Short Crack Research Articles

Improving contact durability of polycrystalline systems by controlling the parameters of large-angle grain boundaries

Polycrystalline metal systems in the samples made of steel 40X with different morphology and parameters of the distribution of large-angle grains boundaries for energy have been examined. The effect of the structural-energy state of grains boundaries on the operational reliability of improved steel 40X has been established. Based on the hierarchical modeling of polycrystals structure, new approaches have been proposed and algorithms have been developed for defining relationships between the structure that is formed in the technological processing of materials and stages in the life cycle of parts. It has been revealed that it is advisable to use, as a digital prototype of the structure of polycrystalline alloys that describes their performance under conditions of contact loads, the matrix representation of a system model that would incorporate the quantitative characteristics of grains. By using the devised procedures, the ways to execute technological control over the energy state of grain boundaries in the structural components have been defined in order to improve durability of parts exposed under contact loads. An estimation-experimental method has been developed to assess the effect of quantitative characteristics of the structure on the parameters of strength of the grains boundaries and their ability to form intragrain damage under external loads. The energy level of grains boundaries and triple joints between the groups of small and large grains is higher than that between grains of the same size. The boundary surfaces with a high level of energy are places where damage occurs at technological processing and under external loads on structural materials. This points to the crucial role of large-angle boundaries placed between triple joints with a high energy gradient in the process of forming microstructurally short cracks and intragrain destruction of polycrystalline systems. The use of hierarchical modeling methods and computational materials science makes it possible to improve the operational reliability of articles by choosing the optimal parameters for internal boundary surfaces. The lower cost of the parts' life cycle is achieved by thermal treatment regimes, which alter the quantitative characteristics of the steel structure

Investigation of the stress corrosion cracking behavior in annealed 5083 aluminum alloy sheets with different texture types

Al–Mg alloys are commonly employed for anti-corrosion parts in marine and rail vehicles. However, the corrosion resistance of these alloys is affected by the alloy microstructure. In this study, 5083 aluminum alloy sheets with different recrystallized textures (a low-intensity texture, a cube-based texture, and a brass-based texture) were obtained by cold rolling and annealing. The stress corrosion cracking and short crack propagation behavior were investigated based on the microstructural characterization, intergranular corrosion, electrochemical corrosion, and slow strain rate testing. The results showed that the tested samples exhibited large differences in stress corrosion properties, although they had a similar resistance to intergranular corrosion. The sample dominated by the brass texture exhibited the best resistance to stress corrosion cracking (SCC) with an ISSRT of 11.7%, while the sample with the low-intensity texture exhibited the worst resistance to SCC with an ISSRT of 29.7%. Moreover, the stress corrosion crack in the sample dominated by the brass texture propagated along a more tortuous path. The crack was deflected at brass texture grains, which was attributed to the combined effects of the relatively large twist angle between the grain boundaries of brass grains and neighboring grains and the low grain boundary energy configuration of {011} orientation grains.

Fractographic Study on Naturally Initiated Short Fatigue Cracks in a Near-Lamellar TiAl Alloy at Room Temperature

Short crack phenomena are considered important for lamellar structures in γ-TiAl alloys and have been thoroughly investigated in the past. However, the short cracks in the previous studies were nearly all introduced artificially. No particular investigations have looked into the initiation of fatigue short cracks. Therefore, naturally initiated short fatigue cracks at room temperature under two different stress ratios (0.1 and 0.5) were investigated in a near-lamellar γ-TiAl alloy (Ti-45Al-2Mn-2Nb) in this study. The observations show that the fatigue crack initiation behaved differently at low and high stress ratios. At low stress ratio, the specimens failed at lower ultimate stress levels (σmax = 450 and 475 MPa), and the crack initiated from the cluster of interlamellar fracture near mode-I orientation or stress concentration areas. At the higher stress ratio, the specimens failed at higher but consistent stress levels (σmax = 560 and 570 MPa), and in the specimen crack initiation areas, the interlamellar fractures were still the primary fracture mode, whereas some were found at tilted angles due to shear deformation. The results suggest that short fatigue cracks can naturally initiate in lamellar γ-TiAl alloys, thus attention should be paid to their microstructure design, surface finishing and cleanliness.

