Small Crack Growth Behavior Research Articles

Small crack growth behavior and correlated microstructure-sensitivity during cyclic deformation of a high-chromium/carbon wrought tool steel

The macro to micro-mechanical mechanisms of cyclic deformation behavior of a carbide-containing ferritic steel was studied, and the exceptional properties obtained in low-cycle regimes were discussed, relying on the sensitivity of the small crack propagation to the microstructural characteristics. In this respect, the evolution of boundaries, sub-boundaries, carbides, and micro-textures in the vicinity of the crack growth path has been characterized in detail. Regarding this matter, the subtle alteration of grain size gradient, resulting in the decrement of incompatibility between mechanical properties of grains, is the main factor that halts the crack growth temporarily. Furthermore, both pre-existed and cyclically developed substructures could tortuous the crack propagation path and induce the crack closure effect, respectively. The advantages and disadvantages introduced by primary carbides, serving as initiators of small cracks, and dispersed high volume fraction of submicron carbides, inducing the formation of an effective strengthening layer (ESL) and acting as deflectors of small cracks, have been investigated, respectively. The {001} <110> oriented grains have been identified as preferred micro-texture contributing to small crack propagation. In total, the small crack sensitivity of the elaborated microstructure has resulted in a prolonged lifetime.

Mechanical properties and growth mechanisms of long and small fatigue cracks in as-deposited bulk cold spray Al-6061

Cold spray depositions are created by accelerating powder particles at a substrate at supersonic speeds to create a deposition with effective metallurgical and mechanical bonding. To progress from coatings/repairs to bulk additive depositions suitable for structural components, fatigue properties and microstructure-scale crack growth mechanisms must be established. In this study, the microstructure, tensile properties, and crack growth behavior were systematically investigated for as-sprayed Al-6061 and compared to those of wrought Al-6061-T6. Long fatigue crack growth behavior was investigated at two stress ratios (R = 0.1 and 0.7) and two crack plane orientations with respect to the deposition direction. Small crack growth behavior was investigated at a stress ratio of R = 0.1 in four crack plane orientations. Fractographic observations were made to identify the crack-microstructure interactions at different growth stages, and interparticular failure was determined to be the lifetime-limiting crack growth mechanism. A lap-shear testing method was developed to evaluate the interfacial shear strength of cold spray depositions at the interface with the substrate and to compare it to the strain energy release rates obtained from fatigue crack growth experiments. Design maps were created to relate loading conditions, crack orientation, and deposition ductility and predict the onset of interparticular growth.

Microstructurally and mechanically small fatigue crack growth behaviors of 316LN stainless steel in high-temperature pressurized water

Corrosion fatigue behaviors at different stages of microstructurally small crack (MSC) and mechanically small crack (MeSC) for 316LN stainless steel were investigated in high-temperature pressurized water by interrupted tests. An improved model about the proportion in total fatigue life and growth rate for the MSC and MeSC has been proposed. It is found that the proportion in total fatigue life for the MSC is lower and the crack growth rate is higher at a lower strain rate due to the high plastic deformation and dislocation density at grain boundary. Mechanism of crack growth in high-temperature pressurized water is discussed.

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

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

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

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.

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.

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.

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.

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.

In-situ SEM investigation and modeling of small crack growth behavior of additively manufactured titanium alloy

The fatigue crack growth of wire and arc additive manufactured Ti-6Al-4V alloys is investigated using in-situ SEM fatigue testing. The small crack initiation and propagation behaviors within grains and across grains are systematically studied, and the inter- and trans-granular crack growth rates are quantified. A synergy and competition among many factors such as persistent slip bands, grain boundaries, and crack branching governs the growth rate. A modified model based on the CTOD is proposed to characterize the growth rate considering the microstructure dissimilitude and local yield strength variations. Results show the proposed model improves the prediction accuracy.

A modelling framework for fatigue-life prediction of selective laser melting additive manufactured Ti-6Al-4V

Selective laser melting (SLM) enables the fabrication of complex, lightweight structural parts that would be impossible to be produced with conventional methods. Despite its advantages SLM process has some intrinsic drawbacks, with the formation of defects in parts being one of the most serious, as defects detrimentally affect the fatigue behavior of SLM parts. In order to consider defects in SLM part design and production, reliable models for defect-based fatigue life estimation models should be developed. The current work presents a modelling framework for the prediction of susceptible areas for defect formation and subsequently fatigue life estimation of SLM components. The ability of this modelling framework to effectively predict the fatigue life was tested on the prediction of fatigue life of SLM Ti-6Al-4V specimens. The small fatigue crack growth behavior of AM Ti-6Al-4V was considered. The prediction results of a fatigue life model were successfully compared with experimental data verifying the current modelling approach. Limitations concerning the prediction results are attributed mainly to the assumptions performed and the considered defect size which will be thoroughly investigated in future works.

Probabilistic Approach for Nondestructive Detection of Fatigue Crack Initiation and Sizing

Early detection of a growing crack is one of the concerns in structural integrity and can be used to predict the remaining useful life of a structure. Acoustic Emission (AE) is a nondestructivetesting method with potential applications for locating and monitoring fatigue cracks. A novel AE signal analysis approach is proposed in this paper to detect crack initiation and assess small crack growth behavior. A probabilistic AE-based model for small fatigue cracks was developed and the uncertainties of the model were estimated. The outcome of this research can be used to evaluate the integrity of structures and assess structural health by estimating the probability density function of the length of detected cracks. This paper discusses the methodology used, experimental approach, results obtained and predictive models developed.

