Creep-fatigue Crack Growth Tests Research Articles

Evaluation of the Testing and Analysis Methods in ASTM E2760-10 Creep-Fatigue Crack Growth Testing Standard for a Range of Steels

Abstract The development of the ASTM E2760-10, “Creep/Fatigue Crack Growth Testing” standard has initiated a phase of testing, analysis, and round robin initiatives associated with high temperature cyclic loading crack growth tests. Creep and fatigue are two complex independent mechanisms which may assist each other to drive the crack. A simplification of the macro creep/fatigue crack growth response of structures is therefore required. Using a linear cumulative damage method, as proposed in ASTM E2760, and other codes of practice, data for a range of steels are analysed. Limited crack growth data, using test methods similar to ASTM E2760, from previous collaborative projects for a range of steels are presented and analysed to identify the difficulties in the process. Creep-fatigue crack growth tests at frequencies ranging from 10 to 0.001 Hz are examined to consider the testing methodologies and in order to assess the effect of data scatter, dwell times, and the interaction region between creep crack growth (CCG) and fatigue crack growth mechanisms. Given the limited data available and the level of scatter, it can still be shown that the linear cumulative summation of static and high frequency data may be sufficient to predict creep/fatigue interaction both in terms of C* and ΔK, and the level of creep ductility will dictate appropriateness of the correlating parameter employed. Finally, in order to show that the level of data scatter can be dealt with in a predictive manner, the crack initiation and growth rates are also analysed using the NSW CCG model.

G030023 ひずみ制御高温クリープ・疲労重畳条件下におけるP92鋼のき裂発生・成長挙動([G03002]材料力学部門一般セッション(2):クリープ損傷)

Testing method under the creep-fatigue conditions has been proposed as the ASTM standard E2714-09. In accordance with the ASTM standard, round bar smooth specimen 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 from a mechanical view point. In this study, the creep-fatigue crack initiation and growth tests for P92 were conducted using circular notched round bar specimens under strain controlled conditions. An attempt is made to measure the crack length during the tests by DCPD method, which results in the establishment of high accuracy crack length measurement. Additionally, the criterions of crack initiation and growth life under the creep-fatigue interaction conditions were discussed.

Code of practice for the measurement and analysis of high strain creep-fatigue short crack growth

There are no procedural standards for the determination of crack growth properties where a reversal of stress is involved, in particular at elevated temperatures. The purpose of this code of practice is to detail the requirements for fatigue and creep-fatigue short crack growth (CFSCG) testing, generally on uniaxial testpieces subject to high strain loading conditions. CFSCG testing may entail the use of a single testpiece to produce data over several strain ranges. Alternatively, data from a number of constant strain range tests may be obtained, as in conventional low cycle fatigue (LCF) endurance, but where a small starter (sharp) defect has been introduced. Procedures for long crack fatigue crack growth and LCF testing are covered by a number of existing codes of practice and standards, and this document does not recommend any alteration to these.CFSCG rates may be determined from testpieces with much larger notch acuities, e.g. uniaxial specimens of the Bridgman type, or reverse-bend specimens with a stress concentrating feature. Thus in addition to recommending best laboratory practice, this document includes sections on engineering requirements and test data analysis. Advice is also given for those circumstances where testpiece material is limited, thus requiring quick methods of data acquisition using block loading techniques. Only isothermal testing at appropriate temperatures under strain-controlled conditions is covered, although the techniques can be extended to non-isothermal (thermo-mechanical) fatigue testing conditions. An appendix covers in more detail: (i) the relation between the shape of short cracks (straight-fronted, thumbnail, etc.) and the specimen area thereby consumed by the fracture surface, and (ii) the relation between this fractional area change and the corresponding change in both the crack-monitoring potential drop signal and drop in peak tension load.

Inelastic Deformation and Crack Growth Behavior of CrMoV Cast Steel after Long-Term Service at High Temperature

For the purpose of residual life prediction of high-temperature components after long-term service, both stress-strain behavior for evaluating fracture mechanics parameters and crack growth behavior were examined using CrMoV cast steel taken from a main steam valve served for 155,000 hours at 811K under 216 start-stop cycles. Firstly the influence of cyclic softening and thermal aging history on the stress-strain curve was examined. As a result of comparison between virgin and long-term served material, a possibility of predicting the stress-strain behavior of the long-term-served material from that of virgin material was discussed. Secondly creep and fatigue crack growth behavior of long-term served material were experimented and compared with those of virgin material. Finally creep-fatigue crack growth tests under displacement-controlled condition were carried out with structural model specimens made of the long-term served material. The crack growth behavior of the tests was simulated using the method proposed by the authors on the basis of the monotonic stress strain curve and the stress strain curve of half-fatigue life of the material. Comparison with the test results showed that the simulation using the monotonic stress-strain curve of the material gave the conservative prediction of the crack growth behavior.

