Small Fatigue Cracks Research Articles

Experimental and modeling study on small fatigue crack initiation and propagation behavior of Inconel 617

Small fatigue crack initiation and propagation behavior of Inconel 617 was studied by in-situ SEM observation, electron back-scattered diffraction (EBSD), digital image correlation (DIC), crystal plasticity finite element method (CPFEM) and extended finite element method (XFEM) in this paper. The experimental results indicated that different surface corrosion methods used to display surface microstructure will affect the small fatigue cracking mode of Inconel 617. The local micro-scale strain field of Inconel 617 near the notch and crack tip was quantitatively measured by a new grain scale DIC method. The CPFEM-XFEM simulation model of small fatigue crack initiation and propagation characteristics of Inconel 617 was established. A new framework for small fatigue crack path and lifetime prediction at the polycrystalline scale was constructed, which includes the influences of slip system and grain boundary on small fatigue crack initiation and growth rate.

Effect of Fabrication Processes on Fatigue Strength and Small Fatigue Crack Growth Behavior of A6061 and Al-Si Eutectic Alloys

Effect of Fabrication Processes on Fatigue Strength and Small Fatigue Crack Growth Behavior of A6061 and Al-Si Eutectic Alloys

Detection of small internal fatigue cracks in Ti‐6Al‐4V via synchrotron radiation nanocomputed tomography

AbstractTwo types of synchrotron radiation computed tomography (SR‐CT)—projection CT (micro‐CT) and phase‐contrast imaging CT (nano‐CT)—were used to observe internal fatigue cracks in (α + β) Ti‐6Al‐4V alloy. Micro‐CT detected cracks in the specimen at ~1 μm spatial resolution, and the nano‐CT provided magnified images at ~200 nm spatial resolution. The crack initiation sites were clarified as the α‐phase for both the surface and internal cracks; however, their opening behaviors differed. A sharp crack tip was observed in the surface crack, and the crack tip opening displacement (CTOD) increased with an increase in the applied load. By contrast, a blunted crack tip, similar to that of a crack in a vacuum, was observed for the internal crack, and its CTOD remained almost constant regardless of the applied load. These phenomena are likely to explain the different behaviors of surface and internal cracks, particularly the slower growth rate of internal cracks, which leads to a longer fatigue life in the very high cycle fatigue regime.

Use of the helium ion microscope in small crack fractography

Highly stressed aerospace structures, such as the wing attachment points and load carry-through bulkheads of fighter aircraft, often grow fatigue cracks from micro-scale discontinuities. These cracks spend much of their lives close to these discontinuities in the small crack regime. The accurate prediction of fatigue crack growth in this small crack regime leads to setting realistic inspection thresholds for the aircraft and assisting in accurate risk-based assessment of the lives of such cracks. Fractography can greatly aid the assessment of the growth of small cracks. It has traditionally been carried out using optical and Scanning Electron Microscopes to gather the necessary small crack growth rate data, however, limitations on the resolution and the surface sensitivity of these instruments restricts the gathering of crack growth rate data at small crack lengths. As an extension to the current capabilities, the newly introduced Helium Ion Microscope offers improved nano-scale resolution and topographical sensitivity. As a result, the Helium Ion Microscope offers the potential for both quantitative and qualitative analysis of small fatigue crack growth about nucleating discontinuities at resolutions previously unobtainable. This paper presents an overview of the technology and gives three different fractographic examples of its possible applications to failure analysis. These examples include extension of small crack fractography, extension of near initiation fractographic feature analysis, and extension of nano-scale feature analysis. To demonstrate these capabilities, fatigue fractures produced by experimental loading spectra that were applied to the aluminium alloy AA7050-T7451 have been investigated.

