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Damage generated from creep deformation is pervasive in rock, which is assumed to influence the mechanical performance of rock. However, the influence of creep damage on the dynamic characteristics of rock has not been well understood. Here, a rock creep-impact test apparatus is used to investigate the preexisting creep damage on the dynamic performance of sandstone. The results demonstrate that a higher creep stress results in more initial microcracks in sandstone, leading to a higher damage-cumulation rate. The impact number is more sensitive to the creep-stress level than the creep duration. During the dynamic impact, a higher creep stress leads to a larger peak axial strain and a larger residual axial strain, whereas a sandstone sample tends to dilate with increasing constant stress and load duration. Moreover, the larger the constant stress level and creep duration, the smaller the proportion of the second stage of damage of sandstone under impact loading. These results are helpful to the understanding of the damage and failure process of deep rock structure exposed to dynamic loading.

The creep response and stress relaxation of X20 CrMoV12-1 steam piping under diverse operating conditions were simulated using finite element analysis (FEA) code, Abaqus alongside fe-safe/Turbolife software. In the study, steady-state creep and creep analysis characterized by 24 hours daily cycle consisting of a total of 6 hours peak, 4 hours transient and 14 hours off-peak period was considered. Modified hyperbolic sine creep model used in the analysis was implemented in Abaqus via a special creep user-subroutine to compute the stress relaxation and creep behaviour, while the useful service life and creep damage was estimated using fe-safe/Turbolife. The optimum creep strain, stress, damage, and worst life were found at the intrados of the piping, with the steady-state analysis having a higher useful creep life and slower creep damage accumulation. Furthermore, slower stress relaxation with faster damage accumulation was observed in the analysis involving cycles. Finally, a good agreement was obtained between the analytical calculated and simulated rates of the piping.

Creep is a fundamental process responsible for the long-term crustal evolution and the stability of rock engineering. However, the influence of initial microcracks on the creep of rock has not been determined. Here, the influence of initial microcracks generated by repetitive impacts on the creep of sandstone is quantitatively investigated. Results show that the increase in the initial microcracks density reduces not only the compressive strength but also the stress of the crack damage threshold. Under the same creep stress, i.e. 24.6 MPa, the axial strain rate increases from 1.8×10−9 s−1 to 3×10−6 s−1 with the initial microcracks density increasing from 0.07 to 0.8. Both the axial strain and the cumulative acoustic emission (AE) energy at the onset of the tertiary creep phase increase with increasing initial microcracks density. The creep strain rate exhibits a quasi-linear relationship with the difference between the creep stress and the stress of the crack damage threshold. Based on the experimental results, a creep model considering the initial microcracks density is proposed. These results are helpful to better understand the long-term evolution of crust and stability of rock engineering with initial microcracks.

Shear creep is one of the most important mechanical behaviors of rock discontinuities. The creep mechanism and prediction of starting point of the accelerating creep stage are vital for establishing the creep model and predicting creep failure. In this study, a series of multi-step creep tests are conducted. The three creep stages of shear creep tests are investigated in detail, and a method for predicting the accelerating creep stage is proposed. Distinct nonlinear and local fluctuations caused by cracking are observed in the creep curve. To describe the transition creep stage and steady creep stage, an empirical creep model is established, and the creep characteristics related to the joint roughness coefficient (JRC) and the normal stress are explored in detail using the model’s parameters. The creep process can be described as involving the JRC resistance weakening and frictional resistance compensation, and a model also established to describe this process. The frictional resistance cannot compensate for the loss of JRC resistance; consequently, creep failure occurs. The starting point of the accelerating creep stage can be predicted by combining the JRC weakening and frictional mobilization model and the empirical creep model. A new method for determining long-term strength is also proposed based on the relationships between the starting point creep deformation and the shear creep stress.

