Investigation Of Fatigue Crack Growth Research Articles

Investigation of microstructural evolution and mechanical properties for in-service nickel-based superalloy

In this study, the microstructural evolutions are systematically investigated in the IN718 alloy that has been in-service for eight years. Correspondingly, the effect of in-service process on mechanical properties are explored in terms of tensile properties, fatigue performance and crack growth behavior. Results show that the grain sizes at different sampling locations vary from 9.8 to 15.5 μm, exhibiting the inhomogeneous microstructures for the in-service IN718 alloy. The volume fraction of δ phase and kernel average misorientation (KAM) at outermost layer are the maximum ones, followed by the middle and innermost layers. In addition, the high magnitude of δ phase is caused by the transformation from strengthening phase γ′′. From the perspective of mechanical properties, the in-service IN718 alloy can maintain excellent yield strength and elongation at the high temperature of 650 °C, while the tensile performance is degenerated at room temperature (RT) compared with those of new IN718 alloy. The fatigue life at 650 °C is similar to that at RT under the stress levels lower than 600 MPa. As the stress level increases to 800 MPa, an obvious decrease in fatigue life at 650 °C compared with that at RT. This special fatigue performance is revealed based on the fractography examinations under different loading conditions. Moreover, the investigation of fatigue crack growth for the in-service IN718 alloy is carried out under different stress ratio and temperatures.

Investigation of Fatigue Crack Growth in TA2 Cruciform Specimen with an Inclined Crack, Considering Stress Ratio and Biaxial Load Ratio

The biaxial fatigue crack growth behavior of commercial pure titanium TA2 of cruciform specimens with different crack inclination angles (β = 90°, 60°, 45°) under various biaxial load ratios (λ = 0, 0.5, 1) and different stress ratios (R = 0, 0.1, 0.3) is studied by an IPBF-5000 biaxial testing machine. The test results prove that the maximum tangential stress criterion is suitable for predicting the initiation angle of uniaxial and biaxial mixed-mode I–II fatigue cracks. The fatigue crack growth rate of a cruciform specimen with mode I and mixed-mode I–II cracks decreases with the increase of biaxial load ratio and increases with the stress ratio. The Walker model and Kujawski model have better compression effects on fatigue crack growth data than the Paris model.

Numerical analysis of fatigue crack growth using a plastically dissipated energy factor based model

The investigation of fatigue crack growth (FCG) behavior may contribute to the assessment of damage tolerance of components. To study the FCG behavior considering the elastic–plastic behavior at the crack tip, a numerical simulation scheme based on compact tension (CT) specimen is developed. Also, an effective plastically dissipated energy (PDE) factor composed of maximum PDE ([Formula: see text]) and PDE range ([Formula: see text]) is proposed to establish the expression for evaluating the FCG rate. The simulation results show good agreement with the results of test under same load conditions. Also, the mesh sensitivity analysis and the comparison with test results confirm the validation of proposed model. Based on the proposed numerical simulation scheme, the FCG behavior is studied by analyzing the influence of plastic wake, mean load, load range, overload, underload, load sequence, and cyclic compression load on FCG from the perspective of crack driving force, FCG rate, and crack opening displacement (COD). It is found from the analysis results that the developed numerical simulation scheme can consider the load history effect and crack closure effect. The mechanisms of these factors on the influence of FCG rate are analyzed in detail.

Investigation of Fatigue Crack Growth in Full-Scale Railway Axles Subjected to Service Load Spectra: Experiments and Predictive Models

In this paper, a series of experimental investigations was performed on full-scale railway axles to analyze the fatigue crack growth behavior of EA4T steel under load spectrum derived from real operating conditions. The experimental results were compared to life predictions carried out adopting two models: (i) the conventional Nasgro equation and (ii) the cyclic R-curve concept implemented in the Modified Nasgro equation for describing the crack growth behavior of an arbitrary crack length. The results show that the life predictions performed by means of the Modified Nasgro equation coincide well with the experimental results with an underestimation of the residual lifetime less than 32%, while the traditional Nasgro equation leads to significant overestimation (≈120%) of the residual lifetime for load spectra close to the in service scenario.

