Small Crack Growth Rates Research Articles

An investigation of small fatigue crack behavior in titanium alloy TC4 under different stress levels

In this paper, the small fatigue crack behavior in titanium alloy TC4 under different stress levels was investigated. Single-edge-notch tension specimens were axially fatigued with stress ratio of 0.1 at room temperature. Results show that the naturally initiated cracks nucleate from the α/β interfaces. When the crack is below a critical length of ∼200 µm, crack growth rates exhibit large fluctuations and temporary retardations due to microstructure effects. But once the crack length exceeds ∼200 µm, the fluctuations in crack growth rates die down and the crack grows more rapidly. There is no distinct effect of stress level on small crack growth rate. A linear relationship exists between the total fatigue life and crack initiation life.

Probabilistic prediction of minimum fatigue life behaviour in α + β titanium alloys

AbstractThe objective of this work was to develop and demonstrate a probabilistic life prediction method for the prediction of minimum fatigue lives that are typically used in the design of fracture critical rotating turbine engine components. A Monte Carlo analysis was used to predict the variability in fatigue lives based on the distribution of microstructural features that lead to early crack initiation as well as the variability in small fatigue crack growth rates. Two titanium alloys, both with bimodal microstructures, were tested and analysed in this study. The distribution of critical microstructural features was calibrated based on test results and understanding of microstructure neighbourhood effects. Testing was conducted on both alloys and included both smooth and notched specimens. The predictions are presented and compared with the data for smooth and notch geometries for the various loading conditions. A parametric study was performed to identify the importance of several model inputs and to identify areas for future improvement.

Retardation of Small Creep–Fatigue Crack in Gr. 91 Steel Through the Combined Effects of Stress Relaxation, Microstructural Evolution, and Oxidation

This investigation reports an unusual effect of hold time (up to 10 seconds) on retardation in the growth of creep–fatigue small cracks at 550 °C in Grade 91 steel. The observed phenomenon was interpreted by elucidating multiple processes that are active in the plastic zone at the crack tip. To this effect, microstructural and mechanical responses of the crack tip plastic zone were compared with the mechanical and microstructural responses during low cycle fatigue/creep–fatigue. It is proposed that the stress relaxation that occurs during the hold time can reduce the stress intensity in the plastic zone of crack tip thus, contributing to retardation in the small crack growth rate. Aside from stress relaxation, stress intensity in the crack tip can be further reduced by the phenomenon of cyclic softening that occurs because of plasticity-induced microstructural coarsening. Separately, the contribution of oxidation-induced crack tip shielding is also considered to explain the observed effects of hold time on crack growth rates. Taken together, a combination of stress relaxation, enhanced rate of cyclic softening, and higher degree of oxidation with the introduction of a hold time is demonstrated to be responsible for reduction in the crack growth under creep–fatigue conditions.

Comparison of small and long fatigue crack growth behavior in AA 7050-T7451

Fatigue crack growth behaviors of small and long cracks are compared in terms of crack closure, from near-threshold to final fracture, in over-aged aluminum alloy AA 7050-T7451. This is done experimentally to quantify inputs for crack propagation models such as the Navarro-Rios model. Growth rate of small cracks are determined as a function of stress intensity factor range using replicas taken from etched specimens loaded at high and low stress amplitudes. Similar data for long cracks are obtained using a compact tension C(T) specimen. Recrystallized grains are found to be the most significant microstructural barrier to propagation when cracks are small. In addition, small cracks typically appear to behave as closure-free long cracks. Transition from apparent closure-free to crack closure behavior is observed and characterized in terms of grain size. Finally, analysis of crack closure effects on C(T) specimens shows that long cracks propagating in the studied aluminum undergoes roughness- and oxide-induced closure.

