Short Crack Propagation Research Articles

Retarding effect of submicron carbides on short fatigue crack propagation: Mechanistic modeling and Experimental validation

High-strength steels generally contain a large number of spherical carbides with the size ranging from several hundred nanometers to several micrometers, which have an important impact on fatigue performance. Most studies believe that the carbides, similar to inclusions as fatigue crack initiators, should be refined as much as possible to improve fatigue performance. However, when the carbide size reaches submicron scale (several hundred nanometers to one micrometer), the traditional theory regarding the effect of carbides on fatigue performance may not be applicable. In this paper, it is found that smaller carbides are not always better, and an optimal submicron carbide size exists. By means of experiment and simulation, the mechanism as to how submicron carbides affect the fatigue performance of high-strength steels is systematically studied. It is found that a kind of microstructural transition occurs around carbides as a result of local stress concentration. This leads to the formation of Effective Strengthening Layers (ESLs), which force the short fatigue cracks to propagate along the ESL-matrix interface and decelerate crack propagation. The stress concentration required to generate ESL decreases with carbide size. Therefore, the competition between increasing the specific area of ESL and decreasing stress concentration with regard to decreasing carbide size yields an optimum carbide size. Based on this finding, a novel quantitative fatigue performance evaluation model with experimental validation is proposed for high-strength steels, providing a theoretical guidance for the microstructure design of submicron carbides for fatigue performance improvement.

Short crack propagation near coherent twin boundaries in nickel-based superalloy

Short crack propagation near a coherent twin boundary in polycrystal nickel alloy is investigated with 3D crystal plasticity extended finite element modelling (CP-XFEM), utilising reconstructed experimentally characterised microstructures and crack path observations. Short 3D cracks at coherent twin boundaries are shown to grow on parallel (111) slip planes at very high rate so as to be an intrinsic part of the nucleation process. This is found to occur predominantly because of the stress states established by twin/parent constraint driven by the local elastic anisotropy. If elastic isotropy is modelled, crack growth is found to be non-planar and inclined to the twin boundary. The twin/parent twist angle is also found to influence local stress state, such that the actual 60° twist angle gives rise to the highest local stresses, preferential crack nucleation and rapid growth parallel to the twin boundary, and the deviations from 60° change the crack morphology and rate of growth.

Improved high-temperature fatigue performance of laser directed energy deposited Ni-based superalloy by regulating the heat treatment

The hot-section components are prone to fatigue failure due to severe working condition of high rotational speed and high cyclic load at high temperature. This study investigates the fatigue behavior of GH4169 Ni-based superalloy at 650 °C fabricated by laser directed energy deposition (L-DED) and proposes an improved high-temperature fatigue resistance by regulating the heat treatment. To this purpose, three conditions of solution and aging heat-treated fatigue specimens, namely 980STA, 980S5hDA, and 1050STA, were assessed. Results show that the fatigue resistance of the L-DEDed GH4169 with different heat treatments is mainly determined by the short crack propagation behavior, which is influenced by the combined effect of grain size and morphology. Finer grains with less texture strength of the 1050STA fatigue specimen has the longest short crack propagation life, while a cluster of large grains oriented to the 〈001〉 orientation of 980S5hDA leads to the most significant short crack propagation rate. Besides, the long fatigue crack propagation under the effect of strength and precipitates also minorly influence the fatigue resistance. The microstructure of interconnected precipitates including the needle-like δ phase and slender-stripe shaped Laves phase in 980STA and 1050STA specimens has a better hindering effect on the fatigue crack propagation than that of the discrete δ phase in 980S5hDA specimen, and the 1050STA specimen has the best strength. As a result, the 980S5hDA specimen has the worst fatigue performance, while the 1050STA specimen has the best. Compared to longer solution time, higher solution temperature can improve the fatigue resistance of the L-DEDed GH4169.

