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The widely used GH4169 alloy can meet various application through different heat treatment methods. In order to investigate the effect of heat treatment method on crack resistance at elevated temperature, fatigue crack growth tests of direct aging heat-treated GH4169 alloy (DA-GH4169) and standard heat-treated GH4169 alloy (SHT-GH4169) were performed at 650 °C in air. Under 10 Hz sinusoidal waveform, the fatigue crack growth rate of DA-GH4169 is slightly higher than that of SHT-GH4169 at low- Δ K Paris regime. The rates become close as Δ K increased. Under 0.5 Hz triangular waveform, the fatigue crack growth rate of DA-GH4169 is always higher than that of SHT-GH4169 in the tested Δ K scope. The change of fracture morphology from intergranular to transgranular in the Paris regime was observed for two GH4169 alloys. It is attributed to the interaction of cyclic plastic damage and dynamic embrittlement at crack tip, which is influenced by load, microstructure and frequency. Completely different propagation modes were found in the near-threshold region. While intergranular δ phases mildly inhibit grain boundary oxidation in the Paris regime, the transgranular propagation induced by intergranular δ phases in the near-threshold region is responsible for lower threshold. [ABSTRACT FROM AUTHOR]

• FFM to model life estimation and crack growth prediction in open-hole composites. • Implementation of revision crack direction as a key approach to determine a realistic crack length. • Application of the Paris law to establish a translaminar crack propagation model. • Influence of fibre orientation on damage propagation mechanisms and crack direction change. • Attribution of broom and sawtooth patterns to fibre orientation followed by splitting failure mechanisms. The demand to capture translaminar crack growth under fatigue loading scenarios led this work contribution to carry out the Finite Fracture Mechanics (FFM) method in fatigue damage growth and the application of the Paris model to generate the translaminar damage propagation prediction. The purpose of this study is to analyse the effect of fibre orientation on translaminar crack propagation rate using the FFM model, which includes cycle damage increment estimation and fractographic analysis. The results confirm the feasibility of FFM in predicting crack growth and estimating life under cyclic loading. However, C-scan analysis and the revised crack propagation direction are critical in determining the realistic crack length, considering adhesive failure along the fibre direction. Additionally, this work contribution is also related to the application of the Paris model (based on dL/dN vs ΔK) to generate the translaminar damage propagation prediction model. The most dominant damage mechanism was the splitting pattern, which changed the aspect of failure for each laminate architecture as a function of fibre orientation. The laminate with multidirectional fibre orientation exhibited higher resistance to translaminar crack propagation due to the growth of splitting and delamination in multiple directions. The fibre orientation changed the propagation path, which influenced the fracture toughness and crack propagation rate behaviour. [ABSTRACT FROM AUTHOR]

One of the ways to identify the type of failures and to estimate the lifetime of components is to examine the fracture surface by fractographic analysis. The application of this investigation can be introduced in the prediction of the fatigue lifetime of the vehicle parts, aircraft and marine components, and all other structures that are under cyclic loading. In this study, fracture properties of the aluminum-silicon-magnesium alloy, under strain-controlled low-cycle fatigue loading, were studied considering striations on the fracture surface. This material has been widely used in engine cylinder heads. For this purpose, at the first step, relations and formulations were extracted, including the Paris crack growth law and the distance of the fracture surface striations. After measuring striation spacing from the experimental data of the low-cycle fatigue test for aluminum alloys (with and without heat treatment), the material constants of the Paris law were calculated and calibrated. The obtained results of the lifetime estimation, as compared to the experimental fatigue lifetime, stated that the scatter-band was 1.4X, which indicated the accuracy of the proposed model with an average error of 15%. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mixed-mode I&II FCG tests are conducted on 16MND5 steel using CTS specimens. • Crack driving force and crack closure both have significant impacts on the FCG curves. • Fatigue fractography reveals similar microscopic features in the Paris region for all CTS tests. • A unified method for characterizing FCG rates is developed. Although numerous fatigue crack growth (FCG) tests have been conducted, the conclusions drawn regarding the mixed-mode FCG rate are inconsistent and even contradictory. A reasonable conjecture is that crack driving force and crack closure effects may play a crucial role in causing this situation. In this paper, the 16MND5 steel, commonly used reactor pressure vessel (RPV) steel, was employed to investigate the mixed-mode I&II FCG behaviors through the compact-tension-shear (CTS) experiments. The influence of crack driving force and crack closure effect was analyzed and discussed. The results show that the crack propagation path tends to maximize the tangential stress component at the crack tip, and the effect of Δ K II can be negligible once the Paris region is reached. Moreover, when considering the crack closure corrected Δ K I , named Δ K I eff , all the FCG rate data under different loading conditions can be collapsed into a narrower band of distribution. The premise for arriving at this result is to use the stress intensity factors solution that are applicable to specimens with real slanted propagating cracks, rather than applying equivalent methods based on single-edge straight crack assumptions. Finally, the fatigue fractography further confirms that the FCG mechanism during the Paris region all follow a similar quasi-mode I behavior. [ABSTRACT FROM AUTHOR]

