Fatigue Crack Growth Rate Tests Research Articles

Experimental investigation on the fatigue and fracture properties of a fine pearlitic rail steel

This study reports an experimental investigation of the fatigue and fracture resistance of R350HT, a heat-treated pearlitic rail steel with refined microstructure used in rails. Monotonic tensile, rotating bending, linear elastic plane strain fracture toughness, and fatigue crack growth rate tests are presented. The results are used to outline the basic properties and are corroborated by fractographic investigation. This enables the identification of the dominant type of fracture. Regarding fatigue and fracture resistance, the investigated material shows similar properties as other pearlitic rail steels, such as R260. At room temperature, the dominating fracture is of brittle cleavage type, showing some ductile regions associated with pro-eutectoid.

CO2 effect on the fatigue crack growth of X80 pipeline steel in hydrogen-enriched natural gas: Experiment vs DFT

Utilizing the existing natural gas grid presents a promising method for transporting hydrogen on a large scale. However, the effects of natural gas and its impurities on hydrogen-assisted cracking in pipeline steel have not been adequately studied. This study aims to investigate the fatigue performance of X80 pipeline steel in hydrogen-enriched natural gas (HENG) environments and in mixtures containing the impurity CO2. This is achieved through fatigue crack growth rate (FCGR) tests and density functional theory (DFT) calculations. The experimental results show that the FCGR in H2 is slightly faster than that in HENG, whereas it is slower than that in the N2/CO2/H2 mixtures. The enhanced FCGR by CO2 further increases with the increasing CO2 content. DFT computational results indicate that the adsorbed CO2 on the iron surface significantly accelerates the migration of H atoms from surface to subsurface. This promotes the entry of hydrogen into the steel.

A nonlinear kinetic rate model for unifying fatigue crack growth rates of nickel-based superalloys at room and elevated temperatures

This study presents a nonlinear kinetic rate model to describe the temperature-dependence of fatigue crack growth rate (FCGR) of nickel-based superalloys across a wide range (20 to 650 °C). FCGR testing was performed using compact tension specimens of GH4742 superalloy. Metallography analysis on crack growth paths of two tested specimens suggests there is a local resistance to pure model I cracking at 20 °C, which can greatly be weakened at 650 °C. Compared to the classical Arrhenius equation, the proposed model captures the nonlinear temperature-dependence, and yields more accurate results by reducing the scatter factor from 5 to 3.

Crack growth in high strength mooring line steel under variable amplitude loading

Mooring lines are of key importance for the safe and reliable operation of numerous floating offshore structures. Due to the constantly changing wind and wave conditions acting on such structures, any fatigue crack growth in a mooring line will occur under cyclic loading of distinctly variable amplitude. This paper considers a long-term load history for mooring lines on a North Sea offshore platform. For fatigue testing purposes, a characteristic load sequence representing the actual load history in a condensed and simplified form is developed and presented. Fatigue crack growth rate test results for a grade R4 high strength mooring chain steel subjected to this variable amplitude load sequence is furthermore presented, including tests performed in air as well as under free corrosion conditions in artificial seawater. Based on comparison testing performed under variable- and constant amplitude conditions, the validity of a linear damage rule hypothesis for stable crack growth under the characteristic load sequence is investigated. This damage rule converts the load spectrum to an equivalent constant load range, making it a potentially useful tool for estimating fatigue crack growth in mooring chains when test data for a representative characteristic load sequence are unavailable. Results from the comparison testing are used to show that under stable fatigue crack growth conditions, while subjected to the characteristic load sequence developed for this offshore platform, the linear damage rule can be a reasonable assumption.

