Creep-fatigue Crack Growth Tests Research Articles

Crack tunnelling analysis of P91 steel under creep-fatigue loading with various force ratios and hold times at 600 ºC

ABSTRACT The advent of non-conventional and renewable energy sources places a tremendous strain on generic sources of power generation, which have been historically conceived and designed for constant-load criteria, as they are compelled to conform to a ‘fluctuating’ mode of operation so as to comply with modern-day energy requirements. Power plants undergo a starting schedule, followed by sustained operation at intermediate loads when the energy requirement is mediocre. This renders the evaluation of material behaviour under creep-fatigue conditions, a critical component of failure prevention and prediction. ASTM has come up with a standard E-2760 for creep-fatigue crack growth (CFCG) testing, wherein Compact tension, C(T) specimens, are subjected to trapezoidal loading waveforms. In this article, the crack tunnelling phenomenon has been objectively evaluated along with exploring the propagation of crack growth fronts and the crack tip opening angles (CTOA).

Creep, fatigue, and creep-fatigue crack growth behaviours of P92 steel at 600 °C

High-temperature components in power generation plant are exposed to creep, fatigue, and creep-fatigue environment during service. The components are usually under multiaxial state of stress condition. Understanding how the material behaves under these loading and environment is essential in order to sustain and keep the plant safe. The present paper aims to investigate the creep rupture and crack growth behaviours of P92 steel at 600 °C. For creep rupture test, notched bars with two different notch radii were prepared and tested under creep condition between 250 and 3500 hours at 600 °C, while the C-shaped specimen was prepared for fatigue and creep-fatigue crack growth tests. The material tested under creep condition showed notch strengthening effect where the life of notched bar specimen increased compared to smooth specimens when the net stress against creep time data was plotted. The effect was more significant as the notch radius decreased. It was also observed that the rupture life of all specimens was von-Mises stress controlled. Based on the fatigue test, it was found that the frequency in a range of 0.1 Hz–10 Hz was insignificantly affecting the crack growth rate. Under creep-fatigue, however, the material showed frequency-dependent behaviour. Observation on the fracture surface revealed that the ductile dimple associated with plasticity was dominant for all creep specimens. In addition, frequency independent specimen was associated with the transgranular fracture, thus flat appearance was evident, while fracture surface of frequency dependent specimen was roughly associated with intergranular fracture.

Results of a Pilot Study in Support of a Round-Robin on Creep-Fatigue Crack Growth Testing of a Creep-Brittle Material for ASTM E2760

Abstract This paper describes pilot test results from a round-robin conducted in support of the standard ASTM E2760, Standard Test Method for Creep-Fatigue Crack Growth Testing, on creep-fatigue crack growth testing. Results from the pilot study are recommended to participants to develop their individual experimental testing. This round-robin is an international effort with 10 participants from five countries from Asia, North America, and Europe. All participants are testing compact type (C(T)) specimens according to the procedure described in the standard. A few participants are evaluating the Kb-bar (surface crack) specimen configuration. The test material is Inconel-718 (IN718) nickel-based alloy, which is a creep-brittle alloy. Pilot creep-fatigue crack growth tests were conducted at 600°C using C(T) specimens under constant load amplitude conditions. The tests were conducted at hold times of 0, 60, and 600 s. Creep-fatigue crack growth rates were analyzed using the methods proposed and included in the test standard. It was found that the procedures followed throughout the pilot test program were sufficient in generating the desired outcomes. As a result, it is proposed that the various participants follow the procedures and test matrix as close as possible and document any deviations. This will allow data from the various participants to be assessed and determine the levels of precision and bias in creep-fatigue crack growth data obtained using the ASTM standard.

Effect of creep-fatigue loading condition on crack tip fields of grade 91 steel at crack initiation and growth

In this paper, the effect of creep-fatigue loading condition on crack tip deformation and stress fields of Grade 91 steel at 600 °C at crack initiation and growth is studied. By using finite element debond analysis method, creep-fatigue crack growth tests using C(T) specimens under various load ratios and hold times are simulated, and the effect of creep-fatigue load ratio and hold time on the crack opening profile, equivalent stress, crack-opening stress and triaxiality at crack initiation and growth is presented. It is found that the crack tip deformation and stress fields under tension–compression creep-fatigue loading are overall different from those under tension-tension creep-fatigue and pure creep loading, due to re-sharpening of the crack tip.

