High Fatigue Crack Growth Research Articles

On the role of flat-top beam in pore elimination and fatigue performance of Ti-6Al-4V fabricated by laser powder bed fusion

It's unavoidable to produce defects in the repeated layer-by-layer manufacturing process of laser powder bed fusion (LPBF). This limits the application of LPBF for safety–critical parts. The formation of defects is closely related to the interaction between the laser beam and material in the molten pool. In this paper, beam shaping was used to improve the building quality of LPBF. Process parameter optimization was performed to obtain samples with the fewest defects. Heat treatment was also employed to further improve the mechanical properties. The results showed that the obtained flat-top beam significantly reduced the propensity for keyhole pore formation and enlarged the process windows. In-situ heat treatment induced by high heat input led to the decomposition of martensite and improved the fatigue crack growth (FCG) threshold (∼3.7 MPa m0.5) of the as-built samples. After heat treatment (annealed and solution/aged), the FCG threshold further increased to 5.5/6.3 MPa m0.5, respectively. Reduced defects and high FCG resistance resulted in a fatigue limit above 600 MPa (N = 1e7, R = 0.1), which is comparable to wrought and aged Ti-6Al-4V parts. This study demonstrates the feasibility of improving molten pool stability and reducing defects through beam shaping, providing a new approach to manufacturing high-quality LPBF parts.

A combined cyclic viscoplasticity and entropy generation approach for modelling fatigue crack growth behavior of a nickel-based superalloy at high temperature

This paper developed an entropy-based approach to estimate the fatigue crack growth (FCG) behavior of GH4169 at high temperature. Firstly, systematic FCG experiments of GH4169 at 650 °C were conducted to obtain the experimental data. Then, a finite element analysis combined with Chaboche viscoplasticity and node release technology was developed to obtain cyclic stress–strain responses and entropy generation fields. Based on thermodynamic analysis, numerous discrete material elements as sub-systems were divided along the crack growth direction in simplified representative 2D middle section. Fracture process zone (FPZ) was defined equal to the size of discrete material elements (ρ) which can be determined by distribution of normal stress perpendicular to the crack plane. Subsequently, an effective cyclic entropy generation related to ρ was defined as the crack driving force. Effective cumulative entropy generation was calculated when crack increment was ρ and the results indicated that effective cumulative entropy generation float within a certain range. The average of all effective cumulative entropy generation values was defined as crack growth entropy. Finally, an entropy-based FCG rate model was established to estimate the FCG behavior of GH4169, and a good correlation between experimental results and model predictions are achieved for all the high temperature FCG tests.

High cycle fatigue properties and fatigue crack propagation behavior of a bainite railway axle steel

Carbide-free bainite (CFB) steel with metastable austenite and refined microstructures has been demonstrated to be a promising alternative to conventional pearlitic rail and wheel steels, due to its outstanding static mechanical properties and performance in service. However, the exploration on its potential application in railway axles is scarce, and the effect of microstructures on the fatigue crack growth (FCG) behavior of CFB steels is not well understood. In this work, a CFB axle steel containing retained austenite (RA) and refined bainite is designed by austempering and low-temperature tempering. The static mechanical properties, high cycle fatigue (HCF) properties and FCG properties of the conventional high-speed railway axle steel (grade DZ2) and the CFB steel are evaluated. The tensile strength, uniform elongation and HCF strength of CFB steel are about 87%, 24% and 60% higher than that of the DZ2 steel, respectively. The FCG behavior of the two axle steels is almost equivalent in the Paris law region. In the DZ2 steel, crack deflection and secondary cracks inhibit the main crack growth, the grain bridging caused by the intergranular crack growth delays the crack opening at the crack tip. The crack propagates transgranularly in the CFB steel. When the crack propagation direction is parallel to the long axis of the bainite lath, the crack propagates preferentially along this direction. However, when the directions are nearly perpendicular, a zig-zag crack path formed near the fine bainitic lath can effectively prevent crack growth. Furthermore, the deformation induced martensitic transformation ahead of the crack tip enhances the crack-tip shielding.