The different creep-aging forming processes of 2524 aluminum alloy were taken as the research object, and the effects of creep-aging temperature and creep stress on the fatigue-crack propagation properties of the alloy were studied. The research results showed the following under the same sintering time of 9 h, at creep-aging temperatures of 100 °C, 130 °C, 160 °C, and 180 °C, respectively, with an increase in creep-aging temperature: the fatigue-crack propagation rate was promoted, the spacing of fatigue striations increased, and the sizes of dimples decreased while the number was enlarged; this proves that the fatigue property of the alloy was weakened. Compared with the specimens with creep deformation radii of 1000 mm and 1500 mm, the creep deformation stress was the smallest when the forming radius was 1800 mm, with a higher threshold value of fatigue-crack growth in the near-threshold region of fatigue-crack propagation (ΔK ≤ 8 MPa·m1/2). Under the same fatigue cycle, the specimens under the action of larger creep stress endured a longer fatigue stable-propagation time and a faster fracture speed. Comparing the effect of creep-aging temperature and creep stress, the creep-aging temperature plays a dominant role in the fatigue-crack propagation of creep-aged 2524 aluminum alloy.

Load and hold conical indentation responses calculated for materials having creep stress exponents of 1.15, 3.59 and 6.60 are regarded as input ‘experimental’ responses. A Bayesian-type statistical approach (Zhang et al. 2019 J. Appl. Mech. 86 , 011002 ( doi:10.1115/1.4041352 )) is used to infer power-law creep parameters, the creep exponent and the associated pre-exponential factor, from noise-free as well as noise-contaminated indentation data. A database for the Bayesian-type analysis is created using finite-element calculations for a coarse set of parameter values with interpolation used to create the refined database used for parameter identification. Uniaxial creep and stress relaxation responses using the identified creep parameters provide a very good approximation to those of the ‘experimental’ materials with stress exponents of 1.15 and 3.59. The sensitivity to noise increases with increasing stress exponent. The uniaxial creep response is more sensitive to the accuracy of the predictions than the uniaxial stress relaxation response. Good agreement with the indentation response does not guarantee good agreement with the uniaxial response. If the noise level is sufficiently small, the model of Bower et al. (1993 Proc. R. Soc. Lond. A 441 , 97–124 ()) provides a good fit to the ‘experimental’ data for all values of creep stress exponent considered, while the model of Ginder et al. (2018 J. Mech. Phys. Solids 112 , 552–562 ()) provides a good fit for a creep stress exponent of 1.15.

The paper presents a procedure for assessing the condition of the current structure of boiler tubes. First, a modified version of the Garofalo equation is proposed to model the strain-and-stress st...

In the present paper, we have studied the temperature and pressure dependent creep stress analysis of spherical shell. The review is critical to enhance the warm resistance of spherical shells in high-temperature conditions. The effect of different parameters was studied and it was noticed that the parameter n has a significant influence on the creep stresses and strain rates. Creep stresses and strain rates are ascertained on the premise of summed up strain measures and Seth’s transition hypothesis. This investigation is completed to demonstrate the impacts of temperature on the creep stresses and strain rates in the spherical shell. The resulting quantities are computed numerically and depicted graphically. It has been watched that the spherical shell made of an incompressible material is on more secure side of configuration when contrasted with the shell made of a compressible material.

The low-temperature creep deformation characteristics and primary creep behaviour of TA2 were analysed by the small punch creep (SPC) test in this paper. Uniaxial creep and SPC tests were conducted to estimate the temperature and load influences on the creep deformation. At room temperature and 373 K, an increase in the creep load and temperature caused a rapid increase in the creep deflection, exhibiting a greater primary creep deformation. Considering the non-linear deformation process, a modified membrane stretch model was introduced and a deformation analysis was performed. The creep deformation evolution process of a SPC test specimen was discussed. An apparent decline was observed in the equivalent creep stress and uniform thickness reduction was found. The material characteristics and stress reduction caused by the non-linear deformation were believed to be the cause of the primary creep behaviour of TA2. The results showed that SPC characteristic differs from that of the uniaxial creep test. To simplify the creep curve, a modified power relation between the creep deflection and time was established. Based on the constant strain rate test results, consideration of the strain rate sensitivity was applied to rationalise the low-temperature creep behaviour of TA2 in the uniaxial and multiaxial creep tests. Finally, a modified creep analysis model for the SPC test was proposed to predict the low-temperature creep deformation.