Results of the ASTM Round Robin on Creep-Fatigue Crack Growth Testing of a P91 Steel

Abstract This article summarizes the results of a round robin (RR) study begun in 2013 to evaluate ASTM E2760-10, Standard Method for Creep-Fatigue Crack Growth Testing (Superseded). Thirteen laboratories from around the world conducted testing and reported results. The test material was a creep-ductile, ASTM Grade P91 9Cr-1Mo steel. All testing was performed using nominally 50-mm-wide compact tension specimens at a test temperature of 625°C using trapezoidal waveforms with hold times of 0, 60, and 600 s. The 600-s hold time condition was the primary condition used for assessing interlaboratory variability among crack growth rates. Loading/unloading times were 2 s each. All laboratories tested duplicate specimens that were identical in all respects except for the force amplitude levels. Results from prior studies that included tensile, creep deformation and rupture, and low-cycle fatigue properties were available for the analysis of tests conducted as part of this RR testing. Crack growth rates at 600-s hold time reported by six laboratories fall within a 95 % confidence band (interlaboratory variability) separated by a factor of 6.16 when correlated with ΔK, 10.76 when correlated with measured vales of (Ct)avg, and 7.17 when correlated with calculated values of (Ct)avg.

Performance of Alloy 709 under creep-fatigue at various dwell times

A comprehensive experimental evaluation of the creep-fatigue behavior of Alloy 709 at 750 °C is reported in this study. Alloy 709 is a 20Cr–25Ni austenitic stainless steel, with high temperature creep strength and corrosion resistance, which can potentially be used in structural components of nuclear power plants. Creep-fatigue crack growth (CFCG) experiments were conducted using an in-situ heating-loading and Scanning Electron Microscope (SEM) equipped with Electron Backscatter Diffraction (EBSD) detector. To study the “real-time” CFCG behavior of Alloy 709 at 750 °C with varying dwell times in vacuum, flat dog bone samples were prepared. A starter notch was added, and a pre-crack was introduced by high frequency fatigue cycles at room temperature. Prior to loading and heating the entire area ahead of the crack tip was mapped using EBSD. These maps were utilized to generate a set of Coincident Site Lattice (CSL) boundary maps from the area ahead of the crack tip. Upon completion of the EBSD and CSL mapping, the heating and loading of samples took place in the SEM. During the experiment, crack growth was monitored on the surface of the sample using SEM imaging and data was transferred over to the CSL maps, to highlight the crack path with respect to the grain boundary and precipitations arrangement in the sample. Some samples went through EBSD-CSL mapping before heating and loading along with Transmission Electron Microscopy (TEM) imaging post heating and loading. Comparing the CSL maps before and after crack growth provided additional details about the dependence of crack path and crack growth mode on microstructure, primarily grain boundary character, and dwell time. TEM analysis of the microstructure after the crack growth was employed to validate the findings of the in-situ heating and loading SEM data. Real-time monitoring of microstructural phenomena, such as void nucleation, grain boundary cavitation, slip activation, and resistance of twin boundaries to cracking during CFCG tests, sheds a new light on the crack growth mechanism. The results indicated that at lower dwell times, the crack mainly propagates in a transgranular fashion, with the aid of slip lines. At higher dwell time, intergranular cavitation dominates the crack growth. However, as more than 50% of grain boundaries are coherent twin boundaries, which are low energy boundaries resistant to cavitation, crack growth is delayed when reaching such boundaries and hence twin boundaries impart some resistance to crack propagation in Alloy 709 at high temperatures.

Judgment criterion of the dominant factor of creep-fatigue crack growth in a nickel-based superalloy at elevated temperature

Creep-fatigue crack growth (CFCG) tests on FGH97 at a temperature range from 650 °C to 750 °C with 90-s dwell time and at 750 °C with dwell time range 0–1500 s were performed. Based on the trinomial model including fatigue item, creep item, and creep–fatigue interaction item, the contributions of these three items to CFCG behavior at different stress intensity factor range (ΔK) under different loading conditions, including different temperatures for a given dwell time and various dwell time for a given temperature, were discussed. The time-dependent component (creep item + creep-fatigue item) and cycle-dependent component (fatigue item) were used to describe the test data and reveal the influence of ΔK levels on crack growth mechanism. The method to determine the dominant factor of CFCG behavior was discussed, and the judgment criterion of the dominant factor related to the ΔK level was built. The microscopic morphology of fracture surfaces under different loading conditions was observed by scanning electron microscope to confirm the results.

