Fatigue Crack Initiation Mechanism Research Articles

In-situ investigation of the fatigue crack initiation and propagation behavior of Zircaloy-4 with different hydrogen contents at RT and 300 °C

In this paper, the effects of hydrogen content and temperature on the fatigue crack initiation and propagation behavior of Zircaloy-4 are investigated by in-situ scanning electron microscope observation. The results show that, at room temperature, with the increase of hydrogen content, the tensile strength increases and the elongation decreases, but the fatigue lifetime (fatigue crack initiation lifetime and fatigue failure lifetime) first increases and then decreases. The cut-off point is at about 200 ppm hydrogen content. The fatigue crack growth rate increases with the increase of hydrogen content. At 300 °C, with the increase of the hydrogen content, tensile strength, elongation and fatigue lifetime of Zircaloy-4 alloys exhibit the same variations as those at room temperature. However, the fatigue crack growth rate is less affected by the hydrogen content. The results also show that, with the same hydrogen content, the fatigue lifetime at 300 °C is longer than that at room temperature, and the fatigue crack growth threshold at 300 °C is lower than that at room temperature. The fatigue crack propagation path is strongly affected by the hydride and grain boundaries. The hydrogen content has a significant effect on the fatigue crack initiation mechanism. For the un-hydrided sample, the fatigue cracks initiate at the sub-surface. With the increase of hydrogen content, the fatigue cracks tend to initiate at the surface of the sample.

Investigation of crack initiation mechanism of a precipitation hardened TC11 titanium alloy under very high cycle fatigue loading

TC11 alpha-beta titanium alloy (Ti-6.5Al-3.5Mo-1.5Zr-0.3Si) is widely used in the construction of aero-engine components. In service, these components are subjected to cyclic loading due to vibrations. This work aims to assess the fatigue properties of a TC11 alloy and to reveal the effect of silicide precipitates and α2 phase (Ti3Al) on fatigue behavior including failure mechanism in very high cycle fatigue (VHCF) regime. For this purpose, tension-compression fatigue experiments were carried out with an ultrasonic fatigue testing system. Fatigue crack initiation and early propagation mechanisms were finely investigated using Field Emission Gun Scanning Electron Microscopy (FEG-SEM). The results show that this alloy exhibits a high fatigue strength at 109 cycles with a fatigue limit up to 618 MPa. Fracture surface observation indicates that the fatigue failure can be categorized into three modes: (a) surface failure in short life regime, (b) subsurface failure and (c) internal failure with the presence of a fish-eye pattern for long life regime. The internal and subsurface crack initiation regions are characterized by microvoids, dimples and peaks without the presence of crystallographic facets in the VHCF regime. This signifies that ductile damage mechanism instead of brittle one is the dominant failure mode for this alloy in the VHCF regime. Detailed observation of fracture surfaces seems to indicate that this ductile damage mechanism can be attributed to the combined effect of the silicide precipitates and α2 phase. Moreover, the results show that the cuboid micro-silicide precipitates significantly deteriorate the fatigue life of this studied alloy.

First-principles study of phase stability and temperature-dependent mechanical properties of (Cr, M)23C6 (M = Fe, Mo) phases

Although Cr-rich (Cr, M)23C6 (M = Fe, Mo) carbides as ineluctable precipitates dramatically affect the mechanical performances of advanced materials such as refractory steels and superalloys, their high-temperature material properties are difficult to be determined in experiments. In the present work, the first-principles method is applied to comprehensively investigate the phase stability and thermodynamic and temperature-dependent mechanical properties of Cr23-xMxC6 (M = Fe, Mo; x = 0–23). The results indicate that Cr23-xFexC6 presents thermodynamic phase stability under a broad occupying concentration of Fe (i.e., x = 1, 3, 9, 12, 13, 14, 15). Amongst which, the Cr8Fe15C6 with 4a, 8c and 48 h sites co-occupied by Fe is the most stable. Whereas Cr21Mo2C6 is the only stable phase among Cr23-xMoxC6. The mechanical properties of (Cr, M)23C6 are found to exhibit a strong doping composition-position dependence and a negative temperature dependence. Doping a low concentration of M leads to an increase of the elasticity of Cr23C6. The occupation of 4a or 8c sites alone plays an effective role in enhancing mechanical properties. The present work provides necessary information for understanding the heterogeneous deformation process and the fatigue crack initiation mechanism for alloys with the precipitate of M23C6 inclusions.