Effect of a large population of seeded alumina inclusions on crack initiation and small crack fatigue crack growth in Udimet 720 nickel-base disk superalloy

Crack initiation, crack coalescence and small crack growth behavior were monitored for over 400 seeded inclusions during interrupted low cycle fatigue testing conducted on the P/M Udimet 720 nickel disk alloy at 650 °C. Two types of seeded alumina inclusions with average sizes of 54 µm and 122 µm were used in the study performed at varying loading conditions resulting in LCF lives ranging from 2,000 cycles to over 1,000,000 cycles. The fatigue behavior was sub-categorized into four groups. Visual maps detailing inclusion size/cycle history were developed. The effect of surface residual stresses on the fatigue life was also investigated.

A normalized equivalent initial flaw size model to predict fatigue behavior of metallic materials

A normalized equivalent initial flaw size method is proposed based on the updated Kitagawa–Takahashi diagram considering the small crack growth behavior, which is essential to accurately predict the fatigue crack initiation and propagation. Meanwhile, the updated Kitagawa–Takahashi diagram indicates that small cracks can propagate even with an applied stress below the fatigue limit, which is consistent to the experimental observations. A new fatigue life prediction model is proposed by considering the fatigue crack growth from the normalized equivalent initial flaw size to normalized final crack length when the failure occurs. The parameters of the proposed model have clear physical meaning and can be determined from experiments. Experimental data for aluminum alloys and steels under different test conditions are adopted to verify the proposed model, the theoretical predictions show good agreement compared with the experimental results.

The effect of cubic-texture on fatigue cracking in a metastable β titanium alloy subjected to high-cycle fatigue

In this work, stress-controlled high-cycle fatigue tests were performed on Ti-7Mo-3Nb-3Cr-3Al alloy with β-cubic texture to investigate fatigue crack-initiation mechanisms and microstructurally small fatigue crack growth behaviors. The S-N curve shows a general feature of monotonic increase in the mean lifetime with decreasing stress level. Fatigue crack-initiation sites revealed the contiguous β facets. The fracture surface observations and the slip trace analysis show that most faceting occurred in {110} 〈111〉 slip system with the highest Schmid factor. Besides, a higher Kernel average misorientation (KAM) was found in fatigued microstructure. This strain localization would contribute to the fatigue crack-initiation process.

A nature-inspired interface design strategy of carbon fiber composites by growing brick-and-mortar structure on carbon fiber

Poor interfacial interaction and damage-tolerance restrict practical application of carbon fiber (CF) composites. The brick-and-mortar structure meets the requirements of high strength and small crack growth behavior of composites. However, uniformly fabricating the three-dimension brick-and-mortar structure as an interface layer on CF surface remains a challenge. To fill up this technical gap, herein, 3D brick-and-mortar structures were uniformly constructed on carbon fiber surface via in-situ growth. Ni(OH)2 nanosheets which the morphology matched the geometry of fiber were used as the “brick”, while multifunctional polydopamine (PDA) was employed as “mortar” that guarantees the powerful bonding interactions with “brick”. The synergistic effects of composites were demonstrated through interlaminar shear strength (ILSS) and flexural tests, and the optimize ILSS and flexural performance were obtained from the composites with superior interface, which showed respectively 53.5% and 95.6% enhancement compared to the pristine CF/EP, presumably due to the brick-and-mortar structure arising from stress dissipation, plastic deformation and physical and chemical entanglements. This system could be applied on other three dimensional materials, overcoming the limitation of mimic nacre-like materials only achieved in the form of films.

Fatigue crack nucleation and microstructurally small crack growth mechanisms in high strength aluminum alloys

Characterization of microstructurally small fatigue crack growth behavior for two aluminum alloys, AA7065 and AA2099, were quantified using a surface replication process for the first time. In addition, scanning electron microscopy analysis revealed that for AA7065, crack initiation was caused by either voids or intermetallic particles. Whereas, for the AA2099, crack initiation was caused by persistent slip bands and intermetallic particles. From electron backscatter diffraction results, the grains at the crack initiation site for the AA7065 exhibited high misorientation boundaries, while the grains at the crack initiation sites for the AA2099 exhibited both high and low misorientation boundaries.

Small fatigue crack growth behavior of Ti-6Al-4V produced via selective laser melting: In situ characterization of a 3D crack tip interactions with defects

Small fatigue crack growth (SFCG) behavior is examined for additively manufactured Ti-6Al-4V specimens with optimal and trial build conditions (representing an increased degree of defects) through a combination of in situ tomography and in situ energy dispersive X-ray diffraction. The results showed slower crack growth rates for the SFCG samples compared to the long cracks (which was not expected). In the case of the trial conditions, the increased levels of porosity deterred crack growth by blunting the crack, as evident by the in situ tomography characterization.

Simulation of fatigue small crack growth in additive manufactured Ti\u20136Al\u20134V material

Additive manufacturing (AM) offers design freedom and ability to fabricate parts of complex shapes which are not often possible with the conventional methods of manufacturing. In an AM part, even with optimum build parameters, a complete elimination of defects is not possible and this makes it hard to fully deploy the AM technology to build load bearing parts operating under cyclic loading conditions. Many of these defects are < 1 mm in size and are categorised as ‘small cracks’. Local interaction of cracks with microstructural features and closure effects at the wake of the crack tip are some of the factors which make the growth behaviour of small and long cracks different. A crack growth life prediction method, which effectively considers the small crack growth behaviour, has been discussed in this paper. This proposed method includes a detailed finite element-based crack growth simulation using the ANSYS SMART fracture technology. The lifing calculations utilise the modified NASGRO equation and small crack growth data which was obtained from the published long crack growth data, corrected for closure effects. The predicted stress versus number of cycles curves were compared against the fatigue test results for the AM specimens in Ti–6Al–4V material. A good correlation between the predictions and test results suggests that the proposed method can be used to assess the small crack growth life of AM parts where the fatigue effects of cyclic loading can be quite significant.