High-temperature tensile-hold crack-growth behavior of HASTELLOY® X alloy compared to HAYNES® 188 and HAYNES® 230® alloys

The creep–fatigue crack-growth tests of HASTELLOY® X alloy were carried out at the temperatures of 649°C, 816°C, and 927°C in laboratory air. The experiments were conducted under a constant stress-intensity-factor-range (ΔK) control mode with a R-ratio of 0.05. In the constant ΔK tests, a ΔK of 27.5 MPa $\sqrt{\mathrm{m}}$ and a triangular waveform with a frequency of 0.333 Hz were used. Various tensile hold times at the maximum load were imposed to study fatigue and creep–fatigue interactions. Crack lengths were measured by a direct current potential drop method. In this paper, effects of hold time and temperature on the crack-growth rates are discussed. Furthermore, the crack-growth rates of the HASTELLOY® X alloy are compared to those of the HAYNES® 188 and HAYNES® 230® superalloys.

Role of nonlinear fracture mechanics in assessing fracture and crack growth in welds

An experimental study was conducted to assess the structural performance of repair welds in an ex-service 1Cr–1Mo–0.25V steam turbine casing material. Material from two weld techniques, one involving a post-weld heat-treatment that produced undermatched welds and the other involving a temper bead welding technique that produced overmatched welds were tested. Both welding techniques were implemented in two base metal conditions giving rise to four different welds and two different base metal conditions. The tests conducted included tensile tests, creep tests, fracture toughness tests, fatigue crack growth tests, creep crack growth tests, and creep–fatigue crack growth tests on the base metal, weld metal and the weldment region. The yield strength of the weld metal in the undermatched condition was approximately 10% lower than the base metal, while the weld metal in the overmatched condition had a yield strength that was 30% higher than the base metal at 565 °C. The creep deformation rates in the undermatched welds were 60 times faster than the base metal at a stress of 207 MPa. In the overmatched welds, the creep rates at 207 MPa were about 2.8 times faster in one case and 2.8 times slower in the other. The crack path in fracture toughness specimens followed the interface between the transition layer and the weaker of the weld metal and the base metal. The J-resistance curves for the weldments at 565 °C showed significant variability among duplicate samples from the same welds. This scatter was caused by the variability in the location of the precrack with respect to the fusion line and the location of the low fracture toughness region in the weldment. This behavior was explained using a novel approach for characterizing the fracture of welds. The creep–fatigue crack growth rates at equivalent ( C t ) avg values in undermatched welds was higher than the crack growth rates in the overmatched weld samples. In all cases under creep–fatigue, the crack appeared to grow in the weaker of the base metal and the weld metal. Recommendations for future work are provided to enhance the theoretical underpinnings of the nonlinear fracture mechanics frame-work to rigorously address fracture and crack growth in welds.

Detection and Evaluation of Localized Damage under Creep-Fatigue Loading at 1000°F Using Acoustic Emission Technique

Acoustic emission (AE) technique was applied as a non-destructive test method to detect and evaluate the localized damage at high temperature environment. The creep-fatigue crack growth tests were carried out with the acquisition of AE signal at 1000°F. Under trapezoidal waveform loadings, AE results showed different features according to each damage mode. During the creep period, low and steady emissions were shown, while emissions were burst and high counts rate was recorded during the fatigue loading. Based on these characteristics, damage contribution was expressed in terms of acoustic emission parameter as a part-time monitoring method. Comparisons of damage contribution with respect to lifetime showed the transient behavior from cycle-dependent to time-dependent process. In case of full-time monitoring, bilinear behavior between AE counts and life was represented. From both monitoring results, it was confirmed that creep and fatigue damage can be characterized by means of emission features and AE is possible way to evaluate the localized damage at elevated temperature.

クリープ疲労寿命評価に必要な材料パラメータの遺伝的アルゴリズムによる迅速決定

The present paper investigates the applicability of Genetic Algorithms (GA) to accelerated determination of the material parameters required in order to evaluate creep-fatigue life and remaining life of Mod. 9Cr-1Mo steel. The GA program, which was written in Visual C++, was developed in order to estimate the values of 23 material parameters required to describe the partitioned inelastic strain range versus life equations and the creep-fatigue damage growth model based on the strain range partitioning concept. Three types of test, constant-strain amplitude creep-fatigue tests, constant-amplitude creep-fatigue crack growth tests and variable-strain waveform creep-fatigue tests, were needed in order to determine the above-described parameters experimentally. The GA analysis was able to determine the parameters without using the creep-fatigue crack growth test data. The effects of the number of data used in the GA analysis for the estimation accuracy of the material parameters and the creep-fatigue life were evaluated. The obtained results suggest that the number of tests required for GA determination of material parameters is half of the number of creep-fatigue tests (175) that must be conducted in order to determine the material parameters experimentally.