Influence of microstructural deformation mechanisms and shear strain localisations on small fatigue crack growth in ferritic stainless steel

Microstructurally small fatigue crack growth (FCG) rate in body-centred cubic (BCC) ferritic stainless steel is investigated by using a novel domain misorientation approach for EBSD microstructural deformation analyses, in conjunction with in situ digital imaging correlation (DIC). The DIC analyses revealed that shear strain localisations occur ahead of the crack tip during propagation and correlate well with the FCG rate retardations. Grain boundaries can be found at both peaks and valleys of the FCG rate curve and alter the interaction between crack growth and shear strain localisations. At the microstructural level, the deformation is associated with the dislocation-mediated plastic deformation process, showing increased formation of grain sub-structures in the regions of the strain localisation. Consequently, material experiences local hardening causing the FCG retardation events. If the crack avoids the hardened material region through a macroscopic cross-slip mechanism, retardation is minor. On the contrary, if the crack penetrates the hardened region, retardation is significant.

Study on small crack growth behavior of laser powder bed fused Ti6Al4V alloy

AbstractThe small fatigue crack growth (FCG) behavior in Ti6Al4V alloy manufactured by selective laser melting (SLM) was experimentally investigated on an in situ fatigue testing machine. A semi‐elliptical artificial defect was introduced into the specimen to simulate the pore defects induced during additive manufacturing (AM) process. The results indicated that the fatigue life was mainly consumed in the small FCG stage. The FCG rate was significantly affected by microstructure at the early small FCG stage with great fluctuations. With the increase of crack length, the effect of microstructure decreased. Compared with long crack growth data, it was found that small crack can still grow below the threshold value of long crack, and at the same stress intensity factor (SIF) ΔK, the FCG rate of small crack was higher than that of long crack. Therefore, the small crack growth behavior should be considered in fatigue life prediction for SLM Ti6Al4V.

The effect of ultrasonic surface rolling process on tension-tension fatigue limit of small diameter specimens of Inconel 718 superalloy

A high cycle tension-tension fatigue test was conducted on specimens of different diameters of aged Inconel 718 superalloy after an ultrasonic surface rolling process (USRP). The results show that although all USRP treated specimens had similar compressive residual stress and hardening layer, the fatigue limit of small diameter specimens (3.4 mm) decreased considerably than that of non-USRP specimens with the same diameter, while that of larger diameter specimens (7.0 mm) increased. The larger the diameter was, the higher the fatigue limit (The diameter of specimens: φ3.4 mm ∼ φ7.0 mm). The fatigue life first exhibits an increasing tendency and then a decreasing tendency with increasing distance between the fatigue crack initiation position (FCIP) and the surface of specimen. Focused ion beam (FIB) analysis of the dislocation configuration in the wake regions of small fatigue cracks at different FCIPs showed that when the FCIP was close to the centre of the specimens where high stress triaxiality hindered emission and movement of the dislocation at the crack tip. The dislocation density and the size of the plastic zone were greatly reduced, and the stress concentration increased. These factors reduced the critical condition required for small crack propagation, and thus diminished the fatigue limit.

Small fatigue crack propagation rate and behaviours in a powder metallurgy superalloy: Role of stress ratio and local microstructure

The role of stress ratio and local microstructure on small crack propagation (0.065–1.0 mm) behaviours in superalloy FGH4096 were investigated using the Photomicroscopy method. ΔK correlated the growth rates of small cracks with different stress ratios well, which is attributed to the low crack closure level. Statistical and crystallographic analysis revealed that grain boundaries resulted in crack temporary arrests, which is independent of loading conditions and crack length. Temporarily arrested cracks propagated further by connecting to the secondary crack that initiated on the slip plane that parallels to the twin boundary or in the grain with a local highest Taylor factor.