Predicting the 3D fatigue crack growth rate of small cracks using multimodal data via Bayesian networks: In-situ experiments and crystal plasticity simulations

Small crack propagation accounts for most of the fatigue life of engineering structures subject to high cycle fatigue loading conditions. Determining the fatigue crack growth rate of small cracks propagating into polycrystalline engineering alloys is critical to improving fatigue life predictions, thus lowering cost and increasing safety. In this work, cycle-by-cycle data of a small crack propagating in a beta metastable titanium alloy is available via phase and diffraction contrast tomography. Crystal plasticity simulations are used to supplement experimental data regarding the micromechanical fields ahead of the crack tip. Experimental and numerical results are combined into a multimodal dataset and sampled utilizing a non-local data mining procedure. Furthermore, to capture the propensity of body-centered cubic metals to deform according to the pencil-glide model, a non-local driving force is postulated. The proposed driving force serves as the basis to construct a data-driven probabilistic crack propagation framework using Bayesian networks as building blocks. The spatial correlation between the postulated driving force and experimental observations is obtained by analyzing the results of the proposed framework. Results show that the above correlation increases proportionally to the distance from the crack front until the edge of the plastic zone. Moreover, the predictions of the propagation framework show good agreement with experimental observations. Finally, we studied the interaction of a small crack with grain boundaries (GBs) utilizing various slip transmission criteria, revealing the tendency of a crack to cross a GB by propagating along the slip directions minimizing the residual Burgers vector within the GB.

Microstructurally small fatigue crack growth rates in aluminium alloys for developing improved predictive models

The fatigue or durability life of a few critical structural metallic components often sets the safe and/or economic useful life of a military airframe. In the case of aluminium airframe components, growth rates, at or soon after fatigue crack nucleation are being driven by near threshold local cyclic stress intensities and thus are very low. Standard crack growth rate data is usually generated from large cracks, and therefore do not represent the growth of small cracks (typically <1mm). Discussed here is an innovative test and analysis technique to measure the growth rates of small cracks growing as the result of stress intensities just above the cyclic growth threshold. Using post-test quantitative fractographic examination of fatigue crack surfaces from a series of 7XXX test coupons, crack growth rates and observations of related growth phenomenon in the threshold region have been made. To better predict small crack growth rates under a range of aircraft loading spectra a method by which standard material data models could be adapted is proposed. Early results suggest that for small cracks this method could be useful in informing engineers on the relative severity of various spectra and leading to more accurate predictions of small crack growth rates which can dominate the fatigue life of airframe components.

Measured fatigue crack growth increments versus predictions for small cracks in 7XXX aluminium alloys

Whilst crack growth retardation following the application of an overload during variable amplitude loading is a well-known spectrum effect, the order of smaller amplitude cycles can also affect crack growth. One method of accounting for such spectrum effects in crack growth prediction algorithms is by correcting the estimated effectiveness of each cycle or group of cycles. Such predictions are usually verified by comparison to demonstrated total fatigue lives. These corrections have been shown to be suitable for cracks that are at least several millimetres long. However, for physically small cracks: here defined as less than a few mm in depth, such predictions are often far less accurate. In the case of aluminium alloy aircraft structures, where cracks typically grow approximately exponentially from small discontinuities, these predictive algorithms should also be evaluated for their ability to predict small crack growth rates, rather than total life alone.To examine the more subtle crack growth acceleration and retardation effects from individual load cycles in typical aircraft structural 7XXX aluminium alloys, this paper presents the results of quantitative fractography measurements of very small fatigue crack growth increments produced under the application of specially engineered spectra. One such spectrum that contained short segments of constant amplitude loads of various mean stresses, was designed to examine the extent of crack growth retardation, such as from closure, at a range of small crack lengths. Other spectrum effects were also explored through use of test spectra with constant amplitude cycles that were either separate to or embedded within small segments of variable amplitude loading. The local growth produced in each case was then measured and compared to predictions produced by two closure/retardation correction methods; based on wake and plastic zone related phenomena. The results showed that the prediction methods, although they made allowances for the presence of subtle retardations and accelerations, did not accurately predict these effects over the increments of growth examined.

