Small Crack Growth Research Articles

Characterization of Surface Fatigue Crack Nucleation and Microstructurally Small Crack Growth in High Strength Aluminum Alloys

It is well established that fatigue crack nucleation and small crack growth in high strength aluminum alloys are highly influenced by the surrounding microstructure including grain boundaries, texture, inclusion barriers, among other factors. As such, specific and targeted experimental and computational methods are necessary to accurately capture and predict the discrete behavior of microstructurally small fatigue cracks. In this study, surface fatigue crack nucleation and microstructurally small crack growth in high strength aluminum alloys, commonly used in aerospace applications, are evaluated through a holistic approach encompassing fatigue testing, crack measurement, and computational prediction of crack growth rates. During fatigue testing, crack shapes and growth are quantified using a novel surface replication technique that is applied to investigate crack nucleation, as well as to collect validation data that includes an accurate description of crack shape during crack propagation, a challenging and essential component in predicting crack growth. Computational simulation of fatigue crack growth in non-straight, complex surface crack arrays typically requires high fidelity analysis using computationally expensive methods to account for the mathematical and geometrical complexities inherent in the solution. A dislocation distribution based technique has been previously demonstrated to rapidly and accurately predict the stress intensity factors for through cracks of complex shape. This method was expanded and investigated as an approach for rapidly predicting the crack growth rate of kinked and tortuous surface crack arrays, using the crack configuration and bulk material properties as inputs. To investigate the accuracy and effectiveness of this characterization approach, surface crack growth in AA7075-T7351 was experimentally analyzed and modeled under high cycle and low cycle fatigue conditions. This comprehensive approach was determined to be an expedient and applicable method for characterizing and evaluating the nucleation and crack growth rate of non-planar microstructurally small and short crack configurations.

Consideration to the mean stress based line method for estimating the notch fatigue limit

To consider a mechanistic explanation of the mean stress based line method (called LM) for estimating the notch fatigue limit, the limit values predicted by the LM are investigated from both the energy condition required to create fractured surfaces and the crack propagation condition of the linear elastic fracture mechanism.Based on the energy condition, an energy release criterion (ERC), was introduced to estimate the notch fatigue limit of Δσnp for crack propagation. It is found that Δσn|ERC,1.58L≅Δσn|LM,2L holds for all the investigated notches. Here, Δσn|ERC,1.58L denotes the predicted value by the ERC with the intrinsic crack length lc=1.58L, and Δσn|LM,2L denotes the predicted value by the LM with the characteristic distance dLM=2L (L is the El Haddad’s material characteristic length). The validity of the LM in predicting the fatigue limit of Δσnp based on stress condition was demonstrated by the correspondence relation between the average stress and the energy release in the relevant zone.Also, the growth of small cracks near the notch root is investigated using the R-curve approach. It is found that, as a universal criterion applicable to various notch root radiuses ρ, Δσn|LM,2L gives a critical condition for crack to overcome the typical resistance force represented by the simplified Kitagawa-Takahashi diagram, the values of which is denoted by ΔKth(l)|Kitagawa here. However, to each specified notch root radius ρ,Δσn|LM,2L may be a conservative solution, and theoretically Δσnp⩽Δσn|LM,2L holds. The reason is that: (1) most of the curves of crack driving force at a nominal stress of Δσn|LM,2L are above the resistance curve of ΔKth(l)|Kitagawa; (2) the actual resistance curve of the material is generally below the resistance curve of ΔKth(l)|Kitagawa.

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.

Direct observations and characterization of crack closure during microstructurally small fatigue crack growth via in-situ high-energy X-ray characterization

This study provides direct observation of the crack closure mechanism of a naturally occurring, tortuous, 3D microstructurally small fatigue crack (SFC) in additively manufactured Inconel 718. In-situ non-destructive characterization is performed using high-energy X-ray diffraction techniques to capture the evolution of the 3D microstructure and micromechanical response in the vicinity of the crack front. Based on the stress state of twelve grains analyzed at the crack tip, the crack closure events of the SFC front was found to be spatially heterogeneous with respect to loading progression governed by the local stress state of the grains (specifically the stress reversal from compression to tension). From this analysis, three grains that displayed different degrees of crack closure were further investigated, based on the orientation of the crack relative to the grains and the associated modality of crack growth. The stress normal to the crack plane and the associated degree of Mode I crack behavior were correlated with events of the crack opening earlier during the loading cycle. This was further corroborated by diffraction spot spreading analysis that quantified the crystallographic lattice distortion caused by the opening crack. The detailed characterization of the opening behavior of the grains located at the crack tip and their associated states of stress elucidates the mechanism governing crack closure and will inform future modeling efforts of this phenomenon.

