Creep Specimens Research Articles

The correlation between prior fatigue damage and remaining creep properties in a high chromium steel welded joint

Strain cycling prior to a constant load exposure is expected to influence the creep properties. The present work aimed to evaluate the effect of prior low cycle fatigue (LCF) damage on the remaining creep properties of P92 steel welded joints. Strain amplitudes of 0.25%, 0.4% and 0.6% and fatigue cycles ranging from 10% to 80% of the number of cycles to failure (Nf) were introduced in creep specimens to represent fatigue damage. A critical analysis of the creep test responses revealed that a high strain amplitude and high fatigue cycles induced a clear reduction in the remaining creep life; conversely, prior LCF also led to a significant increase in the creep rupture strain. It was found that prior LCF loading produced its saturated effect after fatigue cycles at 70% Nf or a strain amplitude of 0.4%. It is important to note that the degradation of the remaining creep life of a welded joint was more severe than that found in homogeneous base metal. It was also observed that the failure location and development of creep voids were dependent on both the number of fatigue cycles and strain amplitude. Additionally, the prior fatigue damage was evaluated using a defined damage parameter that revealed two nonlinear stages of fatigue damage.

Stress Relaxation and Creep of a Polymer-Aluminum Composite Produced through Selective Laser Sintering.

This article discusses the rheological properties (stress relaxation and creep) of polymer-aluminum composite specimens fabricated through the selective laser sintering (SLS) from a commercially available powder called Alumide. The rheological data predicted using the Maxwell–Wiechert and the Kelvin–Voigt models for stress relaxation and creep, respectively, were in agreement with the experimental results. The elastic moduli and dynamic viscosities were determined with high accuracy for both models. The findings of this study can be useful to designers and users of SLS prints made from the material tested.

An efficient implicit time integration method for discrete dislocation dynamics

Plastic deformation of most crystalline materials is due to the motion of lattice dislocations. Therefore, the simulation of the interaction and dynamics of these defects has become state-of-the-art method to study work hardening, size effects, creep and many other mechanical properties of metallic specimens. Lot of efforts have been made to make the simulations realistic by including specific dislocation mechanisms and the effect of free surfaces. However, less attention has been devoted to the numerical scheme that is used to solve the equations of motion. In this paper we propose a scheme that speeds up simulations by several orders of magnitude. The scheme is implicit because this type is the most efficient one for solving stiff equations that arise due to the long-range nature of dislocation interactions. The numerical results show that the method is not only faster than other approaches at the same numerical precision, but it can also be efficiently applied even without dislocation annihilation. The suggested method significantly increases the achievable volume and/or duration of discrete dislocation dynamics simulations and can be generalized for complex 2D and 3D simulations as well.

Microstructure, Thermal Stability during Creep and Fractography Study of Friction-Stir-Processed AA2024-T3 Aluminum Alloy

Friction stir processing (FSP) makes it possible to obtain a stir zone with very fine grain size with the aid of severe plastic deformation. Yet using FSP, it is impossible to obtain a uniform cross section as far as the microstructure and mechanical properties are concerned. To reduce the effect of this limitation, in the current study, the material was processed on both sides, thus yielding a wider, rectangular and more homogenous stir zone. In a recent publication, the authors focused on the mechanical properties, thermal stability and transmission electron microscopy (TEM) study of friction-stir-processed AA2024-T3, comparing this alloy to the parent material. While the previous study mentioned focused on the parent and on the as friction-stir-processed material, the current study focuses on post-creep specimens and hence completes the previous one with microstructural processes occurring during creep. The current paper completes the above precipitate analysis using electron-dispersive x-ray spectroscopy (EDS) mapping of the various precipitates reported after exposure to creep temperatures. The TEM study reported in the current paper revealed the formation of dislocation structures during creep, in parallel to the dynamic recrystallization (DRX) reported by the authors in the past. In addition, fractography study indicated premature cracking as the prevailing failure mechanism as well as in the case of friction stir welded creep specimens.