Material damage integration approach for efficient modelling of high cycle fatigue

The recognition of the risk of fatigue failure and its study, particularly fatigue crack growth (FCG) behaviour of engineering materials, is not neoteric, and the majority of approaches for investigating the problem are empirical and dated. With recent computational advances, the cohesive zone modelling (CZM) approach for FCG analysis has become popular especially amongst researchers owing to its flexibility of use particularly within the finite element framework. However, the use of the CZM for explicit cycle-by-cycle high cycle FCG analysis of real structural components is still largely computationally prohibitive. Thus, this study presents a novel material integration (MI) approach to accelerate fatigue crack propagation within the cyclic cohesive zone modelling (CCZM) framework. Using a bilinear cohesive law, the proposed technique is compared with the Linear Extrapolation (LE) technique and assessed for three models of different bulk-interface element discretisation and deformations. The results show that the MI technique offers a more consistent approximation to the accelerated fatigue damage computation and, more importantly, better convergence characteristics for the different models under tension, mixed mode and bending deformations. These outcomes underline the computational benefits of the proposed MI technique in assessing the FCG behaviour of real structural components within the CCZM framework.

Numerical simulation of plasticity identification and discussion of parameters

Numerical studies play a major role in the understanding and prediction of plasticity. The numerical models simplify the reality by considering discrete crack propagations, a relatively high fatigue crack growth, sharp cracks and that propagation occur at a well-defined load. Besides, there are a great number of numerical parameters affecting the accuracy of the predictions. The aim of the present paper is to discuss the numerical simulation of PICC. The numerical parameters, affecting the accuracy of the numerical simulations, and the dependent parameters, adequate to characterise the plastic wake and the closure level, are identified. The influence of the parameters of the numerical algorithm, the radial size of crack front elements and the extent of crack propagation for stabilization is analysed. An intrinsic uncertainty is associated with the number of load cycles between crack increments and the definition of crack closure level. Finally, the model developed is used to study the effect of stress ratio (R) on crack closure level.

Fatigue crack growth behavior of friction stir processed super duplex stainless steels (SAF- 2507)

The effects of friction stir processing (FSP) on the fatigue crack growth (FCG) behavior of super duplex stainless steel (SDSS) SAF 2507 was investigated. Both base metal (BM) and friction stir processed (FSPed) compact tension (CT) samples were fatigue tested under constant amplitude load; at a stress ratio (R) of 0.1 and frequency of 10 Hz. Crack length was monitored using a video camera. The crack growth behavior for both BM and FSPed CT specimens was characterized using stress intensity factor and the J-integral computed using elastic-plastic finite element analysis (FEA). Optical and scanning electron microscopes (SEM) were used to analyze the fracture surfaces. The results of this study show that FSPed specimens have higher fatigue crack growth (FCG) resistance and fatigue life as compared to base material. In addition, the crack propagation under the above-mentioned conditions has revealed changes in fracture mechanisms, due to FSP. The microscopic analyses of the fracture surface of FSPed specimens revealed numerous crack initiations sites and complicated crack propagation paths which contributed to the retardation of the crack propagation.

Fatigue Crack Growth Behavior of Al-4.41Cu-0.69Mg-0.64Si-0.52Mn Alloy Forged at Different Temperatures

The fatigue crack growth (FCG) behavior of T6-aged Al-4.41Cu-0.69Mg-0.64Si-0.52Mn alloys forged at 350°C and 450°C were analyzed. FCG rate tests revealed that the alloy forged at 350°C exhibited a higher fatigue crack threshold (ΔKth) and lower FCG rate in the near-threshold region but a higher FCG rate in the Paris and instability regions than the alloy forged at 450°C. Examination of the crack paths suggested that intergranular propagations were predominant in the near-threshold region, and more crack deflections at grain boundaries contributed to the lower FCG rate of the recrystallized sample forged at 350°C. In the Paris and instability regions, transgranular propagations were predominant, and the lower FCG rate of the non-recrystallized sample forged at 450°C was attributed to the crack deflections between the dislocation walls and sub-grain boundaries.

Influences of PPB, PPB affect zone, grain boundary and phase boundary on crack propagation path for a P/M superalloy FGH4096

The fatigue crack growth (FCG) tests for hot isostatic pressed (HIPed), isothermal forged (IFed) and heat treated (HTed) FGH4096 superalloys were conducted in air at 650–750 °C with and without dwell time to investigate the effect of microstructural characteristics on FCG paths. Optical microscope (OM) and scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) were used to observe and analyze the microstructures. After FCG tests, fracture surfaces and central sectioned surfaces were observed to investigate the crack propagation paths. FCG rate curves were also plotted to assist the analysis of FCG behaviors. The results indicated that prior particle boundaries (PPBs) were prevalent in HIPed specimens, but they transformed to PPB affect zones (PAZs) in IFed specimens by isothermal forging. After the standard heat treatment, PAZs were eliminated, and grain boundaries which were decorated with continuously distributed carbides became prevalent in HTed specimens. Grain boundaries in HTed specimens made the greatest contribution to FCG, and the highest FCG rate was attributed to the predominant intergranular failures. PPBs also had a pronounced influence on FCG, and the dominant failures of interparticle resulted in the high FCG rate in HIPed specimens. However, PAZs made the least contribution to FCG mainly owing to their particular microstructures. Thus, the prevalent failures of PAZs resulted in the lowest FCG rate in IFed specimens. Phase boundaries of large γ′ precipitates in PPBs or PAZs also had accelerating effect on FCG, while their contributions were relatively small.