Uniaxial fatigue tests were conducted for a β titanium alloy Ti-22V-4Al up to a very high cycle fatigue (VHCF) regime. The initiation and propagation processes of the internal fatigue cracks were investigated using 3D fractographic analysis. Multiple facets were observed at the crack initiation site. Three facet initiation models were proposed based on the surface appearances and the 3D facet bonding patterns of the multiple facets, and the major facet was determined to be the true crack initiation site. Using the size of the major facet, a Tanaka–Akiniwa model, which can determine the material constants for the Paris law using only conventional fatigue tests, was applied to reveal the propagation process of the internal cracks. A reverse fatigue life prediction was also conducted to evaluate the accuracy of the material constants obtained using the Tanaka–Akiniwa model. When the facet initiation models were applied, the predictions showed less deviation and better agreement than when the facet initiation process was not considered. The findings of this study indicate that the formation of multiple facets in β titanium alloys is sequential rather than simultaneous. [ABSTRACT FROM AUTHOR]

Fatigue delamination growth (FDG) is an important failure in composite structures during their long-term operations. Hygrothermal aging can have significant effects on interlaminar resistance. It is therefore really necessary to explore FDG behavior in composite laminates with hygrothermal aging. Dynamic mechanical thermal analysis (DMTA), mode I FDG experiments and fractographic examinations were conducted to fully investigate hygrothermal aging effects and the corresponding mechanisms on FDG behavior. The DMTA results indicated that environmental aging can induce obvious T g decrease. Mode I experimental fatigue data interpreted via different Paris-type correlations demonstrated that: Bridging has obvious retardation effects on FDG behavior via the Paris interpretations; The modified Paris relation can well characterize the intrinsic FDG behavior around the crack front; The use of the two-parameter Paris-type relation can appropriately account for R -ratio effects, contributing to a master resistance curve in determining mode I FDG behavior. According to these interpretations, it can be concluded that hygrothermal aging can have adverse effects on mode I FDG behavior. SEM examinations demonstrated that moisture absorption can cause fibre/matrix debonding and resin matrix pores/voids in the composite. However, no obvious difference in damage mechanisms was identified in mode I fatigue delamination for composite with/without environmental conditioning. Both fibre/matrix debonding and matrix brittle fracture were identified on fatigue fracture surfaces. Accordingly, it was concluded that fibre/matrix interface and matrix degradation induced by water absorption were the main reasons for a faster mode I fatigue crack growth in environmental aged composite. [ABSTRACT FROM AUTHOR]

An experimental investigation of mode‐I translaminar fracture toughness and fatigue crack growth behavior of a carbon fiber‐epoxy plain weave laminate manufactured by resin infusion under flexible tooling (RIFT) is presented in this article. Pre‐cracked compact tension (CT) specimens were used to perform both quasi‐static and fatigue tests. Different data reduction techniques for fracture toughness calculation were used and compared with each other. The ASTM E399 test method was modified to account for the material orthotropy and specimen geometry effects using a correction function based on a numerical evaluation of the strain energy release rate. The proposed modification shows good agreement against other experimental methods found in the literature and its application was validated for fatigue tests. Fatigue testing shows that failure in undesired modes is likely to occur prior to translaminar fracture, which was attributed to a higher tensile fatigue threshold than compression or shear fatigue threshold presented by the composite in analysis. Increasing the specimens' initial notch length was a solution for avoiding these types of failure. The experimental results were compiled in the form of a Paris curve, and their particularities were discussed. A fractographic analysis was carried out to define damage patterns and its evolution process in both types of tests. POLYM. COMPOS., 40:3791–3804, 2019. © 2019 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

