Constant Load Creep Tests Research Articles

Effect of Load Cycling on High Temperature Creep of 316L Stainless Steel

For components in concentrating solar thermal plants, the creep mechanism will cause a significant portion of material damage to receivers, storage tanks, turbines and pipework. These components will undergo conditions where loads, temperature, or both load and temperature are not constatnly applied. This creep damage process will not resemble the constant load creep tests used to characterise creep of materials. Therefore, creep tests incorporating a load cycle were undertaken to obtain a better understanding of how high temperature materials respond to these cyclic conditions. These tests showed that when time under load was considered, creep undertaken under load cycling conditions were accelerated relative to constant load conditions. A modified Larson-Miller approach was used to assess this effect and determine an equivalent stress where constant load test data agrees with the cycled test data. This found that cycling the load was equivalent to if the system were under 3 and 7 MPa higher stress, for tests which were loaded for 12 hrs and 6 hrs per 24 hrs respectively. This could be a potential approach to simply and easily consider these load cycling effects for design and life analysis.

The High Temperature Strength of Single Crystal Ni‐base Superalloys – Re‐visiting Constant Strain Rate, Creep, and Thermomechanical Fatigue Testing

The present work takes a new look at the high temperature strength of single crystal (SX) Ni‐base superalloys. It compares high temperature constant strain rate (CSR) testing, creep testing, and out‐of‐phase thermomechanical fatigue (OP TMF) testing, which represent key characterization methods supporting alloy development and component design in SX material science and technology. The three types of tests are compared using the same SX alloy, working with precisely oriented <001>‐specimens and considering the same temperature range between 1023 and 1223 K, where climb controlled micro‐creep processes need to be considered. Nevertheless, the three types of tests provide different types of information. CSR testing at imposed strain rates of 3.3 × 10−4 s−1 shows a yield stress anomaly (YSA) with a YSA stress peak at a temperature of 1073 K. This increase of strength with increasing temperature is not observed during constant load creep testing at much lower deformation rates around 10−7 s−1. Creep rates show a usual behavior and increase with increasing temperatures. During OP‐TMF loading, the temperature continuously increases/decreases in the compression/tension part of the mechanical strain‐controlled cycle (±0.5%). At the temperature, where the YSA peak stress temperature is observed, no peculiarities are observed. It is shown that OP‐TMF life is sensitive to surface quality, which is not the case in creep. A smaller number of cycles to failure is observed when reducing the heating rate in the compression/heating part of the mechanical strain‐controlled OP‐TMF cycle. The results are discussed on a microstructural basis, using results from scanning and transmission electron microscopy, and in light of previous work published in the literature.

The influence of microstructural heterogeneities on high-temperature mechanical properties of additively manufactured γ'-forming Ni-based alloys

AbstractAdditive manufacturing (AM) of metallic materials yields distinctive hierarchical and heterogeneous microstructures owing to the complex thermal conditions during the build-up process. Consequently, the knowledge gained from creep properties of conventionally manufactured (CM) Ni-based alloys cannot be directly applied to AM-processed alloys. Furthermore, insufficient creep life has posed a significant challenge in the development of Ni-based superalloys fabricated by laser powder bed fusion (LPBF), one of the most important AM techniques. Nevertheless, limited research has been conducted to understand their creep behavior due to the time-consuming nature of creep testing and extended research cycles. This study delves into investigating the creep behavior of an additively manufactured, precipitation-strengthened Ni-based alloy (NiCrAl) in comparison to its CM counterpart, focusing on the structure-property relationships. Constant-load creep tests were conducted at temperatures of 750 °C and 950 °C up to a maximum duration of nearly 1500 h. Although both the AM and CM states demonstrated high creep activation energy and creep exponents, indicative of a dislocation climb mechanism, the AM state demonstrated inferior creep life and ductility compared to the CM state for creep times below 500 h. To gain deeper insights into the underlying mechanisms, multi-scale microstructural characterization was performed to understand the effect of the AM-inherent microstructure. Overall, this study provides a comprehensive understanding of the creep behavior of Alloy 699XA after AM and CM processes, emphasizing the significance of AM-specific microstructural heterogeneities.

