Crack Initiation Mechanism Research Articles

Microstructure and mechanical behavior of Al–TiAl3 composites containing high content uniform dispersion of TiAl3 particles

The microstructure, mechanical behavior and deformation mechanisms of Al–TiAl3 composites containing a wide range of volume fraction of TiAl3 (Vp: 24.6–89.0%) with uniform distribution were investigated. Microhardness and strength of the composites increase with increasing Vp of TiAl3 particles, reaching ~11.0 times and ~8.0 times those of the matrix respectively. While the strength of the composites can be correlated well to the comprehensive contributions of various strengthening mechanisms of the matrix for low Vp, that for high Vp was found to fit to direct contributions from both the matrix and particle following the law of mixture. Besides the deformation of the matrix, evident plastic deformation of TiAl3 particles by dislocation activities as well as twinning was observed in the composites upon loading at room temperature. Two mechanisms for crack initiation inside the TiAl3 particle were identified, namely by intersection of twins and by interaction between dislocations and twins. Microcracks tend to propagate along the twin boundaries in TiAl3 particles and were found to be suspended at the particle/matrix interface. The unique deformation behavior of TiAl3 particles in the composites can be attributed to the uniform distribution of TiAl3 particles that ensures a continuous network of surrounding matrix even at very high Vp.

High cycle fatigue behavior and numerical evaluation of Alloy 690TT steam generator tube

High cycle fatigue behavior of Alloy 690TT steam generator (SG) tube under different maximum stresses was investigated at stress ratio R = 0.1 through experimental and numerical evaluation. The morphology of fatigue fracture surface was observed by SEM, and the crack initiation and propagation mechanism of Alloy 690TT SG tube was analyzed. The fatigue crack of Alloy 690TT SG tube initiated at the intrados surface edge, then propagated along the direction of maximum shear stress and penetrated through the side of the specimen, and finally transient fracture occurred in the direction perpendicular to the loading axis. With the increase of maximum stress, the number of fatigue crack initiation sources increased, but they were all distributed on the edge of the intrados surface, and a large number of persistent slip bands (PSBs) were formed on the intrados surface. The fracture mechanism of the tubular specimen was different from that of the bar specimen, and its fracture mode affected the life evaluation of the tubular specimen. The finite element analysis software ABAQUS and advanced fatigue durability analysis software FE-SAFE were first used to evaluate the fatigue life of Alloy 690TT SG tube. Compared with the experimental results, the error in predicting fatigue life by numerical method was acceptable.

Exploring factors controlling pre-corrosion fatigue of 316L austenitic stainless steel in hydrofluoric acid

While the low-cycle fatigue performance of metals is significantly influenced by corrosion, the corrosion damages and therefore the fatigue mechanisms induced by different solutions are different. In this paper, the pre-corrosion fatigue behavior of 316L austenitic stainless steel in hydrofluoric acid (HF) are studied. To characterize the corrosion type as well the extent of corrosion, the corrosion behavior of 316L austenitic stainless steel at different HF concentrations are investigated. Combined uniform corrosion and pitting corrosion are observed, with grooves formed by the superposition of several etched pits. Two methods are proposed to characterize corrosion damages, and the fatigue life is found to negatively correlate with the corrosion damage. To characterize fatigue crack initiation mechanisms, fracture surfaces of corroded fatigue specimens are examined with an emphasis on crack initiation sites. It is found that the crack initiates from multiple sources caused by multiple pits, and the surface average annular width of the pitting is found to be a more important factor controlling the fatigue life than the depth of pits. The results in this study are expected to provide insights into the understanding of failure mechanisms of equipment in fluorine chemical industry.

Influence of processing parameters on mechanical and fatigue properties of 316 L steel manufactured by selective laser melting

The Selective Laser Melting process involves multiple factors that are detrimental to fatigue life. It is crucial to select optimal processing parameters to avoid internal porosity, source of early failure through crack initiation mechanisms. Surface condition, especially high roughness values, may also be a critical factor for crack initiation. Influence of surface condition must be studied in the industrial context, where parts may not be completely machined before implementation. This work first focused on finding optimal parameters for producing low-porosity 316 L parts by the SLM process. Once a suitable set of parameters was found, a set of samples was characterized mechanically. Fatigue samples were then produced and submitted to various surface treatments (machining, polishing, …), while some were left as-built. They were then tested in high-cycle fatigue and S-N curves produced for each sample type. This study indicates that high-cycle fatigue life of SLM 316 L is generally inferior to wrought 316 L, with as-built samples exhibiting lower fatigue limits. This was explained by surface roughness acting as crack initiation sites. Porosity was also found to be a very significant factor in fatigue life of SLM 316 L.