Hydrogen-assisted fatigue crack growth in ferritic steels – a fractographic study

Fatigue crack growth (FCG) behavior of a Fe-3wt.%Si ferritic alloy under different environmental conditions using in-situ electrochemical (cathodic) hydrogen (H) charging has been investigated. Three frequencies have been applied. Results clearly show that the FCG rate increased by a factor spanning from 20 to 1000 times, depending on the loading frequencies, when compared to the reference test in air. Lower frequency leads to higher FCG rate. A comprehensive fractographic analysis was carried out: the area fraction of different fracture surface features was measured and taken into statistical analysis. Based on these investigations, the possible mechanisms of H-enhanced FCG are discussed. Similar tests in high-pressure H gas from other studies were also compared and discussed. These results give a preliminary understanding of H effect in fatigue crack propagation procedure in ferritic alloys.

Oxidation and creep interactions during high temperature high-R fatigue crack growth threshold determination

While knowledge relating to the determination and practical application of fatigue crack growth threshold stress intensity factors (ΔKth) for defect assessment is relatively well established for many circumstances, this is not the case for materials and conditions which are sensitive to time dependent mechanisms. In particular, current testing standards do not address determination of the property for very high R (Kmin/Kmax) ratios, and those conditions when the environment can be influential and ΔKth is sensitive to the adopted da/dN(ΔK) criterion. The incidence of creep crack growth in association with very slowly growing high-R high cycle fatigue cracks is not always discernable by the direct examination of near-to-threshold da/dN(ΔK) test records, but can be predicted by means of a time dependent failure assessment diagram analysis. In addition to a general state of knowledge review, particular attention is paid to circumstances relating to high-R ΔKth in power plant steels at high temperatures for which oxide induced closure and creep cracking can be influential. Evidence for a low alloy 1%Cr steel, a martensitic 9%Cr steel and an austenitic 18%Cr steel is considered, and the practical implications are examined.

Very high cycle fatigue strength and crack growth of thin steel sheets

For basic observations or for industrial applications it is of interest to use flat specimens at very high frequency in the gigacycle regime. In this work, thin flat sheet, with 1.2 mm thickness of a complex phase ferrite-martensitic steels were considered for carrying out fatigue tests at high frequency (20 kHz) up to the gigacycle regime (>109 cycles). The crack initiation tests were carried out with water cooling, while the crack growth test were carried out in laboratory air at room temperature. All the tests were carried out under loading ratio R=-1. To do that, special designs of specimens were made and computed using FEM for defining the stress amplitude for endurance tests. Special attachments for specimens to the ultrasonic system’s horn were enhanced. A particular FEM computing of the stress intensity range on crack growth specimens was carried out for determining the specimen dimensions and an equation that defines the stress intensity range as a function of the harmonic displacement amplitude, dynamic Young’s modulus, material density and crack length. Detailed procedures and fatigue results are presented in this paper.

Fatigue Crack Growth Behavior of Bonded Aluminum Joints

The current paper presents a research aiming at characterizing the fatigue behavior of adhesively bonded aluminum joints. In particular, Double Cantilever Beam (DCB) specimens and End Notch Flexure (ENF) specimens built using the 6061-T651 aluminum alloy substrate and the Araldite 2015 epoxy adhesive were used to evaluate the pure mode I and mode II fatigue crack propagation rates. Besides the fatigue crack growth rates the monotonic quasi-static fracture behaviors of the adhesive joints are also characterized for both loading conditions. Numerical finite element models of the joints are also proposed for the evaluation of the compliance calibration curves, avoiding the need of direct fatigue crack growth measurements, which is a very complex task mainly for the ENF tests. Critical fracture energies (GIc and GIIc) from experimental tests were in the same order of magnitude of the values published in the literature. Concerning the fatigue crack growth behavior, it was verified a higher fatigue crack growth resistance under pure mode II loading. A good agreement was found between the experimental fatigue crack growth data and the modified Paris law that accounts for fatigue crack propagation regimes I and II.