Creep behavior and fracture mechanism of an additively manufactured 316L stainless steel with extraordinary creep resistance

The creep behaviors of laser powder bed fusion (LPBF) additively manufactured (AM) 316L stainless steel (SS) and its recrystallized (Re) counterpart were investigated via uniaxial constant-load creep tests at 600 °C with nominal stress levels ranging from 235 to 360 MPa. Anisotropic creep behavior was observed in AM 316L SS, with superior creep resistance but inferior creep ductility in the horizontal sample (loaded perpendicular to the build direction (BD)) compared to the vertical sample (loaded parallel to the BD). This superior creep resistance was likely resulted from shorter dislocation slip distance and the inferior creep ductility was due to faster propagation of cracks along the columnar grain boundaries. Compared with both the Re counterpart and conventional 316L SS, AM 316L SS in this study exhibited an extraordinary creep resistance at various stress levels, with the minimum creep rate being two to three orders of magnitude lower and much longer creep life. This exceptional creep resistance of AM 316L SS was attributed to the presence of dislocation cells that impeded the deformation-induced dislocations. This led to a remarkably low rate of creep deformation and delayed the creep crack initiation, ultimately resulting in a long creep life. The gradual development of the precipitate films enriched with Mo, Si and Cr along high-angle grain boundaries, following prolonged exposure to high temperatures, was found to restrict the creep ductility in AM 316L under low stress conditions. Nevertheless, the study demonstrates that the stable dislocation cells are beneficial in enhancing the high-temperature creep resistance of AM 316L SS.

Probabilistic analysis for the reinforced fill over void problem

Analytical and numerical solutions for the problem of geosynthetic-reinforced fills over a void have been the subject of investigation for the last four decades. A common feature of this prior work is that all methods have treated the analytical solutions as deterministic. While the treatment of some input parameters must be taken as deterministic, there are other parameters that have uncertainty. Furthermore, the underlying mechanistic models for load and resistance terms in the limit state equations for the reinforced fill over a void problem can be expected to have different accuracy. This paper revisits the problem of geosynthetic-reinforced fills over voids from a probabilistic point of view for reinforcement tensile strain, tensile strength, and geosynthetic stiffness limit states. Particular attention is paid to the method used to select the isochronous stiffness of the reinforcement and the associated uncertainty in the magnitude of that value. The paper demonstrates how the factor of safety from deterministic past practice can be linked quantitatively to the reliability index used in contemporary probabilistic design. Finally, the paper demonstrates the advantage of using product-specific constant-load creep test results to maximise margins of safety for strength and stiffness limit states in both deterministic and probabilistic frameworks.

Tertiary creep behavior for various rate- and state-dependent friction laws

Forecasting the acceleration of slow landslides to the point of catastrophic failure is crucial. It follows the Voight power-law model with the power exponent α, which is typically close to 2 but can be significantly smaller. Understanding the underlying mechanisms may improve landslide warnings. A previous study applied a rate- and state-dependent friction (RSF) law in the form of the aging law to the creep behavior of an underlying shear zone of landslides, and showed an α value of 2. The aging law is one of the conventional forms of RSF law, and we extended the analysis to other representative laws: the slip law, Perrin-Rice-Zheng (PRZ) law, composite law, and Nagata law. We showed that the acceleration is expressed in terms of the slip rate using the state-evolution equation. As the slip rate increases, α decays to 2, regardless of the frictional parameters, following a power law for the aging and Nagata laws and logarithmically for the slip and the composite laws. For the PRZ law, the asymptotic value of α is between 2 and 3 and depends on frictional parameters. In typical RSF laws, the logarithm of the slip rate is proportional to the friction coefficient or normalized shear stress f minus frictional strength Θ. The logarithmic direct effect and a linear increase with slip in f−Θ independent of the slip rate, which is a characteristic of aging and Nagata laws under constant stress conditions, leads to α=2. By contrast, a purely time-dependent increase in f−Θ would lead to α=1. If these two effects coexist, α increases from 1 to 2 with acceleration. In laboratory experiments of investigation of RSF law, constant-load creep tests in the tertiary-creep stage have been rarely conducted, but they could provide new insights on the form of the state-evolution equation and deserve future study.

Creep life estimation of reformer alloy using θ-projection method - A neuro fuzzy approach

This paper deals with creep life assessment and deformation behavior of 11 years service exposed primary hydrogen reformer tube made up of HP40 grade of steel using the θ-projection method. Constant load creep tests have been performed at 870 °C under the stress range of 50–68 MPa. The generated creep data exhibit a significant amount of scatter under identical stresses. The existence of scatter in the creep deformation data leads to substantial amount of uncertainty in the estimation of θ-parameters and hence in the assessment of creep life. An adaptive neuro fuzzy based model is proposed to study the effect of variation of these parameters on creep life prediction. The model has been compared with the experimental findings.