Investigation of fatigue crack growth in engineering components containing different types of material irregularities by XFEM

The extended finite element (XFEM) has been applied to investigate fatigue crack growth in engineering components containing different types of material irregularities. The standard approximations of finite element method are enriched with additional functions by employing the partition of unity approach. In order to keep track of discontinuities in the domain, level set method is employed. Fatigue crack growth in presence of different material irregularities, such as elliptical, rectangular, hexagonal and octagonal discontinuities, has been the main focus of the study. The effect of geometry, position and material properties of these discontinuities on fatigue behavior of the cracked specimen has been addressed. The fatigue life of the cracked component has been estimated by the generalized Paris law. The mixed mode stress intensity factors have been evaluated by the domain-based interaction integral approach. Several numerical problems are solved by XFEM to study the fatigue crack growth behavior of cracked specimens containing different types of material irregularities.

In-phase and out-of-phase mixed mode loading: Investigation of fatigue crack growth in SEN specimen due to tension–compression and torsional loading

Numerous cases of damage by fatigue in structures are related to mechanical stresses due to mixed mode loading conditions. To prevent such cases of failure a precise knowledge of the corresponding theoretical background is indispensable. Unfortunately, it is not yet possible to satisfactorily describe the crack propagation by considering a superposition of all three crack modes. For this purpose, experiments on single-edge notched specimens made of 34CrNiMo6 under tension-torsion with different mixed mode ratios and phase shift angles were performed. The focus of the investigations is especially on the kinking and the twisting angle of the crack as well as on the influence on the residual lifetime. Furthermore, existing concepts concerning the prediction of the crack growth under mixed mode are evaluated.

Uticaj neusklađenosti na granične krive širenja zamornih prslina osnovnih materijala tipa S690KL, S960KL i S960TM i njihovih, elektrolučno u zaštiti gasa, zavarenih spojeva

Nowadays, one of the basic trends in the industry is the environmental impact reduction, in other words the weight decreasing of structural elements and structures, which can be approached by applying different high strength steels. In case of different steel structures, the main manufacturing and joining technology is the welding, the conventional and advanced methods of fusion and pressure welding processes. Beside the weight decreasing, the reliability and safety requirements according to steel structures have significant grown. During the welding process, the welded parts are affected with heat-effect and mechanical loads, which result in inhomogeneous welded joint. The inhomogeneity of the welded joints appears both in microstructural (local) and in geometrical (both local and global) aspects. The changes in microstructure and geometry appear in deflections (basically acceptable), or rather in failures (basically unacceptable); and these influence both the behaviour and the loadability of welded joints. Discontinuities in base materials and their welded joints have especially high danger in case of cyclic loading conditions, which are typical for different structures and structural elements (e.g. bridges, vehicles). There are different standards and prescriptions containing fatigue crack propagation limit curves and rules for the prediction of the crack growth; simple and two-stage crack growth relationships can be found in the literature, most frequently based on the Paris-Erdogan law. The paper summarizes and presents the results according to our fatigue crack growth investigations on Weldox 700E and Weldox 960E quenched and tempered (Q+T) and on Alform 960M thermomechanically treated (TM) high strength steel base materials and their gas metal arc welded joints. The mismatch effect has also been studied; matched, overmatched, undermatched and matched/overmatched (mixed-matched) welded joints were investigated. 15 mm thick plates were used for the investigations, statistical aspects were applied both for presenting the possible crack locations in the real plates, as well as for processing the measured data. Furthermore, the results will be compared with each other, and fatigue crack propagation limit curves will be derived using simple crack growth relationship.

Analysis of fatigue crack growth under mixed mode (I + II) loading conditions in rail steel using CTS specimen

The paper is focused on the investigation of fatigue crack growth in rail steel. Mode I and its combination with mode II are one of the frequent reason for rail failures. Therefore, mode I and mixed-mode (I + II) experiments were carried out in order to obtain the fatigue crack growth curves for typical rail steel. In experimental campaign were used CTS (Compact Tension Shear) specimens. The Stress Intensity Factors (SIF) were determined for all load cases using Finite Element Method (FEM) with developed and automated Authors’ tool ABAQUS environment. During the experimental and numerical study, the impact of different elastic mixity level on fatigue crack growth rates and fatigue crack growth directions has been discussed. As it is noticeable the decrease of the mode I domination (under mixed-mode loading condition) caused an increase of the specimen fatigue lifetime. This observation was also discussed in the light of the detailed fatigue crack paths study using Scanning Electron Microscope (SEM).