On the behaviour of non-propagating cracks in steel and aluminium castings

Casting defects as well as sharp notches can serve as initial points for fatigue cracks under cyclic loading. On the contrary, the cyclic material crack resistance counteracts to crack propagation. Hence, an incipient crack can arrest completely, even though a constant cyclic load is applied. Considering a short propagating crack, the local stress intensity factor, which represents the crack growth driving force, increases due to the crack extension, yet crack arrest may occur. Such an arresting of growing cracks can be traced back to an increase of the materials resistance in terms of crack closure effects. This paper deals with the evaluation of the cyclic short crack behaviour and the assessment of non-propagating cracks for cast aluminium and cast steel in terms of the Frost diagram. At first, single edge notched bending specimens are machined out of both cast materials and investigated in regard to their fracture mechanical short and long crack growth behaviour under varying stress ratios. Moreover, the data is analysed in terms of the short crack resistance (R-curve) and the subsequent long crack growth behaviour. Secondly, detectable surface porosity defects, regarding the aluminium material, and sharp surface notches, as in case of cast steel, are investigated by a linear elastic finite-element simulation to evaluate the local stress concentration factors numerically. Finally, Frost diagrams are set up for both materials and well approved by the detected non-propagating cracks. A final comparison of the investigated fatigue test data points and the Frost diagrams reveals a sound agreement validating the applicability of the presented approach.

On the numerical modeling of nucleation and growth of microstructurally short cracks in polycrystals under cyclic loading

Abstract

An improved method of characterizing fracture resistance of asphalt mixtures using modified Paris’ law: Part II—Establishment of index for fracture resistance

The modified Paris’ law has been employed to characterize the fracture resistance of asphalt mixtures. However, the correlation between the coefficients of the modified Paris’ law has lacked theoretical justification. This study formulated the coefficients of the original Paris’ law in terms of the stress intensity factor for a long crack whose failure zones were much smaller than the crack size. These formulations were then utilized to derive the formulas of the coefficients of the modified Paris’ law in terms of the pseudo J-integral for the multitude of short cracks in a viscoelastic material such as an asphalt mixture. A theoretical model was developed between the two coefficients of the modified Paris’ law. This theoretical relation provided solid theoretical support to the empirical relation reported in the literature that was identified through regression analysis based on the results of hundreds of tests. The experimental data in this study further justified the theoretical relation.An index for fracture resistance of asphalt mixtures was established through rigorous theoretical derivation and experimental justification. The established index for fracture resistance serves as an excellent indicator of fatigue properties of asphalt mixtures. This index can be utilized in mix designs for comparing different mixtures, in field core evaluations for quantifying the fracture resistance of in-service asphalt pavements, in multiscale analyses of asphalt mixtures for linking mechanical properties at a macro level to interface adhesion at a micro level, and in other potential applications.

Stability of fatigue cracks at 350 °C in air and in liquid metal in T91 martensitic steel

The low cycle fatigue behaviour of a 9Cr1MoNbV martensitic steel has been investigated at 350 °C in air and in lead–bismuth eutectic (LBE) with low and high oxygen content. Total strain controlled tests were performed from Δεt = 0.40% to 1.2%. The material exhibited a pronounced cyclic softening in both environments. LBE reduced the fatigue resistance. LBE accelerated the formation of the long crack by promoting the growth of the first short cracks. A clear change in propagation mode was observed. In air, ductile fatigue striations were observed while in LBE a brittle fracture decorated by voluminous and largely spaced striations were visible. From EBSD analysis, it was concluded that in LBE, the long crack advanced quickly by repeated and discontinuous cleavage.