Fire resistance and seismic performance are the two of the most important considerations for the design of construction steel. The fire resistance of the high-strength construction steel, Q460FRE, which contains low Mo, was investigated by means of creep test under different experiment conditions. Then, the seismic performance of the Q460FRE steel, after being subjected to various fire temperatures, was characterized by internal friction analysis. The Q460FRE steel exhibited excellent fire resistance, and the yield strength at 600 °C was 67.4% of the yield strength at room temperature. The constitutive equation of creep stress and fracture time at 600 °C was established. The bearing tensile stress of the Q460FRE steel should be lower than 337 MPa to ensure at least 1 h to evacuate from the fire. The creep stress and alternating temperature significantly affected the serration behavior of the creep strain. The internal friction analysis indicated that the seismic performance of the Q460FRE steel decreased with the increase in fire temperature, being consistent with the analysis results of the seismic performance characterized by the yield ratio.

This paper addresses the viscoelastic behaviour of a commercially available cold-curing structural epoxy adhesive, under different preparation, curing and hygrothermal conditions. The main parameters studied were the preparation method (influence of the degassing and the temperature of the initial curing), the creep stress level, and the hygrothermal conditions. Tensile creep tests last up to 2400 hours. Test results revealed that the preparation method has great influence on the tensile properties of the adhesive, particularly on the viscoelastic response where degassing and curing at 20 ºC showed lower creep deformations. Specimens under 98% of relative humidity faced tertiary creep and then rupture. For the adopted levels of creep stress, the adhesive shows a linear creep behaviour, being parameterized using the Burgers and the modified Burgers equations., This work was carried out in scope of the project FRPLongDur POCI-01-0145-FEDER-016900 (FCT PTDC/ECM-EST/1282/2014) funded by national funds through the Foundation for Science and Technology (FCT) and co-financed by the European Fund of the Regional Develop-ment (FEDER) through the Operational Program for Competitiveness and Internationalization (POCI) and the Lisbon Regional Operational Program and, partially financed by the project POCI-01-0145-FEDER-007633. This work was also partly financed by FCT/MCTES through national funds (PIDDAC) under the R&D Unit Institute for Sustainability and Innovation in Structural Engineering (ISISE) , under reference UIDB/04029/2020. The authors also like to thank the S&P Clever Reinforce-ment Iberica Lda.com pany for providing the materials. The first author wishes also to acknowledge the grant SFRH/BD/131259/2017 provided by Fundacao para a Ciencia e a Tecnologia (FCT) , while the fourth author acknowledge his sabbatical grant SFRH/BSAB/150266/2019, provided by FCT, financed by European Social Fund and by national funds through the FCT/MCTES.

Improving the graphitic structure in carbon nanofibers (CNFs) is important for exploiting their potential in mechanical, electrical and electrochemical applications. Typically, the synthesis of carbon fibers with a highly graphitized structure demands a high temperature of almost 2500 &deg, C. Furthermore, to achieve an improved graphitic structure, the stabilization of a precursor fiber has to be assisted by the presence of tension in order to enhance the molecular orientation. Keeping this in view, herein we report on the fabrication of graphene nanoplatelets (GNPs) doped carbon nanofibers using electrospinning followed by oxidative stabilization and carbonization. The effect of doping GNPs on the graphitic structure was investigated by carbonizing them at various temperatures (1000 &deg, C, 1200 &deg, C, 1500 &deg, C and 1700 &deg, C). Additionally, a stabilization was achieved with and without constant creep stress (only shrinkage stress) for both pristine and doped precursor nanofibers, which were eventually carbonized at 1700 &deg, C. Our findings reveal that the GNPs doping results in improving the graphitic structure of polyacrylonitrile (PAN). Further, in addition to the templating effect during the nucleation and growth of graphitic crystals, the GNPs encapsulated in the PAN nanofiber matrix act in-situ as micro clamp units performing the anchoring function by preventing the loss of molecular orientation during the stabilization stage, when no external tension is applied to nanofiber mats. The templating effect of the entire graphitization process is reflected by an increased electrical conductivity along the fibers. Simultaneously, the electrical anisotropy is reduced, i.e., the GNPs provide effective pathways with improved conductivity acting like bridges between the nanofibers resulting in an improved conductivity across the fiber direction compared to the pristine PAN system.

