Creep-fatigue Crack Growth Research Articles

Simulation of creep-fatigue crack growth in aluminum alloy layers of hydrogen storage cylinders

ABSTRACT Hydrogen storage cylinders are crucial to hydrogen production and storage systems. During the service of the hydrogen storage cylinders, they undergo frequent hydrogen charging, dwelling, and discharging. The ceep-fatigue problems in the liner of the cylinders typically made of AA6061-T6 should be taken into account. In this paper, the creep-fatigue crack growth experiments on the compact tension specimens of AA6061-T6 are conducted. Based on it, simulations of creep-fatigue crack growth in the liner under six different dwell conditions are performed. Comparative analysis of the stress intensity factor range curves reveals that under the same working conditions, as the dwell time and initial crack depth increase, accelerating the crack growth rate leads to the life decreasing of the liner, thereby hastening the failure of the liner and the overall structure, which should be considered during life assessment.

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).

Crack growth behaviour of P91 steel under trapezoidal loading at high temperature

The increased incentives to utilize renewable energy sources so as to supplement the energy requirements of the present civilization pose a challenge to the power plants running on fossil fuels, as they are required to operate in a ‘flexible’ manner instead of operating at a constant load as intended during their design and inception. This flexible mode of operation at elevated temperature causes a combined creep and fatigue type of loads to be applied on the components. This allows for a comprehensive and effective understanding of synergistic creep-fatigue crack growth behaviour to assess long term failure of components and accessories in plant. The use of P91 steel has been widely prevalent in power plant components owing to its high creep resistance, superior mechanical properties and excellent thermal conductivity, cost-effectiveness, and minimal coefficient of thermal expansion when compared to austenitic steels and Ni-based alloys. The American Society for Testing and Materials (ASTM) has proposed a standard for experimentally evaluating creep-fatigue crack growth, E 2760-19, wherein compact specimens, C(T) are tested under load-controlled conditions. The creep-fatigue crack growth regime and mode of fracture undergone by P91 steel at 600 °C were studied using C(T) specimens at two different force ratio 0.1 and 0.5 and with two different hold time 10 and 600 sec. The Creep fatigue crack growth behaviour has been characterized using stress intensity factor range parameter ∆K and (Ct)avg parameter to study the effect of hold time and force ratio.

An investigation into the influence of variable amplitude creep-fatigue loading on plasticity-induced crack closure

Finite element simulations of fatigue crack growth (FCG) and creep-fatigue crack growth (CFCG) under constant amplitude (CA) and variable amplitude (VA) loading waveforms are performed to investigate the mechanics of crack opening and closing, which has implications on crack growth rate. The material of study is the ferritic-martensitic steel 9Cr-1Mo (modified 9Cr-1Mo) at 625 °C. Two-dimensional finite element analyses of compact tension (CT) specimens are performed to simulate crack growth under several VA loading scenarios, considering elastic–plastic and creep deformations of the material at the crack tip. In this study, the relevant variable influencing crack growth rate is the crack opening load, caused by the phenomenon of plasticity-induced crack closure. A series of loading conditions were simulated using the finite element method, i.e., fatigue overload (OL) in either CA FCG or CFCG waveforms, creep-fatigue OL in CFCG waveforms, and underload (UL) in CFCG waveforms. The crack opening loads obtained from this study for CA loading in both FCG and CFCG, agree with data from published studies. This study also found that a significant reduction in crack opening loads is experienced by the growing crack when an OL or UL cycle was introduced. Immediately after the OL or UL cycle is applied, the crack opening load increases during both fatigue OL and creep-fatigue OL to values larger than those recorded during CA FCG and CFCG loading, whereas UL waveforms result in consistently lower crack opening loads. The increase in crack opening loads is proportional to both the extent of OL and the duration of hold time in the OL cycle. However, following the application of a UL, the crack opening loads in the subsequent cycles were not influenced by the magnitude of the UL or the duration of hold time in the UL cycle.