Detection and analysis of high temperature fatigue fracture surface oxygen diffusion of GH4133B superalloy used in turbine disk of aero-engine

The chemical contents on the fatigue fracture surface of GH4133B superalloy piece suffering tension-tension fatigue and operated at high temperature 600 ºC is analyzed using an energy dispersive spectrometer, and the element content percentages at the position of carbide inclusion and that of second-phase particle in the fatigue source region are measured. It is shown that under the conditions of high temperature and air environment, the carbon element in the carbide inclusion can be easily oxidized, which results in accelerating fatigue crack nucleation and microcrack generation, reducing the fatigue crack initiation lifetime. The oxygen concentration is detected on the fatigue fracture surface along the crack propagation direction, the oxygen diffusion model is constructed, and the oxygen concentration gradient equation is derived. It is found that the oxygen content decreases gradually along the crack propagation direction, suggesting that at the crack initiation and short crack propagation stage, the alloy element reacts sufficiently with oxygen for a long time, and the oxidation accelerates the crack initiation, while at the long crack growth stage, the crack propagation rate is faster, so the alloying elements do not have adequate time for reacting with oxygen, resulting in the low oxygen content, and the oxidation has a small influence on the long crack propagation rate.

Dislocation emission and crack growth in 3D bcc iron crystals under biaxial loading by atomistic simulations

This paper is devoted to the study of the ductile-brittle behavior of a central nanocrack (1¯10)[110] (crack plane/crack front) under biaxial loading via free 3D molecular dynamics (MD) simulations, as well as the comparison of MD results with continuum predictions concerning T-stress. The so called T-stress is a constant stress component acting along the crack plane, which should be considered (together with the stress intensity factor K) in the assessment of brittle-ductile behavior, namely, in the case of the short cracks. Previous 2D atomistic simulations under plane strain conditions indicated that the level of T-stress (controlled by the biaxiality ratio σB/σA from the external loading) affects dislocation emission from the crack and can cause the ductile-brittle transition. The plane strain simulations using the periodic or translational boundary conditions in the bcc lattice have certain limitations: they enable the in-plane dislocation emission (Burgers vector lies in the observation plane), but they do not allow the complete dislocation emission on the all slip systems favored by the shear stress. As presented, our new free 3D atomistic simulations (without periodic or symmetry conditions) enable the activity of the all favored slip systems. Thus, they offer a more realistic insight into the microscopic processes generated by the crack itself in dependence on the T-stress level.

Micromechanical modeling of short crack nucleation and growth in high cycle fatigue of martensitic microstructures

High cycle fatigue (HCF) is a frequently limiting failure mechanism of machine elements and modern high strength steels. Present day design rules rely on semi-empirical methods, guidelines and utilization of macroscopic analysis means in origin, such as fracture mechanics. The resulting challenge is that short crack regime, critical for HCF in terms of lifetime of components and products, is somewhat poorly handled. This is an outcome of the fact that the present means and methodologies do not explicitly account for effects arising from material microstructure, an oversight micromechanics aims to rectify.Micromechanical modeling operating on fatigue at the scale of material microstructure necessitates the introduction of suitable means to describe the mechanisms of cyclic plastic deformation and microstructural morphologies, considered critical for HCF especially at the early stages of micro-crack nucleation and damage evolution towards and within the short crack regime. In current work, a crystal plasticity based approach with combined hardening is utilized to capture the respective deformation response utilizing full field modeling. The modeling is carried out for both simplified prior austenite grain like microstructures as well as complex imaging based martensitic quenched and tempered steel microstructural models. A fully coupled damage modeling scheme is introduced to track damage nucleation and evolution at the scale of the studied microstructures. Crack closure is included within the approach to track behavior of microstructure scale defects under, e.g., fully reversed loading, more realistically. Model calibration is addressed and application cases involving damage and crack growth both under monotonic and cyclic loading are presented.The results demonstrate how the coupling of damage to crystal plasticity modeling can be utilized to identify and track the evolution of microstructure scale damage mechanisms in complex martensitic microstructures. Interactions between strain localization and damage accumulation are presented as well as transition from micro-cracking to short crack growth. The results show that the proposed approach can interpret the intricate dependencies and relations between complex microstructures, their (cyclic) deformation mechanisms and evolution of damage, the outcomes regarding crack formation and behavior are found to be in line with similar experimental studies.The proposed framework for modeling damage in polycrystalline microstructures is quite general in its capabilities. By solely introducing a suitable crystal plasticity based deformation model and a damage model describing nucleation and softening can plastic slip and damage interactions be studied in complex microstructures, and in principle, on any system where similar constitutive models are utilizable. The exploitation of the resulting micromechanical modeling and simulation capabilities lies both in simulation driven design of fatigue resistant components and high strength steels.