Long-term strength (LTS) of rock materials is important for the long-term stability analysis and the failure prediction of structures in rock engineering. Numerous studies have been carried out on the LTS for various kinds of rock; however, the effects of initial damage on the LTS and creep failure time of rock have not been conducted. In the present study, the creep experiment with controllable initial damage state of rock was designed. Then, the LTS of rock specimens with different initial damage was determined by four methods (i.e., the isochronous stress-strain curve method, the steady creep discriminated method, the volumetric strain inflexion point determined method, and the intersection of the steady creep rate method). The results show that, with the increase in the initial damage, the LTS of rock decreases and the relationship between the initial damage and the LTS of rock can be described as a linear function. Finally, an evaluation method for predicting the creep failure time of rock under a single stress level was proposed. In addition, the creep failure time of rock with different initial damage under different creep stress levels was obtained by the method. The results indicate that both the initial damage and the creep stress levels have a great influence on creep failure time, i.e., greater initial damage or creep stress leads to a shorter period for rock failure. Thus, for analyzing the long-term stability of rock mass structure, not only the influence of in situ stress but also the initial damage state of the surrounding rock should be considered.

The present work aims at investigating the coupling between thermal ageing/degradation and creep behavior of PEKK and C/PEKK composites at 200 °C, above the glass transition temperature (which is close to 160 °C). Ageing tests were carried out at 200 °C, under 5 bar of oxygen conditioned atmosphere. DSC scans and DMA tests were carried out. Tests results discussion were supported by indentation tests and image analysis of the aged specimen cross-section. Coupled oxidation-thermomechanical-creep tests were carried out on C/PEKK samples at 200 °C under a 5 bar O2 environment. The resulting strain response was compared to creep tests carried out on C/PEKK samples at 200 °C under air at similar creep stress. Such comparison allowed to investigate the presence of degradation phenomena in C/PEKK samples. The analysis of X-rays µ-tomography scans allowed investigating the possible occurrence of degradation, and to discriminate between pure thermal and thermomechanical coupling with degradation phenomena.

This study was carried out to investigate the creep life at the high temperature of the Alloy 718 for automotive engine components using the initial strain parameter method (ISPM). Creep tests have performed at elevated temperatures in the range of 550 oC to 700 oC in this work. We also carried out constant stress creep tests. The initial strains were measured during 1 minute after loading. Both the creep stress and rupture time depend on the initial strain. We calculated the creep life of Alloy 718 by using the creep life prediction equations obtained from the ISPM. Then, we compared the creep life predicted by the ISPM to the Larson-Miller parameter (LMP). The experimental rupture time and the calculated rupture time by using the ISPM agreed with a confidence level of 95 %. The creep life predicted by using the ISPM was in very good agreement with the creep life predicted using the LMP method.

