Fatigue Crack Growth Rate Behavior Research Articles

Investigations of fatigue crack growth rate behaviour and life prediction of Si3N4/TiB2 reinforced hybrid metal matrix composites

Many engineering structures and components experience complex fatigue loading throughout their operational lifespan. Economically, it is crucial to forecast the remaining lifespan to prevent catastrophic failure by implementing appropriate inspection schedules. The aim of present work is to investigate the fatigue, and fracture performances of AA7075 alloy-based hybrid MMCs containing Si3N4/TiB2 reinforcement. The composite samples are made by a liquid-state stir-casting process. The study examines the fatigue characteristics of manufactured Hybrid Metal Matrix Composites (HMMCs) by analyzing fatigue crack propagation and fracture toughness under constant amplitude loading with a uniform load ratio of 0.1. The potential influence of reinforcement particles on the fatigue durability of HMMCs was predicted through the application of an 'Exponential Model'. A comparison between the model's performance and experimental findings was carried out. The study suggests that the exponential model accurately predicts fatigue life, showing a maximum percentage deviation of −4.96 %, in close agreement with experimental findings. The fatigue crack growth rate (da/dN) and crack opening displacement (COD) of HMMC indicate that the fatigue life and fracture toughness improve with increase in TiB2 content, up to 6 wt%. The fabricated HMMC with 4wt% Si3N4 and 6 wt% TiB2 particulates demonstrated the highest resistance to fracture and longest fatigue life among others HMMCs and base alloy.

Application of Machine Learning (ML)-based multi-classifications to identify corrosion fatigue cracking phenomena in Naval steel weldments

Corrosion fatigue (CF) crack detection in welded ship hull structures is quite crucial as well as challenging for ensuring their structural integrity. This is conventionally done by offline ship hull survey and inspection methods, which have large time and cost implications. Data-driven approaches of ship hull crack detection are promising in this regard. Corrosion fatigue crack growth rate (CFCGR) behaviour of shipbuilding steel was studied for its base metal (BM), fatigue-prone heat affected zones (HAZs) of Submerged Arc Welding (SAW), and shielded metal arc welding (SMAW) joints of Naval-grade steel. The results indicated the highest CFCGR for BM, intermediate for SAW, and smallest for SMAW welds. This was attributed to 44% lath martensite observed in the SMAW HAZ microstructure as compared to 33% in SAW HAZ. A novel ML-based method has been proposed for CF crack location identification by a multi-classification approach using CFCGR data for structural health monitoring (SHM) applications. Prediction of crack location (either BM, SMAW HAZ, or SAW HAZ) was investigated using K-nearest neighbour (KNN), linear support vector machine classifier (LSVC), Naïve Bayes (NB), decision tree (DT), random forest (RF), and artificial neural network (ANN) models. DT and RF models were found to perform very well even without scaling and hyper-parameter tuning operations and exhibited 98–99% classification accuracy. Scaling and tuning operations resulted in significant improvements in accuracy of KNN models from 43% to 84% (minmax scaling) and 98% (standard scaling). Significant improvement in the accuracy from 37% to 98% ANN model was achieved by standard scaling. The proposed method shall be useful in data-driven real-time crack location identification in ship hull structures.

Assessing fatigue crack growth thresholds for a Ti-6Al-4V (STOA) alloy using two experimental methods