Fatigue performance and life prediction of cast aluminum under axial, torsion, and multiaxial loadings

Many industrial components made of cast aluminum alloys are subjected to multiaxial loadings, while most of research on these materials are on uniaxial loading condition. This article investigates the fatigue performance of A356-T6 aluminum alloy produced by high pressure die cast (HPDC) under different loading conditions. In addition to obtaining monotonic and cyclic deformation behaviors, fatigue tests were performed under axial, torsion, in-phase and out-of-phase combined loadings. In order to study the fatigue crack growth mechanism, few tests were also conducted under cyclic torsion with static tension or compression. It was observed that the fatigue cracks in all specimens initiated from defects, mainly porosity, and grew on the plane of maximum principal stress. Smith-Watson-Topper (SWT) damage parameter was used to correlate the fatigue data. The experimental fatigue life data were predicted using small crack growth models and from the crack initiating defect on the fracture surfaces. The fatigue lives under different loading conditions were also predicted using the maximum defect size estimated by extreme value statistics. Very good agreements with the experimental lives under different loading conditions were obtained.

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.

Failure behavior of aerial bomb lifting lug under variable amplitude loading: Failure analysis and life prediction

The lifting lug is one of the most important structures of aerial bomb, which mainly bears the variable amplitude cyclic loading during the working process. In this paper, the failure analysis of several lifting lugs which failed in fatigue test was carried out. Firstly, through the mechanical analysis, fatigue cumulative damage analysis and fracture surface analysis of the lifting lug structure, it was determined that the fracture failure was not caused by the issue of structural design, but by the Al2O3 inclusion defect in the casting process. Next, the crack source defect was equivalent to the initial crack, and the fatigue full-life was predicted by the method of fatigue crack growth (FCG). The FCG process was divided into two stages: small crack growth stage and long crack growth stage for life prediction respectively. Finally, the validity of this method was verified by comparing with the experimental results, and it was suggested to adopt advanced lifting lug casting and detection technology to prevent similar faults.

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.

Fatigue life prediction of cast aluminum alloy based on porosity characteristics

Fatigue life prediction is one of the most important problems in design of industrial components made of defect-containing materials. Not only can defects existing in these materials significantly affect the mechanical properties especially fatigue behavior, but they also lead to considerable scatter in fatigue life. Therefore, a robust design methodology for these materials should consist of statistical evaluation of defects and prediction of fatigue life using an appropriate defect-based model. In this study fatigue life analysis is performed using crack growth models. A small crack growth model is proposed which can consider the effect of mean stress as well as overall plastic deformation. Fatigue life predictions are performed using the defect size measured from the fracture surfaces, as well as the maximum defect size estimated by extreme value statistics within the stressed volume. The predicted fatigue lives based on the proposed model are in very good agreement with the experimental fatigue lives. Discussions of fatigue limit, mean stress, and the effect of defect parameters on fatigue life are also included.

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.

Creep–fatigue interaction behavior of nickel-based single crystal superalloy at high temperature by in-situ SEM observation

Creep-fatigue interaction behavior of nickel-based single crystal superalloy was studied in an in-situ SEM platform at 900 °C. Dwell times of 15, 30, 60, 90, 120 s were introduced at the tensile peak stress of each cycle. Experimental results showed that the life cycles firstly decreased rapidly then slightly increased with the increasing dwell time, which was explained by the accumulated inelastic strain. Dislocation microstructure and inelastic strain were correlated for different dwell time by TEM analysis. Crack propagation mode transition was analyzed. Comparing with the stress intensity factor amplitude, crack tip opening displacement was more suitable for characterizing the creep-fatigue small crack growth rate.

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.