The authors’ research group has investigated the close relation between the time-dependent behaviors under various loading conditions through laboratory tests, theories, and constitutive equations. It is well known that water accelerates the time-dependent behaviors of rocks; water enlarges the loading-rate dependence of strength and shortens the time to failure (creep lifetime) under constant stress level (ratio of creep stress to strength). This paper introduced the theory in the previous studies based on the rate process theory and the stochastic process theory, and found that the theory disregarded the effect of a change in water conditions on the strength and creep lifetime of rocks. The theory was briefly modified and then applied to the strength, creep lifetime, and their variability in both dry and wet conditions. To validate the modified theory, strength tests and creep tests were conducted under uniaxial compression with Sanjome andesite. The constants in the modified theory were calculated from the strength, and then the creep lifetime collected from the creep tests was examined with the formulae derived from the modified theory. The test results in dry and wet conditions were consistently elucidated by the modified theory, which indicates that the strength and creep lifetime of Sanjome andesite are approximated by the simple formulae in the theory. Creep lifetime was estimated from strength, and the results in dry conditions were estimated from those in wet conditions and vice versa, based on this study.

In this paper, a two-parameter approach and finite element method were conducted to investigate the constraint effect of six different types of cracked specimen geometries on the creep crack initiation (CCI) time. The theoretical enhanced model of the C∗–Q∗ approach, which considers the load-independent constraint parameter Q∗, was proposed to predict the CCI time around a sharp crack tip. The order of Q∗ values and the creep damage accumulation rate for the different specimen geometries was C(T) > CS(T), SEN(B) > SEN(T) > DEN(T) > M(T). Whereas the CCI times were ordered conversely. The influence of the constraint on hydrostatic stresses, triaxiality and multiaxial strain factor was discussed. The suitability of the two-parameter approach was verified due to the comparison of CCI times between theoretical and simulated results, which demonstrated that the C∗–Q∗ two-parameter prediction approach under stress intensity factor—Riedel–Rice (K-RR) control (initially by K, then by transient creep stress or Riedel–Rice conditions) and Hutchinson–Rice–Rosengren—Riedel–Rice (HRR-RR) control (initially by plastic HRR control, then by RR conditions) could conservatively and effectively predict the CCI times. The K-RR solutions were more accurate when initial stress intensity factor K 6 MPa m1/2.

A method for calculating the creep damage during low cycle fatigue (LCF) (tensile plasticity reversed by compressive plasticity, PP) controlled by strain rate e was proposed. According to the equivalent principle of creep damage with the same reduction in cross section, a creep damage model considering a continuously changing stress was established based on the equivalent creep stress σ en , which was determined by the total creep damage, the actual creep time (the total time of tensile loading stage in the whole process of fatigue), and the fatigue life during LCF (PP). For Sanicro 25 alloy, σ en first increases quickly, and then increases linearly and slowly with increasing e . Based on the creep damage model, the total creep damage and fatigue life under different e were predicted accurately. To study the application of the method for calculating the creep damage, the creep damage model for 316 stainless steel was established and verified. At a constant e , when γ decreases or K increases, the creep damage decreases and the fatigue life increases. Based on the variation trend, the engineering application of the creep damage model was discussed.

The tensile and creep behaviour of a high-density polyethylene grade used to manufacture pipes were evaluated as a function of the time of exposure to UV radiation. The effect of the creep stress level was also analysed. A threshold radiation dose was determined, above which UV radiation causes loss of the macroscopic mechanical behaviour for this particular HDPE grade. Below the threshold value, macromolecular modifications were also taking place, but the deleterious effect of UV radiation was masked by the development of a more rigid structure. Scanning electron microscopy, melt flow index and thermogravimetric analysis were used to support the results of the mechanical analysis.

This study focused on the shear strength of unsaturated soil subjected to creep stress loading for large deformation (i.e. leading to large reduction of matric suction). Solving the practical problems regard to unsaturated soils is considering the properties of unsaturated soil that are subjected to creep progress before occurrence instability of natural slope. This testing program consisted of a creep test and a static compression triaxial test under undrained condition with matric suction control. A silty soil was used and growing of excess pore-water pressure (i.e. reduction of suction) was closely related to large deformation. Effective stress paths of creep behaviour for unsaturated soil were described with that of apparent saturated soil. The failure line was confirmed using effective stress paths of undrained triaxial compression test for apparent saturated soil. Behaviours around the failure line between unsaturated soil subjected to creep stress and apparent saturated soil were discussed and creep failure according to increase of creep stress was possible to interpret using the failure line defined by apparent saturated soil.