Fatigue crack growth rate behavior in the threshold and near-threshold regimes for a Ti-6Al-4V solution-treated and over-aged (STOA) alloy was investigated using two test procedures: i) accelerated load reduction (ALR) utilizing faster load-shed rates than recommended in the ASTM standard E647 load-reduction (LR) test procedure, and ii) compression pre-cracking constant-amplitude (CPCA) or load-increasing (CPLI) and load-reduction (CPLR). Fatigue crack growth tests were conducted on compact, C(T), specimens (B = 6.35 mm) with two different widths (W = 51 and 76 mm) at two stress ratios (R = 0.1 and 0.7). The crack-growth-rate test data were compared to previous test data produced from the same batch of material using the ASTM LR and CPCA test procedures that were tested on C(T) specimens (W = 25, 51 and 76 mm) over a wide range in stress ratios (R = 0.1, 0.4, and 0.7).Although none of the load-reduction test procedures provided a good representation of the threshold value (ΔKth) at low R, the CPCA test procedure was the most promising. The LR, ALR, and CPLR test procedures were influenced by prior loading history, leading to higher ΔKth values and slower crack growth rates in the threshold regime due to a rise in the crack-opening loads. The LR and ALR tests at R = 0.1 and shed rates of −0.08, −0.32, and −0.95 mm−1 yielded ΔKth values that were 15 to 38 % higher than the estimated threshold stress-intensity factor range (ΔK*th)R=0.1 based on the crack-closure model (FASTRAN). The compression pre–cracking test procedures are more accurate alternatives for developing threshold and near-threshold fatigue crack growth rates. Specifically, the CPLR test procedure produced a ΔKth value that was 8–18 % higher than (ΔK*th)R=0.1. Additionally, LR and ALR tests produced different ΔKth values as a function of the specimen width for a given load ratio, which violates classical fracture mechanics principles. In addition, the spread in the ΔK-rate data in the low-rate regime was larger than the spread in the mid-rate regime as a function of R, a term referred to as “fanning”. In contrast, the CPCA test procedure produced consistent crack growth rates over the same range of ΔK values examined, independent of the specimen width and these data did not exhibit “fanning” with R. Further research is necessary to develop standard test procedures capable of providing a more definitive representation of the ΔKth value in the threshold regime. However, FASTRAN can serve as a crucial tool in aligning the actual ΔKth values with those obtained through CP testing, thereby bridging the gap and enhancing the reliability of the results.

Microstructure, fatigue crack growth rate and low-cycle fatigue behavior of an Al[sbnd]Li alloy sheet with different aging

The microstructures, fatigue crack growth (FCG) rates and low-cycle fatigue (LCF) behaviors of 1445 Al-Li alloy sheet with T6 aging at 170 °C, single-step T8 (S-T8) aging at 150 °C and double-step T8 (D-T8) aging at 170 °C following aging at 150 °C for 4 h were investigated. The main precipitates in T6 and S-T8 samples are δ′ phases, but strain strengthening causes 40 MPa enhancement in the S-T8 sample yield strength (YS). Due to coherent shearable δ′ precipitates, the FCG rate curves of S-T8 samples are highly coincident. With the 2nd-step aging time extension, δ′ precipitates in D-T8 sample gradually coarsen, semi-coherent S′ phases form and their amount increases. The interaction between δ′ precipitates with dislocations transites from shearing to by-passing, leading to the YS enhancement and the FCG resistance decrease. Meanwhile, cyclic-hardening decreases and transfers to cyclic-softening during LCF at starin amplitude of 0.6% ∼ 0.9%. At 0.9% strain amplitude, due to greater cyclic-softening and cleavage fracture of S′ precipitates, the D-T8 sample with 2nd-step aging time of 120 h has a shorter LCF life. The D-T8 sample with 2nd-step aging time of 24 h possesses a comprehensive property of higher strength, lower FCG rate and better LCF life.

Correlating stress ratio effects on the fatigue crack growth rate behavior of a nickel-based superalloy GTM718

Damage tolerance evaluation under service loads requires a unified constant amplitude fatigue crack growth law for the material which takes care of stress ratio effects. Several types of crack driving force parameters have been proposed in the past to correlate stress ratio effects on fatigue crack growth rates. In the present investigation, a unified crack growth law is derived for a nickel-based superalloy by considering various crack driving force parameters to eliminate stress ratio effects. Initially, the constant amplitude fatigue crack growth rate (FCGR) behavior of GTM718 alloy was experimentally determined at various stress ratios, R ranging from R = 0.1 to 0.7. As expected, increasing the stress ratio, increased crack growth rates, and decreased threshold stress intensity factor range, ΔKth. The conventional crack growth rate, da/dN vs stress intensity factor range, ΔK data was then modified and re-plotted as a function of different crack driving force parameters viz., (a) Elber’s parameter, ΔKeff, (b) Walkers’ parameter, Kw (c) Kujawski’s parameter, K*, (d) Forman’s parameter, Kf, and (e) Two-parameter crack driving force, ΔK*. It was observed that Kf and ΔK* crack driving force parameters were correlating stress ratio effects better than other models in all three regimes of crack growth rates. Further, a unified crack growth law was derived based on the Kf and ΔK* crack driving force parameters which is useful in predicting FCG behavior under service loads.