Correlation between microstructure and fatigue properties of hot-rolled thick-plate complex-phase steel

Complex-phase (CP) steels, with a multiphase microstructure, offer an excellent combination of high strength, ductility, and formability, making them an attractive alternative to conventional high-strength low-alloy (HSLA) steels in the automotive industry. However, the microstructure and fatigue property relation in CP steels is complex. This limits the full exploitation of CP steels in applications, such as heavy-vehicles, where excellent fatigue performance of thick-plates after punching holes is the critical parameter. In this work, we initiate the study of the relation between microstructure and fatigue properties of a commercial CP steel (800CP) and compare it with a conventional HSLA (500MC) steel. Fatigue property, tensile property, and fatigue crack growth rate (FCGR) testing are conducted and the performance of the two steels is rationalized using detailed microstructure characterization, before and after fatigue testing. FCGR testing shows that, despite a higher yield strength of the 800CP, both steels have a similar propagation rate due to a more tortuous crack propagation path and a higher quantity of secondary crack formation in the 800CP microstructure. The high cycle fatigue (HCF) testing shows that the fatigue limit in the 800CP is 25% higher. This increase in fatigue limit is attributed to the improved resistance to fatigue crack initiation in the 800CP due to its larger fraction of bainite.

Enhanced hydrogen embrittlement of steel by the premature hydrogen dissociation with the increasing inert gas pressure in hydrogen-containing mixtures

Regarding hydrogen embrittlement (HE) in the mixtures of hydrogen and inert gas, the results were understood mainly based on the partial hydrogen gas pressure. On the other hand, the effect of the total gas pressure has not been fully investigated. In this study, fatigue crack growth rate (FCGR) tests were conducted in high-purity hydrogen gas at different gas pressures. Also, the FCGR tests were conducted in hydrogen and nitrogen mixtures at different total gas pressures. The hydrogen partial pressure in the mixture was the same as the high-purity hydrogen. This was the first time that acceleration of the FCGR in the mixtures compared to that in the high-purity hydrogen was observed. To elucidate the mechanism of the enhanced HE in the mixtures, hydrogen permeation and hydrogen desorption experiments were carried out. Based on the results of these tests, the acceleration of the FCGR in the mixtures could be interpreted by the fast hydrogen entry rate, which may result in a higher local hydrogen concentration near the crack tip during a limited time. Density functional theory (DFT) calculations were performed to consider the total gas pressure effect on an atomic scale. The thrust force of other gas molecules in the mixtures made the hydrogen molecules enter the potential field of the iron surface sooner, advancing the dissociation of the hydrogen. This will increase the surface concentration of the hydrogen atoms.

Fatigue crack growth behavior of SS316L deposition from wire arc additive manufacturing

Wire arc additive manufacturing has shown the potential to build large metal parts with high deposition rates and low cost of production compared to other additive manufacturing techniques. Austenitic stainless steel 316L is known for corrosion resistance and weldability, which makes it a strong candidate for building and repairing marine and offshore parts, with wire arc additive manufacturing. These parts experience fluctuating loads during service, which necessitates the fatigue performance study of wire arc additively build stainless steel. This work investigates the fatigue crack propagation behavior of stainless steel 316L depositions from wire arc additive manufacturing. Single beads of stainless steel 316L were deposited layer by layer to fabricate the wall structures. From the wall structures, the specimens were prepared for the tensile and fatigue crack growth rate tests along the traverse and build directions. In conjunction with mechanical testing, electron back-scattered diffraction and scanning electron microscopy techniques were employed to characterize the microstructure. Microstructural observations showed the presence of columnar dendrites along the build direction. The electron dispersive spectroscopy revealed the elemental diffusion at the layer interfaces due to the remelting of the layers. Differences in mechanical properties were observed across the two directions. Along build directions, the ultimate tensile strength was 8 % higher and yield strength was 7 % lower than the respective strengths along the traverse direction. The build direction also exhibited 46 % higher fatigue life cycle than the traverse direction. The difference in properties across different orientations was closely associated with the microstructural variations. Overall, the mechanical performance of the wire arc additively deposited SS316L is better than the previously reported selective laser melted SS316L.

The influence of printing strategies on fatigue crack growth behavior of an additively manufactured AISI 316L stainless steel

AbstractSelective laser melting (SLM) is an additive manufacturing powder‐bed fusion process that allows producing complex metallic parts with a relative density of up to 99.9%. Nevertheless, the mechanical properties of these components depend on numerous process variables, and there is a lack of systematic studies focused on SLM stainless steels. In the work herein presented, three specific printing strategies were X‐Y patterned to manufacture Compact Tension specimens transversally, longitudinally, or chess‐oriented, without any post‐processing heat treatment. The specimens were made of AISI 316L austenitic stainless steel. Thereafter, fatigue crack growth rate (FCGR) tests were carried out at room temperature, at constant amplitude loading (R = 0.2), to determine the fatigue crack propagation constants of the Paris Law associated with each print strategy. It was possible to conclude that transversal additively manufactured specimens showed the lowest FCGRs, and all results were well below the fatigue design curve given in ASME BPVC, Section XI.