Characterization of irradiation crosslinked polyamides for solar thermal applications – Fatigue properties

In this research, the influence of electronic beam irradiation on the fatigue crack growth (FCG) behavior of unreinforced polyamide 66 (PA 66) and two glass fiber reinforced PA 66 (30 w% glass fiber content, 35 w% glass fiber content) was investigated. In total, five different doses in the range of 0 kGy and 200 kGy were applied. While previous research covered the influence of irradiation on basic thermo-analytical characteristics and thermo-mechanical properties of these PA material grades, the present paper deals with the effect of irradiation crosslinking on the FCG performance. To cover the relevant range of service environmental conditions, the FCG investigations were performed for two different application near material states “as-received” and “water saturated” at 23 °C and 80 °C, respectively. For the water saturated material states, the tests were conducted with specimens under simultaneous water immersion. For this purpose, specific test devices and test arrangements were designed, built and implemented on an electro-dynamic test machine. In general, a change in the material state and test conditions from 23 °C in air with as-received specimens (fatigue testing occurs below the materials glass transition temperature TG) to 80 °C in water with water saturated specimens (fatigue testing occurs above the materials TG) led to a significant reduction in the FCG resistance. Furthermore, while for unreinforced PA 66 no significant effect of irradiation on FCG kinetics was detected for the as-received state at 23 °C (i.e., below TG), irradiation had a detrimental influence on the fatigue behavior at 80 °C for the water saturated material state (i.e., above TG). Interestingly, quite the opposite effect of irradiation on FCG resistance was found for glass fiber reinforced Pas. The irradiation caused a reduction in the FCG resistance for as-received material states at 23 °C, while hardly any effect of irradiation was detected for the water saturated materials at 80 °C.

Investigation of fatigue crack growth under loading sequence effects using in-situ SEM testing

Fatigue damage is one of the most important failure mechanisms in engineering components. The excited structures are usually subjected to spike loads in the fatigue weakness area during their service lives. The nonlinear loading sequence effects due to overloads are significant in the crack propagation process. In this paper, an in-situ scanning electron microscope (SEM) testing is performed to analyse the mechanisms of nonlinear fatigue crack growth affected by the load sequence. The crack tip behaviours under constant amplitude loading cycles superimposed by tensile overload were observed. The SEM experiment results reveal that the overload effects include the transient weakened area (shear bands and micro-cracks) and the relatively long-term retardation. Additionally, the observation loading sequence influence region is larger than the theoretical value. According to these SEM testing analyses, the Willenborg is modified considering the nonlinear loading sequence effects. In this approach, the damage zone concept is introduced to account for the instantaneous acceleration. Moreover, the loading sequence effect area is defined as the whole plastic zone due to overload rather than part of it. The proposed algorithm is validated by experiment data of 350WT steel and Al 2024-T351 specimens under constant loading with overloads. Good agreements are observed.

Crack path predictions in anisotropic structures considering residual stresses

AbstractDifferent mechanical processes induce a directional microstructure (texture) during the production of a material, e.g. during the hot rolling of aluminum plates or the injection molding process of natural fiber reinforced bio‐polymers. In such materials the anisotropic mechanical features are related to the texture and have a decisive impact on the fracture process. Rolling or shot peening processes, on the other hand, induce an irreversible plastic deformation and residual compressive stresses for the purpose of a surface toughening. Similarly, residual stresses are induced if a circular core with interference fit is inserted into a drilled hole. The measurement of crack paths in specimens with residual stresses due to a core reveals an influence of the core's interference with respect to the crack deflection and the resulting paths [1,2]. Numerical investigations of fatigue crack growth show that conventional crack deflection criteria are not valid in such specimens, as the deflection angle depends on the magnitude of the external loading, going along with a rotation of the principal stress axes and thus the Jk‐vector [2]. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental and numerical investigation of fatigue crack growth in the cracked gear tooth