Very high cycle fatigue properties at 973 K of additively manufactured and conventionally processed intermetallic TiAl 48-2-2 alloy

The fatigue lives of 2nd generation γ-TiAl alloy Ti–48Al–2Cr–2Nb manufactured both by additive manufacturing using electron beam powder bed fusion (EB-PBF) as well as conventional casting process by vacuum arc remelting for comparison (REF) were investigated. Both batches (EB-PBF and REF) were subject to the common treatment process, i.e. hot isostatic pressing and heat treatment to achieve a duplex microstructure. As a result, a fine- and a coarse-grained microstructure consisting of γ-grains and colonies of α2/γ lamellae have evolved for the EB-PBF and conventionally cast material, respectively. Uniaxial fatigue tests were performed in the very high cycle fatigue (VHCF) range up to 109 cycles at a load ratio of R = −1 using ultrasonic fatigue testing equipment at an elevated temperature of 973 K. The SN data of both batches were discussed with respect to the influence of microstructure and temperature on the processes of crack initiation and propagation. The additive manufactured material showed superior fatigue strength compared to the conventional material due to its smaller grain size. It was shown that the fine-grained duplex microstructure of the batch EB-PBF is particularly appropriate to decrease short crack propagation at elevated temperatures due to its microstructural characteristics.

The role of plasticity-induced crack closure in the non-propagation prediction of surface defect-initiated cracks near fatigue limit

The purpose of present work is to deepen the understanding of crack propagation, non-propagation, and fatigue limit. Quantitative prediction of fatigue crack non-propagation in constant amplitude loading is attempted through extensive 3D elastic–plastic finite element analysis of near-threshold propagation of short cracks nucleating from surface defects. Level of plasticity-induced crack closure is used to estimate the effective stress intensity factor range, which is evaluated in terms of non-propagation potential. It is shown that plasticity-induced crack closure alone is adequate in producing a local minimum in the development of effective stress intensity factor range, which gives a prerequisite to crack arrest.

Crack initiation and short crack propagation of friction stir welded TC17 alloy joint

Heterogeneities of microstructure, tensile and fatigue behavior of FSWed titanium alloy joint were in-situ studied by utilizing replica, DIC and electron back-scattered diffraction methods. Banded TMAZ consisting of needle-like α precipitates and elongated αp within DRXed β grains bridges the HAZ and SZ. Fine β grains in SZ contribute to the highest tensile strength in the 3rd layer. Fatigue cracks prefer to nucleate from HAGBs and {112} 〈111〉 slip deformation of β structure in the 2nd layer of SZ below 210 MPa and 3rd layer of HAZ otherwise. Transgranular crack propagation along the {112} planes exhibits a higher crack growth rate in HAZ than SZ.

The high-cycle fatigue behavior of carburized M50NiL steel with high-energy modification and compound infiltration: A comparative study

In this study, ultrasonic rolling (C + USRP) and plasma nitriding (C + N) is performed on the surface of M50NiL steel after carburizing， the tension-compression fatigue test of R = −1 on different samples. The results show that the maximum fatigue limit of C + N, C + USRP, and single carburizing are 770 MPa, 676 MPa, and 412 MPa, respectively. The residual stress relaxation in the fatigue process of C + N and C + USRP is tested and found that the initial residual compressive stress of C + N is lower than that of C + USRP, but its attenuation value is considerably smaller. The microscopic characterization of samples is conducted after the fatigue tests. The nanometer nitrogen compounds in the composite permeability layer showed a strong pinning effect, however, microplastic deformation limits the fatigue process. The C + N demonstrates a much lower residual compressive stress relaxation, consequently, the C + N short crack propagation effective driving force ΔKeff value decreases. As a result, due to the higher crack closure effect, the C + N fatigue limit is observed higher than the C + USRP.