The available models for estimating the creep strains of concrete generally assume that concrete is a homogeneous material. Since concrete is a composite, such models are incapable of taking into a...

From experimental creep rates of high temperature alloys, it is found they can be classified into three types: with creep rate decreasing only, with creep rate deceasing firstly but accelerating after a certain time, and with creep rate increasing slowly but accelerating after a certain time. These three typical creep types can be characterized in a unified form, by dividing creep process into two creep stages distinguished by the critical creep time. Before the critical time, the first stage has decreasing or slowly increasing creep rate only, after the critical time, the second stage also has accelerating creep rate besides the previous creep rate. A new generalized creep constitutive relationship with creep time and four stress-dependent parameters has then been proposed. The stress-dependent parameters can be interpolated from test results under various stress levels. It is found that the interpolation functions should be expressed in different styles according to the stress level distinguished by a critical creep stress. The critical creep stress can be determined just by the glass strain variation under various stress levels. More meaningfully, critical creep stress corresponds to the cross point in the bi-linear model of creep failure life in fact. It is also found there is a non-creep stress below which creep deformation can be neglected.

An evaluation was conducted on the impact of a Pt–Al bond coat on the creep behaviours of a second-generation Ni-based single crystal superalloy at 750 °C/820 MPa, 850 °C/630 MPa, 1038 °C/137 MPa and 1100 °C/112 MPa, respectively. As revealed by the creep results, the creep behaviours of the coated superalloy were inferior to those of the bare superalloy to some extent under all testing conditions. The maximum deterioration in the strain-to-fracture and time-to-rupture was observed at 1100 °C/112 MPa (7.3%) and 750 °C/820 MPa (74.7 h), respectively. Following the creep test, deconvolution calculations were performed, which indicated that the creep stress for all core superalloys was as low as approximately 1.7–1.9% of the applied stress while the bond coat stress was 4.0–10.4% that at the minimum creep rate. Moreover, TEM analyses revealed that within the superalloy substrate adjacent to the bond coat, there were fewer stacking faults and an abundance of dislocation debris observed within γ′-Ni3Al at 750 °C/820 MPa. The misfit of the γ/γ′ phase was insignificant and the dislocation networks became irregular at 1100 °C/112 MPa. With an increase in temperature from 750 °C to 1100 °C, the fracture mechanism of both the substrate and bond-coat was found to shift from quasi-cleavage to micro-void coalescence. The quasi-cleavage cracks occurring within the bond coat increased the number of connections with creep cracks to cause spreading from the interior of the samples, thus resulting in premature failure at 750 °C/820 MPa and 850 °C/630 MPa. However, the degradation at 1038 °C/137 MPa and 1100 °C/112 MPa was ascribed to the increasing thickness of the inter-diffusion zone, as well as the volume of the blocky γ′-Ni3Al phase inside the bond coat and topologically close-packed phases in the superalloy near the bond coat.

Static annealing of (100)-oriented NaCl single crystals at a homologous temperature of ~0.92 showed a significant reduction in the etch-pit dislocation density during the first 100 h, which remained constant during subsequent annealing up to 225 h. The existence of the frustration of the dislocation density, at a value of ~109 m-2, in NaCl is similar to the observations in the high purity Al and LiF single crystals. The implication of the frustration dislocation density on the observation of a creep stress exponent of >1 in the Harper-Dorn creep regime is discussed.