Characterising the early stages of crack development in environment-assisted cracking

ABSTRACTAn overview is given of recent NPL research to characterise the early stages of crack development in environment-assisted cracking, focusing on the nature of the crack precursors, the pit-to-crack transition, and the impact of solution chemistry on growth rate in the small and long crack growth rate regime. The impact on corrosion and cracking of surface grinding-induced changes in near-surface material and mechanical properties is highlighted, noting that awareness of this aspect in the corrosion community still remains limited. A major feature is the remarkable insight into the evolution of small surface cracks, from dealloyed layers and from corrosion pits, provided by advanced 3D imaging. Finally, further evidence for the concept of the solution conductivity-dependent crack-size effect in stress corrosion cracking and corrosion fatigue is put forward.

Effects of macro-scale corrosion damage feature on fatigue crack initiation and fatigue behavior

The remaining fatigue life for aluminum aerospace alloys is greatly reduced by the presence of corrosion damage; prediction of this deleterious influence via standard fracture mechanics methods is non-trivial. Several corrosion morphologies can develop depending on the local chemistry, pH and electrochemical potential along the geometry of the structure. Previous studies demonstrate that such variations in corrosion morphology can affect the fatigue behavior. This study aims to quantify the effect of corrosion morphologies relevant to a fastener-hole geometry of AA7050-T7451 on the initiation life, propagation life and total fatigue life. Four corrosion morphologies are considered: shallow and deep discrete pits widely distributed on the corroded surface, deep fissure-like features, and general corrosion with surface recession. Corrosion morphologies are carefully characterized using optical microscope, white light interferometer and X-ray computed tomography. The samples are subjected to fatigue using loading protocols that create identifiable marks on the fracture surface at known cycle count. These marker bands are used to determine the small scale crack growth rate (da/dN) of the samples, as well as the cycles to initiate a 10μm crack. Results showed that initiation life plateaus to near constant values for varied corrosion morphologies, and crack growth rates converge to comparable values. The macro-scale corrosion metrics are considered and correlated to the location of the fatigue initiation. Severe macro-scale metrics such as pit or fissure depth, pit mouth surface area, pit volume, pit density, root mean square, peak density, maximum valley depth, do not uniquely correlate with the location of fatigue crack initiation. This suggests a strong role of micro-scale corrosion features and underlying microstructural features on the location of the fatigue crack initiation.

Crack Initiation and Growth from Pre-corroded Pits in Aluminum 7075-T6 Under Laboratory Air and Salt Water Environments

Small crack growth behavior, including the pit-to-crack transition, of pre-corroded aluminum alloy 7075-T6 was investigated under ambient air and saltwater environments. Through pit and corner crack pits were created prior to constant stress amplitude fatigue testing. The small crack growth life (SCGL) and crack initiation life (CIL) under salt water condition were about 5-8 times less when compared to those under ambient air. The ratio of SCGL to CIL was significantly smaller with large uncertainty (variation) under salt water environment. The small crack growth rates under salt water were 2-12 times larger than corresponding rates under laboratory air at identical stress amplitude and crack length. The crack lengths required to attain Paris region growth rates (Paris region threshold crack lengths) were about 1-3 mm for stress amplitudes less than 45 MPa. Sub-millimeter cracks attained 10−4 mm/cycle fatigue crack growth rates (FCGR) for stress amplitudes greater than 45 MPa. The crack growth rates of 0.25-mm-long cracks were about 2 times higher for salt water environment when compared to identical stress conditions in ambient air. Empirical crack growth rate versus stress intensity factor range equations similar to Paris law were developed for small crack growth and crack initiation under ambient air and salt water condition, for fatigue crack prognostics applications. For the design applications, a modified Kitagawa-Takahashi (K-T) diagram for corrosion-pitted aluminum 7075-T6 is presented.