Microstructurally short crack growth simulation combining crystal plasticity with extended finite element method

Accurate description of microstructurally short crack growth remains a long-standing challenging task, due to the fluctuation of crack growth rate induced by grain size and orientation, and grain boundary impediment. Combining crystal plasticity with extended finite element method, a novel framework for short crack growth prediction is proposed, where a new criterion for short crack propagation is established in terms of accumulated shear strain on specific slip system. Compared with three classic empirical models, the proposed method exhibits substantial advantage on the prediction of the fluctuation in FGH96 superalloy, serving as a promising tool for precise estimation of short crack growth rate in polycrystalline materials.

The synergetic effects of shot peening and laser-shot peening on the microstructural evolution and fatigue performance of a medium carbon steel

The synergetic effect of shot peening (SP) and laser shot peening (LSP) on the fatigue behavior of a medium carbon steel was investigated. A fatigue life transition was observed in the SN curve, where the fatigue performance of the SP + LSP specimen was improved only at lower stress levels. This result is consistent with the short crack propagation behavior observed in the near-threshold region. The heterogeneous structures comprised by ultra-fine grains, dislocation tangles, and more high-angle grain boundaries were responsible for this behavior which effectively retard the short crack propagation.

Study on the short fatigue crack initiation and propagation behavior of 42CrMo

A micro-tensile fatigue test of 42CrMo alloy steel was carried out. The result shows that the crack initiation and propagation of the material could be characterized with randomness and localization. There existed two sorts of short fatigue crack propagation behaviors for the tested material, that is, self-propagation and joint propagation with other short cracks. A short fatigue crack growth rate function was built considering the propagation characteristics. Moreover, a simulation model of short fatigue crack evolution was built. Total three samples were tested and simulated. The experimental result was in good consistent with that from simulation, both showing that the crack growth rate accelerated first and then decelerated during propagation.

3D CP-XFEM modelling of short crack propagation interacting with twist/tilt nickel grain boundaries

Short fatigue crack growth across grain boundaries of differing tilt and twist combinations has been investigated in three dimensions using coupled crystal plasticity and extended finite element methods. Crack path selection and growth rate are mechanistically determined by considering crystallographic planes containing the highest shear strain and the achievement of a critical stored energy density respectively. Tilt angle is found to have weak influence on crack growth rates approaching the grain boundary but increasing twist angle was shown to lead to higher growth rate retardation, as observed in experiments. Large twist angle was also shown to lead to multi-slip system activation as the crack tip traversed the grain boundary, potentially giving rise to multiple crack planes and hence faceted cracks. Elastic stored energy density was shown to capture observed sensitivity of crack growth rates and retardations to grain boundary tilt and twist, and in combination, thereby offering an improved metric over residual Burgers vector and geometric compatibility.

Effects of ultrasonic shot peening on fatigue behavior of TA15 titanium alloy fabricated by laser melting deposition

Due to the influence of tensile residual stress and pores, the fatigue resistance of titanium alloy fabricated by laser melting deposition (LMD) still needs to be improved. In this study, ultrasonic shot peening (USP) was utilized to strengthen the TA15 titanium alloy manufactured by LMD. The surface topography, microstructure, microhardness, residual stress and fatigue behavior were analyzed. After USP treatment, the root-mean-square deviation of the three-dimensional profile (Sq) and the local stress concentration factor (Kst) increased. High-density dislocations (dislocation tangles, dislocation walls, dislocation cells) and deformation twins were generated on the specimen subsurface. The existence of high-density dislocations improved the surface microhardness. Meanwhile, changes in microstructure affected the state and distribution of residual stress. The surface tensile residual stress was completely transformed into compressive residual stress. The fatigue behavior was significantly influenced. Under 720 MPa and 760 MPa stress levels, the safe fatigue life (reliability R = 0.95, confidence level C = 0.95) of TA15 titanium alloy manufactured by LMD increased by 200 % and 43 %, respectively. In the pore-profile valley competition, the pore showed greater fatigue sensitivity. The high-density dislocations and compressive residual stress accounted for the improvement in fatigue life. The high-density dislocations hindered the crack nucleation and short crack propagation at pores near the surface. The compressive residual stress reduced the crack propagation rate in the stable crack propagation stage. The results indicated USP as an effective method to improve the fatigue performance of the TA15 titanium alloy manufactured by LMD.