Computing the Growth of Small Cracks in the Assist Round Robin Helicopter Challenge

Sustainment issues associated with military helicopters have drawn attention to the growth of small cracks under a helicopter flight load spectrum. One particular issue is how to simplify (reduce) a measured spectrum to reduce the time and complexity of full-scale helicopter fatigue tests. Given the costs and the time scales associated with performing tests, a means of computationally assessing the effect of a reduced spectrum is desirable. Unfortunately, whilst there have been a number of studies into how to perform a damage tolerant assessment of helicopter structural parts there is currently no equivalent study into how to perform the durability analysis needed to determine the economic life of a helicopter component. To this end, the present paper describes a computational study into small crack growth in AA7075-T7351 under several (reduced) helicopter flight load spectra. This study reveals that the Hartman-Schijve (HS) variant of the NASGRO crack growth equation can reasonably accurately compute the growth of small naturally occurring cracks in AA7075-T7351 under several simplified variants of a measured Black Hawk flight load spectra.

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.

Performance-Related Characterization of Forming-Induced Initial Damage in 16MnCrS5 Steel under a Torsional Forward-Reverse Loading Path at LCF Regime.

Forming technology and in particular cold forward rod extrusion is one of the key manufacturing technologies with regard to the production of shafts. The selection of process parameters determines the global and local material properties. This particularly implies forming-induced initial damage in representation of pores. On this background, this study aims on describing the influence of these pores in the performance of the material 16MnCrS5 (DIN 1.7139, AISI/SAE 5115) under a torsional load path in the low cycle fatigue regime, which is highly relevant for shafts under operation conditions. For this purpose, the method of cyclic forward-reverse torsional testing was applied. Additionally, intermittent testing method and the characterization of the state of crack growth using selective electron microscopy analysis of the surface were combined. A first attempt was made to describe the influence of forming-induced initial damage on the fatigue performance and the crack growth mechanisms. The correlation of fatigue performance and initial damage was contiguous in the sense that the initial damage corresponds with a decrease of material performance. It was concluded that the focus of further investigations must be on small crack growth and the related material changes to identify the role of initial damage under cyclic loads.

Distinct fatigue limit of a 6XXX series aluminum alloy in relation to crack tip strain-aging

In this study, a conventional precipitation-hardened A6061-T6 alloy (base alloy) and a modified alloy containing excess solute magnesium (Mg) compared to the base alloy were compared in terms of high-cycle fatigue property. Particular attention was paid to the emergence of a distinct knee point on the fatigue life diagram together with a time-dependent strengthening (coaxing) effect in the modified alloy under various test conditions. An attempt using specimens with different defect size successfully revealed that a distinct knee, which was absent in the base alloy, was attributed to the threshold against small crack growth. The clear coaxing effect in the modified alloy was also explicitly confirmed in terms of the high ΔK value achieved by the arrested cracks regardless of the defect size. The comparative tests conducted under different environments and temperature conditions further confirmed that the crack tip strengthening, essentially caused by the strain-aging capability endowed to the modified alloy, operated most prominently in the ambient air at room temperature. Such results were rationally interpreted by incorporating a competitive interaction of solute Mg and hydrogen (H) with dislocations at the crack tip.

Development of a Method for Evaluating Microstructurally Small Fatigue Crack Initiation and Growth by Using an Automatic System for <i>In Situ</i> Observation in Conjunction with a Digital-Image Correlation Technique

Development of a Method for Evaluating Microstructurally Small Fatigue Crack Initiation and Growth by Using an Automatic System for <i>In Situ</i> Observation in Conjunction with a Digital-Image Correlation Technique

Role of retained austenite with different morphologies on sub-surface fatigue crack initiation in advanced bainitic steels

We describe here the distinct high cycle fatigue properties of advanced bainitic steels to elucidate the significant role of retained austenite with different morphologies on sub-surface fatigue crack initiation process. The inter-plate film-like retained austenite at the straight front of small crack tip transformed to martensite, leading to the arrest of small crack growth and altered the active slip systems. However, the blocky retained austenite or martensite/austenite islands located at prior austenite grain boundaries induced intergranular fatigue cracking because of ease of activation of slip in retained austenite.

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.