Predicting the behavior of commercially available building foundation isolation materials using a four-parameter fractional Zener model

In this paper we apply a four-parameter fractional Zener model to describe the short and long term creep behavior of a viscoelastic building foundation isolating material currently in use globally in noise and vibration mitigation applications. Building from the classical rheological models composed of springs and dashpots suitably combined (Maxwell, Kelvin-Voight, Zener), we include a fractional derivative component in the form of a Scott-Blair element composed of a Riemann-Liouville derivative of non-integer order. The parameters are identified empirically through measured stress-strain results and hysteresis curves for the material in compression. The model is then compared to the results of a 168h constant load creep test of three commercially available materials.

A Novel Design of Constant Load Creep Test Machine

Polymer materials have been used in a wide area from our homes to industry today. For this reason, creep curves from the mechanical properties of polymer materials are an important factor for the correct use of polymers. This article presents a new constant load creep test machine design which is a mechatronic system to perform creep testing of polymers. A low-cost creep test machine was developed by mounting clamps, pulleys, weights and steel cable on top of a main frame. With the measurement unit, the linear movement of the steel cable was detected and recorded with the data logging system together with the ambient temperature. Tests were made with a 15% calcium carbonate reinforced polypropylene specimen. It was observed that the experimental data obtained were compatible with a standard creep test curve. These findings confirm that the presented constant load creep test machine design works efficiently.

Dwell Fatigue Behavior of Two-Phase Ti-6Al-4V Alloy at Moderate Temperature

AbstractFatigue life of titanium and titanium alloys is frequently reduced if the load holds at the maximum stress are introduced, which is termed dwell debit. Many factors affect dwell fatigue of titanium alloys, like stress state, level and ratio, alloy chemistry, microstructure and microtexture, holding time and temperature. Dwell sensitivity of titanium alloys is usually attributed to the phenomenon of load shedding, which is a time-dependent redistribution of stress from so-called ‘weak’ to ‘strong’ grains possessing different crystallographic orientations. Stress concentration leads to crack initiation and formation of quasi-cleavage facets in ‘strong’ grains. Another factor contributing to dwell sensitivity of titanium alloys is time dependent strain accumulation, which is observed even at room temperature. In the paper, the effect of load holds and volume fraction of primary α phase on the fatigue behavior of Ti-6Al-4V alloy at the temperature of 150°C was investigated. Two variants of bi-modal microstructure of the alloy were obtained by means of the heat treatment. Stress controlled fatigue tests with and without hold time at maximum load and constant load creep test were carried out.

Damage and Fracture in Creep of a Cast B1914 Superalloy

Damage and fracture processes in high temperature creep of an investment cast B1914 Ni-based superalloy with the increased amount of boron to 0.08wt.% for high temperature applications were analysed. Constant load creep tests in tension were conducted at temperatures from 800 to under applied stress ranging from 150 to 700 MPa. The microstructure of fractured specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. Microstructure investigation showed that the microstructure of the B1941 superalloy consists of a gamma (γ) phase with a dendritic structure and gamma prime (γ ́) phase with a cuboidal shape. Precipitates of γ ́and a lamellar eutectic, composed of γ/(Mo,Cr,Ni)3B2, were identified in the interdendritic region. Creep damage and fracture are closely connected with decohesion of the interface between M3B2 boride and matrix.

Influence of Cryo-Processing and Post-SPD Annealing on Creep Behavior of CP Titanium.

The commercial purity of VT1-0 titanium was processed by the rolling process and executed at elevated, room, and cryo-temperatures. These processings led to the formation of an ultrafine-grained microstructure, with the mean grain size at a nanometer level. Some of these materials were statically annealed at a temperature of 823 K for 1 h, which led to significant subgrains and grain coarsening. The constant load creep tests in tension were carried out in argon on all states of materials, at temperatures of 648–723 K and different ranges of applied stresses. From the value of the steady-state creep rate, the control creep mechanisms were determined. The microstructure analyses were carried out via SEM and TEM. It was found that titanium prepared at elevated and room temperatures have a higher creep strength than titanium prepared at cryo-temperatures. Furthermore, the post-SPD —annealing led to a significant decrease in the creep properties. The influence of the preparation temperature on the difference of the creep behavior were discussed and explained using the microstructure analyses of the tests’ samples.