AbstractThe numerical simulation was conducted to analyse the fatigue crack growth in gear with the finite element codes ansys (ANSYS, Inc. Canonsburg, Pennsylvania, USA.) and franc3d (Fracture Analysis Consultants, Inc. Ithaca, New York, USA.), and the corresponding fatigue test was also carried out. During the simulation, the location of maximal stress induced by the external force was first determined by the code ansys, and then the obtained results were imported into the franc3d to analyse the crack growth. The analysed results were input into the codes ansys and franc3d again to compute the stress and the stress intensity factor in the following steps. After several rounds of analysis, the results of the fatigue crack propagation were obtained. The investigations show that the crack mode I is dominant during the crack growth and the stress intensity factor KI raises with increase of crack growth length and a series of quarter‐elliptical cross sections of the ruptured gear tooth are obtained. The simulation results are in good accordance with experimental findings.

Investigation of fatigue crack growth in a power plant steel under elevated temperatures

The definition of inspection intervals on the basis of fracture mechanics requires beside the knowledge of the load e.g. in terms of stress intensity solutions also the knowledge of the material parameters e.g. in terms of crack growth data. For this reason, fatigue crack growth curves were determined for a ferritic-martensitic power plant steel at elevated temperatures. Therefore, an experimental setup was developed. The fatigue crack growth tests are performed on C(T)-specimens, which are manufactured with varied orientations from a high pressure bypass valve (HP-Bypass) which was exchanged at a revision.The isothermal test for determining the threshold values as well as the fatigue crack growth data in the Paris-regime have been performed at room temperature and at temperatures between T = 300 °C and T = 600 °C. Beside the temperature, the R-ratio, the specimen orientation, the frequency and the normalized K-gradient are varied. The influences of all investigated parameters are evaluated and the main factors are identified. For statistical evaluation of the crack growth data quantile curves are created from the crack growth data as a function of the main factors.

Investigation of Fatigue Crack Growth under Pure Mode-I and Mixed Mode

Investigation of Fatigue Crack Growth under Pure Mode-I and Mixed Mode

Experimental and numerical investigation of fatigue crack growth in aluminum plates repaired by FML composite patch

Purpose – The purpose of this paper is to use the fiber metal laminates (FML) composites as a patch for repairing a single notched specimen made of AL1035 aluminum alloy. The FML composite patch was bonded on one side of the cracked specimens by adhesive Araldite 2015. Then the fatigue crack growth tests were conducted on the specimens and the effects of both FML patch lay-up sequence and pre-crack angle on the fatigue life were investigated. Finally, the effect of repairing on the fracture parameters (SIF and crack propagation direction) at the crack front has also been calculated using three-dimensional finite element analysis. Design/methodology/approach – The fatigue crack growth tests were conducted on the specimens and the effects of both FML patch lay-up sequence and pre-crack angle on the fatigue life were investigated. Findings – The results show that the fatigue life of the patched specimens with inclined crack increased approximately 2-6.02 times compared to the un-patched specimens. In addition, the fatigue crack growth rate decreased significantly when the patch was used. Generally, the FML patch with Plate-Fiber-Fiber-AL lay-up has more efficiency than other lay-up sequences. Originality/value – Recently, composite patches are used in the structure repair processes to increase the service life of cracked components. The bonded patch method is one of the efficient methods among repairing methods. Today, the FMLs are used in the aircraft structures as a replacement of high-strength aluminum alloys due to their lightweight and high-strength properties. Many researches have been performed on single and double side repaired panels using composite patches. In this study, the FML composites have been used as a patch for repairing a single notched specimen made of AL1035 aluminum alloy.