Fatigue Property and Improvement of a Rounded Welding Region between the Diaphragm Plate and Closed Rib of an Orthotropic Steel Bridge Deck

By means of finite element modeling (FEM) and fatigue experiments, we study the fatigue performance of the rounded welding region between the diaphragm plate and closed rib of orthotropic steel bridge deck in this work. A local sub-model of the rounded welding region from the orthotropic steel bridge deck was developed to analyze the stress distributions. Based on the analysis results we designed the fatigue specimen for the fatigue test of this detailed structure. The fatigue experimental results revealed that the crack initiates from the weld toe of the rounded welding region and the stress concentration at the rounded welding region is the main mechanism of fatigue crack initiation. In addition, we propose three improvements to reduce the stress concentration of the rounded welding region, and the local structure optimization scheme of the diaphragm–rib weld can effectively improve the fatigue resistance of the detailed weld structure.

Fatigue behavior of AW7075 aluminum alloy in ultra-high cycle fatigue region

Advanced electron microscopy methods were used with the aim to explain the differences in the response of strengthened AW 7075 – T6511 aluminum alloy to fatigue loading at 5 Hz and 20 kHz. The shift of the S–N curve to higher number of cycles to fracture for the specimens tested at high frequency was experimentally determined. This effect is not connected with a change of the fatigue crack initiation mechanism and site from the surface to material interior. The absence of slip markings and cracking of primary intermetallic particles on the surface of cycled specimens were characteristic features. A difference in the dislocation density and dislocation arrangement in the vicinity of the fatigue crack initiation sites was shown to be the only observable effect. However, consistent with the strengthened structure of the alloy, no specific dislocation structure due to the cyclic loading was observed, regardless of the loading frequency and stress amplitude.

Fatigue Crack Initiation and Propagation Mechanism of FGH96 PM Superalloy

FGH96 is regarded as a promising powder metallurgy superalloy for high-temperature / pressure turbine disc in aerospace industries due to its high resistance / defect tolerance and high working temperature up to 750 °C. The initiation and propagation mechanism of FGH96 PM superalloy is researched by in-situ fatigue experiments. The result shows that the crack of gap samples initiates and propagates at the same stress level at 25 °C and 300°C. At 300°C, there are no creep characteristics such as the intergranular characteristics found in the sample fracture. The crack at 300°C has a faster initiation and propagation process compared to the crack at 25°C, especially in the late stage of fatigue. The increase of the crack propagation rate of the sample is unrelated to the creep, mainly due to oxidation. Oxidation reduces the fatigue life of materials because it accelerates the initiation and propagation of transgranular cracks.

Near-Threshold Fatigue Crack Growth Rates of Laser Powder Bed Fusion Produced Ti-6Al-4V

When Ti-6Al-4V is processed by laser powder bed fusion (LPBF), a material with a martensitic microstructure is obtained. Moreover, the presence of internal stresses, an outer surface with relatively high surface roughness and the presence of remnant porosity all influence the fatigue life of high cyclically loaded components. The majority of investigations on LPBF produced Ti-6Al-4V have focussed on a fatigue life method providing valuable, albeit limited, insight into fatigue failure mechanisms. Near-threshold fatigue crack growth rates are vital for describing fatigue crack initiation mechanisms and how these are influenced by residual stress, a martensitic microstructure and build orientation. This study investigates near-threshold fatigue crack growth rates of LPBF produced Ti-6Al-4V. The study makes use of full-size compact tension specimens for fatigue crack growth rate investigations, the contour method for residual stress measurement, and scanning electron microscopy with electron backscatter diffraction to consider both morphological and crystallographic texture. Results show anisotropic near-threshold fatigue crack growth rates that are dependent on residual stress levels and load-ratios. Fracture is predominantly governed by transgranular quasi-cleavage mechanisms, and the fracture path is directed by the columnar prior beta-grain structure resulting in orientation-dependent crack closure effects. Residual stresses result in crack opening that causes a shift of near-threshold fatigue crack growth rates. An intrinsic ΔKth of ~1.6 ± 0.2 MPa√m and critical Kmax of ~ 3 MPa√m is measured that is independent of the stress state but dependent on orientation. It is shown that this anisotropy is linked to morphological texture and to a lesser extent the crystallographic texture of LBPF produced Ti-6Al-4V.