Crystal plasticity analysis of temperature-sensitive dwell fatigue in Ti-6Al-4V titanium alloy for an aero-engine fan disc

This study investigates the temperature-dependent dwell fatigue behavior of Ti-6Al-4V alloy using crystal plasticity finite element method. The dislocation mechanism-based crystal plasticity parameters are calibrated with the data of quasi-static tensile and constant load creep tests at ambient and intermediate temperatures. In particular, the rate-dependent parameters related to slip property are employed to establish the relationship between strain rate sensitivity of a soft α-titanium single crystal and temperature. The variation of strain rate sensitivity with temperature influences the stress redistribution occurring within the hard-soft grain combination, which in turn affects the dwell fatigue sensitivity. Further, the structural analysis for a Ti-6Al-4V fan disc provides the stress fields at takeoff and cruise phases to examine the in-service stress redistribution. The highly localized hoop stress and presence of large macrozones at the bore of the fan disc, rather than the fair rate sensitivity of soft macrozones at working temperatures, are responsible for triggering basal stress enhancement and dwell facet nucleation in hard macrozones.

Investigations on Creep Behavior and Fractal Derivative Constitutive Model of Sandstone under Different Drying-Wetting Cycles

Sandstone has been regarded as an optimal medium for underground gas storage (UGS). To better understand effects of drying-wetting (D-W) cycles on creep behavior of sandstone, comprehensive experimental investigations including uniaxial compression test (UCT) and uniaxial constant-load creep test (UCCT), were conducted on sandstone specimens under different D-W cycles. Experimental results demonstrate that the UCS and E, instantaneous strain, and steady creep rate of sandstone are greatly influenced by D-W treatment. Steady creep rate of sandstone increases with the increase in stress and D-W cycles, while both the UCS and long-term strength (σ∞) of sandstone specimens gradually decrease with the increase in D-W cycles. A novel fractal derivative creep constitutive model of sandstone considering the variations of viscosity coefficient and D-W cycles is established, which could accurately describe the creep characteristics of sandstone after different D-W cycles in the whole creep process. The influence of D-W cycles on the sensitivity of creep parameters is analyzed to discuss the influence of D-W cycles on creep and damage behavior of sandstone.

Modified θ projection model-based constant-stress creep curve for alloy 690 steam generator tube material

Steam generator (SG) tubes in a nuclear power plant can undergo rapid changes in pressure and temperature during an accident; thus, an accurate model to predict short-term creep damage is essential. The theta (θ) projection method has been widely used for modeling creep-strain behavior under constant stress. However, many creep test data are obtained under constant load, so creep rupture behavior under a constant load cannot be accurately simulated due to the different stress conditions. This paper proposes a novel methodology to obtain the creep curve under constant stress using a modified θ projection method that considers the increase in true stress during creep deformation in a constant-load creep test. The methodology is validated using finite element analysis, and the limitations of the methodology are also discussed. The paper also proposes a creep-strain model for alloy 690 as an SG material and a novel creep hardening rule we call the damage-fraction hardening rule. The creep hardening rule is applied to evaluate the creep rupture behavior of SG tubes. The results of this study show its great potential to evaluate the rupture behavior of an SG tube governed by creep deformation.

Microstructural Evolution of 9CrMoW Weld Metal in a Multiple-Pass Weld

9CrMoW steel tubes were welded in multiple passes by gas-tungsten arc welding. The reheated microstructures in the Gr. 92 weld metal (WM) of a multiple-pass weld were simulated with an infrared heating system. Simulated specimens after tempering at 760 °C/2 h were subjected to constant load creep tests either at 630 °C/120 MPa or 660 °C/80 MPa. The simulated specimens were designated as the over-tempered (OT, below AC1, i.e., WT-820T) and partially transformed (PT, below AC3, i.e., WT-890T) samples. The transmission electron microscope (TEM) micrographs demonstrated that the tempered WM (WT) displayed coarse martensite packets with carbides along the lath and grain boundaries. Cellular subgrains and coarse carbides were observed in the WT-820T sample. A degraded lath morphology and numerous carbides in various dimensions were found in the WT-890T sample. The grain boundary map showed that the WT-820T sample had the same coarse-grained structure as the WT sample, but the WT-890T sample consisted of refined grains. The WT-890T samples with a fine-grained structure were more prone to creep fracture than the WT and WT-820T samples were. Intergranular cracking was more likely to occur at the corners of the crept samples, which suffered from high strain and stress concentration. As compared to the Gr. 91 steel or Gr. 91 WM, the Gr. 92 WM was more stable in maintaining its original microstructures under the same creep condition. Undegraded microstructures of the Gr. 92 WM strained at elevated temperatures were responsible for its higher resistance to creep failure during the practical service.