An investigation of the effect of pre-straining on the creep behaviour of a P91 steel at 600 °C using impression creep testing

This paper investigates the effect of high temperature tensile strain on subsequent creep strength in a grade 91 steel using impression creep testing. The grade 91 material investigated has been tested in two different microstructural conditions, in the normal martensitic condition and in an aberrant mis-heat treated condition in which the microstructure is 100% ferrite. The latter condition is of interest because of its widespread occurrence on operating power plant. The two microstructural conditions were confirmed by hardness mapping and Electron Back Scattered Diffraction (EBSD). Previous investigations have used pre-strained uniaxial creep specimens to investigate this effect, but the present work has utilised the specialised small-scale impression creep testing technique to test material obtained at a number of positions along the axes of failed hot tensile specimens. This allowed impression creep samples to be extracted at various pre-strains for investigation and for a wide range of hot tensile pre-strain to be investigated. The two microstructural conditions have shown a divergence in behaviour, with the normal martensitic material showing little change in creep strength with increasing pre-strain and the aberrant material increasing markedly in creep strength with increasing pre-strain.

An Anomaly in Creep Property Dependence on Grain Size for Ultrafine Grain Nanocrystalline Nickel at Higher Creep Temperature

In this present study, molecular dynamics simulation of creep for ultrafine grain NC Ni specimens with different grain sizes have been carried out under a constant 1 GPa applied load for various creep temperatures to study the dependence of grain growth on creep temperature and grain size during creep process and its influence on creep properties. It is observed that the extent of grain growth in ultrafine grain NC Ni during creep deformation process is more if creep in creep temperature is higher. A noteworthy anomaly, that is NC Ni with smaller grain exhibits better creep property compared to NC Ni with larger grain, is observed in case of higher creep temperatures (i.e. around or greater than 1400K).

Proposing an Image Analysis to Study the Effect of Lightweight Aggregate on Shrinkage and Creep of Concrete

Abstract Creep and shrinkage, particularly in lightweight concrete, are known as complex phenomena that highly affect the serviceability of concrete. This may be due to the fact that creep and shrinkage behaviors are highly dependent on the behavior of the surface layer at the interfacial transition zone of the aggregate and surrounded cement paste. Despite the importance of the influence of creep and shrinkage on concrete behavior, however, no simple measuring methods were accepted by researchers until now. In the present article, the shrinkage and creep of both normal and lightweight concrete, made by expanded clay, were studied via image analysis and traditional methods. A load control system equipped with a camera was designed to measure the strains of the cured specimens induced by shrinkage and creep. The water/cement ratio was 0.3. The results were also verified by direct measuring via embedded strain gages on the concrete surface. Both measuring methods showed that the expanded clay improved the transition zone of the lightweight specimens, and the induced creep and shrinkage strains of the lightweight specimens were smaller than those for normal specimens. Lower creep and shrinkage values were obtained by a visual measuring method rather than by direct measuring methods.

The usability of Burger body model on determination of oriented strand boards’ creep behavior

In this work, the usability of the Burger body model (BBM) for determining the behavior of oriented strand boards (OSBs) under long-term loads was evaluated. The actual bending strain data and predicted strain data as a function of different stress levels and load durations under constant environmental conditions (25 ± 2°C and 50% relative humidity) were compared. Two test groups, short-term bending tests and long-term creep-rupture bending tests, were performed according to relevant ASTM standards. Specimens were randomly assigned to three groups and loaded at 47% (132.2 kg), 51% (137.4 kg), or 55% (154.9 kg) of the mean static short-term flexural strength. Specimen creep was monitored for 10,000 h using an automated measurement system. The four-parameter BBM parameters were obtained for all specimens at 2000-h time intervals, providing five different estimates. Measured strain values were compared with strain predictions from the BBM and with the goal of evaluating length of experiment on prediction accuracy. Each stress level provided statistical differences based on the error between the actual strain and predicted strain values. Group 3 provided minimum error compared to group 1 and group 2. The 10,000 and 8000 h loading provided the most accurate predictions compared to 6000, 4000, and 2000 h of data. Overall, the longer the actual data is collected the more accurate predictions were obtained. As a result, the BBM was found useful tool for predicting the creep behavior of OSBs under different loads and load durations. It was also shown that the increased duration of practical loading minimizes the error between the prediction. Therefore, the BBM is suggested for use predicting the creep behavior of OSBs over 8000 h load durations.