Calculational models for mode II and mode III small fatigue crack growth in structural elements

Calculational models for mode II and mode III small fatigue crack growth in structural elements

Initiation and propagation of small fatigue crack in beta titanium alloy observed through synchrotron radiation multiscale computed tomography

Synchrotron radiation computed tomography is an emerging nondestructive method for fracture analysis in materials. Herein, the in situ small-crack growth of a beta titanium alloy, Ti–22V–4Al, was monitored by a combination of microtomography (micro-CT) and nanotomography (nano-CT). The 3D characteristics of small cracks and the corresponding stress intensity factor range (ΔK) were obtained by micro-CT; the cracks initiated at ∼10% fatigue life. In the low-ΔK regime, the scatter of the crack aspect ratio and the deviation in the crack propagation rate were significant; however, they decreased with increasing ΔK, reflecting the microstructural effect on small-crack propagation. The microstructure of the beta titanium alloy was successfully visualized by nano-CT. Consequently, the crack-surrounding-microstructure interactions evidenced that the fatigue cracks initiated from the center of the beta grain wherein the alpha-phase was less precipitated.

In-situ observation of microstructurally small fatigue crack initiation and growth behaviors of additively-manufactured alloy 718

To reveal the effects of microstructural features on fatigue property, microscopic fatigue crack initiation and growth behavior were successively observed for rolled plate and additively-manufactured nickel base 718 alloy. Three types of additive manufacturing (AM) methods: powder bed Electron Beam Melting (EBM), Selective Laser Melting (SLM), and wire-fed Electron Beam Additive Manufacturing (EBAM) were adopted for specimen preparation to create various microstructures for comparison. The results showed that the fatigue lives of the EBM and EBAM specimens, which have a robust (001) texture, were shorter than those of the rolled plate and SLM specimens. This fatigue life difference could not be explained by the defects, because the fatigue cracks in the part of EBAM specimen were initiated along a slip plane. On the other hand, acceleration of the early stage fatigue crack growth rate (FCGR) was found in the EBM and EBAM specimens. Cross-sectional electron back scatter diffraction (EBSD) observations also showed that early stage cracks initiated and grew along the {111} slip plane; but in the EBAM specimens, the cracks continuously propagated straight to an adjacent grain due to their (001) texture. It thus appears that the low-angle grain boundaries which tend to be generated as a result of the (001) texture do not act as barriers to the penetration of dislocations; instead, they accelerate the shear mode fatigue crack growth rate in the early stages and decrease fatigue life.

Effect of underloads in small fatigue crack growth

There is considerable cost incurred within the aircraft industry due to the inability to accurately predict fatigue crack growth rates for complex loading scenarios. This is particularly the case for small cracks. This paper aims to facilitate the improvement of fatigue crack growth by investigating the effects of underloads on small cracks grown in the common airframe structural aluminium alloy; AA7050-T7451. Six novel loading spectra were used to quantify how underload spacing, magnitude, and consecutive application impact crack growth. Quantitative fractography of post-test fractures enabled crack growth measurements to be made at crack lengths as small as 100 µm. It was found that underloads accelerate growth rates most significantly for small cracks, though this effect appears to progressively diminish as the crack grows. The results also suggest that underloads may have both short and long-term effects on crack growth. This research paves the way for accounting for the acceleration of crack growth rates caused for small cracks in predictions, which if not considered, could lead to non-conservative fatigue life predictions. Ultimately, this should allow for more accurate growth rate prediction of high strength aluminium structures subject to underloads, as well as provide quantitative basis for future mechanistic studies.

On small fatigue crack growth rates in AA7085-T7452

Aluminium alloy 7085-T7452 is a material developed by Alcoa (now Arconic) for its favourable mechanical properties in thick, forged aluminium aerospace structural parts. Currently reported fatigue crack growth rate data for 7085-T7452 in the open literature does not fully characterise the small crack growth behaviour of this material. The growth rates of small fatigue cracks are important since they often govern the useful service life of large primary structural parts in the aerospace industry. In this paper, small fatigue crack growth rates were measured experimentally using two methods and compared with data available in the open literature for longer cracks. The fractography based method was found to develop fatigue crack growth data that better characterised the small crack regime and so these results were then developed into a fatigue crack growth model which was evaluated for its ability to predict an independent set of 7085-T7452 specimens tested with a complex aircraft wing bending plus buffet loading spectrum. The fractography method based small crack growth rate model was able to robustly predict fatigue crack growth under this complex variable amplitude sequence from small, realistic initial discontinuity sizes better than long crack data, yet it is not without need of further refinement.