ICONE23-1713 WELDING JOINT DESIGN OF ITER TOROIDAL FIELD COIL STRUCTURE UNDER CRYOGENIC ENVIRONMENT

Application of partial penetration welding (PPW) to ITER Toroidal Field Coil structure has been proposed because of limited access for weld due to complex shape and low stress components. In order to obtain fatigue crack growth (FCG) behavior of PPW joint in cryogenic environment, Japan Atomic Energy Agency performed FCG test at 4K by using Compact Tension (CT) specimens having as-weld notch of PPW. These CT specimens were sampled from mockups having one of actual joint shape of PPW, double J-groove. As the result of these tests, it was observed that crack propagated in weld metal having inclination from as-weld notch. Moreover it was shown that as-weld CT specimens had high FCG rate region in early stage of crack propagation due to residual stress distribution. In addition, it was proposed and verified application method of this FCG rate to designing of PPW joint.

Thermal fatigue crack growth behaviours of laser deposited Ti60A near α titanium alloy

Thermal fatigue damage of high temperature titanium alloys and thermal fatigue crack growth (TFCG) behaviours are of great concern for severe temperature fluctuating environments. Cyclic thermal fatigue tests of heating to 700°C for 55 s followed by water cooling for 3 s at 20°C per cycle were performed with laser deposited Ti60A (Ti–5·54Al–3·38Sn–3·34Zr–0·37Mo–0·46Si) alloy. Thermal fatigue crack growth paths including crack length and crack growth directions were examined, as were their correlations with microstructure changes during the thermal fatigue process. Results show that local coarsened lamellar α developed by the interaction between cyclic thermal stress and oxygen atom interstitial solution effect occurs in the alloy after 800 cycles. Thermal fatigue cracks initiate after 800 cycles. Subsequently, total crack length increases with the volume fraction of local coarsened lamellar α from 800 to 2000 cycles. The enhanced oxygen penetration process plays a key role in the TFCG process.

Effect of Hydrogen Gas on the Growth of Small Fatigue Crack in JIS-SCM435

The fatigue crack growth (FCG) from a small hole in a low alloy steel JIS-SCM435 round bar was investigated using tension-compression fatigue tests in 0.7 MPa hydrogen gas and ambient air. In the higher FCG rate regime (e.g. da/dN > 108 m/cycle), FCG was accelerated in hydrogen gas compared to in air. On the other hand, in the lower FCG rate regime (e.g. da/dN < 108 m/cycle), FCG in hydrogen was rather slower than that in air. There was no noticeable difference in fatigue limits between these two atmospheres. The FCG in the respective atmospheres showed a typical small crack behavior, i.e. the da/dN for small cracks were much greater than those for large cracks obtained by compact tension (CT) specimen when they were compared at the same ΔK level. In order to unify such a discrepancy of FCG behavior between small crack and large crack, the strain intensity factor range ΔKε was adopted. As a result, the da/dN data for various crack sizes was gathered in a narrow band, i.e. the small crack effect was successfully evaluated with the strain intensity. Moreover, the crack growth life was predicted based on the da/dN-ΔKε relation. The reproduced S-N curve showed a conservative agreement with the fatigue life obtained by experiments.

Fatigue crack propagation behavior in railway steels

Purpose – The present paper aims to characterize the fatigue crack propagation behavior of wheel and rail steels, in particular the AVE wheel steel and an UIC60 rail steel, including several R-values and near threshold behavior. To accomplish this objective, mode I fatigue crack growth tests were performed according to the ASTM E647 standard on C(T) specimens taken from a Spanish high-speed AVE train used wheel and a UIC60 rail, tested with 0.1, 0.4 and 0.7 load ratios. Design/methodology/approach – In the present study, the two different methodologies presented in the ASTM E647 standard were used to characterize the fatigue crack propagation behavior of the two studied materials. The K-decreasing test procedure was used to characterize fatigue crack propagation near the threshold, whereas the K-increasing with constant load range method was used in the Paris law regime. Findings – It was observed that for the wheel a small influence of R-ratio was found, with greater R implying higher fatigue crack growth rates. For the rail, the influence is small, and for large values of ΔK, it is slightly reversed. The near-threshold results obtained indicate lower threshold values for higher R-ratio, a fact that is possibly associated with crack closure phenomena. A scanning electron microscope (SEM) study of fatigue crack propagation surfaces identified a random behavior in the striation orientation for both materials and no correlation was found between striation spacing and actual fatigue crack growth rate. Originality/value – R-ratio and threshold behavior of fatigue crack propagation of a steel used in high-speed train wheels, as well as of UIC60 rail steel, were studied, with the objective of generating data to be used in maintenance and damage tolerance models.