Characterizing crack growth behavior and damage evolution in P92 steel under creep-fatigue conditions

The present paper characterized crack growth behavior and damage evolution in P92 steel under creep-fatigue interaction conditions. Creep-fatigue crack growth tests were conducted at a constant load amplitude with various hold times. To reveal the role of constraint in creep-fatigue regime, specimens with different crack depths and thicknesses were used. Combination with a non-linear creep-fatigue interaction damage constitutive model, the crack growth and damage evolution behaviors were simulated using finite element method. Under creep-fatigue condition, time dependent crack growth rate increased as the duration period was reduced. This was attributed to the role of enhanced fatigue damage with the decreasing of the dwell time. Moreover, the deviation of crack growth rate with different dwell times became small as crack grew. With the crack length increasing, the creep damage level was improved regardless the dwell time, which may be due to the fact that the creep damage dominated crack growth in this stage. Furthermore, the crack growth rate increased as the crack depth became deep and the specimen thickness became large, as a result of the increased constraint level. A load-independent constraint parameter Q* was introduced to correlate the crack growth rate, which provided a good prediction for specimens under different constraint conditions.

Testing and assessment of cracking in P91 steels under creep-fatigue loading conditions

Abstract Future green energy options dictate that renewable energy source must be utilized when available. This poses a challenge to conventional power plant, which has generally been designed for base-load conditions, as they now need to operate in a ‘flexible’ manner. This flexible operation, in high temperature power plant components, could lead to a combination of creep and fatigue crack growth failure. Thus a better characterization of interactive creep-fatigue crack growth behaviour is required especially for assessing long term failure in plant. The industrial codes, such as R5 or BS7910, treat this interaction using linear accumulation of damage. However, this does not consider the degradation of properties and reduction in creep ductility in long term operations and their effect on the subsequent creep/fatigue behaviour. In this work creep-fatigue crack growth (CFCG) tests were performed on compact tension specimen of P91 steel in the as received and ex-service conditions at temperatures ranging between 600 °C to 625 °C, with the hold-times ranging from static to 600 s. The experimental results in addition to appropriate data from the literature have been analysed using stress intensity factor range, Δ K appropriate under fatigue control and the creep fracture mechanics parameter C* relevant under creep control. Scanning electron macroscopy (SEM) analysis confirms the influence of frequency on the mode of cracking. Within the scatter of experimental data for the present short term accelerated tests a linear cumulative damage rule can still predict the creep/fatigue interaction. However the effects due to low frequency cyclic loading as well as degraded the steel under ex-service conditions tending to reductions in creep ductility show factors of two or more faster cracking rates compared to as received static testing. Unavailability of long term tests at low stresses may pose additional problems under creep since creep is stress state controlled but fatigue in not. However using plane strain predictions of crack initiation and growth data using the NSW multiaxial ductility creep crack growth model suggest that conservative predictions of long term cracking can still be made under creep/fatigue and when there is a marked reduction creep failure ductility.

Creep-Fatigue Crack Growth Testing and Analysis of Pre-strained 316H Stainless Steel

Material pre-straining is known to have significant effects of the mechanical response and crack growth behaviour of steels. In this paper, the influence of material pre-straining on the subsequent creep-fatigue crack growth behaviour of Type 316H stainless steel at 550 °C has been examined by performing tests on compact tension specimens that were extracted from blocks uniformly pre-compressed at room temperature. Creep-fatigue crack growth tests on pre-compressed material were performed at the frequency of 0.01 Hz and R-ratio of 0.1. The crack growth data obtained from these experiments have been correlated with the C* and K fracture mechanics parameters and the results are compared with the existing creep crack growth data on the pre-compressed and as-received material at 550 °C. The results obtained have also been compared with the creep-fatigue data from experiments on weldments where the crack tip was located in the heat affected zone (HAZ). The crack growth behaviour in creep-fatigue tests on pre-compressed material has been found similar to that of HAZ material and are higher than that of the as-received material. Moreover, depending on the loading condition and frequency the crack growth data obtained from creep-fatigue tests on pre-compressed material may be characterized using C* or ΔK fracture mechanics parameters.