Fatigue crack growth behavior for gas tungsten arc welded joint of TC2 titanium alloys sheets

Gas tungsten arc welding (GTAW) is a most preferred welding method for titanium alloys. Due to severe inhomogeneity of welded joints resulting from welding heat input and stress concentration, the joints are prone to fatigue crack initiation, and the fatigue crack growth rate (FCGR) and direction have great variability. Thus, it is of great significance to master the fatigue crack growth (FCG) behavior for the evaluation of remaining life. The FCGR tests were performed in different zones of the TC2 titanium alloy joints welded by the GTAW, and the acceleration behavior of the FCG was investigated. The results show that the crack growth is accelerated prematurely at stable crack growth stage (stage II). The precracks in the weld zone (WZ) and heat affected zone (HAZ) present cross-regional growth during the acceleration process. The regions before and after the critical acceleration point correspond to stage IIa and stage IIb, respectively. The FCG life at the stage IIa accounts for 90% of the total life, and the stress intensity factor range at the critical acceleration point is much lower than that at the instability point. It is proposed that the critical acceleration point is taken as the remaining life evaluation point of the TC2 titanium alloy gas tungsten arc welded joint, and the crack should be limited to the stage IIa, where the FCGR decreases according to the base metal, WZ and HAZ. In addition, the control parameters and the critical criterion of the accelerated growth were proposed, which laid a foundation for the remaining life prediction of the TC2 titanium alloy structures welded by the GTAW.

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.

Remaining fatigue life assessment of corroded mooring chains using crack growth modelling

Failures caused by the combined actions of fatigue, corrosion and wear are important safety concerns for mooring chains used on floating structures in the oil and gas industry. Prediction of remaining corrosion fatigue life based on surface condition could therefore be a useful tool for the continued safe operation of corroded chains. This paper investigates the use of crack growth modelling for estimating the remaining corrosion fatigue life of mooring chains that exhibit significant pitting corrosion damage. A crack growth modelling approach is used to produce remaining fatigue life estimates for a selection of severely pitted mooring chains. Using fatigue crack growth rate test results for grade R4 high strength mooring chain steel, empirical crack growth laws are presented for free corrosion and cathodic protection conditions at load ratio R = 0.1. Two different methods for establishing equivalent cracks from surface scans of corrosion damage are presented. The mooring chains are proof loaded as part of their manufacturing process. Residual stresses introduced during this process have therefore been determined by finite element analysis and accounted for in the fatigue crack growth predictions. One of the equivalent crack models, accounting for the single dominant corrosion pit, provided quite accurate fatigue life predictions when compared with full scale test results.

Effect of Mg on Inclusion and High Cycle Fatigue Behavior in Titanium Microalloyed Beam Steel

In this paper, the fatigue behavior of titanium microalloyed beam steels were studied by high cycle fatigue test and fatigue crack growth rate test. The effect of Mg addition on the fatigue behavior in titanium microalloyed beam steel was systematically analyzed. According to the experimental results, the addition of magnesium can effectively modify the inclusions by reducing the size of Al2O3 and TiN and promoting the formation of finer complex inclusions with a MgO·Al2O3 core in titanium microalloyed high-strength beam steel. The number of inclusions in the experimental steels had far less of an impact on the fatigue characteristic than inclusion size. With the heterogeneous nucleation effect of MgO·Al2O3, the inclusions are refined after the Mg addition. The tensile strength of Beam-2 steel decreased by approximately 54 MPa, while its fatigue strength increased by about 33 MPa, showing favorable fatigue resistance. These findings are essential for optimize the fatigue properties of titanium microalloy steel and promoting the development of automobile beam steel with excellent fatigue properties.