Methodical approach for studying kinetics of short and long fatigue cracks growth for irradiated reactor materials. Part 1. Statement of problem. The effect of the initial notch acuity on the fatigue crack rate on small-sized specimens

The paper considers methodical issues in the experimental research of fatigue crack growth kinetics when testing irradiated small-sized specimens. The effect of the initial notch acuity is studied on the long crack growth rate. The stress concentration zone sizes are estimated for notches of various types. A brif literature review of the main problems in the study of the growth kinetics of short fatigue cracks has been performed. The tasks of further research are formulated.

Customized weight function for cracks initiated from semi-circular notches

A new weight function is analytically derived for calculating the stress intensity factor of a crack initiated from a semi-circular notch root. This function is dependent on crack size and notch radius and independent from loading condition. The employed method is based on the solution of the weight function for an edge crack. A proper modification factor is applied to considering the effect of the notch and crack geometry on the weight function of the edge crack. It is initially assumed that the notch is a part of the crack. This assumption is used in an analytical method to obtain accurate solution for the weight function. The modification factor is obtained in terms of dimensionless parameters for the ease of application. The provided weight function is examined for different geometries under different loading conditions, and the results are compared with finite element solutions. The weight function is found accurate and reliable. Accuracy is not altered by crack size, notch geometry, and loading conditions. Therefore, the weight function can be used to study the different stages of the initiation and growth of short and large cracks with suitable accuracy.

Effects of 3D microstructural distribution on short crack growth behavior in two bimodal Ti–6Al–4V alloys

In situ X-ray micro-tomography was performed to analyze the short crack growth in the 3D microstructure of two bimodal Ti–6Al–4V alloys with predominant α+β and α phase volume fractions. Short crack strongly interacts with the local microstructure and it grows into the predominant phase above the average microstructural fraction in each alloy. The higher volume fraction and size of the α grains in the microstructure induce crack growth with lower variations and larger deflection lengths inside the α grains. As the short crack length increases, the larger deflections left behind the crack front induces higher crack closure that decreases the crack driving forces. As a result, the short crack remains sensitive to the local microstructural features at higher lengths.

Short review on multiscale short fatigue crack growth model

Short review on multiscale short fatigue crack growth model

Effect of crystallographic orientation and geometrical compatibility on fatigue crack initiation and propagation in rolled Ti-6Al-4V alloy

Fatigue crack initiation and short crack propagation was systematically investigated in a rolled α/β Ti-6Al-4V alloy through a combination of in situ optical microscopy observations during fully-reversed uniaxial fatigue experiments and post-mortem electron backscattering diffraction analyses. The effect of microstructural attributes (phase, crystal orientation, grain boundary) on fatigue crack formation and early growth was statistically quantified based on the analysis of a large dataset of grains. Crack initiation was mainly observed on prismatic slip systems which was related to the effect of the rolling texture. Crack propagation was found to be mostly transgranular and crystallographic on both basal and prismatic slip systems. An analysis of the effect of the β phase was proposed to explain the preferential slip systems for propagation in lamellar colonies. Crack deflection was quantified for a large set of grain boundaries. A combination of elastic, plastic and geometric compatibility criteria was identified to consistently predict the preferential slip systems for crack propagation across grain boundary. Finally, a discussion on the transition from microstructure-sensitive to microstructure-insensitive crack propagation was proposed.