Microstructural and load hold effects on small fatigue crack growth in α+β dual phase Ti alloys

Microstructurally small crack growth was monitored in Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo with equiaxed and bi-modal microstructures. The influence of the microstructure was assessed to obtain an improved understanding of the lifetime variability observed in Ti alloys. Primary α grains, basal plane cracking and misalignment across boundaries were identified as key features for high crack growth rates. The origin of life debits encountered under dwell-fatigue loadings was also investigated through crack growth monitoring with the introduction of load holds at peaks stress. Dwell periods were found to induce a substantial small crack acceleration.

Examining the role of water vapor on small fatigue crack growth behavior in Ti-6242S using ultrasonic fatigue and scanning electron microscopy

The influence of water vapor on small fatigue crack growth rates in the near alpha titanium alloy Ti-6242S was investigated using a combined ultrasonic fatigue/environmental scanning electron microscope system (UF-SEM). Small fatigue cracks were grown at a cyclic frequency of 20 kHz in laboratory air, partial pressures of water vapor ranging from 65 to 1330 Pa, and vacuum (1.7 × 10-4 Pa). Fatigue cracks grew at higher rates in all water vapor pressures investigated versus in vacuum, with a nearly identical growth rate observed in laboratory air and 1330 Pa water vapor. A linear dependence of small fatigue crack growth rate on water vapor pressure was observed that is consistent with a pressure dependent transport rate of water vapor molecules to the crack-tip. This work indicates that the amount of water vapor present in the environment (humidity) plays an important role in determining the fatigue life of Ti-6242S and must be accounted for even at very high cyclic frequencies, such as the 20 kHz cyclic frequency commonly employed in ultrasonic fatigue testing.

Microstructurally small fatigue crack initiation behavior of fine and coarse grain simulated heat-affected zone microstructures in low carbon steel

Successive observation of fatigue crack initiation process was conducted on six types of thermally simulated HAZ microstructures to clarify the fundamental reason of fatigue life difference between coarse grain (CG) and fine grain (FG) heat-affected zone (HAZ) microstructures. As a result, regardless of microstructure, fatigue cracks initiated and propagated along shear stress plane when the crack is smaller than prior austenite grain size. In addition, this shear mode fatigue crack growth of CGHAZ were much faster than that of FGHAZ and categorized as microstructurally small fatigue cracks. Although prior austenite grain size of crack initiation life of the FG and CGHAZ were comparable, the remaining fatigue life after the grain size crack initiation of the CGHAZ become shorter than that of the FGHAZ as a function of grain size difference. As a result, total fatigue life of CGHAZ becomes shorter than that of FGHAZ. Therefore, relatively larger part of CGHAZ's fatigue life is consumed by shear mode microstructurally small fatigue crack growth process.

On the behaviour of small fatigue cracks emanating from corrosion pits: Part I – The influence of mechanical factors

The initiation and propagation behaviour of cracks emanating from corrosion pits in a high strength low alloy steel is investigated by testing pre-pitted specimens in the high cycle fatigue regime in an air environment. This was complemented with finite element analysis of electrochemically generated pit profiles. Fatigue cracks were found to initiate from the mouth of pits due likely to strain localisation at this region. Crack initiation lifetime and fatigue endurance decreased with increasing pit depth. Fatigue behaviour of the steel was found to be crack initiation dominated. A fracture mechanics methodology that provides a satisfactory estimation of critical threshold stresses and threshold stress intensity factor required for fatigue failure from corrosion pits is discussed.