Abstract In the present work, the investigation of an early fatigue failure of steel rebar samples, of dimensions of Ø 14 mm, during standard fatigue tests is performed. The rebar failed after approximately 1.67 × 106 fatigue cycles, much below of the specified 5.0 × 106 cycles according to BS 4449 standard. The rebar tested sample exhibited a crack initiation site close to the root of a surface geometric discontinuity ("nipple"), which is considered as a stress concentration point. Light optical microscopy, microhardness testing, SEM/EDS, and numerical simulation were employed as the main investigation methods. Fractographic examination depicted the presence of clearly resolved fine fatigue striations elucidating the continuous crack growth period, under Paris law regime, followed by a dimpled ductile zone as the instant ultimate fracture zone. An evaluation of the steel inclusions (MnS, Fe-Si-Mn-Al-Mg-O) was achieved by optical metallography and SEM/EDS analysis. In addition, the results of the numerical analysis, verified that the existence of manufacturing imperfections or geometric irregularities, such as the "nipple" marking of the bar, imposed a significant stress raise up to 2 to 3.3 times of the applied mean stress level, leading to an effective reduction of the expected lifetime of the specimen. Highlights • Fatigue fracture of B500C steel rebar initiated at a surface discontinuity. • Fracture analysis indicated fine fatigue striations and transverse longitudinal cracks. • Sulfides and stringers of complex oxide macro-inclusions were detected. • Numerical simulation yielded an increased stress field, 2–3.3 times of the applied mean stress, at the discontinuity area. [ABSTRACT FROM AUTHOR]

Highlights • The failure cause of a railway rail was investigated. • We estimate the behavior of fatigue crack growth in rail specimen. • Boundary element model is developed for studying rail fatigue life. • We compare the results of fatigue loading for rail steel and specimens. • Comparison between experimental and numerical results has shown a good agreement. Abstract The aim of the present research is to determine the initiation and propagation of cracks and its geometric specifications. Metallurgical factors, especially the non-metallic inclusions have a significant impact on the fatigue life of railways. Since all non-metallic inclusions in a measure of steel alloy is not dangerous, determining how and which one of them affects the fatigue process of the rails is of the present studies' concern. Afterwards, the results of the research on fracture surface of fatigue specimens will be presented. Numerical and experimental methods are used to investigate the behavior of fatigue crack growth in the specimen. For this purpose, the fatigue crack growth and hardness tests were carried out and have been checked by fractography studies on fractured specimens. The hardness of the specimens was measured in different points by using Rockwell micro-hardness experiment. Afterwards, a three-dimensional boundary element method is used for fatigue crack growth under stress field. The modified Paris model is used to estimate fatigue crack growth rates. Finally, three-dimensional boundary element analysis results obtained show good agreement with those achieved in experimental tests. [ABSTRACT FROM AUTHOR]