Fatigue Crack Prediction of LC9 with Corrosion Damage Considering Effect of Load Frequency

Considering the degradation effect of corrosion damage on fatigue behavior of aero aluminum alloy, the present thesis made a research on corrosion fatigue crack growth rate. Taking into account the effect of load frequency on fatigue crack growth, a concept of corrosion fatigue frequency factor is proposed. Based on the fact that low frequency will lead to high corrosion fatigue crack growth rate, and frequency higher enough will make little difference between corrosion fatigue and pure mechanical fatigue behavior, an exponential expression of corrosion fatigue frequency factor is proposed. The crack growth rate prediction from proposed formula is proved to be in good agreement with experimental results for steadily extended corrosion fatigue crack.

Effect of Processing Layer on Fatigue Strength of Extruded AZ61 Magnesium Alloy

Fatigue Property of an Extruded AZ61 Magnesium Alloy with the Processing Layer Introduced by Machining was Investigated. Rotating Bending Fatigue Tests were Carried out with the Specimen with and without the Processing Layer. According to Results of the Fatigue Tests, Fatigue Life Significantly Increased by Introducing the Processing Layer to the Specimen Surface. Fatigue Crack Initiation and Propagation Behaviors were Observed by Replication Technique during the Fatigue Test. Fatigue Crack Initiation Life of the Specimen with the Processing Layer was Slightly Longer than that of the Specimen without the Processing Layer. Higher Fatigue Crack Growth Resistance was also Observed when the Fatigue Crack was Growing in the Processing Layer in the Specimen with the Processing Layer. the Longer Fatigue Life Observed in the Fatigue Test in the Specimen with the Processing Layer could be Mainly due to the Higher Crack Growth Resistance. it is Speculated that the Fatigue Strength can be Controlled by Change in Condition of Machining Process. it could be Effective way in Industry to Improved Fatigue Strength only by the Cutting Process without Additional Surface Treatment Process.

Stress and Stress Intensity Factor Near Notches in Thick Cylinders

This work investigates the impact of semi-elliptical notches (erosion grooves) at the bore of pressurized and autofrettaged thick cylinders. It provides a robust yet rapid method to determine the rapidly varying stress in the near-notch region prior to crack development and crack tip stress intensity factor after a crack develops at the notch root. The procedure involves a sequence of asymptotic solutions and adjustments. A superposition is presented for the stress concentration factor (SCF) of a small edge notch in a pressurized cylinder with bore pressure infiltrating and acting upon the notch surface. A procedure for adjusting this SCF to account for a varying pre-existing stress field is described. This provides accurate predictions for notch depths of up to 15% of wall thickness. Stress variation beyond the notch root is determined by scaling analytic solutions. Solution accuracy appears to be approximately 5%. Stress profiles were used to calculate stress intensity factor (SIF) for cracks emanating from the notch root and deep into the wall. There are notable differences between SIF behavior in the pressurized tube and in the autofrettaged tube. The main reason for this difference is that compressive reyielding near the notch disrupts the residual compressive stress profile over an extended distance. This leads to the conclusion that a crack originating from a notch in an autofrettaged tube exhibits a much higher cyclic SIF range during pressurization than the same length crack originating from the bore. This will cause higher fatigue crack growth rates and shorter fatigue lifetimes.

Influence of Fatigue Loading on the Engineering Critical Assessment of Steel Catenary Risers in Sour Deepwater Oil and Gas Developments

Steel catenary risers (SCR) are used in deepwater oil and gas developments to transfer produced fluids from the seabed to surface facilities. SCRs can be subject to fatigue loading from a variety of sources including wave and tidal motion, vortex induced vibration (VIV) and operating loads. When the produced fluids are sour (ie contain water and H2S) higher fatigue crack growth rates (FCGR) are expected, and this can have a significant effect on defect tolerance. The aim of this paper is to provide guidance on the current best practice methods for performing engineering critical assessments (ECA) on internal surface breaking defects in SCRs operating in a sour environment and subject to VIV fatigue loads. Example ECA calculations are presented for circumferential girth weld flaws, based on the failure assessment diagram (FAD) approach within the framework of BS 7910 [1]. The influence of certain key input variables is demonstrated, including the FCGR, determined from recent sour test data generated as part of this research.