Influence of supercritical CO2-water on the micromechanical properties of sandstone

The effects of CO2-brine-rock interaction on the physical and macro-mechanical properties of rock have been extensively studied in CO2 sequestration-related research. However, there are few studies focus on mechanochemical effects of the interaction of supercritical CO2 (SC−CO2), water, and rock and its effects on micromechanical properties of sandstone. In this work, we studied the micromechanical mechanism of crack initiation induced by SC−CO2-water saturated sandstone. A micromechanical model including parameters of fracture cohesive strength, friction coefficient, and fracture energy was proposed, which extended the “sliding surface” to include not only the friction, but also the cohesions on the surfaces and the tensile resistance at the crack-tips. To this end, tests of two saturation conditions, water and SC−CO2-water, were conducted on 25 mm diameter by 50 mm length Sichuan sandstone with a porosity of ∼15.57 % for 15 days and 30 days under temperature of 80 ℃ and pressure of 30 MPa. Afterward, samples were subjected to triaxial compression tests with confining pressure up to 24 MPa. The mineralogical alteration and induced crack morphology were examined to better understand the mechanism of mechanochemical coupling on compression failure induced by SC−CO2-water-rock interaction. Experimentally, mineralogical and microstructural changes induced by illite and kaolinite dissolution, weaken the quartz grain contacts in SC−CO2-water saturated sandstone. Compared to water-saturated sandstone, the SC−CO2-water saturated sandstone exhibits a maximum reduction by 18.82 % and 21.21 % in compressive strength and crack initiation stress respectively under unconfined condition. Additionally, reductions of 5%, 50 %, and 37.3 % were observed in friction coefficient, fracture energy, and cohesive strength respectively for SC−CO2-water saturated sandstone. The reductions of these three parameters, especially the fracture energy and cohesive strength, significantly weaken SC−CO2-water saturated sandstone. The results are representative for the partly saturated zone where SC−CO2 is displacing the in-situ pore fluid and could be used to analyze effects of CO2 injection on stability and integrity of storage formation under mechanochemical coupling effects of SC−CO2-water on sandstone.

On the mechanism of fatigue and dwell-fatigue crack initiation in Ti-6Al-4V

In the present study, the mechanism of fatigue and dwell-fatigue crack initiation was investigated in Ti-6Al-4V with a bi-modal microstructure. While substantial dwell-fatigue life debit was evidenced, microstructural configurations associated with crack nucleation are similar and suggest a unique mechanism for fatigue and dwell-fatigue loadings. Pairs of α grains well oriented for basal slip and separated by a (0001) twist grain boundary were found to be critical configurations for crack initiation. Intense slip is localized at the boundary and precedes crack formation along this boundary. Data from a prior fatigue study was revisited and supports the occurrence of this mechanism.

Effect of constraint factor on the thermo-mechanical fatigue behavior of an Al-Si eutectic alloy

The thermo-mechanical fatigue (TMF) behaviors and corresponding damage mechanisms of Al-Si piston alloy were investigated in the temperature range of 120~425 °C under different constraint factors (η). The TMF hysteresis loops exhibit asymmetrical, because the yield strength and elastic modulus decrease significantly with the increasing temperature. Taking the phase angles into consideration, the mean tensile stress for out of phase TMF (OP-TMF, η<0) and mean compressive stress for in-phase TMF (IP-TMF, η>0) can be found. The fatigue life of IP-TMF is longer than that of OP-TMF except for higher constraint factor, and the life of TMF decreases with the increasing absolute value of η. There are two different crack initiation mechanisms under the constrain TMF: primary phases broken mainly for OP-TMF and matrix deboned mainly for IP-TMF. The oxidation may have only little influence on the crack behavior and TMF life, considering the obvious damage by deboned or broken primary phases. The tensile stress is key factor for fatigue crack initiation and growth in primary phases (Si and intermetallic) under cyclic deformation. The TMF life under different constrain factors can be predicted by the strain energy model with good precision.