• Mixed mode (I/II) fatigue crack growth experiments are performed using new specimen. • For the first time, GMTS criterion based Δ K eq model is used for the fatigue studies. • Performance of different Paris constants are evaluated in terms of life prediction. • Performance of Δ K eq models is evaluated in terms of fatigue life prediction. • Fractographic studies have been performed in support of the experimental results. Mixed-mode (I/II) fatigue crack growth experiments in Al 7075-T6 alloy have been performed using a newly proposed single edge cracked circular (SECC) specimen. An equivalent stress intensify factor (Δ K eq) model based on the generalized MTS criterion (GMTS) has been tested for the first time for mixed-mode fatigue crack growth rate correlation and prediction of the fatigue life. Along with this proposed model, other widely used Δ K eq models have also been considered to study the effect of Paris constants on the fatigue life prediction. Three different mode mixity configurations (30°, 45°, and 60°) have been considered for experiments with multiple specimens to study the reproducibility aspect. With the help of detailed fatigue crack growth finite element simulations and experimental results, the performance of the various Δ K eq models has been tested in terms of correlation with the fatigue crack growth rate and prediction of the fatigue life. The results of the present investigation show that using the mixed-mode Paris constants along with Tanaka's and Richard's Δ K eq models accurately and consistently predicts the fatigue life compared to other Paris constants. The GMTS criterion has been shown to exhibit excellent performance in the prediction of crack growth path and thus signifies the importance of T - stress under mixed-mode fatigue loading. Additionally, the results of the present investigation establish the potential application of the SECC specimen for mixed-mode fatigue crack growth studies of metallic materials. Furthermore, fractographic studies also confirm the experimental observations in Al 7075-T6 alloy. [ABSTRACT FROM AUTHOR]

• Fatigue crack growth tests on 13 WAAM steel compact tension specimens presented. • Hardness and tensile strength of examined WAAM steels are approximately proportional. • Influence of deposition strategy on the fatigue crack growth (FCG) behaviour studied. • Test results compared against conventional steels and BS 7910 recommended FCG laws. • Fractography performed to analyse the mechanisms of crack growth. Combining welding technology with robotics, wire arc additive manufacturing (WAAM) is emerging as a viable method of construction. To facilitate its wider application, in particular with structural integrity in mind, an improved understanding of the fatigue properties of WAAM materials is needed. Hence, an experimental investigation into the fatigue crack growth (FCG) behaviour of WAAM plates made of normal- and high-strength steels has been conducted and is reported herein. Following material characterisation, FCG tests on 13 machined compact tension specimens were undertaken, extracted in different directions from WAAM plates built using a parallel deposition strategy. Fractography of the tested specimens was also performed to analyse the mechanisms of crack growth. The material testing demonstrated an approximately proportional relationship between the hardness and ultimate strength. From the FCG tests, Paris' law constants were derived for the WAAM normal- and high-strength steel specimens, revealing consistently lower material constants m for the latter. Similar FCG behaviour to that of equivalent, conventionally-produced steels, with no significant anisotropy, was found. The observed response was shown to be generally well captured by the FCG laws given in BS 7910, with the recommended curve for unwelded steels providing a close match to the experimental data and the recommended curve for welded steels providing conservative predictions. Additional FCG data on WAAM steels printed using the parallel and oscillatory deposition strategies were collected and analysed. It was shown that the two deposition strategies led to similar crack growth rates for WAAM normal-strength steels, but distinct FCG behavioural trends for WAAM high-strength steels. Finally, the fractographic analysis showed that the WAAM specimens exhibited principally brittle behaviour during FCG but somewhat ductile behaviour before fracture. [ABSTRACT FROM AUTHOR]

• Exp. & Numerical investigation of fatigue crack growth in unidirectional composites. • Investigation of Fibre bridging effect in DCB specimens of unidirectional composites. • Evaluation of interfacial fatigue properties for DCB and ENF specimens. • Numerical simulations for fatigue crack growth using combined framework of XIGA-CZM. This article presents an investigation of interlaminar fatigue crack growth in unidirectional (AS4/914) composites. Influence of fibre bridging on Mode-I interfacial fatigue properties is investigated under increased loading displacement and constant R-ratio. Significant shift is Paris law resistance curves represents the influence of fibre bridging in DCB specimen. Mode-II fatigue tests are carried out on precracked ENF specimens. A new set of interfacial fatigue properties are generated and incorporated into numerical formulation. Numerical simulations are carried out with combined framework of XIGA-CZM. Numerical prediction of crack propagation shows good agreement with the experimental results. Fractography is carried out using SEM. [ABSTRACT FROM AUTHOR]