Effects of microstructure and temperature on short fatigue crack propagation behaviour of powder metallurgy superalloy FGH4098 in vacuum

Short fatigue crack propagation (SFCP) behaviour of a powder metallurgy (PM) superalloy (i.e. FGH4098) for aeroengine turbine disc application was investigated at room temperature and 650 °C in vacuum (10−3 pa) by in-situ fatigue test. The SFCP mechanism was analyzed by the microstructural characterization and assessment of the SFCP rate. The results show that the SFCP resistance of fine grained (FG) FGH4098 is slightly better than that of coarse grained (CG) FGH4098 at room temperature due to the hindering effects of grain boundary, and close to that of CG FGH4098 at 650 °C. The temperature exerts more evident influence on SFCP behaviour of FG FGH4098 due to the grain boundary oxidation compared with that of CG FGH4098. In the scenario which grain boundary oxidation is absent or insignificant, SFCP shows transgranular feature and is closely related to slip bands linked to the slip systems with high Schmid factor and/or geometric compatibility factor. Grain boundary oxidation still occurs in both FG and CG FGH4098 alloy and mainly forms Cr oxides at 650 °C, which results in intergranular or mixed-inter-transgranular SFCP and promotes SFCP to some extent for FG FGH4098 alloy.

Influence of Residual Stresses on the Crack Initiation and Short Crack Propagation in a Martensitic Spring Steel

The crack initiation and short crack propagation in a martensitic spring steel were investigated by means of in-situ fatigue testing. Shot peened samples as well as untreated samples were exposed to uniaxial alternating stress to analyze the impact of compressive residual stresses. The early fatigue damage started in both sample conditions with the formation of slip bands, which subsequently served as crack initiation sites. Most of the slip bands and, correspondingly, most of the short fatigue cracks initiated at or close to prior austenite grain boundaries. The observed crack density of the emerging network of short cracks increased with the number of cycles and with increasing applied stress amplitudes. Furthermore, the prior austenite grain boundaries acted as obstacles to short crack propagation in both sample conditions. Compressive residual stresses enhanced the fatigue strength, and it is assumed that this beneficial effect was due to a delayed transition from short crack propagation to long crack propagation and a shift of the crack initiation site from the sample surface to the sample interior.

Microstructural investigation and model of fatigue short crack in EH36 steels

AbstractFor better revealing the behavior and mechanism of fatigue short crack (SC) propagation, five different kinds of notched samples were designed. Based on the crack growth rate‐SC length map, finite element method and electron backscatter diffraction (EBSD) pattern, the evolution theory of geometrically necessary dislocation density (ρGND) versus SC length was proposed to distinguish the physically short crack (PSC) length; Schmid factor was utilized to define the length of microstructurally short crack (MSC). Thus, the whole SC growth was divided into three parts: MSC, transition zone and PSC. Finally, an original piecewise model containing two parameters was proposed to describe the whole SC growth of EH36 steels.

The influence of milling induced residual stress on fatigue life of aluminum alloys

The effect of machining strategies on the surface integrity have received minimal consideration as this fluid situation develops, specifically, the machining induced residual stress generated in the material during milling. This study investigates the presence of machining induced residual stress in Aluminum 7075 and its effects on fatigue life. The testing consists of a layer removal process to measure residual stress followed by a three-point bending fatigue test. The study finds that an increase in feed per tooth in combination with a reduction in cutting speed increases compressive residual stress at the surface in the material. The fatigue testing concludes that this increase in compressive stress improves fatigue life with varying effects on long crack and short crack propagation.

