Fatigue growth of short cracks in Ti-17: Experiments and simulations

The fatigue behaviour of through thickness short cracks was investigated in Ti-17. Experiments were performed on a symmetric four-point bend set-up. An initial through thickness crack was produced by cyclic compressive load on a sharp notch. The notch and part of the crack were removed leaving an approximately 50 μm short crack. The short crack was subjected to fatigue loading in tension. The experiments were conducted in load control with constant force amplitude and mean values. Fatigue growth of the short cracks was monitored with direct current potential drop measurements. Fatigue growth continued at constant R-ratio into the long crack regime. It was found that linear elastic fracture mechanics (LEFM) was applicable if closure-free long crack growth data from constant K_Imax test were used. Then, the standard Paris’ relation provided an upper bound for the growth rates of both short and long crack.The short crack experiments were numerically reproduced in two ways by finite element computations. The first analysis type comprised all three phases of the experimental procedure: precracking, notch removal and fatigue growth. The second analysis type only reproduced the growth of short cracks during fatigue loading in tension. In both cases the material model was elastic-plastic with combined isotropic and kinematic hardening. The agreement between crack tip opening displacement range, cyclic J-integral and cyclic plastic zone at the crack tip with ΔK_I verified that LEFM could be extended to the present short cracks in Ti-17. Also, the crack size limits described in the literature for LEFM with regards to plastic zone size hold for the present short cracks and cyclic softening material.     

ELEVATED TEMPERATURE FATIGUE CRACK GROWTH BEHAVIOR OF Ti-1100

Abstract— The fatigue crack growth behavior of Ti-1100 is analyzed at elevated temperatures to evaluate the effects of mechanical and environmental variables. Experiments conducted over a wide range of frequencies from 0.01 Hz to 200 Hz indicate a strong dependence of the growth rate upon cyclic loading frequency. Superposition of hold time at maximum and minimum loads over a baseline 1.0 Hz cyclic loading frequency produces an insignificant variation in crack growth rate, which may be attributed to the combined effects of enhanced environmental degradation, crack-tip blunting and increased asperity-induced closure level in this material. It is deduced that a hold time at maximum load results in an interaction of the environmental effects with a retardation effect due to crack tip blunting as a consequence of creep under maximum applied load, whereas for hold at minimum loads, extensive crack-branching and micro-cracking appear to enhance crack closure loads resulting in lower crack growth rates. A linear superposition model is employed to account for the complex interactions due to fatigue, creep and environmental degradation.     

Modelling of high temperature fatigue crack growth in Inconel 718 under hold time conditions

Inconel 718 is a frequently used material for gas turbine applications at temperatures up to 650 °C. The main load cycle for such components is typically defined by the start-up and shut-down of the engine. It generally includes hold times at high temperatures, which have been found to have a potential for greatly increasing the fatigue crack growth rate with respect to the number of load cycles. However, these effects may be totally or partly cancelled by other load features, such as overloads or blocks of continuous cyclic loading, and the actual crack propagation rate will therefore depend on the totality of features encompassed by the load cycle. It has previously been shown that the increased crack growth rate found in hold time experiments can be associated with a damage evolution, where the latter is not only responsible for the rapid intergranular crack propagation during the actual hold times, but also for the increased crack growth during the load reversals. In this paper, modelling of the hold time fatigue crack growth behaviour of Inconel 718 has been carried out, using the concept of a damaged zone as the basis for the treatment. With this conceptually simple and partly novel approach, it is shown that good agreement with experimental results can be found.     

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.     