Crack Growth Rate Model Derived from Domain Knowledge-Guided Symbolic Regression

Machine learning (ML) has powerful nonlinear processing and multivariate learning capabilities, so it has been widely utilised in the fatigue field. However, most ML methods are inexplicable black-box models that are difficult to apply in engineering practice. Symbolic regression (SR) is an interpretable machine learning method for determining the optimal fitting equation for datasets. In this study, domain knowledge-guided SR was used to determine a new fatigue crack growth (FCG) rate model. Three terms of the variable subtree of ΔK, R-ratio, and ΔKth were obtained by analysing eight traditional semi-empirical FCG rate models. Based on the FCG rate test data from other literature, the SR model was constructed using Al-7055-T7511. It was subsequently extended to other alloys (Ti-10V-2Fe-3Al, Ti-6Al-4V, Cr-Mo-V, LC9cs, Al-6013-T651, and Al-2324-T3) using multiple linear regression. Compared with the three semi-empirical FCG rate models, the SR model yielded higher prediction accuracy. This result demonstrates the potential of domain knowledge-guided SR for building the FCG rate model.

Analysis of fracture toughness and fatigue crack growth rates of electron beam-welded dissimilar nickel-based superalloys: Mode-I cracks oriented normal to the weld

This paper discusses the experimentally obtained results of fracture toughness and fatigue crack growth rates of dissimilar metal-welded joints of Inconel 718 to Nimonic 80A. The dissimilar metal-welded joints were prepared in butt joint configuration using electron beam welding. Mode-I cracks were oriented normal to the weld. The base metals have different yield strength values (plastic mismatch) and ultimate tensile strength values but have the same elastic modulus and thermal expansion coefficients. The experiments were conducted using single-edge notched bend specimen design. The fracture toughness ( J) and fatigue crack growth rates of dissimilar metal-welded joints were affected by the direction of crack growth, such as cracks moving from stronger metal to weaker metal or vice versa. In comparison to Nimonic 80A (base metal), the J value of dissimilar metal-welded joint with crack direction from Nimonic 80A (stronger) to Inconel 718 (weaker) was lower. Moreover, the J value of dissimilar metal-welded joint was higher than that of Nimonic 80A (base metal) when crack traveled from Inconel 718 (weaker) to Nimonic 80A (stronger). In the fatigue crack growth rate test, the crack grew faster as it approached the weld interface from Nimonic 80A toward Inconel 718 due to crack acceleration. Conversely, the crack grew slowly in the opposite direction due to crack shielding. The onset of these effects began sooner for higher loads than lower loads.

Validation of on-line crack compliance data analysis methods for the residual stress intensity factor

Residual stress can significantly impact the fatigue crack growth rate (FCGR) observed in standard tests. In the present study, implementation of the on-line crack compliance (OLCC) method for determining the residual stress intensity factor, Kres, in FCGR tests is supported by validation data, details of novel data analysis methods, and strategies for data reduction. Slitting measurements on C(T) specimens of quenched (but not stress relieved) and aged aluminum alloy 7050-T74 provide reference profiles of Kres as a function of crack size. The OLCC method quantifies Kres profiles from stress intensity factor control and load control FCGR tests over a range of residual stress states. Novel OLCC data reduction techniques are developed, and data analysis attributes and limitations explored, contributing to an improved methodology for standardization. The results demonstrate that the on-line crack compliance method is a valid means of quantifying Kres during fatigue crack growth rate testing.

Effect of Stress Ratio on Fatigue Crack Propagation Behavior of Single Crystal Superalloy

In order to study the impact of different stress ratio on the fatigue crack growth behavior of DD6 single crystal superalloy, the fatigue crack growth rate test of different stress ratio (R = 0.05, 0.2, 0.5, 0.6) at 530 °C was conducted, the influence of stress ratio on crack growth rate was studied, and the growth of crack extension rate was corrected by normalization method. The results show that the section includes the fatigue fracture section perpendicular to the load and an angle to the load. The flat section is the main section representing the crack extension. The oblique section is formed with the change of the crack extension load state, and its occurrence is related to the dispersion of (da/dN)−Δk data, reflecting the uniqueness of single crystal. The Paris formula better characterized the crack expansion behavior of DD6 alloy, and the stress is more sensitive than the crack growth rate in the early stable growth phase. Crack growth rate normalized characterization equation constructed under different stress ratios are predicted to be within the 1.3-fold dispersion band.