A Phenomenological Model for Creep and Creep-Fatigue Crack Growth Rate Behavior in Ferritic Steels

A model to rationalize the effects of test temperature and microstructural variables on the creep crack growth (CCG) and creep-fatigue crack growth (CFCG) rates in ferritic steels is described. The model predicts that as the average spacing between grain boundary particles that initiate creep cavities decrease, the CCG and CFCG rates increase. Further, the CCG data at several temperatures collapse into a single trend when a temperature compensated CCG rate derived from the model is used. The CCG and CFCG behavior measured at different temperatures is used to assess the effects of variables such as the differences between the base metal (BM), weld metal (WM), and heat-affected zone (HAZ) regions. The model is demonstrated for Grade 22 and Grade 91 steels using data from literature. It is shown that differences between the CCG behavior of the Grade 22 steel in new and ex-service conditions are negligible in the BM and WM regions but not in the HAZ region. The CCG behavior of the Grade 91 steels can be separated into creep-ductile and creep-brittle regions. The creep-brittle tendency is linked to the presence of excess trace element concentrations in the material chemistry. Significant differences found in the CCG rates between the BM, WM, and HAZ regions of the Grade 91 steel are explained.

Oxidation damage zone formed in creep fatigue crack growth of GH4169 alloy at 650 °C

Oxidation damage zone formed in creep fatigue crack growth of GH4169 alloy at 650 °C

Dwell-fatigue crack growth behaviour of Alloy 709

Dwell-fatigue crack growth behaviour of an advanced austenitic stainless steel Alloy 709 has been investigated and compared with that of a conventional Type 316H stainless steel. The test procedure employed alternation of 1 h dwell-fatigue loading and 0.25 Hz fast cycling so that crack growth rates (da/dN) obtained from dwell-fatigue loading can be compared to those purely result from fatigue mechanism on the same test-piece. Tests were conducted at 550, 650 and 750 °C in air using 0.5 T compact tension test-pieces under a fixed maximum load of 8 kN and a stress ratio, R, of 0.1. For the investigated temperature and ΔK ranges, crack growth mechanisms of fatigue alone, creep alone and mixed fatigue-creep have all been observed. Detailed fractographic and metallographic observations were conducted to interpret failure mechanisms and regimes of different failure modes. Compared to crack growth rates obtained under 0.25 Hz fatigue loading, dwell-fatigue produces: no obvious increases in crack growth rates at a test temperature of 550 °C; a moderate increase (∼ 2–5 times) at a test temperature of 650 °C; and, over a tenfold increase at a test temperature of 750 °C. Compared to 316H, Alloy 709 has much improved resistance against creep-fatigue crack growth, here confirmed at the single test temperature of 650 °C.

A new creep-fatigue interaction damage model and CDM-XFEM framework for creep-fatigue crack growth simulations

In this paper, a coupled continuum damage mechanics (CDM) and extended finite element method (XFEM) based methodology is developed to predict the crack growth life under creep-fatigue environment. To predict the life accurately, a new simple nonlinear creep-fatigue interaction (CFI) damage model is presented. In the combined methodology of CDM and XFEM, the XFEM computes stresses in cracked domain and the coined CFI damage equation determines specimen life under creep-fatigue environment. The simulations are performed for different hold time and compared with experiments. The simulated results confirm that proposed CFI damage model along with developed CDM-XFEM framework is a simple and effective methodology for determining creep-fatigue crack growth life efficiently.

A new creep-fatigue crack growth model and a correlation of the creep-fatigue crack growth rate with unified constraint parameter

Creep-fatigue and constraint are important factors affecting the fracture behavior and service life of components in high-temperature service. Some researchers have attempted to describe creep-fatigue crack growth (CFCG) behavior using constraint parameters. However, there is still no consensus regarding the effect of the constraint on the CFCG rate, and a correlation between the constraint and the CFCG rate must be established. Thus, in this study, a new CFCG model was developed and verified by finite element simulations and experiments. Then, the CFCG behavior for single edge-notched tensile (SENT), centre-cracked tension (CCT), and compact tension (CT) specimens under different constraint conditions was studied using the new model to reveal the effect of constraint on the CFCG rate. The relationship between the dwell time and the CFCG rate was also studied. Moreover, a linear correlation between the CFCG rate and the constraint was established for the first time using the unified constraint parameter Ap, and it is shown that the CFCG rate has a strong correlation with the area surrounded by the equivalent plastic strain (εp) contour at the crack tip. This indicates that the constraint effect on the creep-fatigue could be further revealed by Ap. Furthermore, Ap was extended from monotonic loading to creep-fatigue loading, and a method is provided for using CFCG test data to evaluate high-temperature components with defects.

