Creep Damage Evaluation Research Articles

Investigation of creep damage accumulation under variable loading conditions in nickel-based single crystal superalloys

Nickel-based single crystal (SX) turbine blades typically experience varying load conditions during operation. It is essential to consider the effects of changing stress and temperature when evaluating creep damage. The commonly used time-fraction method, based on the linear damage rule (LDR), has been demonstrated to be inconsistent with experimental results under certain loading conditions. There is a lack of research on damage accumulation under variable loading conditions specifically for SX superalloy. This paper conducted two-stage creep experiments for SX superalloy DD6 at 980 °C with stress levels of 230 MPa and 320 MPa, and at 1050 °C with stress levels of 135 MPa and 230 MPa. The load for the experiment was set based on the typical operating conditions of the SX turbine blades. The evolution of the microstructure under variable loading was observed using scanning electron microscope (SEM) and transmission electron microscope (TEM). The result of creep experiments revealed that the damage accumulation behavior of the SX superalloy does not conform to the LDR. Furthermore, in two-stage loading tests, the creep damage accumulation behavior differs from the classical sequence effect of fatigue loading. SEM and TEM analyses suggested that this phenomenon might be associated with the varying degradation rates of microstructures in SX superalloy.

In situ monitoring of high-temperature creep damage in CrMoV high-strength structural steel using acoustic emission

The accurate detection and evaluation of creep damage in structural steel is essential for ensuring the safety and reliability of high-temperature structures. In this work, the in situ monitoring and identification of high-temperature creep damage in CrMoV high-strength steel under different stress levels was conducted using the acoustic emission (AE) technique. A denoising procedure was proposed and applied to raw AE signals to reduce the noise unrelated to the growth of creep damage. To characterize the creep damage both qualitatively and quantitatively, multiple AE features were extracted from the creep-related signals, including peak amplitude, count, and information entropy. The results showed that a sudden increase in multiple cumulative parameters accompanied by high-entropy and high-count AE signals can accurately identify the transition from the secondary to the tertiary stage and can offer an early warning of the final rupture. In the log-log scale, the quantitative relationship between the AE rate and the minimum creep rate was found to be linear. Additionally, the substantial plastic deformation, and the nucleation, growth and coalescence of creep cavities were the dominating source mechanisms contributing to the generation of numerous high-count and high-entropy AE signals in the tertiary creep stage. Findings from this work will offer an approach for in situ monitoring and evaluation of the state of creep damage of high-temperature structures based on AE.

Damage evaluation of the austenitic heat-resistance steel subjected to creep by using Kikuchi pattern parameters

Abstract Austenitic heat-resistance steel is widely employed in ultra-supercritical power generation because of its superior performance. This article reports the evaluation of creep damage of a typical Super304H austenitic heat-resistance steel by using the Kikuchi pattern parameters, such as band contrast (BC), mean angular deviation (MAD), and band slope (BS). It was found that the mean BC and MAD values did not show a monotonic changing trend and then an in-depth research that distinguished grain boundary and grain interior regions of Super304H steel was studied, respectively. The results manifested that, with the increase of creep time, both the mean BC and BS values showed a monotonic decreasing trend, while the MAD did not. Besides, compared with the BC, the change amplitude of BS was larger. The results indicate that the BS could be a useful indicator, which can be utilized to evaluate the creep degree of Super304H steel in a quantitative way.

Nondestructive Evaluation of Long-Term Creep Damage and Its Availability in ASME Grade 91 Steel Welds

Nondestructive Evaluation of Long-Term Creep Damage and Its Availability in ASME Grade 91 Steel Welds

A novel approach for evaluating creep damage and cavitation in copper bicrystals subject to constant load

Creep in metal alloys is an important failure mode for high temperature and stress applications, but despite extensive study it is still not fully understood, particularly the early stage of creep cavity formation. This paper describes a novel constant load cantilever beam test to investigate creep damage and cavitation at grain boundaries in copper bicrystals. Bicrystals of copper have been prepared with the grain boundary oriented normal to the beam long axis, allowing the development of damage at a single boundary to be studied. Tests were conducted at a temperature of 285°C in a vacuum of 10−10 MPa. Creep cavitation is observed in copper bicrystals of {001} and {111} orientation with a 22° rotation at the boundary. We compare these data with observations for a polycrystalline copper specimen.

Creep detection of Hastelloy X material for gas turbine components with nonlinear ultrasonic frequency modulation

Creep damage is one of the main failure modes in hot-gas‑leading components in gas turbines, which results from high temperatures along with mechanical loads. The aim of this study is to clarify the metallurgical creep behaviour of the Hastelloy X material and detect and evaluate creep damage at an early stage with a nonlinear ultrasonic modulation technique. For this purpose, multiple samples were examined to demonstrate that pores and microcracks in grain boundaries spread from the outside to the inside. Inside the specimen, molybdenum was identified as the main precipitation element. In addition, the chromium diffusion in the outer areas led to the depletion of this element and favoured the formation of pores and microcracks. Failures were proven with nonlinear dual-frequency ultrasound technology. Moreover, two different longitudinal waves were sent into the samples to use harmonic and modulated response frequencies for evaluation. As a result, harmonic frequencies offered a favourable prediction of pore sizes, whereas defined sideband frequencies reacted very sensitively to the damage density and area distribution of the failures. This study offers a method for detecting creep damage with nonlinear ultrasound techniques at an early stage as well as for differentiating between pores, microcracks, dislocations and precipitation. Therefore, the design of future gas turbine components made of Hastelloy X can be adapted with regard to the shown metallurgical behaviour and damage signatures.