Assessment of Creep Rupture Properties of Friction Welded Super304H/T92 Steel Weld Joints

The T92 martensitic and Super304H austenitic dissimilar steels welded joints were always encountered in the super critical boilers. The isothermal method was used to assess the creep rupture strength of weld joints. With the higher loading stress, the transcrystalline fracture was the rupture mode of creep specimens, and results in forming dimples. With the higher loading stress, the transcrystalline fracture was the rupture mode of creep specimens and results in forming dimples. With the lower loading stress, creep cavities joined each other along the grain boundary, the creep rupture was the rupture mode of creep specimens and results in cleavage rupture.

Experimental and numerical study on creep and shrinkage effects of ultra high-performance concrete beam

Compared with normal concrete, ultra high-performance concrete (UHPC) is of increasing interest for bridge engineering because of its superior tensile strength, compressive strength, and significantly enhanced durability. The creep and shrinkage effect have a significant influence on the long-term behavior of UHPC. However, the existing research on the time-dependent behavior of UHPC is still limited. In this research, based on numerical integration method, the ABAQUS user subroutine for simulating creep and shrinkage of both normal concrete (NC) and UHPC were developed by the recursive algorithm of adjacent stress increment in the time history. The accuracy of developed user subroutine was validated by creep and shrinkage test of reinforced NC beams. Subsequently, the long-term creep and shrinkage test of UHPC specimens was conducted with stress to strength ratio of 20%, 30%, and 40%, respectively. Based on the test results and the CEB-FIP Model code 1990, the simplified design formulas for predicting creep and shrinkage of UHPC were proposed respectively based on the developed UHPC material. In the proposed design formulas, the final shrinkage strain was 145 με, and the final creep coefficient was 0.46 for the developed UHPC specimens based on the test environment condition. The comparison between the test result and proposed formulas indicated good applicability and accuracy of the proposed formulas. Based on the developed ABAQUS user subroutine and the proposed design formula for creep and shrinkage, the finite element (FE) models of RC beam and UHPC beam were developed. The influence of creep and shrinkage on the long-term mechanical behavior of simply supported prestressed beams were investigated.

Experimental creep behavior and life prediction through observation and numerical analysis for AISI 310

The present study conducted for the first time a life prediction based on numerical analysis of creep deformation and fracture behavior results of special notched specimens of AISI 310 steel. For this purpose, flat creep specimens with double edge notch were tested under stress is of 225, 160 and 100 MPa at temperatures of 700, 675 and 650 °C. Then it was analyzed the creep behavior of the material through observational and numerical analysis. Furthermore, the Q* parameter was used to establish a novel life predication equation for 310 steel. The creep curves were characteristic of the experiment, with a strong influence of the stress in the creep stage II. Using the Q* parameter, a good agreement was observed between the experimental and the calculated data. The characteristic of fracture surfaces was dependent on both stress and temperature with a transition from ductile fracture, with formation of dimples, to brittle intergranular fractures, evidenced by cavities in the grain boundaries. Coalescence and growth of microcracks on the specimen surface were also observed in a region at 45° from the notch tip, characteristic of the damage propagation caused by the creep test.

Creep behaviour of IN 740 alloy after HAZ thermal cycle simulations

In this study, the creep specimens of an age-hardened IN 740 alloy were subjected to heat affected zone (HAZ) thermal cycle simulations using a Gleeble 380 physical simulator. The short-term uniaxial creep tests in tension were carried out at temperatures of 973 and 1023 K and at applied stresses ranging from 220 to 600 MPa to evaluate the effect of welding simulations on the microstructural evolution, creep properties and behaviour of the IN 740 alloy. It was found that the stress exponent, n, of the minimum creep rate is not constant and the evaluated values of n are of the order of ~3 for lower stresses, with the gradual variation of n with an increase of the applied stress up to ~15 at the highest stresses, indicating that the creep testing was carried out in the power-law (dislocation) creep region. Analysis of the creep data leads to the conclusion that the creep behaviour of the simulated HAZ of the IN 740 alloy arises from a synergism between the deformation process that is based on intragranular dislocation processes, involving the glide and climb of mobile dislocations, and grain boundary mediated processes. Finally, an assumption was proposed that both creep deformation and fracture are controlled by the same mechanism(s). Examination of the creep fracture surfaces revealed that the main fracture mode is mixed-mode fracture with dominating intergranular character.