Mechanism and modeling of fatigue crack initiation and propagation in the directionally solidified CM186 LC blade of a gas turbine engine

First-stage blades of gas turbines are elements positioned after the combustion chamber and are responsible for extracting energy from high temperature and high pressure gas. One of the common problems of mechanical components, particularly gas turbine blades, is fatigue cracking which usually occurs under variable loading due to the start and shutdown process and vibrations. Low-cycle fatigues are caused by the stress induced by high centrifugal force due to high rotational speed and thermal loads triggered by temperature gradients in startup and shutdown of gas turbine, as well as operation at critical environmental conditions. The present study aimed at presenting the mechanism and modeling of fatigue crack initiation and propagation in CM186 LC directional solidified material of a first-stage gas turbine blade. To investigate the crack initiation point, a finite element study was carried out using ABAQUS software package. The stress distribution of the blade was calculated in accordance with engine conditions and the fatigue crack growth was then predicted by using ZENCRACK fracture mechanics code. Moreover, the Paris model was used in this study to estimate the fatigue life. The stress intensity factors (KI & KII) of individual mode and fracture mechanics quantities were calculated from CTOD and J-Integral methods, followed by the computation of crack growth direction on the basis of maximum energy release rate criterion.

The Effect of Microstructure and Axial Tension on Three-Point Bending Fatigue Behavior of TC4 in High Cycle and Very High Cycle Regimes.

The effects of microstructure and axial tension on the fatigue behavior of TC4 titanium alloy in high cycle (HCF) and very high cycle (VHCF) regimes are discussed in this paper. Ultrasonic three-point bending fatigue tests at 20 kHz were done on a fatigue life range among 105–109 cycles of the alloys with equiaxed, bimodal and Widmanstatten microstructures. Experimental results without axial tension show that three typical shapes of S-N curves clearly present themselves for the three different microstructures. Moreover, the crack initiation sites abruptly shifted from surface to subsurface of the specimen in the very high cycle fatigue regime for equiaxed and bimodal microstructures. But for the Widmanstatten microstructure, both surface and subsurface crack initiation appeared in the high cycle fatigue regime, and the multi-points crack initiation was found in the bimodal microstructure. The subsurface fatigue crack originated from the αp grains in equiaxed and bimodal microstructures. However, it originated from the coarse grain boundary α in the Widmanstatten microstructure. Additionally, the S-N curve shape, fatigue life and fatigue crack initiation mechanism with axial tension are similar to that without axial tension. However, the crack origin point shifts inward with axial tension.

Revealing the fatigue crack initiation mechanism of a TiB2-reinforced steel matrix composite

The initiation of fatigue cracks in a TiB2-reinforced high-modulus, low-density steel was revealed by in-situ scanning electron microscopy for the first time. The results show that the superior interfacial strength between the TiB2particles and the matrix prevents the initiation of micro-cracks at the interfaces. Fatigue micro-cracks are mainly induced by the brittle fracture of small eutectic TiB2 particles, while the coalescence of these micro-cracks plays the major role in the fatigue failure. This failure mechanism is essentially different from the static tensile failure mode in which the large primary TiB2 particles are the main source of fracture failure.

Tensile and fatigue behavior of electron beam welded TC17 titanium alloy joint

The tensile fracture was originated from the strain concentration in equiaxed grain region adjacent to the fusion center, where a lower micro-hardness occurred when compared to the columnar grain region in the fusion zone. A higher stress level was more potential to fabricate a fatal fatigue crack initiating from the intrinsic metallic deficiencies. At a lower stress of 180 MPa, fatigue crack appeared to initiate from the intergranular welding pores other than the intrinsic metallic deficiencies. Furthermore, intergranular welding pores induced fatal crack initiation became the major fatigue crack initiation mechanisms when the fatigue stress amplitude down to 140 MPa.