A creep-fatigue life prediction model considering multi-factor coupling effect

Abstract A creep-fatigue life prediction model based on a novel creep damage evaluation method (NCDEM) considering the multi-factor coupling effect is presented in this paper. Further, to verify the validity and practicability, the creep-fatigue life of GH4169 at 650 °C is calculated to compare with the experimental results. Ultimately, the prediction results are respectively compared with those of the creep-fatigue life prediction models based on the time fraction method (TFM), ductility exhaustion method (DEM), and strain energy density exhaustion method (SEDEM). Consequently, the prediction results are distributed in ±1.5 times dispersion band, which elucidates the creep-fatigue life prediction model proposed based on the NCDEM has the best ability.

Creep damage evaluation via uniaxial miniature testing for multiaxially stressed Mod.9Cr–1Mo steel components

In this study, creep damage in a biaxially creep-damaged Mod.9Cr–1Mo component has been examined via uniaxial miniature creep testing. Using cruciform specimens, biaxial tension creep tests were conducted at principal stress ratios (λ) of λ=0.0 and 1.0 at 923 K. The principal stress ratio is the ratio of the y-directional principal stress (σy) to the x-directional principal stress (σx) in a cruciform specimen (λ=σy/σx). The equi-biaxial creep test was interrupted at a half lifetime, and two miniature specimens were machined from the damaged cruciform specimen in the x- and y-direction. The applicability of the linear cumulative creep damage rule was assessed using the results of the miniature lifetimes. The linear cumulative creep damage rule gave an unconservative estimate to the remaining life of a biaxially damaged cruciform specimen. Using the hyperbolic constitutive relationship, the unconservative estimate according to the damage accumulation was explained.

Creep Rupture Behavior for Welded Pipe of Ni-Based Alloy Using Full Thickness Specimen and Application of Nondestructive Damage Detection

Creep rupture behavior of HR6W weldment using full thickness specimen cut from the circumferentially welded pipe was investigated for evaluating its applicability to advanced USC (A-USC) power plant. Creep tests were conducted at 750°C for durations up to 8,600 hours, and damage morphology of weldment during creep was characterized. The applicability of several nondestructive detection methods to the creep damage evaluation was discussed. It was found that full thickness specimen was broken at the base metal and main crack was inclined approximately at 45 degrees to the axial direction of the specimen. Based on stress at the ruptured location, times to creep rupture of full thickness specimen were comparable with those of the smooth bar specimen of base metal. Among several nondestructive detection methods, a small crack in base metal on the outer surface was first observed at life fraction of 35% by replication. PT can detect the crack up to 0.5mm in length in about half of the life. The crack whose length is longer than 3mm can be detected by UT in latter half of the life.

Development of a structural integrity evaluation program for elevated temperature service according to ASME code

A structural integrity evaluation program (STEP) was developed for the high temperature reactor design evaluation according to the ASME Boiler and Pressure Vessel Code (ASME B&PV), Section III, Rules for Construction of Nuclear Facility Components, Division 5, High Temperature Reactors, Subsection HB. The program computerized HBB-3200 (the design by analysis procedures for primary stress intensities in high temperature services) and Appendix T (HBB-T) (the evaluation procedures for strain, creep and fatigue in high temperature services). For evaluation, the material properties and isochronous curves presented in Section II, Part D and HBB-T were computerized for the candidate materials for high temperature reactors. The program computerized the evaluation procedures and the constants for the weldment. The program can generate stress/temperature time histories of various loads and superimpose them for creep damage evaluation. The program increases the efficiency of high temperature reactor design and eliminates human errors due to hand calculations. Comparisons that verified the evaluation results that used the STEP and the direct calculations that used the Excel confirmed that the STEP can perform complex evaluations in an efficient and reliable way. In particular, fatigue and creep damage assessment results are provided to validate the operating conditions with multiple types of cycles.

An online monitoring method for creep-fatigue life consumption with real-time damage accumulation

Online damage evaluation based on monitored complex cyclic loadings has become an important technique for reliability assessment, especially in high-temperature environments where creep occurs in addition to fatigue. Accuracy and rapidity of calculation are basic requirements for online damage evaluation methods. However, current creep damage evaluation methods seldom consider the fluctuation in stress, which leads to inaccuracy in life-consumption estimates. In addition, traditional cycle-counting methods are not applicable to online use. In this study, an online creep-fatigue damage evaluation method is proposed that accounts for the creep behavior that occurs under fluctuating loads. The cycle-counting method is modified from a rainflow-counting algorithm; it broadens the counting of half-cycles and adopts a new equivalent temperature in the stress-strain response calculation. The proposed method is explained in detail and demonstrated with a case study. The application of this method to a high-temperature, high-pressure pipe demonstrates its online applicability and accuracy. A time-matching algorithm is developed to display the damage evolution in real time, thus revealing the link between the incremental damage and the current load conditions, and yielding an intuitive demonstration of a given component’s state of health.