Welding parameter selection and short fatigue crack growth of dissimilar aluminum alloy friction stir welded joint

The welding process parameters of dissimilar 2024-T4 and 7075-T6 aluminum alloy friction stir welded (FSWy) joints are selected by the metallographic microstructure, the mechanical properties, and the hardness distribution. The joints with rotational speed of 900 rpm and advancing speed of 100 mm/min that have the highest tensile strength are used to carry out the experimental fatigue research. The short crack growth behavior of dissimilar FSW joints is analyzed through the replica technique. The stage of the crack initiation and stable growth accounts for about 60~80% of total fatigue life. The short crack grows rapidly once it reaches the crack size of approximately 350~450 μm. The cracks are prone to initiate at the defects of subsurface in the weld nugget zone and propagate in a quasi-cleavage fracture mode.

Hydrogen induced blister cracking and mechanical failure in X65 pipeline steels

The present work aims to investigate the role of hydrogen induced blisters cracking on degradation of tensile and fatigue properties of X65 pipeline steel. Both tensile and fatigue specimens were electrochemically charged with hydrogen at 20 mA/cm2 for a period of 4 h. Hydrogen charging resulted in hydrogen induced cracking (HIC) and blister formation throughout the specimen surface. Nearly all the blisters formed during hydrogen charging showed blister wall cracking (BWC). Inclusions mixed in Al-Si-O were found to be the potential sites for HIC and BWC. Slow strain rate tensile (SSRT) test followed by fractographic analysis confirmed significant hydrogen embrittlement (HE) susceptibility of X65 steel. Short fatigue crack growth framework, on the other hand, specifically highlighted the role of BWC on accelerated crack growth in the investigated material. Coalescence of propagating short fatigue crack with BWC resulted in rapid increase in the crack length and reduced the number of cycles for crack propagation to the equivalent crack length.

A comparison of recent models for fatigue crack tip deformation

Fatigue crack propagation occupies much of the life of engineering components, particularly in the short crack regime. It is important to understand the mechanisms of propagation in order to carry out damage tolerance assessment and to predict component service life. The paper describes experiments carried out at macro- and micro-scale using digital image correlation to measure near-tip displacements. From these, various key parameters governing crack growth are extracted, and different models of fatigue crack deformation are validated. In particular, it is concluded that the Pommier and Hamam and the CJP models for fatigue crack displacement and stress fields have rather similar approaches to capturing the effects of crack tip plasticity.

First Report of Dodder (Cuscuta australis) Parasitizing the Invasive Jerusalem Artichoke (Helianthus tuberosus) in China