Cyclic fatigue of a silicon nitride: influence of frequency and fatigue mechanism

An investigation has been carried out on the slow crack growth behaviour of an advanced Si₃N₄ ceramic material at room temperature at different loading frequencies. The results clearly show a detrimental effect of cyclic loading on crack growth rate in terms of time and a reduced crack growth resistance with increasing cyclic frequency. Crack growth rates can be described by the Paris power-law expression for both static and cyclic loading, but the exponent n increases with decreasing loading frequency. Further support for the existence of mechanical fatigue in this material is provided from experiments involving alternate cyclic and static fatigue using the same specimen, which show substantial differences in crack growth rate in terms of time. Removal of crack wakes resulted in an unchanged crack growth rate under sustained load, which suggests that the crack wake does not play a key role in enhanced crack growth under cyclic loading. The likely crack growth mechanism is discussed.     

Accounting for crack tip cyclic plasticity and creep reversal in estimating (Ct)avg during creep‐fatigue crack growth

Creep‐fatigue crack growth (C‐FCG) rates in a P91 steel at 625°C were correlated as the average time rate of crack growth during hold time, (da/dt)_avg , with (C_t)_avg. At 60‐second hold time, the rates were lower than for 600‐second hold time. At 600‐second hold time, the crack growth rates converged on to the creep crack growth rate (CCGR) trend. Thus, the CCGR trend represents the upper bound for time‐dependent crack growth rates in P91 materials. The analytical expressions based on considering just the elastic and secondary creep deformation rates overestimated the magnitudes of (C_t)_avg by as much as a factor of 10 for the 600‐second hold time tests. After accounting for the effects of cyclic plasticity during unloading, and accounting for only partial reversal of creep strains accumulated during hold time, the estimates of (C_t)_avg compared well with the measured values. C_R represents the extent of crack tip creep strain reversal, and t_pl is the time required for the crack tip creep zone during the hold time to become equivalent in size to the cyclic plastic zone in terms of stress carried by that region. Together, these parameters accurately account for the effects of crack tip cyclic plasticity on the magnitude of (C_t)_avg. Both t_pl and C_R depend on material properties, and the latter also depends on the hold time. A parameter ? is introduced that is dependent only on material properties and from which C_R can be estimated for a given hold time. t_pl and ? can be reported as part of the test results from C‐FCG testing.     

Low frequency fatigue crack growth behavior of a470 class 8 rotor steel at 538°C (1000°F)

Fatigue crack growth rate data were developed at various frequencies and hold times at maximum load for A470 Class 8 steel at 538°C (1000°F) by using an accelerated test method which involves alternating test frequency and temperature. These data were consistent with fatigue crack growth rate data obtained from the same material and developed according to the ASTM specification E-647-T78. This result suggests that there is no transient effect associated with the alternating test frequency and temperature and that the accelerated testing procedure can be used to expedite the development of elevated temperature fatigue crack growth rate data at very low frequencies and long hold times. At 538°C (1000°F) fatigue crack growth properties with hold time developed from both 1T-CT and multiple-edge-craek tension specimens fall in the same scatter band on the da/dN vs ΔK plot. This result indicates the applicability of ΔK to characterize the fatigue crack growth behavior with hold time at elevated temperature. Also, the model proposed by Saxena et al. was found to successfully predict the fatigue crack growth rate properties with 28 min hold time of the A470 Class 8 rotor steel at 538°C (1000°F).     

EFFECTS OF TEMPERATURE, CYCLIC FREQUENCY AND ENVIRONMENT ON FIBRE DEGRADATION AND FATIGUE CRACK GROWTH RESISTANCE IN A FIBRE-REINFORCED TITANIUM METAL-MATRIX COMPOSITE UNDER DISPLACEMENT-RANGE LOADING