Experimental study on the fatigue crack growth rates of welded ultra-high strength steel plates

The assessment of the fatigue crack growth rate behavior of welded structural components made of ultra-high strength steels (UHHS) is very important to gain a comprehensive understanding of these materials under high-cycle fatigue loadings and to enhance their applications in the construction industry. The literature on the fatigue crack growth rates of welded ultra-high strength steel plates with nominal yield strengths higher than 690 MPa is very scarce and only very limited research has been done thus far to provide the Paris’ Law fatigue material constants for welded UHSS components in terms of the base metal (BM), the weld metal (WM), and the heat-affected zone (HAZ) regions, which are essential for fatigue life prediction. Hence, the fatigue crack growth rates of butt-welded UHSS plates with three grades (S700, S960, and S1100) are experimentally investigated in this study using Paris’ Law for these three different regions of the welded components. Gas metal arc welding (GMAW) and laser welding (LW) methods are adopted to analyze the influence of welding methods on each steel grade with respect to their static and fatigue strength, microstructural changes, and hardness. In the fatigue crack growth rate test, it was observed that the S700 base material shows the lowest fatigue crack propagation resistance amongst these three grades. In welded components, laser welding outperforms gas welding in terms of the fatigue crack growth resistance. Comparisons of fatigue crack growth behaviors are made among all three tested UHSSs as well as with those from the literature.

A study on XFEM simulation of tensile, fracture toughness, and fatigue crack growth behavior of Al 2024 alloy through fatigue crack growth rate models using genetic algorithm

AbstractThis study aims to numerically validate and simulate the fatigue crack growth rate (FCGR), tensile, and fracture toughness properties of T3 and T351 tempered aerospace grade Al 2024 alloy. The extended finite element method (XFEM) coupled with a genetic algorithm (GA) was used to predict FCGR behavior in the framework of an in‐house developed MATLAB code. The numerical simulation was performed using three different FCGR models, i.e., Paris, Forman, and Nasgro. The FCGR prediction ability of simulation models was improved and compared further using GA. The Nasgro model with genetically optimized parameters shows the highest FCGR prediction accuracy compared to other FCGR models. The critical stress intensity factor and von Mises stress distribution under static loading were also predicted for C(T) specimen. A similar XFEM approach was used for analyzing the tensile behavior using a center cracked specimen at the gage section of a dog bone‐shaped specimen.

A robust probabilistic fatigue crack growth model based on walker’s crack growth rate equation for metallic materials

The fatigue crack growth data inherits statistical variation despite carefully controlled test procedures. The available probabilistic models rely extensively on fatigue crack growth test data of the coupon material under replicate loading conditions. In view of the often unavailability of such a database, this work proposes a new probabilistic model based on Walker’s crack growth rate equation and considers the variability in both the stress intensity factor range, ΔK and stress ratio, R. The optimum estimates of the model parameters are established using the maximum likelihood estimation (MLE) method. The model is examined using selected sets of measured fatigue crack growth curves of 7075-T6 and 2024-351 aluminum alloys. It demonstrates acceptable predictions for fatigue crack growth data on replicate and variable R-ratio loadings with failure probabilities in the range of 0.10 ≤pF≤ 0.90. The error on the observed fatigue crack growth rate data is normally distributed. The criterion of the minimum number of replicate tests for a valid probabilistic assessment of the fatigue crack growth data is established. Based on the mean squared error (MSE) parameter, the model provides a better representation of the mean fatigue crack growth rate behavior of the material for fatigue loading with variable R-ratios.