Interface fatigue crack propagation in sandwich X-joints – Part I: Experiments

In this study, face/core fatigue crack growth in sandwich X-joints is investigated numerically and experimentally. The work presented here covers Part I of the study which includes an experimental investigation of fatigue crack growth in sandwich X-joints and characterization of the face/core interface of the joints. Sandwich tear test specimens with a face/core debond representing a debonded sandwich X-joint were tested under cyclic loading. Fatigue tests were conducted on the sandwich tear test specimens with H45, H100 and H250 PVC cores and glass/polyester face sheets. The Digital Image Correlation technique was used to locate the crack tip and monitor the crack growth. For the specimens with H45 core, unstable crack growth took place initially. Following the unstable propagation, the crack propagated in the core underneath the resin-rich cell layer approaching the interface. However, the crack did not kink into the interface. For the specimens with H100 core, the crack propagated initially in the core and then returned into the interface and continued to propagate in the interface. For the specimens with H250 core, the crack initially propagated in the core and then kinked into the interface. The interface crack eventually kinked into the face sheet, resulting in large-scale fiber bridging. Finally, mixed mode bending tests were conducted to measure crack growth rates of the face/core interface at mode-mixity phase angles similar to those calculated for the sandwich tear test specimens. The measured crack growth rates have been used in Part II of this study to simulate fatigue crack growth in the sandwich tear test specimens.

A study of specimens from solebar material with boxlike cross sections using the acoustic-emission method

The results are presented of the investigation of fatigue crack growth in 200 × 100 × 10 mm specimens made from the upper chord of rejected solebars with boxlike cross sections. The specimens were placed in an MTS-50 electrohydraulic machine and alternately loaded with cyclic and static loads from the instant of fatigue-crack nucleation until specimen damage occurred. The data on nucleation and the development of fatigue cracks in the specimens were recorded using the method of acoustic-emission (AE) strain measurements. A piezo antenna consisting of four acoustic emission transducers (AET) was mounted on each specimen. Four wire strain sensors were glued near the saw cut that served as a stress concentrator. After specimen damage, chemical analysis of the material was performed and nonmetallic inclusions and the degree of impurity were evaluated. Micro- and macrofractographic analyses, which allowed determination of the character of damage, were performed.

Fatigue crack growth simulations of interfacial cracks in bi-layered FGMs using XFEM

An investigation of fatigue crack growth of interfacial cracks in bi-layered materials using the extended finite element method is presented. The bi-material consists of two layers of dissimilar materials. The bottom layer is made of aluminium alloy while the upper one is made of functionally graded material (FGM). The FGM layer consists of 100 % aluminium alloy on the left side and 100 % ceramic (alumina) on the right side. The gradation in material property of the FGM layer is assumed to be exponential from the alloy side to the ceramic side. The domain based interaction integral approach is extended to obtain the stress intensity factors for an interfacial crack under thermo-mechanical load. The edge and centre cracks are taken at the interface of bi-layered material. The fatigue life of the interface crack plate is obtained using the Paris law of fatigue crack growth under cyclic mode-I, mixed-mode and thermal loads. This study reveals that the crack propagates into the FGM layer under all types of loads.

Probabilistic Fatigue Crack Growth in Compact Tension Specimens from FSWed 7075-T651 Aluminum Alloys

The process of fatigue crack growth is generally random nature, due to many sources of variability, such as external loading, material properties and geometry conditions. Therefore is is important to investigate the statistical variability of fatigue crack growth in order to development the methodology for probabilistic fatigue crack growth life prediction. This paper is concerned with the randomness investigation of fatigue crack growth for the FSWed 7075-T651 aluminum alloys. In this investigation, the fatigue crack growth tests were conducted for five compact tension (CT) specimens of the FSWed 7075-T651 aluminum alloys, base metal (BM), heat affected zones (HAZ) and weld metal (WM), especially three different constant stress intensity factor range (SIFR) controls.

Experimental Investigation of Fatigue Crack Growth in the Welding Nugget of FSW Joints of a 6060 Aluminum Alloy

The work investigates the fatigue behavior of friction stir welded butt joints by means of fracture mechanics techniques. FSW joints of artificially aged AA6060 T6 aluminum alloy were studied. The welding was performed on 8 mm thick butt joined sheets by means of a CNC machine tool. Welding speed in the range between 117 and 683 mm/min and tool rotational speed between 838 and 1262 rpm were considered. Fatigue crack growth tests were performed according to ASTM E647 standard on CT specimens, under constant load amplitude conditions, at 0.1 minimum to maximum load ratio, with the notch placed in the dynamic recrystallization zone of weld nugget, oriented along the welding direction. The comparison of results demonstrates the crack growth rate is always equal or lower than the base material at low values of stress intensity range. At ΔK values above 12 MPa√m, crack growth rate was found to be higher than base material for high feed, low speed and high feed/speed ratio.