Life prediction method based on short crack propagation considering additional damage under axial-torsional non-proportional loading

A total life prediction method based on short crack propagation considering additional damage from non-proportional loading is proposed under multiaxial fatigue loading. Firstly, a new calculation method of equivalent stress range is proposed to consider the effect of shear stress on crack propagation under tensile or compressive stress, then it is further modified to consider the additional damage. Combined with Paris formula and crack closure correction, a total life prediction method is proposed. The life results were used to verify the proposed method for two materials, which showed that most errors are within a factor of 3.

Effect of macrozones on fatigue crack initiation and propagation mechanisms in a forged ti-6Al-4V alloy under fully-reversed condition

In the present study, the mechanisms of fatigue crack initiation and propagation was investigated in a forged Ti-6Al-4V alloy with a bimodal microstructure through a combination of experiments and crystal plasticity finite element simulations. Transgranular crack initiations were observed on both basal and prismatic planes, in grains exhibiting high plastic activity. A fatigue indicator parameter based on the Tanaka-Mura model was found to correctly identify these critical grains. Intergranular crack initiations were found in pairs of grains sharing the same basal plane. An analysis of the grain boundary configuration suggested that these grains were separated by a (0001) tilt subgrain boundary as the c-axis was argued to belong to the boundary plane. Short crack propagation was found to be transgranular and crystallographic on either basal or prismatic plane depending on the microtextured regions they belong. Crack path was more serrated with high number of deflection and branching in regions favoring prismatic activity. However, no significant difference in terms of crack propagation rate was noticed between the different macrozones which was attributed to a relatively similar plastic activity in both regions. An analysis of the M-index finally suggested that the presence of a macrozone was not a necessary condition for crack initiation but appeared necessary for its propagation.

Research on evolution behavior of 42CrMo microscopic fatigue short crack based on microstructure effect of material

To investigate the causes of damage dispersion in fatigue life of alloy materials, fatigue tests on 42CrMo are carried out under microscope with the aid of a micro fatigue testing device. The fatigue crack initiation and propagation evolution process of 42CrMo are studied on the micromechanical scale. The fatigue short crack and short crack growth rates are measured respectively under loads of 1400, 1550, and 1650 N. The experimental result shows that the initiation of fatigue short cracks is related to the randomness of microstructure defects. The initiation and propagation of fatigue short cracks on the surface of samples are random and local, while the propagation is characterized by first deceleration and then acceleration.

In-situ SEM investigation on the fatigue behavior of Ti–6Al–4V ELI fabricated by the powder-blown underwater directed energy deposition technique

Underwater directed energy deposition (UDED) is a promising technology for on-site maintenance and repair of Ti–6Al–4V alloy. The microstructurally short fatigue crack initiation and growth behaviors of Ti–6Al–4V repaired by UDED were investigated through in-situ SEM fatigue testing. The results were compared with those of the Ti–6Al–4V repaired by in-air directed energy deposition (DED) and a substrate. The experimental results show that the microstructure of the Ti–6Al–4V repaired by UDED was dominated by α′ martensite. The α′ martensite in the Ti–6Al–4V repaired by in-air DED decomposed into α+β due to the intrinsic heat treatment. Both the strength and microhardness of the Ti–6Al–4V repaired by UDED were higher than those of the Ti–6Al–4V repaired by in-air DED. Compared with the Ti–6Al–4V repaired by in-air DED, the Ti–6Al–4V repaired by UDED presented a short micro crack initiation time and a poor fatigue crack propagation resistance. The short fatigue crack initiation and propagation behaviors of the as-deposited Ti–6Al–4V were primarily controlled by the structures and morphologies of the microstructures rather than the residual stresses. In addition, the prior-β boundaries in the Ti–6Al–4V repaired by UDED had a resistance effect on the fatigue crack growth rate. However, the resistant effect of prior-β boundaries in the Ti–6Al–4V repaired by in-air DED was not obvious and the local colony α+β played a dominant role in the resistance of short fatigue crack propagation.