Investigation on the creep-fatigue crack growth behavior of 316H welded joints in sodium-cooled fast reactors

The failure analysis of 316H steel welded joints in sodium-cooled fast reactors at 550 °C under creep-fatigue load with different hold times has been systematically investigated through this work. For the first time, the reason for the difference of cracking failure mechanism between base metal and weld metal was illustrated from the perspective of microstructure evolution. Under various hold times, different damage mechanisms led the fracture surfaces of the samples to exhibit different degrees of tunneling. Meanwhile, due to the existence of a δ-ferrite network in the weld metal, the cracks were deflected, resulting in higher resistance to crack propagation and a more tortuous crack propagation path than those in the base metal. In addition, the nucleation and growth of voids at the grain boundaries were the primary reasons that base material exhibited intergranular crack propagation, and the evolution of ferrite in the weld was a critical reason for the formation of intergranular pores in the welded metal.

Effect of force ratio on creep-fatigue crack growth (CFCG) of P91 steel

Effect of force ratio on creep-fatigue crack growth (CFCG) of P91 steel

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.

A Concise Binomial Model for Nonlinear Creep-Fatigue Crack Growth Behavior at Elevated Temperatures.

The creep-fatigue crack growth problem remains challenging since materials exhibit different linear and nonlinear behaviors depending on the environmental and loading conditions. In this paper, we systematically carried out a series of creep-fatigue crack growth experiments to evaluate the influence from temperature, stress ratio, and dwell time for the nickel-based superalloy GH4720Li. A transition from coupled fatigue-dominated fracture to creep-dominated fracture was observed with the increase of dwell time at 600 °C, while only the creep-dominated fracture existed at 700 °C, regardless of the dwell time. A concise binomial crack growth model was constructed on the basis of existing phenomenal models, where the linear terms are included to express the behavior under pure creep loading, and the nonlinear terms were introduced to represent the behavior near the fracture toughness and during the creep-fatigue interaction. Through the model implementation and validation of the proposed model, the correlation coefficient is higher than 0.9 on ten out of twelve sets of experimental data, revealing the accuracy of the proposed model. This work contributes to an enrichment of creep-fatigue crack growth data in the typical nickel-based superalloy at elevated temperatures and could be referable in the modeling for damage tolerance assessment of turbine disks.

Fatigue and creep-fatigue crack growth in aviation turbine disk simulation models under variable amplitude loading

This study analyzes the simulation model configuration and its loading conditions in fatigue and subsequent crack growth tests. Following the simulation principles, a full-size 3D finite element analysis of a representative GTE turbine disc was performed. With the aim of reproducing the simulation model, the in-service critical zone of both the stress–strain state and damage accumulation is studied. Based on the parametric computations, the block type loading conditions of the simulation models are determined and verified. Experimental study of the surface crack growth rate in the simulation model under harmonic and block loading at elevated temperature is performed for the fatigue and creep-fatigue interaction conditions. To interpret the experimental data, we calculate the nonlinear stress intensity factor distributions along the semielliptical crack front for flaws growing on the inner surface of the hole in the simulation model using numerical procedures. It is found that in terms of the nonlinear fracture resistance parameters, the sections of the fatigue fracture diagrams form one common curve representing the previous and subsequent increasing stages of block loading in pure fatigue and creep-fatigue interaction loading conditions. The possibility of using the proposed simulation model, tested under variable amplitude loading, to assess the structural integrity of GTE turbine disks is discussed.

