep and creep stress relaxation (elastic follow-up) tests, including the link to the experimental procedure. The creep stress relaxation tests have been performed with two rigs to give elastic follow-up factors of 1.17 and 1.7. Both time hardening and strain hardening approaches have been considered where the strain hardening model provided more accurate predictions compared to time hardening; except at relatively low stress levels. The difference between stress relaxations predicted using strain hardening and time hardening approaches are considerable. The model predicts the creep stress relaxation accurately in the early stage of relaxation, indicating that the majority of stress relaxation occurs where primary creep needs to be taken into account. This study highlights the importance of stress dependent creep model to predict stress relaxation, especially with high level of initial residual stresses.

One-zone rolling of composite materials

The energy–force parameters of free rolling of a strip without its tension and rolling with one backward or forward creep zone in the deformation zone are compared. The limiting backward or forward tensions are determined, and the change in the linear sizes of a composite billet during deformation in a rolling mill is considered.

Critical analysis of the prediction of stress relaxation from forward creep of Type 316H austenitic stainless steel

This work evaluates the effectiveness of using forward creep deformation laws to predict the observed relaxation behaviour of 316H austenitic stainless steel. A summary of empirical creep methods, forward and stress relaxation experimental data were given. A primary and secondary creep strain rate (RCC-MR) model and an average creep strain rate model are combined with derived material constants to predict a group of stress relaxation tests at temperatures from 475°C to 600°C. The RCC-MR primary creep model predicted the stress relaxation behaviour soundly and the average creep strain rate model underestimated all of the stress relaxation data. The difference between stress relaxations predicted using strain hardening and time hardening versions of the RCC-MR equations are small. Factors affecting the predictions; including limitations in data range, data scatter, selection of a forward creep model, application of a forward model to stress relaxation and difference between forward creep and stress relaxation and are discussed.

Creep behavior of nanocrystalline Au films as a function of temperature

Freestanding nanocrystalline Au films, subjected to nominally elastic loads at 25–110 °C, demonstrated high primary (10−7–10−4 s−1) and steady-state creep rates (10−8–10−5 s−1). The deformation mechanisms for creep were strongly temperature dependent: grain boundary sliding-based creep dominated at room temperature and 50 °C, while the contribution of dislocation-mediated creep increased at 80 and 110 °C. The effect of applied stress on primary and steady-state creep strain at different temperatures was captured well by a non-linear model that was based on the kinetics of thermal activation. Multi-cycle creep experiments showed that at room temperature virtually all the primary strain accumulated during each forward creep cycle was recovered upon complete unloading. As the contribution of dislocation-mediated creep increased with temperature, the ratio of strain recovery to primary strain accumulated during each cycle was reduced due to the accumulation of plastic strain at higher temperatures. Notably, at all temperatures, the steady-state creep rate decreased after the first creep cycle. Moreover, the entire creep response remained virtually unchanged in all subsequent cycles, which implies that the first creep cycle resulted in mechanical annealing. This conclusion was further supported by calculations of the activation entropy: A reduction in its magnitude between the first and all subsequent creep cycles at all temperatures pointed out to mechanical annealing of initial material defects during the first loading cycle. The negative values of the calculated activation entropy indicated that entropy changes due to annihilation of defects-dominated entropy changes associated with the generation of new defects. Finally, the activation entropy for steady-state creep was temperature insensitive, but increased with stress, which is consistent with an increase in defect generation at higher stresses.

Prediction of Forward Creep Behaviour from Stress Relaxation Data for a 10 % Cr Steel at 600 °C

To systematically characterise the effect of plasticity on the creep resistance of a 10 % Cr steel, strain-controlled monotonic and cyclic/hold tests have been isothermally conducted at 600 °C. In particular, this work focuses on the influence of the magnitude of plastic strain transient(s) preceding the first peak tensile hold for two types of loading sequence. Along with the use of Norton–Bailey formulations assuming time- or strain-hardening, the effectiveness of a method developed by Woodford is explored for converting stress relaxation profiles to equivalent forward creep curves. For quantitative comparison and model validation, conventional creep tests have also been conducted on the same 10 % Cr steel at 600 °C for two different stress levels. Experimental results show that the creep deformation resistance of this steel is significantly reduced if the loading transient immediately preceding the hold is plastic. Furthermore, reduction in creep resistance is found to be more pronounced with increasing plastic strain amplitudes. Forward creep curve construction results indicate that all three assessment methodologies tend to offer qualitatively similar trends although the quantitative predictions differ slightly due to their respective underlying formulations. The creep-rate versus time plots for the constructed creep curves (until the attainment of steady-state conditions) also lead to a similar conclusion when compared with those from conventional creep tests. Finally, the successful implementation of the Woodford method to stress relaxation data from cyclic/hold tests broadens its current scope of applicability and its versatility can thus be further improved.