Study of the coupled phenomena involved in the oxidation assisted intergranular cracking of Ni based superalloys

High temperature fatigue and creep-fatigue crack growth tests were carried out at 650°C on a Ni-hase superalloy. During the tests, mechanical as well as chemical perturbations were used in order to determine the critical scale of interactions and the relevant mechanical and chemical local components of the damaging process in relation with the alloy microstructure. In this study a P/M Ni base superalloy (alloy N18) with two different microstructures and creep properties was used. The results obtained can be summarised as follows (i)Intergranular embrittlement occurs under specific local coupling conditions i.e. high tensile stresses at the crack tip in phase with a simultaneously growing cationic oxide. Defects injection (dislocations, vacancies) related to interface reactions cutting in the oxidation process are likely involved as a major component of the embrittlement process. Thus, when these two local mechanical and chemical loading conditions are acting out of phase no embrittlement is observed (ii) Under creep-fatigue loading conditions, an unloading of several percent from the maximum load applied at the beginning of the holding time period can inhibit embrittlement process. Therefore, by varying the unloading amplitude, it is possible to estimate the size of to interaction zone. In our case, the range of interaction is of the order of the grain size (iii). The cracking resistance depends in the creep properties. When the relaxation rate at the crack tip increases the cracking resistance increases. Finally, conclusions can be drawn from this study in terms of physical basis for modelling fatigue crack growth processes and microstructural modifications which might improve the cracking resistance of these alloys at intermediate temperatures.

Description of creep and creep fatigue crack growth in 1% and 9% Cr steels

Creep crack growth tests on a 1CrMoV steel are presented, covering the aspects of specimen size, geometry and service-like stresses. To consider nonstationary loading in modern plants creep fatigue crack growth tests have been started. As test materials a 1CrMoV steel and a modern 9%Cr-steel were used. By means of a comparison of creep crack and creep fatigue crack results the effectiveness of the fracture mechanics parameters K1, ΔK1, and C* could be evaluated.

Influence of prior cyclic hardening on high temperature deformation and crack growth in type 316L (N) stainless steel

For power generating equipment subjected to cyclic loading at high temperature, crack growth could arise from the combinations of fatigue and creep processes. There is potential for the material to undergo hardening (or more generally changes of material state) as a consequence of cyclic loading. Results of an experimental study to examine the influence of prior cyclic hardening on subsequent creep deformation are presented for type 316L(N) stainless steel at 600°C. Experiments were also carried out to explore creep crack growth at constant load, and crack growth for intermittent cyclic loading. For the as-received material there is substantial primary creep (hardening) at constant load, while for the cyclically hardened material at constant load the creep curves show recovery, and increasing creep rate with increasing time. Specimens subjected to prior cyclic hardening were also used for a series of creep and creep-fatigue crack growth tests. These tests demonstrated that there was accelerated crack growth compared to crack growth in as-received material.

Characterization of creep-fatigue crack growth behavior in 2.25 Cr-1 Mo steel using (C t)avg

Time-dependent creep-fatigue crack growth (CFCG) is an important consideration in the design and remaining life estimation of high temperature components. CFCG tests were carried out on compact type (CT) specimens of 2.25 Cr-1.0 Mo steel and its behavior, for hold times ranging from 10 seconds to 50 seconds, at 594°C (1100°F) was characterized using the average value of the C t-parameter, (C t)avg. The trends in the creep-crack growth (CCG) data for this material are also compared with the CFCG data. The analytically estimated values of (C t)avg are compared with the experimental values of (C t)avg obtained from the measured values of load-line deflection rates. It is also shown that even in the absence of accurate creep deformation constants, accurate estimates of the measured values of (C t)avg can be obtained in CT specimens

The effect of porosity and γ-γ′ eutectic content on the fatigue behaviour of hydrogen charged PWA 1480: Gayda, J. Dreshfield, R.L. and Gabb, T.P. Scr. Metall. Mater.25 11 (Nov 1991) 2589–2594

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.

Corrosion and compatibility: Berry, W.E. CAB Curr. Aware. Bull. (194), 6 Oct. 1989

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.