Purpose This paper aims to investigate the creep properties of the bulks of low-Ag Cu/Sn-Ag-Cu-Bi-Ni/Cu micro solder joints from 298 to 358 K. The creep constitutive modelling was developed. Meanwhile, the creep mechanism of the bulks of Cu/Sn-Ag-Cu-Bi-Ni/Cu micro solder joints was discussed. Design/methodology/approach The creep properties of the bulks of low-Ag Cu/Sn-Ag-Cu-Bi-Ni/Cu micro solder joints from 298 to 358 K were investigated using the nanoindentation method. Findings The results of the experiments showed that the indentation depth and area increased with increasing temperatures. At the test temperature of 298-358 K, the creep strain rate of the bulks of the micro solder joints increases with the rising of the tested temperature. The values of creep stress exponent and activation energy calculated for the bulks of Cu/Sn-Ag-Cu-Bi-Ni/Cu micro solder joints were reasonably close to the published data. At the tested temperatures, dislocation climb took place and the dislocation climb motion was controlled by the dislocation pipe mechanism, and the second-phase particles enhancement mechanism played a very important role. Originality/value This study provides the creep properties of low-Ag Cu/Sn-Ag-Cu-Bi-Ni/Cu solder joints at different temperatures. The creep constitutive modelling has been developed for low-Ag Cu/Sn-Ag-Cu-Bi-Ni/Cu solder joints.

Extensive finite element analyses have been conducted to investigate the effect of creep properties of materials on the creep crack-tip constraint parameter R*. The results show that the parameter R* increases with increasing power law creep stress exponent n of materials, and it is sensitive to lower n values and lower in-plane and out-of-plane constraints. In the engineering estimations of the creep constraint parameter R*, only the creep exponent effect needs to be considered for cracked components with lower exponent n and lower constraint, and the Norton’s coefficient A of power law creep materials can be ignored due to its insensitivity to the R*. The R*-n relation formulae have been established for single-edge-notched tension specimen (it has similar constraint level to pressurised pipes) with different in-plane and out-of-plane constraints, and they may be used to estimate the creep crack-tip constraint levels for cracked pipes with lower exponent n and lower constraint.

This paper presents equations for estimating the crack tip characterizing parameters C(t) and J(t), for general elastic-plastic-creep conditions where the power-law creep and plasticity stress exponents differ, by modifying the plasticity correction term in published equations. The plasticity correction term in the newly proposed equations is given in terms of the initial elastic-plastic and steady-state creep stress fields. The predicted C(t) and J(t) results are validated by comparison with systematic elastic-plastic-creep FE results. Good agreement with the FE results is found.

This paper explores the behavior of steel components operating at high temperature subjected to combinations of applied and residual stresses. Two series of experiments are summarized that used Type 316H stainless steel samples tested at 550 °C. First, creep stress relaxation under displacement control with elastic follow-up is explored. Experimental results are compared with theoretical predictions and show that accounting for stress history during relaxation is necessary. A second set of experiments explores how combinations of applied and residual stress influence creep crack incubation, again with elastic follow-up. Incubation times with elastic follow-up close to conventional displacement control are longer than constant load.

Co-base superalloys show attractive properties for the strengthening of γ'-Co3(Al,W) precipitates. Directional coarsening of γ' precipitates under external stress forms the rafting morphology, which plays a crucial role in the creep property. The microelastic theory, coupled with the crystal plasticity model, is utilized to study the rafting behavior during the early stage of creep in Co-Al-W superalloys. High W content and large eigenstrain can produce a lower creep strain of early stage, for the subdued plastic deformation in the γ matrix. Driving forces of element redistribution during rafting are compared among the chemical, interfacial and elastic contribution, in which the elastic contribution is dominant in rafting. With the change in creep stress from 197 MPa to 297 MPa, the element redistribution is strengthened, and the rafting is accelerated in Co-10 Al-9 W (at.%) alloy. Elements redistribution in the rafting process widens the γ/γ' interface. In contrast, the width of γ/γ' interface along the rafting direction begins to decrease when the coalescence of γ' precipitates happens. The straight-forward insight on the element diffusion during rafting and the related creep properties guides the composition design and rafting behavior prediction of this novel Co-Al-W superalloy.