Ductile cast irons: Microstructure influence on the fatigue initiation mechanisms

AbstractIn the last decades, the combination of high mechanical performances and low production costs increased the industrial interest on ductile cast irons. These grades are often used for applications where the fatigue resistance can be a critical issue (eg, machine frames for the wind‐power industry or crankshaft used in trucks) and the investigation of the main damaging mechanisms during both the crack initiation and the crack propagation stage could offer new perspectives about these alloys.Ductile cast irons can be considered as a natural composite, being characterized by graphite elements (nodules) embedded in a more or less ductile matrix (ranging from fully ferritic to pearlitic, from martensitic to austempered). In this work, the fatigue crack initiation mechanisms were investigated considering different matrix microstructure and the presence of a mechanical properties gradient in the graphite nodules.

Role of retained austenite in hydrogen trapping and hydrogen-assisted fatigue fracture of high-strength steels

The Interaction of hydrogen with retained austenite under fatigue loading of dual-phase and complex-phase high-strength steels with strength of about 1200 MPa was studied. A load-controlled fatigue test was performed in the air with the maximum applied tensile strength of 900 MPa, which is just below the offset yield point of the studied steels. The trapping of hydrogen accumulated into the studied steels under the fatigue loading was studied by thermal desorption spectroscopy. Measurements of hydrogen trapping evolution and microstructure changes during fatigue testing reveal a complicated hydrogen trapping behavior driven by hydrogen interaction with deformation defects and retained austenite. Hydrogen concentration increases in the studied steels during the fatigue testing in the air without preceding hydrogen charging. The fracture surfaces were studied by scanning electron microscopy evidencing the relationship between the hydrogen concentration increase related to retained austenite and initiation of the intergranular fatigue fracture. The role of retained austenite in hydrogen-assisted fatigue cracking is discussed and a possible mechanism of the hydrogen-assisted fatigue crack initiation in the high-strength steels is proposed.

Brittle coating effects on fatigue cracks behavior in Ti alloys

In order to study the coating effects on fatigue crack initiation mechanism in Ti-alloys, two types of brittle coatings, CrAlN and TiN, were deposited on the surface of TC4 titanium alloy by physical vapor deposition. The tension-tension fatigue tests and the in-situ observations of fatigue crack morphology were performed to study the coating effects on the fatigue crack initiation and propagation in the Ti-6Al-4V alloy. It was found that the 510–530 MPa TC4 fatigue limit is reduced to 315–330 MPa due to the CrAlN coating. The brittle coatings impeded the deformation of the TC4 samples at the beginning stage of fatigue tests, while coating cracking promoted the elongation of the tested samples. Fatigue crack was found to be initiated in the brittle coatings and propagated to the coating-substrate interface, inducing micro-damage of the substrate surface. The fracture surface of coated and uncoated samples was quite different, and the formation of non-propagating fatigue cracks was also observed. The coating cracking-induced low cyclic stress substrate damage model was proposed. This study should be of significance for the coating improvement and provides a theoretical basis for improving fatigue properties of coating materials.

Microstructural characteristics and strengthening mechanisms in a polycrystalline Ni-based superalloy under deep cold rolling

Surface modification is an essential process route to improve the fatigue performance of aerospace components. Microstructural evolution in Ni-based superalloy Udimet720Li processed by deep cold rolling (DCR) was investigated experimentally using X-Ray diffraction, electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). Deep cold rolling produces hardened surface due to a range of microstructural changes associated with grain refinement, low angle grain boundaries (LAGBs) formation, and pile-up of dislocations around γ' precipitates and across twin boundaries. The defect structures within the deformed subsurface comprised of equiaxed and elongated dislocation cells at grain boundaries, mutual interactions of slip bands, slip bands- γ' precipitate at grain boundaries and multi-variant modes of twinning. The plastic deformation is predominantly driven through slip and dislocation multiplication mechanism during DCR. Surface compressive residual stresses, FWHM, micro-hardness, the fraction of LAGBs and the depth of plastically strained region increased with DCR hydrostatic pressure. These fundamental understanding on process-microstructure-property could provide a deep insight into the fatigue crack initiation mechanism of surface modified Ni-based superalloys.