Ti-6Al-4Vにおける内部微小疲労き裂進展速度の非破壊計測

Ti-6Al-4Vにおける内部微小疲労き裂進展速度の非破壊計測

Initiation and Propagation Behaviors of Small Crack in a Polycrystalline Ni-Base Superalloy under Thermo-Mechanical Fatigue Loading

In this work, the initiation and propagation behaviors of naturally initiated small crack in a polycrystalline Ni-base superalloy under the several kinds of thermos-mechanical fatigue (TMF) conditions. The effect of phase angle between temperature and strain cycles in TMF loading on the initiation and propagation behavior of small crack was discussed. The experimental results revealed that the initiation and propagation morphologies of fatigue small crack were affected on the phase angle; the crack initiation mode was changed from the intergranular mode to the transgranular mode with the increasing of the phase angle in this study. On the other hand, when the crack growth rates of naturally initiated small crack are correlated with fatigue J-integral range, the effect of the phase angle on the small crack growth rates became negligible even if the crack propagation mode depended on it.

Small internal fatigue crack growth rate measured by beach marks

Internal fatigue crack growth rates were measured using the beach marks created by repeated two-step fatigue tests on a high-strength steel. This material revealed internal fractures originating from oxide-type inclusions whose sizes ranged from 14 to 40µm. Large and small beach marks were observed depending on the conditions under which the repeated two-step fatigue tests were carried out. Small beach marks indicated small internal cracks just after crack initiation, while large beach marks indicated internal cracks in the final stages. The small internal cracks showed extremely slow growth rates that were much smaller than the lattice length: the measured growth rates were very close to those calculated using fracture mechanics. The large internal cracks showed conventional growth rates, larger than the lattice length, and the border between the large and small internal cracks was almost equal to the threshold stress intensity range ΔKth. These results show the validity of evaluating the fatigue lives by calculating the crack propagation lives of the small internal cracks as in the Tanaka-Akiniwa model. However, it has been suggested that the Tanaka-Akiniwa model overestimates the effects of inclusion sizes, meaning that the Tanaka-Akiniwa model has room for improvement. To correct this problem, we propose a new model, based on a new crack growth law, in which the crack growth rate also depends on crack size. The new model generates more realistic predictions.

Investigation of small fatigue crack initiation and growth behaviour of nickel base superalloy GH4169

AbstractSurface replication method was utilized to monitor the small fatigue crack initiation and growth process of single‐edge‐notch tension specimens fabricated by nickel base superalloy GH4169. Three different stress levels were selected. Results showed that small fatigue cracks of nickel base superalloy GH4169 initiated from grain boundaries or surface inclusions. The small fatigue crack initiation and growth stages took up about 80–90% of the total fatigue life. Multiple major cracks were observed in the notch root, and specimen with more major cracks seemed to have smaller fatigue life under the same test conditions. At the early growth stage, small crack behaviour might be strongly influenced by microstructures; thus, the crack growth rates had high fluctuations. However, the stress level effect on the small fatigue crack growth rates was not distinguishable for the three different stress levels. And no clear differences were found among the crack initiation lives by using replication technique.

Effect of Mean Stress on Small Fatigue Crack Growth Rate on Low Carbon Steel with Several Simulated HAZ Heat Treatment

In this study, to clarify the effect of microstructure of Heat Affected Zone (HAZ) and residual stress of weld on small fatigue crack growth behavior, uniaxial fatigue testing with surface small fatigue crack growth observation were carried out for low carbon steel with three simulated HAZ heat treatment under several mean stress conditions. As a results, εna type small crack growth law was also effective to HAZ microstructure under zero mean stress. Under tensile mean stress condition, fatigue fracture life and small fatigue crack growth life were decreased. In addition, it is clarified that small fatigue crack growth rate acceleration under tensile mean stress was able to be evaluated by using Smith-Watson-Topper equivalent strain εeq which is obtained from mean stress dependency of fatigue fracture life for smooth specimen and εeqna type modified small crack growth law was proposed. Furthermore, it is clarified that εeq was proportional to εeff which was evaluated by crack opening point measured with Digital Image Correlation technique. It is considered that equivalent strain εeq implicitly includes small crack growth life and effect of crack closure.