The fatigue crack growth resistance of a [0/90°]_2s cross-ply SCS6 fibre-reinforced Ti–6Al–4V alloy metal-matrix composite has been assessed under displacement range control (i.e. under load shedding conditions with crack extension) to investigate potential fibre degradation and the process of crack extension at room temperature, and at 450°C, in air and in vacuum. Attention is focused on initial conditions that will promote crack arrest at room temperature. Under the test conditions employed here, regions of crack growth can occur where the applied nominal stress intensity factor range (ΔK) is relatively constant. This 'constant'ΔK range is the result of a fortuitous balance between the particular test-piece geometry, loading conditions utilized, matrix crack growth and the rate of fibre fracture. It allows the influence of environment, cyclic frequency and temperature on fatigue crack growth resistance to be analysed more easily than for tests carried out under load control.
The crack growth rate remained almost constant but with some steep local retardations in growth rate in the constant ΔK region at a temperature of 450°C, while crack arrest occurred at room temperature for the same initial ΔK. The average crack propagation rate in this 'constant ΔK region' at a temperature of 450°C in air was much greater than that at a temperature of 450°C in vacuum. This indicates that environment plays an important role in the process of fibre degradation. The effect of cyclic frequency is saturated at a frequency of less than 1  Hz. The process of crack growth at various frequencies is also discussed.     

CREEP, FATIGUE AND CREEP-FATIGUE CRACK GROWTH RATES IN PARENT AND SIMULATED HAZ TYPE 321 STAINLESS STEEL

Abstract— Crack growth rate data are presented from a range of fully reversed displacement-controlled fatigue and creep-Fatigue tests and from static load-controlled creep crack growth tests on aged 321 stainless steel (parent and simulated HAZ) at 650 ° C. In the creep fatigue tests, constant displacement tensile hold periods of 12–192 h were used. Crack growth rates comprised both cyclic and dwell period contributions. Cyclic growth contributions are described by a Paris-type law and give faster crack growth rates than those associated with pure fatigue tests. Dwell period contributions are described by the C* parameter. The total cyclic crack growth rates are given by summing the cyclic and dwell period contributions. Estimates of C* using a reference stress approach together with the appropriate stress relaxation creep data are shown to correlate well with experimentally measured C* values. Crack growth rates during static load-controlled tests correlate well with C* . Good agreement is obtained between crack growth rates during the static tests and those produced during the hold period of the creep-fatigue tests.     

A probabilistic model for fatigue crack growth under random loading

A model for predicting fatigue crack growth rate and life probabilistically under random load history is presented. It allows for random growth per cycle, and is based on experimental results of constant amplitude cyclic loads. Predictions of the model are on the conservative side at the same time avoiding overdesigning. The reliability is included in the model thereby avoiding the need for using a factor of safety or ignorance in estimating a fatigue life or a crack length after N cycles of load application. The model is computer-oriented.     

Effect of cyclic overload on the crack growth behavior during hold period at elevated temperature

Effect of tensile overload on elevated temperature crack growth behavior during the subsequent load hold period has been studied by numerical and experimental methods. Finite element analysis of compact specimens shows that when the tensile overload precedes the load hold period, C _t during the hold period is significantly smaller (i.e. retarded) compared to the case without the overload. This is due to crack tip stress relaxation associated with large crack tip plasticity generated by the overload. A modified C _t estimation scheme is proposed by introducing a new equation for t _pl. Using this scheme, the retardation behavior of C _t due to the overload is successfully modeled.Creep-fatigue crack growth data for an ex-service 1.25Cr-0.5Mo steel at 538°C (1000°F) were generated in air. The hold times are 10 seconds, 98 seconds and 10 minutes. Time-dependent crack growth rate during the load hold period, (da/dt)_avg, is correlated with (C _t )_avg estimated by the new estimation scheme. (da/dt)_avg data from all the tests with overload are higher than those from the tests without overload. The peak stress associated with the overload seems to have enhanced void nucleation and to incrase the time-dependent crack growth rate due to creep. This argument is supported by microscopic observations.     