On the evolution of mechanical properties and microstructure of ferritic-bainitic (FB) 2.25Cr-1Mo (Grade 22) steel during high-temperature creep

The development of creep damage in ferritic-bainitic 2.25Cr-1Mo (Grade 22) steel was studied by following the evolution of mechanical properties and microstructure by characterisation of a series of interrupted creep specimens (605 °C/80 MPa). The change in mechanical properties of the partially crept specimens was evaluated via hardness measurements, while the microstructural changes were characterized by a range of microscopy techniques. The nano-hardness measurements combined with a statistical analysis allowed for analysis of the strength of the present constitutive phases as a function of creep time (tc), and imparted creep strain (ɛc). In addition, a machine learning methodology was used to identify ferrite and bainite regions in Electron Back-Scatter Diffraction (EBSD) orientation maps, which then allowed for a study of the evolution of dislocation densities (Geometrically-Necessary Dislocations, GNDs) in each phase separately. It is shown that the amount of GNDs in the bainite is reduced significantly during the high-temperature creep, while there is a moderate increase in the amount of GNDs in ferrite grains. These trends are consistent with the observed changes in the strength (hardness) of each phase – bainite is significantly softening, while ferrite slightly hardening. The reduction of the amount of dislocations in the bainite and its consequent softening is caused by the progressive decomposition of the needle-like bainite present in the initial microstructure into polygonal fine-grained ferrite-like bainite. Transmission electron microscopy (TEM) confirmed the changes in dislocation density observations and was also used to identify changes in carbides during the high-temperature creep.

Effect of Boron on the Thermal Stability of the Precipitate Phase of FC92B and FC92N Steels During Aging and Creep

In this study, precipitation strengthening alloying elements (C, N, V, Nb, Ta) and solution strengthening alloying elements (Cr, W, Mo, B) were appropriately combined to fabricate FC92B and FC92N alloys. Long-term aging heat treatments at 650 °C of up to 20,000 h were conducted to compare the high-temperature properties of each alloy, investigate the effects of microstructure stability on creep performance characteristics, and analyze the precipitate growth behavior in relation to the long-term thermal stability of the precipitate phase. Transmission electron microscopy and nanoscale secondary ion mass spectrometry were carried out to investigate microstructural changes according to the aging treatment time and the microstructure of creep specimens in relation to the addition of trace alloying elements. Although FC92B contains a higher B content than FC92N, FC92B did not exhibit significant changes in hardness or tensile strength regardless of the aging time, which extended to a maximum of 20,000 h. The FC92B alloy with an adequate addition of B showed the highest creep performance and exhibited rupture durations that were more than twice those of FC92N under stress conditions of 100 MPa and 140 MPa at 650 °C. The precipitate size of FC92N was significantly coarser than that of FC92B. In FC92B aged for 20,000 h, B was distributed in the grain boundaries, as shown by Nano-Secondary Ion Mass Spectrometry. B content was found to be high within the M23C6 precipitate.

Investigating creep rupture and damage behavior of 41Fe-25.5Cr-23.5Ni alloy small punch creep specimens using a novel microstructure meshing approach

The Voronoi tessellation technique was used to model an idealized polycrystalline microstructure. This novel meshing approach was also implemented to study the crack initiation and growth behaviors of Sanicro25 alloy through small punch creep tests using a modified creep damage constitutive model. Furthermore, the influence of specimen thickness on crack fracture mechanics was examined. The suitability of this novel method was validated on the basis of the accuracy of simulation results. Transgranular fracture patterns were found at the primary stage, which were transformed into intergranular cracks as the creep time progressed. Cracks in small punch specimens were predominantly initiated on the outer surface, where high stress triaxiality occurred. Simultaneously, the high stress and stress triaxiality deviation controlled crack growth behaviors were determined by considering the various stress states.

Creep deformation related to γʹ phase cutting at high temperature of a [111] oriented nickel-base single crystal superalloy

The configurations of superdislocations in a [111]-oriented nickel-base single crystal superalloy have been investigated after creep deformation at 1100 °C/150 MPa. The superdislocation arrays and hexagonal superdislocation networks are formed in irregular rafted γʹ phase, which are rarely observed in [001] crept specimens. It is believed that the superdislocation networks are formed by reactions of superdislocation arrays and another kind of superdislocations. Careful diffraction contrast analysis shows that the Burgers vector of the superdislocation arrays is a[011¯], which is not the favorable slip system when loaded along [111]. The formation of the arrays is supposed to be related to the matrix dislocations cross slipping in two {001} channels and the stress concentration caused by inhomogeneous rafted structures. The networks in γʹ phase may prevent dislocation movement and slash the increase of creep rate to some extent.