Fatigue crack initiation site and propagation paths in high-cycle fatigue of magnesium alloy AZ31

Plane bending fatigue tests were conducted under stress ratios of −1, −0.1, 0.1, and 0.5 at room temperature in a laboratory atmosphere to elucidate the fatigue crack initiation mechanism of an extruded AZ31 magnesium alloy. The fatigue life can be expressed as a unique function of the equivalent stress amplitude based on Smith–Watson–Topper theory independent of the stress ratio, and the dependence of the fatigue limit on the mean stress can be expressed by Morrow’s equation, both of which were proposed for conventional metals without twinning. In addition, the specimen surface was observed by optical microscopy and scanning electron microscopy (SEM), and the surface near the crack initiation site was analyzed by electron backscatter diffraction (EBSD) analysis to discuss the fatigue crack initiation mechanism. On the basis of the results of EBSD analysis, it is concluded that the existence of large grain with large Schmid factor of the basal slip system is essential for crack initiation, and the crack initiation mechanism is based on irreversible slipping and unrelated to twinning under the alternating stress condition (R = −1).

Mechanism of Fatigue Crack Initiation and Propagation in Commercially Pure Titanium and Titanium Alloy with Bimodal Harmonic Structure

Mechanism of Fatigue Crack Initiation and Propagation in Commercially Pure Titanium and Titanium Alloy with Bimodal Harmonic Structure

Thickness effect in laser shock processing for test specimens with a small hole under smaller laser power density

In order to consider the influence of the thickness difference of the small hole sample or member on the effect of laser shock processing and the optimization of the process parameters, the differences in residual stress distribution, fatigue source location, fracture characteristics and crack growth rate and their correlations were investigated by simulation and experimental methods after laser shock processing of small hole specimens with different thicknesses under certain power densities. There was a significant difference in fatigue life after laser shock of small hole specimens with different thickness. And there were differences of the fatigue crack initiation and growth mechanism of the laser shocked specimens with different thickness, indicating the small hole specimens after laser shock processing had a thickness effect. This thickness effect was closely related to the distribution of the residual stress due to the difference in plate thickness.

Mechanisms of fatigue crack initiation and early propagation in cyclically strained nickel superalloy

In the present work, the mechanisms of fatigue crack initiation and early crack propagation of a γ′ strengthened nickel-based superalloy have been investigated. Low cycle fatigue tests have been performed on polycrystalline superalloy MAR-M247 under symmetrical strain controlled loading at ambient temperature in air. The nature of early fatigue damage, crack initiation and subsequent fatigue crack propagation was assessed for applied total strain range of 1%. Test was interrupted at given numbers of cycles corresponding to 1%, 2%, 5%, 10%, 20%, 40%, and 80% of fatigue life. The evolution of pronounced surface relief, also known as persistent slip markings (PSMs) was observed using atomic force microscopy (AFM) and high resolution scanning electron microscopy (SEM-FEG). The kinetics of extrusion growth was quantitatively measured. The regions of fast and slow extrusion growth were identified. Sharp surface PSMs arise in areas where lamellae of localized cyclic slip – persistent slip bands (PSBs) – intersect the original flat surface. Thin PSMs cut both γ matrix and γ′ precipitates. PSMs consist of extrusions accompanied by parallel intrusions. Fatigue cracks were initiated at individual PSMs. Focused ion beam (FIB) revealed fatigue crack nucleation from sharp intrusion and its subsequent propagation parallel to one of the active {111} 〈110〉 slip systems. Transmission electron microscopy (TEM) observation showed plastic strain localization into dislocation rich slabs running parallel to {111} slip planes.

Fatigue crack growth rate in axial, torsional and multiaxial mode in 316L austenitic steel

Austenitic stainless steel 316L was tested at room temperature in three different cycling modes – axial tension/compression, pure torsional and multiaxial in phase mode with the same equivalent strain amplitude. Two types of hollow cylindrical specimens were used in this study. The first type of specimen with smooth surface was used for studying the mechanism of initiation of fatigue crack and for determination of fatigue life. The second type of specimen with artificial hole was used for measuring of crack growth rate. The mechanism of crack initiation was observed by scanning electron microscopy and by focused ion beam technique. Detailed observations of microstructure were done by transmission electron microscopy. The mechanism of fatigue crack initiation is following – persistent slip bands are formed and fatigue cracks initiate from them. Crack growth rate was determined from micrographs taken by light microscope during cycling. It was found that the cycling in axial mode leads to the fastest crack growth rate and the shortest fatigue lives. On the contrary, cycling in torsional mode leads to the slowest crack growth rate and the longest fatigue lives. One of probable causes of such behavior can be found in phase transformation from austenite to α´ martensite which is much more intense in the case of pure torsional loading than in other modes.