Effect of texture and banded structure on the crack initiation mechanism of a friction stir welded magnesium alloy joint in very high cycle fatigue regime

Friction stir welding (FSW) produces heterogeneous microstructure in nugget zone of its joint, which should affect the fatigue responses. This work revealed the unique texture and banded structure of the Mg-FSW joint and their decisive roles on crack initiation behaviors. At higher stress levels, failures occur along the AS NZ/TMAZ boundary (AS: advancing side, NZ: nugget zone, TMAZ: thermo-mechanically affected zone) due to the inhomogeneous deformations near the boundary. Severe plastic deformations concentrate in the softer layers of banded structure. However, at lower stress levels, plastic deformations become weak and localized, but accumulate in the alternate layers of banded structure simultaneously due to the same soft texture orientation, and induce random failures in banded structure. The failure mechanisms are verified by semi-quantitative mechanical models.

Internal crack characteristics in very-high-cycle fatigue of a gradient structured titanium alloy

Gradient structure (GS) is commonly designed and processed in engineering materials to improve mechanical properties especially fatigue performance by taking advantage of the strengthened surface. However, whether the very-high-cycle fatigue (VHCF) property can be improved by GS is questioning due to the different crack initiation mechanisms between low-, high-cycle and VHCF. In this paper, GS of a Ti-6Al-4V alloy is generated by pre-torsion and characterized by electron backscatter diffraction. Then the VHCF behavior of the GS specimen is studied. The fractography and synchrotron radiation X-ray microtomography presented detailed characteristics of the internal crack initiation region in VHCF of the titanium alloy with GS. The results indicated that, in contrast to the low- and high-cycle regimes, the VHCF strength is reduced for the specimens with GS. Thus, the GS induced by pre-torsion cannot enhance the VHCF strength of the titanium alloy. This implies that VHCF test (property) is an important consideration for the microstructural designed materials. The graphical abstract is available in Supplementary information.

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.

Crack initiation mechanisms during very high cycle fatigue of Ni-based single crystal superalloys at high temperature

Crack initiation mechanisms in the very high cycle fatigue (VHCF) regime of nickel-based single-crystal (SX) superalloys have been investigated at high temperature (1000 °C) and under fully reversed conditions (R = −1). For all Ni-based SX alloys tested, a discernible area called rough zone has been identified around the crack initiation site on the fracture surface and is for the first time described. By means of electron microscopy and atom probe analyses, localized and severe plastic activity occurring in the rough zone is evidenced. The high dislocation density provided by the slip bands induces single-phase recrystallized grains and phases precipitation (intermetallics or carbides) via the redistribution of interacting solutes in the rough zone. A crack initiation mechanism based on these observations and on the stress intensity parameter determined around the rough zone – which has been demonstrated to be a constant threshold dependent on the material – is finally proposed.

Investigating the crack initiation and propagation mechanism in brittle rocks using grain-based finite-discrete element method

Fracturing process and possible factors influencing crack initiation, propagation and coalescence of granitic rocks are investigated using a grain-based finite-discrete element method (GB-FDEM). In contrast to conventional methods, the GB-FDEM used herein consists of dual-scale contact models ensuring grain breakage, and pre-processing scheme for reproducing the realistic micro heterogeneity of rocks. A standardised calibration procedure is proposed after an analysis of uncertain parameters. An optimized model is built according to calibration results of benchmark experiments including uniaxial compression test, confined compression test and Brazilian disc test in laboratory. The compression testing under different end friction, slenderness, loading rate and confining stress are systematically performed to investigate the external influences on rock fracturing. Boundary constraint is revealed in association with the macro failure pattern, in which the shearing slide leads to slight extension on ends and causes limited influence on the stress distribution in the far field. Effects on crack stresses are consistent with that of uniaxial compressive strength when the end friction takes effect. Slenderness affects the stress distribution and in turn changes the fracture pattern of rocks. Slight influence on the stress level of crack initiation and crack damage can be caused by the change of height-to-width ratio. Loading rate dramatically increase the rock strength based on two underlying mechanisms that the increase in overall number of micro cracks and the transition from intergranular fracturing to transgranular fracturing. These effects on crack initiation and crack damage stresses are inconsistent because the responses of normalized crack initiation and crack damage stresses subject to strain rate have inverse responses. The proportion of different micro cracks is shown to characterize the unchanged micro fracturing when the confining stress is increased. The enhancement of crack initiation and the change of the pattern of crack coalescence are attributed as the mechanism of confinement effect.