Investigation on the validity of creep damage mechanics for the life time prediction of T92 welded joint

This paper reports the damage evolution in ASME T92 welded joints during creep process. The creep test was conducted at 650℃ with applied stress of 90 MPa. The creep specimen was ultimately fractured at the fine-grained heat affected zones after creep for 1560 h. The metallographic results show that the cavity number and size in fine-grained heat affected zones increase with the creep time. The coalescence of creep cavities happened at the late stage of the creep life, which depending on the adjacent voids grows and propagates into the micro-crack. Besides, the deterioration in fine-grained heat affected zones of T92 steel welded joint with various creep time can be simulated based on the continuum damage mechanics with modified Kachanov-Rabotnov constitutive equation. The result of simulated creep rupture life is in good agreement with the experimental value, which indicates that the continuum damage mechanics can be used to predict creep life and evaluate creep deterioration in a T92 steel welded joint.

Creep properties of single crystal Ni-base superalloys (SX): A comparison between conventionally cast and additive manufactured CMSX-4 materials

The present work compares the microstructures and the creep properties of two types of single crystal Ni-base superalloy CMSX-4 materials (SXs). One was produced by conventional directional solidification Bridgman processing. The other was manufactured by selective electron beam melting (SEBM). The microstructures of the two types of materials are compared with emphasis placed on the large (dendritic/interdendritic regions) and small scale (γ-matrix/γ′-precipitates) microstructural heterogeneities, which characterize SX microstructures and their evolution during processing, heat treatment and creep. It is shown that heat treated SEBM materials have creep properties, which match or even outperform those of conventionally processed SX materials. Creep properties were assessed using a miniature creep test technique where [001] miniature tensile creep specimens were tested in the high temperature/low stress (1050 °C, 160 MPa) and in the low temperature/high stress (850 °C, 600 MPa) creep regimes. The creep behavior is interpreted based on microstructural results, which were obtained using analytical scanning and transmission electron microscopy (SEM and TEM).

Creep behavior of Alloy 709 at 700 °C

Alloy 709 is an austenitic stainless steel with potential for structural applications in advanced nuclear reactors. In this study, creep tests on Alloy 709 specimens were performed at a temperature of 700 °C under applied stresses in the range of 125–250 MPa. The alloy exhibited a brief primary, insignificant secondary and prolonged tertiary creep stages. A Norton stress exponent (n) of 5.9±0.7 was obtained from the double-logarithmic plot of minimum creep rate versus stress. Microstructural analyses were carried out for both as-received and crept specimens using optical microscopy, electron backscattered diffraction, and transmission electron microscopy. The microstructures of the as-received and crept specimens were compared. The mode of fracture was transgranular as revealed by scanning electron microscopy of the creep-ruptured specimens. Thermodynamic and precipitation modeling was performed in order to obtain information on the phase types and the kinetics of precipitation for the sigma−phase and the M23C6 carbide. Based on the creep data, microstructural observations and relevant modeling, creep deformation and failure mechanisms at the considered temperature are elucidated.

COMPARISON OF THE LONG-TERM PROPERTIES IN COMPRESSION OF DIFFERENT SIZE FOAMED CONCRETE

Foamed concrete has been used as a building material since the early 1920s. In the beginning, it was used as an insulation material with very low density. Since then there have been attempts to make this material more load-bearing and structural. In the present-day foamed concrete is being used in soil reinforcement, manufacturing of building blocks and other sorts of construction materials. [1] The aim of this article is to determine long-term properties and strength of foamed concrete specimens as well as compare the results between two differently sized foamed concrete specimens. The size of creep and shrinkage specimens were Ø46x190 mm and Ø75x180 mm. The creep properties of the specimens were determined by loading them with 20% of the ultimate stress value. [2] The compressive strength, creep and specific creep of specimens were determined as well as specimen size factor to creep deformations.