The energy density exhaustion method for assessing the creep-fatigue lives of specimens and components

This paper attempts to link the energies to failure involved in low-cycle fatigue at elevated temperature with those involved in forward creep. These energy levels are connected by a suitable sliding strain-rate scale. The principle is then applied to (i) dwell tests on 1CrMoV, 304 and 20Cr25Ni/Nb steels at 550, 650 and 750°C respectively and (ii) service failure of a thick-walled ferritic steam vessel at 570°C. Calculated creep-fatigue endurance using the linear damage summation rule is then plotted on Damage Diagrams. Sensitivity analyses are carried out as regards the linking of energy levels in fatigue (upper-shelf) with those in creep (lower-shelf). These involve knowledge of creep laws or stress relaxation relations and the methods of connecting energy levels (power law or those giving an S-shaped curve). Within this paper, sufficient information has been provided for investigators to undertake their own analyses on relevant laboratory tests or service assessments.

Forward creep grazing of lambs to increase liveweight gain and post‐weaning resistance to endoparasites

An experiment was conducted in New Zealand to assess the effect of creep grazing of lambs on their liveweight and resistance to endoparasites. Comparisons included creep versus non‐creep grazing at two herbage allowances and regular an‐thelmintic drenching. Treatments commenced with 6‐week‐old twin reared lambs, and ceased when they were weaned at 13 weeks, with the exception of the drench treatment which continued post‐weaning. Pastures used contained sheep endoparasites. At 8 weeks post‐weaning one lamb from each twin set was slaughtered, remaining lambs were then drenched, and 8 and 9 days later were orally dosed with 180 Trichostrongylus colubriformis L3/kg liveweight each day (averaging 12 650/lamb). Three weeks later these lambs were also slaughtered. Measurements included liveweight, faecal worm egg count (FEC) and adult endoparasite numbers. Neither herbage allowance nor creep grazing affected FEC or adult endoparasite numbers. Fortnightly drenching of creep grazed lambs on the high allowance had no effect on weaning liveweight, but it improved live‐weights post‐weaning. Neither creep grazing, pre‐weaning herbage allowance, nor drenching, affected lamb resistance to endoparasites. Creep grazing and the higher allowance both increased weaning liveweights. The response to creep grazing was highest at the lower allowance, increasing weaning liveweights by 4 kg. Pre‐weaning treatment had no effect on post‐weaning growth rate.

Forward and backward creep during screw rolling

The common features of lengthwise rolling, cross rolling, and screw rolling (which is the most general case) are found. A mathematical model is used to obtain the values of velocity coefficients at any point in the deformation zone, to reveal the position of the neutral line and the conditions affecting the relation between forward and backward creep zones, and to find the cases where a forward creep zone is absent.

Anisotropic strain hardening following load reversal during high-temperature creep testing of superalloy single crystals

Abstract A critical examination and qualitative evaluation of anisotropic creep behavior in nickel-based superalloy CMSX-4 single crystals is provided with respect to previously established models describing pure shear creep of specimens deformed at high temperatures. Using double-shear creep deformation samples, a complete reversal of applied creep stress results in a large increase in creep rate in the opposite direction. During forward creep, dislocation networks build up that are trapped in γ channels along γ/γ′ interfaces. The sense reversal in applied stress effectively releases the dislocation networks, resulting in a large jump in measured creep rates.

The value of supplementary feeding to pre-weaned and weaned lambs grazing Italian ryegrass

The variation in, and often the disappointingly poor weight gains and the inability of lambs to achieve desirable carcass grades on ryegrass pastures have long been a concern to researchers, extension officers, advisors and farmers. The objectives of this study were to investigate whether concentrate supplementation to lambs on Italian ryegrass (Lolium multiflorum cv Midmar) pastures would improve the performance of lambs, and what the optimum level of supplementation is to lambs on the pasture. The investigation extended over the pre- and post-weaning growth phases of lambs and was conducted during two consecutive seasons at the Cedara Research Station in the KwaZulu-Natal Mistbelt. The pre-weaning stocking rate applied, was 20 South African Mutton Merino ewes with lambs/ha and the following treatments were applied: Control: continuous grazing - no creep feed; forward creep grazing by the lambs in a rotational grazing system where the lambs were allowed to graze paddocks allocated to the ewes, plus the next paddock in their series of grazing paddocks; 100 g creep feed/lamb/day; 250 g creep feed/lamb/day and creep feed ad libitum. The post-weaning stocking rate applied, was 50 lambs/ha and treatments were: Control - no supplement; 250 g of supplement/lamb/day (only the second season); 500 g of supplement/lamb/day and supplement ad libitum. From 42 days of age to weaning, supplementation significantly improved the live weight gain of suckling lambs. Creep feed intake varied between 300 and 350 g/lamb/day. The average daily gain of the weaned lambs improved significantly with supplementation. No significant advantage in terms of growth was gained by supplementing the weaned lambs at levels of higher than 500 g/day. South African Journal of Animal Science Vol.32(1) 2002: 30-37