Near threshold fatigue crack propagation in railways’ steels: Comparison of two testing techniques

Mode I fatigue crack growth thresholds of steels for rail and wheel used in railways were determined according to two different procedures. Although fatigue crack propagation data in the Paris regime is usually straightforward to obtain, the same does not apply to the threshold regime, where crack growth rates are very small up to inexistent, and therefore tests are very time consuming. Furthermore, the very small fatigue crack growth rates involved in testing for threshold determination require very thorough and careful experimental procedures if extraneous effects – as for example interaction effects such as crack retardation or acceleration – are to be avoided. Two methods for evaluation of fatigue crack growth threshold value of the range of stress intensity factor were used: load shedding and constant maximum stress intensity factor with increasing load ratio R (R=minimum load/maximum load), taking into account the ASTM standard E647 prescriptions. The obtained results and lessons learned in conducting these tests are discussed, and the data generated is compared with data published.

Novel concepts on the growth of corrosion fatigue small and short cracks

Corrosion fatigue small, short and long crack growth rates have been determined for a 12Cr steam turbine steel in aerated 300 ppb Cl‐ + 300 ppb SO42‐ solution and in air at 90 °C. The crack growth rate for short and long cracks was monitored by direct current potential drop (DCPD) and for the small cracks by combining high resolution optical microscopy and DCPD. Comparison of the fatigue growth rate demonstrated that in solution the short crack growth rate was remarkably enhanced in comparison to long cracks, when the crack size is smaller than 250 μm. This enhancement was attributed to the electrochemical crack size effect associated with greater anodic polarisation of the short crack in such low conductivity solution. However, such enhanced growth was not observed for small cracks, which was rationalised on the basis of additional contribution of current from the pit limiting crack‐tip polarisation.

Grain size effect on multi-scale fatigue crack growth mechanism of Nickel-based alloy GH4169

The mechanism of multi-scale fatigue crack propagation of Nickel-bases alloy GH4169 with different grain sizes was identified in this paper by using the specimens with micro-notches. Results showed that specimen with coarser grain exhibited longer fatigue life. Grain size effect on the growth rate of small crack cannot be distinguished due to the fluctuation of crack growth rate. The small fatigue crack stage accounted for a larger fraction of the total fatigue life for the specimen with finer grain size. An empirical formula was proposed to predict the relationship between the growth rate of small crack and crack length.

Small and short crack growth and the solution-conductivity dependent electrochemical crack size effect

Corrosion fatigue small crack growth rates have been determined for a 12Cr steam turbine steel in aerated 300ppb Cl− and 300ppb SO42− under simulated two-shifting conditions. The remarkably enhanced crack growth rate observed previously for a short crack (relative to that for a long crack) was not evident for small cracks initiated from pits. In contrast, the stress corrosion small crack growth rate determined in aerated 35ppm Cl− was markedly increased with respect to long crack growth. The concept of the solution-conductivity dependent electrochemical crack size effect is introduced to provide a fundamental framework to rationalise the disparate results. The engineering implications for two-shifting operation are discussed.

Fatigue Crack Growth in Blade Attachment of Turbine Disks: Experimental Tests and Life Prediction

Abstract To assess residual life for those component parts where low cycle fatigue (LCF) design is adopted, especially for those critical regions where high stress concentrations cause cyclic yielding of the material, it is necessary to consider the presence of potential defects and their propagation during service. In order to represent the high stress concentration at the root of blade attachments in a gas turbine and the typical elastic-shakedown conditions, a notched specimen has been designed and tested. A series of experimental tests in three regimes have been carried out with the presence of small semi-circular micronotches and crack growth rates have been measured with plastic replicas. The results have shown that the initial over-load impressed by the spinning tests is able to double the propagation lifetime. The experimental results have then been successfully analyzed with Newman's crack growth model applied to the elastic shakedown stress fields at the notch root.