Creep Damage Evaluation of Ni-base Single Crystal Superalloys by X-ray Diffraction

Creep Damage Evaluation of Ni-base Single Crystal Superalloys by X-ray Diffraction

Application of Machine Learning and Statistical Analysis to Creep and Creep-Fatigue Damage Evaluation for Austenitic Stainless Steel

Application of Machine Learning and Statistical Analysis to Creep and Creep-Fatigue Damage Evaluation for Austenitic Stainless Steel

Simplified approach for creep evaluation in superheaters

This paper is focused on the estimation of temperature distribution on superheater tube bundles for the consequent creep damage evaluation. The precision of estimated temperatures is very important because it significantly affects residual creep life. Conditions in the tube bundle can be simulated using CFD, however, a full 3D simulation would be computationally intensive and not fit for practical use. In order to mitigate this issue, the new approach considered in this paper uses a series of 2D CFD simulations in multiple sections using known inlet flow conditions. Those conditions have been investigated in previous work using 3D CFD simulation of flue gas flow starting in the combustion chamber and ending just before the superheater in the second pass. The paper also deals with possible temperature differences that may arise due to simplifications in the proposed approach. Lastly, the residual creep life is estimated using obtained temperature distribution on the tube bundle.

Nonlinear Lamb wave for the evaluation of creep damage in modified 9Cr–1Mo steel

Higher harmonic generation using nonlinear ultrasonic is known to be very sensitive to microstructural degradation. This technique relies on nonlinear stress-strain relation i.e. nonlinear Hooke's law. The quantitative measurement to characterize the materials degradation is done by the nonlinear ultrasonic parameter, β. The objective of this work is to use nonlinear ultrasonic using Lamb wave to evaluate creep damage in modified 9Cr–1Mo steel. Relative change in β was seen to be dependent on the higher order elastic constants that evolve with microstructure as creep progresses as well as sub-structural changes. The extent of damage in the material due to creep exposure was evaluated by Kernel Average Misorientation (KAM) from EBSD and co-related with nonlinear acoustic parameter.

Experimental Study on High Temperature Creep Damage of 1Cr5Mo Steel Based on Magnetic Parameters

Aiming at the evaluation of creep damage at elevated temperature of ferromagnetic materials in engineering application, the hysteresis loop measurement technology was adopted to study the creep damage behaviour of 1Cr5Mo steel. The creep testing at 600°C/90MPa of 1Cr5Mo steel were carried out to prepare specimens with various degrees of creep damage. The variation of magnetic parameter including coercivity (HC) and remanence (Br) with creep damage was analyzed. The microstructure evolution of 1Cr5Mo steel with different degrees of creep damage was observed by optical microscope. The results show that the coercivity and remanence show certain regularity with creep damage at elevated temperature. Moreover, the regularity is repeatable which indicates that the measurement technology based on magnetic parameters can be used for the evaluation of high temperature creep damage.

Creep Void Formation and Rupture Lifetime in Multiaxial Stress States

This paper discusses creep void formation and rupture lifetimes in multiaxial stress states for a SUS 304 stainless steel at elevated temperatures. Biaxial and triaxial tension creep tests were performed using a cruciform and a cubic specimen, respectively. These two types of the specimens were designed to achieve uniform equi-biaxial and equi-triaxial stress distributions by a finite element analysis in the gage parts. Void formation at grain boundaries was observed by intermitting biaxial creep tests and by interrupting triaxial creep tests. Creep rupture lifetimes were also obtained in biaxial and triaxial creep tests. Biaxial stresses increase the void formation but give a little influence on a creep rupture lifetime in the correlation with von Mises equivalent stress. Triaxial stresses also increase the void formation and drastically reduce a creep rupture lifetime in the correlation with von Mises equivalent stress. Evident void formation in an equi-triaxial stress condition demonstrates that von Mises equivalent stress is not a suitable measure to evaluate creep damage development in multiaxial stress states. A new equivalent stress is proposed to evaluate creep rupture lifetimes in biaxial and triaxial stress states.

Evaluation of Creep damage in Welded Joint for High Cr Ferritic Heat Resisting Steels with Nonlinear Ultrasonics

Evaluation of Creep damage in Welded Joint for High Cr Ferritic Heat Resisting Steels with Nonlinear Ultrasonics

Creep damage evaluation at stress concentration portion on a 304 stainless steel

Creep damage evaluation at stress concentration portion on a 304 stainless steel

A study on creep damage evaluation using a miniature specimen for pre-biaxial-creep-damaged materials

A study on creep damage evaluation using a miniature specimen for pre-biaxial-creep-damaged materials