Jerusalem artichoke (Helianthus tuberosus L.; Asteraceae), a tall perennial herb native to North America (Shoemaker 1937), has become a significant invasive weed in Eurasia. The wild neophyte first recorded in Shandong Province in 1918 is now present in 26 provinces in China, except Heilongjiang, Inner Mongolia, Guizhou, Taiwan, Guangxi, Xizang, Hongkong, and Macao (Ma 2013). This invasive weed easily forms dense monospecific stands in disturbed habitats such as farmlands, wastelands, bared slope lands, roadsides, ditches, and river embankments, threatening local biodiversity in China. To determine its distributions and natural enemies in Liaoning Province, field surveys were conducted in summer 2018. Dodder was first found on Jerusalem artichoke along the Xinkai River (N 41°49′, E 123°35′, 62 to 65 m elevation) in Shenyang (temperate continental monsoon climate), Liaoning. Orange leafless dodder was observed mostly wrapped around affected artichoke plants and was also occasionally present on native Humulus scanden, Artemisia lavandulaefolia, and Chenopodium album plants in the swards. Artichoke plants parasitized by dodder were chlorotic on the upper half of the stems, and necrotic on the lower half. Some parasitized plants were also stunted. Haustoria were observed clearly penetrating Jerusalem artichoke stems, and some stem tissues surrounding the haustoria connections developed grayish lesions with short longitudinal cracks. The dodder stems on Jerusalem artichoke were 0.9563 ± 0.146 mm in diameter (mean ± s.d., n = 15). The dodder inflorescences were lateral, compact cymose, and subsessile. Bracts were scale-like. Corolla were creamy white and cupular. Capsules were enclosed by persistent corolla, depressed-globose, not circumscissile, and containing 3 to 4 seeds. Seeds were brownish, ovoid, and scabrous. Based on inflorescence, bract, corolla, capsule, and seed characteristics, the dodder was tentatively identified as Cuscuta australis R. Br (Wu 1979; Zhao et al. 2015). The internal transcribed spacer 2 (ITS2) region of the dodder rDNA was amplified with S2F-S3R primers resulting in a 437-bp product (GenBank accession no. MK070145). This product had 100% similarity with C. australis (MK063811) reference, confirming its identify as C. australis, albeit the ITS2 of C. chinensis (JF421489) was also identical. The dodder, C. australis, is broadly distributed from Central, East, and South Asia southward to Malaysia, Indonesia, and Oceania (Wu 1979). It has been reported parasitizing other noxious invasive weeds such as common ragweed (Ambrosia artemisiifolia) (Zhao et al. 2015) and Canada cocklebur (Xanthium strumarium L. var. canadense) (Wang et al. 2019). To our knowledge, this is the first report of C. australis on the invasive Jerusalem artichoke in China. In addition, few natural enemies have been found on this invasive artichoke, except an introduced insect Ophraella communa, of which only its adults were observed feeding in the laboratory (Hu and Meng 2007). The dodder, as one of a few reported natural enemies on the weed, may alleviate the detrimental impacts of the aggressive Jerusalem artichoke in China, especially in the two Northeast provinces, Jilin and Liaoning.

Desiccation Characteristics and Desiccation-Induced Compressive Strength of Natural Fibre-Reinforced Soil

In this study, desiccation characteristics of natural fibre-reinforced soil were investigated. The soil was mixed with sisal fibres (25 mm) at the fibre content of 0.5% and 1%. The effects of soil thickness on the desiccation characteristics were investigated using specimens with thickness of 6, 12 and 24 mm. Digital image analysis was used to study variation of crack morphology, width, surface crack area ratio, crack growth rate and shrinkage strain of the desiccated soil. The gravimetric measurements of moisture content were used to investigate rate of moisture loss and evolution of cracks with change in moisture. The effects of saturation and desaturation on the compressive strength of the soil were investigated by subjecting the compacted soil to wet and dry cycles. The results showed that crack morphology of the reinforced soil was characterised by small cell areas of irregular shapes, short and thin cracks and non-orthogonal crack intersections. Specimens with fibre content of 0.5% showed lower rate of moisture loss than unreinforced soil. This was attributed to the reduction of free moisture caused by fibre moisture absorption. Specimens with fibre content of 1% indicated higher rate of moisture loss than unreinforced soil due to an increase in the volume of water pathways. Crack width and surface crack area ratio showed reduction of 74% and 35%, respectively, with 1% fibre content. The crack growth rate and shrinkage strain significantly reduced with fibre inclusion. Increase in the soil thickness caused an increase in crack width and evaporation for both reinforced and unreinforced soil. Increasing number of wet and dry cycles up to 15 caused compressive strength degradation of 41% and 66% for compacted reinforced and unreinforced soil, respectively. Natural fibre inclusion can effectively be applied to control desiccation cracking of the soil which is beneficial in maintaining integrity of soil for various civil engineering applications.