Fatigue Crack Growth Rate Description of RF-Plasma-Sprayed Refractory Metals and Alloys

A fitting method capable of describing the fatigue crack growth rate (FCGR) data in all stages of crack propagation by a simple Forman-style analytical formula was developed. To demonstrate its robustness, this method was used to quantify the fracture behavior of RF-plasma-sprayed W, Mo, W-Mo composite, and four selected Ni-based tungsten heavy alloys (WHA). The fitted FCGR parameters categorized the studied materials into two distinct sets. W, Mo, and W-Mo composite deposits made from inherently brittle refractory metals that contained a range of defects inherent to plasma spray process represented the first class. This class was characterized by low fracture toughness and a relatively wide range of fatigue crack growth thresholds. The second class of materials was represented by WHA. Here, the deposit defects were suppressed by liquid state diffusion that formed a typical WHA structure consisting of a Ni-rich matrix and large spherical W reinforcement particles. The WHA generally showed higher fatigue crack growth thresholds, but differed in fracture toughness values based on the W particle concentrations. The obtained fracture mechanical data represent a reference dataset of plasma-sprayed refractory materials, and their classification into groups clearly demonstrates the capabilities of the developed method to capture a wide range of different types of FCGR behavior.

A damaging function ΔKd for analyzing FCG and R-ratios in metallic materials

This paper presents a damaging stress intensity function ΔKd for analyzing R-ratio effects on fatigue crack growth (FCG) in metallic materials under constant amplitude loading. The proposed formulation is based on the sum of strain and complementary energy and its role in FCG rate behavior in threshold and Paris region at R-ratios ranging from −2 to 0.97. It doesn’t invoke a crack closure assumption or fitting parameters for R < 0.6. For a high R > 0.7 it utilizes an experimentally determine correction factor, which accounts for the excessive forward plastic zone (FPZ) size and its role on FCG process.

Mechanisms of fatigue crack growth in Ti-6Al-4V alloy subjected to single overloads

The relative importance of crack closure and residual stresses on fatigue crack growth (FCG), which is controversial, is studied here. FCG is predicted numerically in CT specimens of Ti-6Al-4V alloy submitted to single overloads, assuming that crack tip plastic strain is the driving force. The numerical procedure, which includes the effects of crack tip blunting, material hardening, crack closure and partial crack closure, was found to be very robust and to give correct trends. The high strength material studied showed relatively small overload affected zones (<0.5 mm), which is even more pronounced under plane strain conditions. The comparison of numerical results with and without contact of crack flanks showed that crack closure is responsible for the effect of overloads on FCG rate behaviour, while the effect of residual stresses is not relevant for the material studied. The transient behaviour obtained after an overload is unattainable without considering the contact of the crack flanks.

Fatigue and Static Crack Growth Rate of Alloy 718 Under Cathodic Polarization

Fatigue crack growth rate was developed on three heats of Alloy 718 (UNS N07718) under cathodic polarization, over a wide range of loading conditions. Fatigue crack growth rate increased with decreasing frequency over a range of Kmax and ΔK conditions. In most cases, there was no evidence of a plateau in fatigue crack growth rate at low frequencies. The fatigue crack growth rate over the range of conditions evaluated was influenced by static crack growth rate at Kmax. The principle of superposition of fatigue crack growth and static crack growth was used to rationalize the observed crack growth rate response. Static crack growth rate of Alloy 718 measured under constant K conditions was lower than that measured under rising displacement conditions. A crack tip strain rate-based model was used to rationalize the fatigue crack growth rate behavior and the static crack growth rate behavior under constant K. However, the formulation of the model for the rising K was not able to rationalize the crack growth rate under rising displacement conditions.

Damage Tolerance Behavior of a Nickel-Based Super-alloy GTM718 Under Cold-TURBISTAN Variable Amplitude Loads

In this study, the damage tolerance behavior in terms of fatigue crack propagation behavior of an aero-engine material under a standard variable amplitude load sequence was predicted and compared with experimental results. The constant amplitude (CA) fatigue crack growth rate (FCGR) behavior of GTM718, a nickel-based super-alloy, was determined at various stress ratios R = σmin/σmax ranging from R = 0.1 to 0.7. The empirical fatigue crack growth law was derived from this data in terms of two-parameter crack driving force, ΔK*. Then, the fatigue crack propagation behavior of GTM718 under a standard cold-TURBISTAN variable amplitude load sequence was predicted using cycle-by-cycle approach and using the CA fatigue crack growth law. Also, experimental fatigue crack growth behavior under the same cold-TURBISTAN variable amplitude load sequence was determined and compared with predicted results. A fairly good correlation was observed with predicted and experimental results.