Creep-fatigue crack growth behaviour of P91 steels

The importance of predicting failure due to combined creep-fatigue crack growth in high temperature power-plant components has become of great importance importance due to the need for plant to ‘load follow’ in response to fluctuations in demands and the availability of renewables. P91 steel has been widely utilized in conventional plant components. Creep fatigue crack growth (CFCG) tests have been performed on compact specimens at temperatures ranging between 600° C to 625° C. The experimental results have been compared to static creep, high cycle fatigue and CFCG test data available in literature on P91 steel. The CFCG data has been characterised using stress intensity factor range parameter, ΔK and C* parameter. The crack growth per cycle and ∆K relationship shows that at high frequency, the CFCG behaviour tends to that of high cycle fatigue crack growth and at low frequency, the contribution of creep becomes increasingly more significant. The correlation between crack growth rate and C* parameter, shows that most CFCG data fall within the creep crack growth (CCG) P91 data band which may indicate that the crack growth behaviour is dominated by creep processes. Fractography has also shown an intergranular, ductile fracture surface indicating creep dominance for the conditions considered. A linear cummulative rule has ben used to predict the CFCG experimental result.

Creep fatigue crack growth and fracture mechanisms of T/P91 power plant steel

High temperature assessment of crack like defects in power plant components is one of the most important task to ensure high energy conversion efficiency. Creep fatigue crack growth tests for P91 power plant steel, conducted at 600°C on standard C(T) specimens with 0·1, 1 and 10 h hold times, are presented. Microscopic features of the creep fatigue damage were observed along the crack path, on polished and etched cross sections of the tested specimens. The dominant damage mechanism is the formation of microvoids and microcracks associated with the crack tip, due to the creep effect during the hold time. In order to predict the experimentally measured crack growth rate a simple summation rule is used. The superposition of fatigue crack growth rate and creep crack growth rate, experimentally obtained on the same batch of material used for creep fatigue crack growth tests, well describes the behaviour under creep fatigue loading.

Characterisation of creep–fatigue crack growth behaviour for Alloy 617 on circular notched specimen

In this study, by conducting the creep–fatigue crack growth tests using circular notched round bar specimens for Alloy 617, the crack growth behaviour was characterized on the basis of crack growth data obtained from the direct current potential drop (DCPD) method, which results in the assurance of the high accuracy of crack length measurement. Furthermore, on the basis of catastrophe theory, an attempt is made to predict theoretically the threshold temperature of creep crack growth related to the diffusion creep mechanism.

Low Cycle Fatigue Crack Growth Behavior of Modified Rotor Steel under High-Temperature

Many components in power plants are subjected to low cycle loading under high-temperature during continuing service. Therefore, creep-fatigue damage has been become a significant concern in the design and operation of these components. This damage mechanism initiates cracks at critical locations and subsequently creates a potential risk to safe and reliable operation. Creep-fatigue crack growth rate data are necessary inputs for assessing the structural integrity of high temperature components in power plants. Creep-fatigue crack growth tests were conducted for a modified X12Cr rotor steel at 600°C under trapezoidal wave shape load amplitude conditions with various hold times using C(T) specimens. A direct current electrical potential-drop was used to monitor crack growth length during the tests.It is shown that crack growth behavior under creep-fatigue conditions is characterized better by the (Ct)avg parameter, and the formula of assessment low cycle fatigue crack growth under high-temperature was fitted by use of ΔK and (Ct)avg parameter.

Round robin on creep fatigue crack growth testing for verification of ASTM standard 2760-10

This paper presents early results from an ongoing round robin study to verify the newly developed ASTM standard E-2760-10 on creep fatigue crack growth testing. This round robin is an international effort with 18 participants from 11 countries from Asia, North America and Europe. All participants are testing compact type, C(T) specimens according to the procedure described in the standard. The test material is P91 taken from a section of retired steam pipe that was heat treated to rejuvenate the microstructure. All background materials data such as tensile, creep deformation and creep fatigue crack formation data are already available on the test material. Creep fatigue crack growth testing was conducted as part of a pilot program at 625°C using C(T) specimens under constant load amplitude conditions at two load levels. The tests are conducted at hold times of 0, 60 and 600 s. Creep fatigue crack growth rates are analysed using the two methods included as part of the test standard. The data from the various participants will be assessed to determine the expected levels of precision and bias in creep fatigue crack growth data obtained using the ASTM standard. Modifications to the current version of the standard will be proposed and balloted as needed from the round robin results.

Crack growth behaviour of P92 steel under creep and creep–fatigue conditions

High temperature creep and creep–fatigue crack growth tests were carried out on standard compact specimens machined from ASME P92 steel pipe. The effects of various loading conditions on crack growth behaviours were investigated. Crack initiation time was found to decrease with the increasing initial stress intensity factor under creep condition and further to decrease by the introduction of fatigue condition. For creep test, the crack growth rate can be well characterised by the facture mechanics parameter C*. For creep–fatigue test, the crack growth behaviour is dominated by the cycle dependent fatigue process when the hold time is shorter, but it becomes dominated by the time dependent creep process when the hold time becomes longer.