Damage tolerance assessment requires the accurate prediction of fatigue crack growth lives. Numerical simulation techniques using mixed mode Paris' law and various equivalent stress intensity factor models (Δ K e q ) are widely employed for fatigue life prediction. In the present work, mixed mode (I/II) fatigue crack growth experiments are performed using the compact tension shear specimens made of AISI 316 austenitic stainless steel for various loading angles. Finite element fatigue crack growth simulations are carried out, and the effect of Δ K e q model in fatigue life prediction is studied. To achieve this, a three parameter double exponential type best fit is proposed for fitting the experimental mixed mode crack length vs fatigue life. The performance and capability of various selected models are assessed by comparing the predicted life with the experimental results. Fractographic studies at different stages of crack propagation for different loading angles are also presented to aid the above assessments. Based on the overall consistent performance, Irwin's and Tanaka models are predicting life close to the experimental data and Richard's and Yan's models provide conservative solutions. • Mixed mode fatigue crack growth tests are performed using AISI 316 stainless steel. • A non-linear regression based best-fit is devised for mixed mode a vs N test data. • Numerical simulations of mixed mode (I/II) fatigue crack growths are performed. • Predictability of equivalent SIF models in fatigue life prediction is assessed. • Fractography studies are made on the fractured surfaces. [ABSTRACT FROM AUTHOR]

• Mixed mode fatigue crack growth tests are performed using Al 6061-T6 CTS specimens. • A double exponential fitting model is proposed for mixed mode (I/II) fatigue test data. • Numerical simulations of mixed mode (I/II) fatigue crack growth is performed. • Evaluated the effect of equivalent SIF models on the fatigue life prediction. • Fractography studies are made on the fractured surfaces to aid in understanding. Damage tolerance philosophy and safe life design principles are widely used for critical structural components, and in such cases, fatigue life prediction using the numerical techniques is essential. In the present work, mixed mode fatigue crack growth experiments are performed using the compact tension shear specimens made of Al 6061-T6 alloy for mode mixity angles of 30°, 45°, and 60°. These experimental studies are supported by numerical and fractographic studies on the selected specimens. A three-parameter double exponential model for fitting the fatigue crack growth curves (crack length vs. the number of fatigue life cycles) is also proposed for both the mode I and mixed mode (I/II) loading conditions. Numerical prediction of mixed mode (I/II) fatigue life is carried out using the Paris' law in combination with various Δ K eq models. The results of the present investigation show that highly satisfactory best fits to the scattered experimental data can be obtained with the help of the proposed double exponential model. The predicted life is compared with the experimental fatigue life using various measures of error. Based on the overall performance of the models during the entire crack growth regime, the results of the present investigation clearly show that Irwin's model and one of the Tanaka's model are consistently found to predict the mixed mode fatigue life close to the experimental data. Whereas, Richard's and Yan's models, again based on the overall performance, are found to be conservative models consistently for the prediction of the mixed mode fatigue life. These conclusions are verified with the published results. [ABSTRACT FROM AUTHOR]

• As-built condition is clearly affected by the build direction, mainly for anisotropies in the microstructure. • Cracks propagating parallel to the layer in the as-built condition exhibit the highest fatigue crack growth rates. • Cracks propagating along planes oriented perpendicular to the layers therefore encounter more resistance. • The heat-affected zones (HAZs) around interlayer boundaries are the weakest link in the material. • T6-heat treatment minimized the dependence of crack growth rate based on crack orientation. Selective Laser Melting is an additive manufacturing technology capable of producing relatively dense parts from metallic and alloy powders using a high-power laser beam. The present study examines the high cycle fatigue crack growth behavior in the linear region of da / dN vs. Δ K diagrams of the AlSi10Mg alloy produced by SLM. Specimen building orientation and notch position have been combined to obtain 4 different crack growth orientations. Further, specimens were tested in both as-built (AB) and T6 heat treated (T6) conditions. The Fatigue Crack Growth (FCG) curves revealed the beneficial influence of the heat treatment on the FCG response, as compared to the as-built case. On the other hand, the fatigue crack growth behavior of the latter is found to be strongly dependent on material orientation. Fractographic analysis have been further performed to understand the mechanisms involved. Additionally, residual stress measurements were also performed to comprehend its contribution on the FCG curves. [ABSTRACT FROM AUTHOR]