Fatigue crack growth behaviour of a nickel base super alloy GTM720 under cold-TURBISTAN spectrum load sequence

Damage tolerance concept is widely used in aerospace industry for designing structural components against fatigue. In this study, an attempt is made to predict fatigue crack growth behavior in an aero-engine material under a standard spectrum load sequence using a closure independent crack driving force parameter, K* and compare with experimental results. Initially, constant amplitude (CA) fatigue crack growth rate (FCGR) behavior of GTM720, a nickel based super alloy was determined at stress ratios, R = σmin/σmax ranging from R = 0.1 to 0.7. Then, empirical fatigue crack growth law was derived from this CA-FCGR data in terms of two parameter crack driving force, K*. Further, the fatigue crack growth behavior was predicted through cycle-by cycle approach in a pre-cracked compact tension (CT) specimen of GTM720 alloy under standard cold-TURBISTAN spectrum load sequence using K* as the crack driving force. Also, experimental fatigue crack growth behavior under the same load sequence was determined in a CT specimen and compared with predicted results. A fairly good correlation was observed with predicted and experimental results. Predicted fatigue crack growth life was conservative and the fatigue crack growth life ratio, Npred/Nexpt was about 0.92.

Fatigue behaviour of a multiphase medium carbon steel: Comparison between ferrite/pearlite and tempered microstructures

AbstractFatigue crack growth is the essential process in the total life of components and is unavoidable in most structures and machines. In this study, the effects of cementite particles on the fatigue properties of medium carbon steel were investigated. A quenching and tempering treatment was developed to obtain uniformly distributed cementite particles with dozens of nanometres. To evaluate the fatigue properties, fatigue strength, fatigue crack growth rate and dynamic mechanical behaviour were examined. It was found that the fatigue strength of the tempered steel is much higher than that of air cooling ferrite/pearlite steel because the cementite particles act as barriers and hinder crack propagation during cyclic loading. More importantly, the high sensitivity of the damping peak to prior fatigue may play an important role on the fatigue history of multiphase medium carbon steel.

MECHANICAL PROPERTIES OF FORGED TUNGSTEN HEAVY ALLOYS

Tungsten heavy alloys are composite materials containing spherical tungsten particles embedded in binder matrix. Their excellent mechanical properties can be further improved by rotary forging. This paper aims to gain deeper understanding of the forging process by investigating the local elastic modulus, hardness, and residual stress of individual phases in W6Ni3Co pseudo-alloy. The resulting global properties of the composite material such as stress-strain behavior, fracture toughness and fatigue crack growth rate behavior are also studied. The results show that sintered and quenched material consists of highly textured matrix containing nearly perfect single crystal spheres of pure W. The rotary forging leads to significant lattice deformations destroying the texture and significantly increasing the hardness of both WNiCo matrix and W particles and making residual stresses in W particles anisotropic with increased compression along the longitudinal axis of the forged part.

Fatigue Crack Growth Rate Behaviour of HSLA Steel at Varying Load Amplitudes

Steels for ship building applications has to possess adequate resistance to propagation of fatigue cracks, as majority of the failures in service are due to metal fatigue. In this work, the fatigue crack growth rate (FCGR) behaviour within the Paris regime of two high strength low alloy (HSLA) steels were studied. In particular the experiments were directed to reveal the effect of load ratio(R) (Tension-Tension) on the Paris law constants for the two grades of HSLA steels. Results indicated that there is an increase in Paris slope ‘m’ and decrease in Y intercept ‘C’ with increase in load ratio for both the steels. Fractography study was carried out using SEM at locations corresponding to various values of stress intensity factor in order to reveal possible reasons for acceleration of crack growth with change in R ratio from 0.1 to 0.5. It was found that secondary cracks are predominant in Steel A at a load ratio of R=0.1 as compared to R=0.5. However for Steel B, secondary cracks were found at both the load ratios. From the plot of crack growth ‘a’ vs no. of cycles ‘N’, it became evident, the number of cycles for the same range of crack length is lesser for R=0.1 than that for R=0.5 for both Steel A and Steel B. Limiting values of ‘∆K’ and ‘K’ max has been obtained for various crack growth rates ‘da/dN’ at different load ratio for steel A and steel B.