Thermal creep fracture of a Zr1%Nb cladding alloy in the α and (α+β) phase regions

The objective of the present study was to provide new relevant information on creep behaviour and fracture processes in a sponge-based modified Zr1%Nb cladding alloy (modified E110 alloy) in the α-Zr and (α+β)-Zr phase regions. To this end, constant load creep tests were carried out in argon at testing temperature intervals from 350°C to 950°C, and applied tensile stresses ranging from 5 MPa to 210 MPa, corresponding to the power-law breakdown creep regime and/or high testing temperatures, and thus to simulate disaster conditions. Creep tests were followed by metallographic and fractographic analyses of the specimens to explain the observed creep behaviour. It was found that in the power-law region (at 350°C) the values of the stress exponent n of the minimum creep rate ε˙m (n = ∂lnε˙m /∂lnσ)T, and the stress exponent m of the time to fracture tf (m = - ∂lntf /∂lnσ)T, were very high and near to each other, indicating a close relationship between creep deformation and fracture. Further support for the idea that creep deformation and fracture are interconnected can be observed by the validity of the empirical Monkman-Grant relationship. Creep tests in the α-Zr phase region revealed creep cavitation near to, and at, the fracture surface. The final fracture is a ductile dimple mode with the synergistic effect of creep cavitation. By contrast, the final fracture in the (α+β)-Zr phase region is caused by a local strain-induced instability of the matrix leading to a loss of an external section of specimen (necking). There is a marked difference between the values of the strain to fracture εf. In the α-Zr region, typical values of εf ~ 0.3-0.5 were found, whereas for the (α+β)-Zr region, the value εf increases with testing temperature up to 850°C. Following drop of εf at temperatures ≥ 900°C was explain by intensive oxidation of the alloy.

Investigation on the microstructure and creep behavior of laser remelted thermal barrier coating

Laser surface modification of the thermal barrier coating was investigated with the aim of increasing creep resistance. Yttria-stabilized-zirconia (YSZ) with a CoNiCrAlY bond coat was deposited by air plasma spraying on equiaxed Ti–6Al–4V substrates. Analysis was carried out comparing uncoated samples with as-sprayed and laser remelted ones. A cross section and detailed characterization of coating surface was carried out by scanning electron microscopy and X-ray diffraction techniques. Mechanical properties in terms of microhardness and creep resistance were evaluated. Constant load creep tests were conducted at stress levels of 125 to 319 MPa at 500 °C and 600 °C. A dense ceramic layer of thickness about 40 μm was formed by laser remelted treatment and its microhardness surface was higher than the other layers. As-sprayed YSZ had higher creep resistance than other samples. Analysis of creep behavior showed that the steady-state creep rate of laser remelted samples had about 42% reduction in 600 °C condition, evidencing a higher creep resistance than uncoated material. SEM images revealed a ductile fracture with presence of equiaxed dimples.

EVALUATION OF CREEP BEHAVIOR OF TI-6AL-4V ALLOY WITH THERMAL BARRIER COATING DEPOSITED BY AIR PLASMA SPRAY

Titanium alloys have attractive properties for application in aeronautical components submitted to high temperature. Yttria-stabilized zirconia (YSZ) of thermal barrier coating systems (TBC) have become an alternative to increase the operational lifetime of turbine blades. The aim of this work was to evaluate the creep behavior of Ti-6Al-4V with TBC deposited by air plasma spray. Constant load creep tests were conducted in air on equiaxed Ti-6Al-4V (uncoated) and Ti-6Al-4V with TBC and the experimental parameters related to secondary creep were determined. All the coated samples showed a reduction of secondary creep rate values and, thereafter, higher creep resistance, which could be associated with oxidation protection and thermal insulation provided by the coating. Creep rate analysis suggests that the dominant creep mechanisms in all cases is the dislocation climb.

Effects of oxidation-resistant coating on creep behavior of modified 9Cr-1Mo steels

Oxidation is a life-limiting factor for material durability. In this study, constant-load creep tests are conducted on modified 9Cr-1Mo steel (F91) with an oxidation-resistant coating, MCrAlY, to evaluate the coating effect on the creep behavior. The creep strain vs. time curves are compared with that of the uncoated material under constant-load conditions. The previously developed deformation-mechanism based true-stress (DMTS) model and the composite stress rule are applied to analyze the creep data for the coated material coupons. This way, the effect of coating and the contributions from material-intrinsic deformation mechanisms, i.e., intragranular dislocation glide and climb, as well as grain boundary sliding, are all delineated quantitatively. The model is shown to describe well the entire creep deformation process consisting of primary, steady-state, and tertiary creep of the F91 with MCrAlY coating.