Stress Intensity Controlled Kinetic Crack Growth and Stress History Dependent Life Prediction with Statistical Variability

Consistent with viscoelastic behavior, a power law form in terms of the stress intensity factor is used to specify crack kinetics (growth rate) in the central crack problem under Mode I conditions. The crack growth rate is integrated to obtain the crack size and thereby the stress intensity factor as a function of time. The crack is allowed to grow in a controlled, load dependent manner until it reaches the size at which it becomes unstable. The corresponding time at which this occurs is taken as the lifetime of the material under the specified load history. The special cases of constant load (creep rupture) and constant strain rate to failure are found to have a very simple relationship with each other. This lifetime relationship is verified through the comparison with corresponding data upon a polymeric composite. Finally the creep rupture case is generalized to a probabilistic formalism. The theoretically predicted lifetime distribution functions are verified with data, also upon a polymeric composite. Possible extension of the entire formalism to cyclic fatigue in metals is discussed. Dedicated to Professor Z.P. Bažant for his many contributions.     

Depth of intergranular oxygen diffusion during environment-dependent fatigue crack growth in alloy 718

The elevated-temperture fatigue crack growth behavior in alloy 718, when subjected to a loading frequency lower than the transitional frequency of this alloy, is viewed as fully environment dependent. In this process, the crack growth increment per loading cycle is assumed to be equal to the intergranular oxygen diffusion depth at the crack tip during the cycle effective oxidation time. In order to identify the trend of this diffusion depth an experimental program was carried out on compact tension specimens made of alloy 718 at 650 °C in which fatigue crack growth measurements were made for cyclic load conditions with and without hold time periods at minimum load level. This work resulted in establishing a relationship correlating the intergranular oxygen diffusion depth and the value of the stress intensity factor range ΔK. This relationship, when integrated over the cycle effective oxidation time, results in a closed-form solution describing the environment-dependent fatigue crack growth rate. A comparison is made between the results of this solution when applied to different loading frequencies and the corresponding experimental results. This comparison shows good agreement between the two sets of results. Furthermore, by combining the parabolic rate law of diffusion and the equation for the intergranular oxygen diffusion depth, an explicit expression for the oxygen diffusivity of grain boundaries is derived. It is found that this diffusivity is both a ΔK- and a frequency-dependent parameter.     

Application of crack tip plasticity based fatigue model on high temperature alloy HAYNES® 282®

Confined crack tip plasticity model is employed to predict time independent fatigue crack growth rate (FCGR) behavior of HAYNES® 282® alloy at temperatures 1200F and 1400F. Crack growth tests were done in lab air, vacuum and steam environments at load ratios R = K_min/K_max ranging from 0.05 to 0.5. Calibrated model predicts average cyclic crack growth rate behavior of the material reasonably well. Predictions do not capture the accelerated fatigue crack growth rates observed in the data at low load levels. Such effects are believed to be caused by environmentally driven factors, which are not expected to be predicted by plasticity based models.     

Effect of frequency on high-temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers, Si₃N₄SiC_W, has been undertaken at elevated temperature during static and dynamic loading at increasing K and ΔK respectively. It is shown that cyclic sub-critical crack growth rates are lower than static crack growth rates. The increased crack growth rate during static far field loading is attributed to the stress relaxation of the inter-granular glass phase which allows time-dependent processes to occur ahead of the crack tip which lead to enhanced sub-critical crack growth rates. During cyclic fatigue the glass phase has insufficient time to relax and glassy ligaments are able to bridge the crack wake thereby shielding the crack tip from the full force of the applied load. Also, at particular temperatures, bridging between the surfaces of the crack wake by the inter-granular glass phase results in increased strength and fatigue retardation. The extent of ‘crack wake healing’ is shown to be time and temperature dependent. The viscosity of the glass phase is directly related to the temperature and the bonding force associated with glass phase bridging is observed to reduce with increasing temperature. The results from a previous study at room temperature are compared to those found during this investigation.     