Prediction of fatigue life of TC4 titanium alloy based on normalized equivalent initial flaw size model

Ti-6Al-4 V (TC4) is the primary and most widely used titanium alloy in the aerospace industry, mainly for the manufacture of engines and high temperature load-bearing elements in aircraft structural components. In this study, fatigue life and fatigue crack growth (FCG) rate tests of TC4 were carried out at different temperatures and stress ratios. The effect of temperature on the fatigue mechanical properties of TC4 titanium alloy materials was investigated analyzing the test results. The fatigue limit and FCG threshold were obtained from experiments. By the using equivalent initial flaw size method, the growth behavior of small cracks were incorporates in macroscopic FCG rate and a method to predict the fatigue life of TC4 at different temperatures was established. The parameters in the proposed model have a clear physical meaning and can be determined by experiment. Finally, the fatigue test data of TC4 titanium alloy under different test conditions are used to verify the proposed fatigue life prediction model and the prediction results have reasonable accuracy with the test results.

Different effects of multiscale microstructure on fatigue crack growth path and rate in selective laser melted Ti6Al4V

AbstractDuring the selective laser melting (SLM) process of Ti6Al4V, a special structure can be formed with columnar prior β grains along the building direction and fully martensitic α′ within the β grain. To investigate the influence of such special structure on the fatigue crack growth (FCG) rate, Ti6Al4V specimens fabricated by SLM were heat‐treated at two different temperatures in this study. The columnar grains were retained, and the martensite was decomposed when heat‐treated below the β transus. It is found that all the SLM features were removed when heat‐treated above the β transus. FCG rate tests were subsequently performed at room temperature, and it was found that the prior β grains affected the macroscopic fracture morphology, but there was no discernible influence on the FCG rate. The morphology of the α phase affected the crack growth path and the FCG rate. Changes in the strength‐toughness relationship induced by heat treatment can help understand the decrease in the FCG rate.

Investigation of the fatigue crack growth behavior of S355 steel weldments of motor hangers of high-speed trains

The behavior of fatigue cracks in welded details of motor hangers is investigated through crack growth numerical simulation under constant amplitude loading. The fatigue crack growth rate (FCGR) parameters of S355J2W steel are measured. FCGR tests are conducted on compact tension (CT) specimens. FCGR increases as the stress ratio increasing. Test data are compared with the reference curve of standards IIW and BS7910. Crack growth calculating results under mode I and mixed-modes conditions are compared. The crack growth path by simulation is in good agreement with the real crack path. The results also revealed that, even though fatigue cracks in the motor hanger are usually caused by mixed-modes conditions, mode I still governs the propagation.

Effect of Internal Hydrogen on the Fatigue Crack Growth Rate in the Coarse-Grain Heat-Affected Zone of a CrMo Steel

The effect of internal hydrogen in the fatigue crack growth rate of the coarse grain region of a 2.25Cr1Mo steel welded joint was analyzed in this work. The microstructure of the coarse grain region was simulated by means of a heat treatment able to provide the same microstructure with a similar prior austenite grain size and hardness to the one in a real welded joint. The fatigue crack growth rate was measured under standard laboratory conditions using compact tensile (CT) specimens that were (i) uncharged and hydrogen pre-charged in a hydrogen pressure reactor (under 19.5 MPa and 450 °C for 21 h). The influence of fatigue frequency was assessed using frequencies of 10 Hz, 0.1 Hz, and 0.05 Hz. Additionally, two load ratios (R = 0.1 and R = 0.5) were applied to analyze their influence in the da/dN vs. ∆K curves and therefore in the fatigue crack growth rate. The embrittlement produced by the presence of internal hydrogen was clearly noticed at the beginning of the fatigue crack growth rate test (ΔK = 30 MPm), obtaining significant higher values than without hydrogen. This effect became more notorious as the test frequency decreased and the load ratio increased. At the same time, the failure mechanism changed from ductile (striations) to brittle (hydrogen decohesion) with intergranular fracture (IG) becoming the predominant failure mechanism under the highest loads (R = 0.5).