Influence of retrogression and re-ageing heat treatment on the fatigue crack growth behavior of 7010 aluminum alloy

Aluminum alloys are widely used in aircraft structural components where light weight, high strength and good corrosion resistance are the primary requirements. These alloys are generally used in peak-aged (T6) condition in which they are susceptible for stress corrosion cracking. In the recent years, retrogression and re-ageing (RRA) treatment on aluminum alloy is carried out to enhance their corrosion resistance maintaining the ultimate tensile strength. The aim of this work was to study the influence of RRA treatment on the fatigue crack growth rate (FCGR) behavior. The 7010 aluminum alloy was heat treated to two different conditions i.e., T6 and RRA. The microstructures of these alloys were characterized by using TEM. Standard compact tension (CT) specimens were prepared and FCGR tests were carried out by using a 100 kN servo-hydraulic test machine as per ASTM E647-15e1. The constant amplitude FCGR tests were carried out at a stress ratio, R = 0.5 using sine wave loading pattern at 10 Hz. Crack length was monitored by following compliance technique. Microstructural studies show that RRA treated alloy contain fine and densely populated precipitates in the matrix along with coarse and discontinuous precipitates in the grain boundary. The fatigue crack growth rate was observed to reduce along with an increase in the threshold stress intensity factor range (ΔKth) for RRA treated alloy compared to the T6 alloy. The mechanisms for reduction in fatigue crack growth rate of RRA treated alloy is attributed to the microstructural modifications. The increased resistance is expected to enhance the damage tolerance capability of the alloy.

Fatigue and Corrosion Fatigue Properties of Additive-Manufactured Nickel Alloy 625 and Ti-6Al-4V

Abstract Specimen blanks of additive-manufactured nickel Alloy 625 and Ti-6Al-4V were produced by the laser powder-bed-fusion process (L-PBF) with the principal test axis in both the Z direction (parallel to the build direction) and the X-Y direction (perpendicular to the build direction). The high cycle fatigue and corrosion fatigue properties of these metals were measured using R. R. Moore rotating cantilever fatigue tests, both in air and with a salt water drip on the test sections. Testing was conducted in order to determine the fatigue and corrosion fatigue limits of these materials at 108 cycles. The fatigue limit for L-PBF Alloy 625 material at 108 cycles in air was roughly 48 ksi (331 MPa), independent of build orientation. This is similar to the air fatigue limit of wrought material. The corrosion fatigue limit for L-PBF Alloy 625 material at 108 cycles in salt water was roughly 39 ksi (269 MPa), which was also independent of build orientation and slightly below the values for wrought material and values obtained by other investigators. The fatigue limit for hot isostatic pressed L-PBF Ti-6Al-4V material at 108 cycles in air was roughly 90 ksi (620 MPa), independent of build orientation. This is significantly better than the air fatigue limit of wrought material. The corrosion fatigue limit for L-PBF Ti-6Al-4V material at 108 cycles in salt water was roughly 78 ksi (540 MPa), which was also independent of build orientation, better than values for wrought material, and comparable to values obtained by other investigators. The fatigue crack growth rate behavior in air was characterized and compared with information available in the literature.

Crack Size and Shape Considerations Related to Near-Threshold Fatigue Crack Growth Rate Behavior

Abstract It is well recognized that crack size or load history or both have an important influence on near-threshold fatigue crack rate behavior at a low stress ratio. This investigation will compare the behavior of a radial crack, such as a surface flaw or a corner crack, to that of a linear crack to see if crack shape plays a role in physically small crack to long crack behavior. Recent research has unveiled a methodology to experimentally partition plasticity from roughness- and oxide-induced crack closure, showing that nonplasticity-induced closure mechanisms have a greater influence at low Kmax by orders of magnitude. Knowledge of this is key to understanding differences between long crack and physically small crack behavior in the near-threshold regime. In this study, the well behaved 2024-T351 aluminum alloy was used to generate threshold data using numerous specimen geometries, flaw geometries, crack sizes, and test procedures to highlight not only the significance of crack size but also the role of crack shape. It will be shown both experimentally and analytically that a radial crack (surface flaw, corner crack) has significantly less remote closure and, therefore, a lower threshold than a through crack (compact tension, middle crack tension), even if the crack size is the same.