Superior fatigue endurance exempt from high processing cleanliness of Metal-Injection-Molded β Ti-Nb-Zr for bio-tolerant applications

Technically optimizing the processing cleanliness of Metal-Injection-Molded titanium alloys (Ti-MIM) is not economically feasible. This problem is common in the materials processing field. In the search for an alternative approach, the work tries to achieve superior high-cycle fatigue (HCF) performance while tolerating very high impurity levels. The concept arose from the large tolerance of β-class Ti-alloys for oxygen-solutes and the feasibility to mitigate detrimental effects of carbide-inclusions, under monotonous loading. In this paper, MIM β Ti-Nb-Zr biomaterials for fatigue-critical applications were deliberately produced with very high O-level and normal/very high C-levels. The impurity-resistant Ti-biomaterials exhibit superior HCF endurance limits beyond 600 MPa irrespective of processing cleanliness, being significantly higher than those of the α-β Ti-reference alloys produced with tight restrictions on impurity levels. This superior fatigue performance while tolerating amounts of impurities stems from the “weak”-microstructural features insensitive to impurities and increased resistance of the Ti-matrix against fatigue small-cracks. Furthermore, a conditional fatigue duality triggered by two competing mechanisms of crack initiation in certain cases, initiating at microscale pore α-platelets and at large pore TiC-inclusions can occur. The success of the present alloy-process development might greatly relax the processing demands for active metals.

Orthogonal sampling moiré method and its application in microscale deformation field measurement

Sampling moiré (SM) method is one of the modern deformation measurement methods in photomechanics. This method realizes the visualization measurement of deformation field by a sparse sampling of grating images, showing the advantages of high efficiency and strong robustness. However, in the traditional sampling moiré method, the unidirectional sampling mode is used with the result that the two-dimensional deformation cannot be acquired at the same time. In this paper, by combining the orthogonal sampling and Fourier phase analysis techniques, an orthogonal sampling moiré (OSM) method is proposed with which the two-directional moiré phase of specimen grating can be separated synchronously. The accuracy of the OSM method for the deformation measurement is verified by numerical experiments and PMMA uniaxial tensile tests. As an application, in a scanning electron microscope (SEM), the deformation field near the crack tip of small fatigue cracks is measured, based on which the crack opening displacement (COD) and the stress intensity factors (K) are obtained. Furthermore, the crack closure effects at different temperatures are characterized. The experimental results show that the crack closure significantly affects the process of small crack growth at both room temperature and high temperature, and the crack opening load at high temperature is greater than that at room temperature, which indicates that small cracks are easier to close at high temperature.

Microstructure evolution, fatigue crack growth, and ultrasonic fatigue in As-fabricated laser powder bed and conventionally cast Al–10Si-0.4Mg: A mechanistic understanding and integrated flaw-sensitive fatigue design methods

Additive manufacturing (AM) enables fabrication of lightweight components with high geometric complexity, and its broad adoption in high-integrity structural applications relies on a fundamental understanding of the processing-microstructure-fatigue performance relationships and development of fatigue design methods. In this study, microstructures, tensile properties, and long and small fatigue crack growth (FCG) behaviors of laser powder bed (LPB) manufactured Al–10Si-0.4Mg were systematically investigated in both orientations with respect to the build direction, and compared to those of an equivalent conventionally cast material. Tensile properties of the LPB material demonstrated orientation dependency, and exceeded on average those of the cast material, with higher yield (>170% increase) and tensile (>150% increase) strengths and enhanced elongation (88% increase in horizontal orientation). In contrast, long FCG properties at various stress ratios showed minor differences associated with orientation (attributed to residual stress), and were lower compared to the cast alloy, particularly in the near-threshold regime. Mechanisms of FCG were identified at all growth stages using fractography and electron backscatter diffraction, and further integrated with the loading conditions by creating dual-parameter load-microstructure-damage mechanisms design maps. Ultrasonic fatigue testing was complementary conducted on the LBP-fabricated materials to study the very-high cycle fatigue regime (for both orientations and different surface conditions), and the results were correlated with the FCG data using Kitagawa-Takahashi diagrams to unify the two fatigue testing methods, and together with the dual-parameter maps, provide a microstructure-sensitive design toolset for fatigue crack initiation and growth resistance.