Evaluating the Critical Temperature of Creep-Fatigue Interaction a Nickel-Based Powder Metallurgy Superalloy

For the components working in high temperature and enduring fatigue loading, the fatigue fracture properties will be reduced remarkably when the working temperature is higher than the critical temperature of creep-fatigue interaction Tc of the material. In consequence, the damage mechanism from creep-fatigue interaction becomes more complex. A method is presented in this paper to determine the Tc of a nickel-based powder metallurgy superalloy. Pure fatigue crack growth and creep-fatigue crack growth tests were conducted in several different elevated temperatures. The fracture mechanism was investigated via observing the fractographic characteristics using scanning electron microscope (SEM). The test results show that the Tc of this superalloy is a little bit lower than a half of the melting temperature Tm.

Crack growth behavior of 9Cr−1Mo (P91) steel under creep–fatigue conditions

Several components operated at high temperature are also subjected to cyclic loading making crack growth behavior under creep–fatigue conditions a significant concern during design and during service. This is especially the case for components designed for long term service. Creep–fatigue crack growth tests were conducted on a modified 9Cr–1Mo (P91) steel at 625°C under constant load amplitude conditions with various hold times using compact type specimens. The crack growth rates per cycle increased significantly with increase in hold time when crack growth data were plotted with the cyclic stress intensity factor, ΔK. It is shown that the creep–fatigue interactions during crack growth for various hold times are represented better by the (Ct)avg parameter implying that the P91 steel behaves in a creep-ductile manner. The test results are also used for assessing the newly developed American Society for Testing and Materials (ASTM) test standard, E2760-10 for creep–fatigue testing.

A double edge notch specimen design for tension–compression fatigue crack growth testing

Compact, C(T), specimens are commonly used for fatigue and creep-fatigue crack growth testing under constant-load-amplitude conditions. The use of the standard C(T) specimens [ASTM] is limited to positive load ratios. They are also limited in the amount of crack growth data that can be developed at high stress intensity values due to accumulation of plastic and/or creep strains leading to ratcheting in the specimen. Testing under displacement control can potentially address these shortcomings of the load-controlled tests for which the C(T) geometry is unsuitable. A double edge notch tension–compression, DEN(T–C), specimen to perform displacement controlled creep-fatigue crack growth testing is developed and optimized with the help of finite element and boundary element analyses. The specimen has threaded cylindrical ends for applying the loads. The test section is rectangular with a half width, W, and half height, H, and a constant thickness, B. The height to width ratio, H/W, of the specimen must be kept low to avoid buckling during compressive loading while ensuring that the stress-intensity parameter values are not strongly dependent on the chosen H/W value. Simple optimization analysis shows that a H/W ratio of 1.2 is likely to best meet these requirements. However, only performing actual experiments will confirm whether this choice of H/W value is optimum.In the finite element analyses, the specimen is modeled using an effective height (heff) to width (W) ratio, heff/W, of 1.46 to account for the fillet regions on the two ends of the specimens. Accurate expressions for estimating the fracture mechanics crack tip parameters such as the stress intensity parameter, K, the crack mouth opening displacement (CMOD), and the load-line displacement (LLD) are developed over a wide range of crack sizes for the DEN(T–C) specimen. Expressions are also developed for crack size as a function of specimen compliance.

Characterizations of Creep-fatigue Crack Initiation and Growth Life for P92 using Circular Notched Round Bar Specimen

Abstract A testing method under creep-fatigue interaction conditions has been proposed as the ASTM standard E2714-09. In accordance with the ASTM standard, round bar smooth specimens have been recommended as a testing specimen. Additionally, creep-fatigue crack initiation and growth life are usually defined as the attainment of a specific rate of decrease in the maximum tensile stress or the modulus of elasticity ratios. However, these criterions of crack initiation and growth life on ASTM standard are not characterized in terms of physical crack size. In this study, the creep-fatigue crack initiation and growth tests for P92 were conducted using circular notched round bar specimens. An attempt is made to measure the crack length during the tests by the direct current potential drop method, which results in the establishment of a high accuracy crack length measurement. Additionally, the life of creep-fatigue crack growth was characterized by linear cumulative damage law combined with the Q*concept which has been proposed as fracture parameter to describe the life of creep crack growth.