A validated approach for probabilistic structural integrity design code implementation

This paper provides a possible approach for incorporating probabilistic structural integrity assessment which is sufficiently simple to encourage its adoption by design engineers. The approach, called the “Three-Term Reference Damage Model (3T-RDM)”, is developed in the context of a particular benchmark problem and a proposed Design Chart where the probability of crack initiation by creep-fatigue in a widely used material in nuclear plants, namely 316H austenitic stainless steel, is estimated based on the R5V2/3 high temperature procedure. In principle, the 3T-RDM is not restricted to a particular failure mechanism, i.e., fracture, creep rupture, creep-fatigue crack initiation and growth are all within its scope, and non-metallic materials may also be addressed. To further validate the accuracy and generality of the proposed 3T-RDM and Design Chart, deterministic and Monte Carlo probabilistic structural integrity assessments are conducted for three EDF nuclear plant case-studies and the results are superimposed on the Design Chart obtained from the benchmark problem. The analysis indicates an excellent consistency between the results, justifying the adoption of the 3T-RDM for an intermediate level of probabilistic assessment in the nuclear sector.

Size effect in creep–fatigue crack growth interaction in P2M steel

AbstractAn investigation of the crack propagation under the creep–fatigue interaction in C(T) specimens of P2M steel was carried out. Numerical calculations included the determination of the stress–strain state fields for the viscoelastic material, as well as the distributions of stress triaxiality, nonlinear stress intensity factors, and the C‐integral along the crack front in specimen with different thickness. The interpretation of the experimental results under fatigue–creep interaction is given by the elastic and nonlinear stress intensity factors in terms of the traditional and new normalized cyclic fracture diagrams. Considering the size effects through the creep‐dominated stress intensity factors caused differences in the crack growth behavior. In specimens with the same geometry, the crack growth rate under the creep–fatigue interaction increased monotonically compared with harmonic fatigue, and the superposition of fatigue and creep contributions shows an order of magnitude increase in the total crack growth rate with respect to the pure creep.

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.

Temperature effect on dwell-fatigue crack propagation behavior of novel tempered martensitic ferritic steel G115

The effect of temperature and holding time on the dwell–fatigue crack propagation behavior of a novel tempered martensitic ferritic steel G115 was studied. The crack growth duration greatly increased with the increase in dwell time at all tested temperatures (625–675 °C). The FCG data and the da/dN for the 60-s dwell tests remained nearly constant when the temperature increased from 625 to 650 °C, whereas da/dN increased remarkably when the temperature increased to 675 °C. Moreover, da/dN increased with increasing temperature for the 600-s dwell tests. These results reflect the comprehensive effects of creep, environmental (oxidation) phenomena, and property degradation. In the (da/dt)avg versus (Ct)avg plot, the crack-propagation data under a specific dwell time fall close to each other, regardless of the temperatures. Scanning electron microscopy observation shows that the fatigue crack propagation has a transgranular cracking mode, regardless of variations in the temperature (625–675 °C) and load level. Meanwhile, the creep-fatigue crack growth mode is transgranular cracking at lower ΔK, whereas it gradually changes to mixed intergranular and transgranular fracture modes with increasing ΔK. The longer the holding time, the more significant the intergranular cracking. The crack growth may change to transgranular mode again when ΔK further increases. The two values of ΔK at which the crack-growth mode change all increase with decreasing dwell time and temperature.

Investigation of creep-fatigue crack growth of G115 steel using a novel damage model

The flexible operation of power-plant structures could cause creep-fatigue crack propagation, which is a major concern for design and maintenance. Therefore, this study investigated the crack propagation in G115 steel under isothermal creep-fatigue loading by using a novel damage model. Creep and fatigue damage were determined by the exhaustion of the creep ductility and the total plastic strain energy during the crack growth process, respectively. The crack propagation behavior was simulated by the finite element method, and the calculated data were compared with experimental results, which demonstrated the high precision of this model. The calculated damage values at the crack tip were analyzed, and the creep damage (cavities) around the main crack under 60 and 600 s hold times were observed based on X-ray micro-computed tomography. The cavity density for the test with 600 s of holding was obviously larger than that for the 60 s dwell test, with the former showing more significant creep damage. Calculations of compact tension specimens with different hold times, specimen thicknesses, and crack lengths were further conducted and analyzed. The results revealed that increasing the hold times, specimen thicknesses, and crack lengths increased the cyclic crack growth rate.