Fracture behaviour of central-flawed rock plate under uniaxial compression

Fault fracture zone is a common engineering geological condition in mountain tunnel excavation, which often brings great difficulties to engineering construction. Thus, uniaxial compression experiments were carried out on the granite specimens with a central hole and hole edge flaws to investigate such engineering problems. And the crack initiation mechanism was studied by theoretical analysis and Extended Finite Element Methods (XFEM) simulation. The experimental results show that the mechanical parameters and crack types of the specimens distinctly vary with the flaw inclination. The mechanical properties of specimens such as strength are greatly weakened by the central hole and hole edge flaws compared with those of intact specimens, and the weakening effect decreases gradually with the increase of flaw inclination angle. When the inclination angle of flaw is small, the proportion of tensile cracks is high, resulting in tensile failure of specimens. While as the inclination angle of flaw increases, shear cracks gradually dominate, leading to shear failure in the specimen. In addition, the results of theoretical analysis and numerical simulation show that the wing crack initiation angle that appeared in the experiment are completely consistent with the theoretical and simulation results. And it is suggested by numerical results that the tensile stress at the flaw tip of specimens with flaw inclination angle of 15° and 75° is lower than that of other specimens, which may cause the absence of wing cracks.

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.

Failure mechanism of S-shaped fissure in brittle materials under uniaxial tension: Experimental and numerical analyses

The behavior and mechanism of crack initiation and coalescence of S-shaped fissure are investigated in the study to shed more light on the failure characteristics of non-straight fissures. The uniaxial tensile tests with 3D-printed samples and numerical simulations are carried out to study the influence of different geometric parameters of S-shaped fissure, including the inclination angle β and effective curvature A/c. The crack coalescence of an S-shaped fissure is observed to occur exclusively through tensile cracks, while the tip cracking and non-tip cracking patterns of S-shaped fissures can be identified based on the crack initiation position. It is found that the inclination angle and effective curvature have a substantial influence on the failure mode of the S-shaped fissure. Based on experimental results, extensive numerical simulations are performed to predict the cracking patterns, e.g., tip cracking/non-tip cracking. A reasonably effective, quick criteria approach for identifying the coalescence patterns in terms of parameters A/c-β for S-shaped fissures under uniaxial tension is derived based on vast numerical calculations. The current study yields a favorable demonstration of the application of 3D printing technology in adept arbitrary geometry formation.

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.

Effects of Postprocess Thermal Treatments on Static and Cyclic Deformation Behavior of Additively Manufactured Austenitic Stainless Steel

As additive manufacturing advances towards use in structural applications which can also include fatigue-critical parts, the process–structure–property relationships must be fully characterized. Currently, most additive manufactured components go through extensive postprocessing including heat treatment to improve their microstructure and resulting fatigue performance. In this study, the effect of stress relief and solution annealing on the tensile and fatigue performance in force- and strain-controlled conditions was investigated. The results reveal that, while the strain–life fatigue behavior was not significantly affected by heat treatment, stress-relieved specimens showed remarkable enhanced stress–life fatigue resistance. Microstructural analysis suggested that the as-fabricated microstructure was beneficial to enhance the crack initiation resistance by shielding process-induced defects from excessive deformation. However, after solution annealing, the crack initiation mechanism shifted from nucleating at twin and high-angle grain boundaries to defects such as lack of fusion and gas-entrapped pores, negatively affecting the fatigue resistance.

Deformation processes near a crack initiation site under dwell-fatigue loading of Ti-6Al-4V

The present article reports an investigation of the mechanism of surface crack initiation of a dwell fatigue tested Ti-6Al-4V alloy with a bi-modal microstructure. Interactions between slip bands and grain boundaries were characterized in order to obtain insights into the crack initiation process and discuss the similarities with models described in the literature. Twinning and local lattice rotation occurred as a result of the slip band blocking at the interface and suggests high local stress concentrations. Nevertheless, crack initiation happened to be intergranular and not transgranular. The crack opened up the basal plane that was located at the interface between two nodules poorly oriented for slip and having a common c axis of the hexagonal unit cell.