Trace Elements in Superalloys

Abstract

In this study, effects of superheated steam on cyclic crack propagation behavior of a heat resistant steel were investigated. Crack propagation experiments were carried out on NF616 (9Cr-0.5Mo-2WVNb) in pressurized superheated steam (600°C/10MPa) under cyclic loading either with or without holding time at constant load. Superheated steam environment has two opposing effects on cyclic crack growth, acceleration and retardation. A modified tarnish rupture (TR) model has been proposed to explain the crack propagation behavior. The crack propagation rate estimated based on the TR-type model well agreed with the experimental data.     

Acceleration and retardation of fatigue crack growth rate due to room temperature creep at crack tip in a 304 stainless steel

Room temperature creep (RTC) at a crack tip and its influence on the fatigue crack growth behavior of a 304 stainless steel have been studied at room temperature. A time-dependent deformation has been observed at the crack tips under various stress intensity factors. The deformation increases with increasing stress intensity factor. Either acceleration or retardation of fatigue crack growth rate is found after holding at K _RTC, which depends on the load pattern. A demarcation line is observed on the fracture surface following the holding period. This implies that the crack propagation root or mode changed after the hold time.     

Effect of cyclic overload on the crack growth behavior during hold period at elevated temperature

Effect of tensile overload on elevated temperature crack growth behavior during the subsequent load hold period has been studied by numerical and experimental methods. Finite element analysis of compact specimens shows that when the tensile overload precedes the load hold period,C _t during the hold period is significantly smaller (i.e. retarded) compared to the case without the overload. This is due to crack tip stress relaxation associated with large crack tip plasticity generated by the overload. A modifiedC _t estimation scheme is proposed by introducing a new equation fort _pl. Using this scheme, the retardation behavior ofC _t due to the overload is successfully modeled. Creep-fatigue crack growth data for an ex-service 1.25Cr-0.5Mo steel at 538°C (1000°F) were generated in air. The hold times are 10 seconds, 98 seconds and 10 minutes. Time-dependent crack growth rate during the load hold period, (da/dt)_avg, is correlated with (C _t )_avg estimated by the new estimation scheme. (da/dt)_avg data from all the tests with overload are higher than those from the tests without overload. The peak stress associated with the overload seems to have enhanced void nucleation and to incrase the time-dependent crack growth rate due to creep. This argument is supported by microscopic observations.     

FATIGUE CRACK GROWTH IN NICKEL-BASED SUPERALLOYS AT 500-700°C. I: WASPALOY

Abstract— The effects of cyclic frequency, hold time, and stress-intensity-factor range (δ K ) on rates of fatigue crack growth in air at 500-700°C have been studied for Waspaloy—a nickel-based superalloy used for gas-turbine engine discs. The main effects observed were: (i) higher rates of crack growth for lower cyclic frequencies at high δ K at 600 and 700°C. and (ii) lower rates of crack growth at low δ K (and higher δ K thresholds) for longer hold times at 700°C, compared with those at a baseline frequency of 2 Hz. Metallographic and fractographic observations suggested that the effects of cyclic frequency and hold time could be rationalised in terms of the competing effects of enhancement of cracking due to creep and inhibition of cracking caused by oxide-induced crack closure, fracture-surface-roughness induced crack closure, and crack-branching/deflection. Possible mechanisms for promoting intergranular and transgranular cracking at low cyclic frequencies or long hold times are discussed.     

A FRACTURE MECHANICS APPROACH TO HIGH-CYCLE FATIGUE CRACK GROWTH UNDER IN-PLANE BIAXIAL LOADS

Abstract Fatigue crack growth under biaxial tensile load conditions is reported for a structural sheet steel. The new test facility can operate at high frequency (0–40 Hz) thereby permitting real-time testing required for threshold investigations; specimens are of the cruciform type.
It is found that crack growth rate is affected by a cyclic tensile load applied in the direction of growth which decreases as the said load increases. The rate however increases if the biaxial loads are increasingly out of phase.
Within the test conditions reported LEFM can be applied to crack growth under biaxial load conditions. The threshold stress intensity range is shown to be a function of load biaxiality, phase difference and stress ratio.