Formation Of Secondary Cracks Research Articles

Insights into fatigue crack propagation behaviour of WAAM produced SS316 stainless steel: Molecular dynamics simulation and experimental analysis

The current work focuses on the crack growth mechanism / behavior of wire arc additively manufactured SS316L. Beside the experimental investigation, a molecular dynamics based polycrystalline model is developed to simulate the crack propagation under cyclic loading. The purpose of this work is to understand the crack growth behavior of wire arc additively manufactured SS316L, particularly the directional dependence of crack propagation. The experiments and molecular dynamics analysis are performed for the crack propagation along the build and traverse direction for the SS316L deposit. The experimental procedure involves fatigue crack growth rate tests, while the molecular dynamics simulations model the crack propagation in polycrystalline structures of different grain sizes. The ultimate tensile strength in build and traverse direction were 535±2 MPa and 495 ± 2 MPa, respectively. The experimental results reveal faster crack propagation along build direction in comparison to traverse direction. The values for Paris law material constants i.e. C and m are 7x10-15 and 6.78 along build direction while 8x10-14 and 6.02 along traverse direction, respectively. The crack growth along the build direction is primarily intergranular, which leads to high crack growth rate. The molecular dynamics simulations reveal the initiation and closure of secondary cracks for the traverse direction, which leads to slower crack growth rate along the traverse direction. The fractography of the fractured surfaces from fatigue crack growth rate test also indicate the formation of secondary cracks for the case of traverse direction. The molecular dynamics simulations performed with three grain sizes and thickness variation indicate the insensitivity of predicted crack growth mechanisms with respect to grain sizes used for simulations.

Thermo-mechanical fatigue crack growth in a nickel-based powder metallurgy superalloy

Fatigue crack growth is studied in a nickel-based powder metallurgy (PM) superalloy (FGH4099) subjected to in-phase (IP) and out-of-phase (OP) thermo-mechanical fatigue (TMF), as well as isothermal fatigue (IF) at peak temperature. The crack growth rate and path are evaluated for both coarse grain (CG) and fine grain (FG) FGH4099, especially the effects of phase angle and polycrystalline microstructure. The results show that the TMF crack propagation is mainly transgranular in OP condition; while in IP condition, fatigue crack propagates intergranularly at low ΔK and transforms to transgranular after passing the transition region. The crack propagation resistance for FG microstructure is lower than that for CG microstructure at elevated temperature, as a result of secondary cracks and grain boundary weakening effect. Crystallographic slip controls the crack propagation process, while the twin boundaries (TB) and special grain boundaries also exert a significant influence on crack deflection. The formation of secondary cracks is closely related to local misorientation and crystallographic deformation during TMF crack propagation. The crack deflection in a single grain reveals a competition mechanism between crystallographic slip and grain boundary effect, which explains the lower crack growth rate for OP when compared to IP or IF.

In-situ X-ray computed tomography of high-temperature tensile behavior for laser powder bed fused Invar 36 alloy

The laser powder bed fusion (PBF-LB) process provides great potential for additive manufacturing of Invar 36 alloy, which possesses a unique low coefficient of thermal expansion. However, the high-temperature tensile behavior of PBF-LB processed Invar 36 alloy has not been explored, severely restricting its applications. Hence, herein, in-situ X-ray computed tomography (XCT) tensile tests were conducted at 200 °C and 600 °C for PBF-LB processed Invar 36 alloy, and the microstructure after heat treatment, fracture morphology, post-mortem microstructure, and nano-precipitates were observed. The in-situ XCT analysis of damage evolution reveals that the tensile behavior at elevated temperatures is sensitive to numerous closely spaced lack-of-fusion (LOF) pores with relatively large equivalent diameters, distributed in the adjacent melt pools or deposited layers. These LOF pores promote the stress concentration and facilitate rapid crack propagation, resulting in a significantly diminished strength and ductility. In contrast, the small number of metallurgical and keyhole pores, possessing large spacing and relatively high sphericity, have negligible influence on the tensile behavior. Therefore, Invar 36 alloy with only metallurgical and keyhole pores can be considered defect-free, displaying an excellent yield strength of 360.0 MPa and a considerable elongation of 65.0% at 200 °C. However, as the temperature increases to 600 °C, both the yield strength and elongation show a marked decrease to 150.0 MPa and 5.4%, respectively. This weakening is accompanied by the observation of a brittle fracture and the formation of secondary cracks. The degradation in mechanical properties can be attributed to the decomposition of Cr-containing SiP2O7, which leads to the formation of numerous small-sized SiO2 and P2O5 precipitates at 600 °C. These precipitates induce embrittlement of the grain boundaries and contribute to the formation of secondary cracks. The anomalous brittle fracture observed is attributed to the intergranular fracture mode, which results from the low grain boundary energy as well as the decomposition of nanoprecipitates. Additionally, the perpendicular orientation of flatter columnar grain boundaries to the loading direction plays a role in the formation of secondary cracks. This high-temperature mechanical performance of strength, elongation, failure mode, and corresponding microcosmic mechanism advances the understanding and widespread application of PBF-LB processed Invar 36 alloy.

Effect of microstructure on hydrogen embrittlement and hydrogen-induced cracking behaviour of a high-strength pipeline steel weldment

Hydrogen embrittlement (HE) and hydrogen-induced cracking (HIC) behaviour of a X65 steel pipeline weldment were investigated using slow strain rate tensile (SSRT) testing of specimens that were specifically extracted from different zones of the weldment (i.e., weld metal (WM), heat-affected zone (HAZ), and base metal (BM)). The WM was found to be the most susceptible zone to HE and HIC, while BM the least. Analysis of microstructure, fracture surface, secondary crack formation, and mechanical behaviour revealed that the high HE susceptibility of WM is correlated to microstructural features including Ti-rich inclusions, martensite/austenite (M/A) constituents, and prior austenite grain boundaries (PAGBs).

Correlation between microstructure and fatigue properties of hot-rolled thick-plate complex-phase steel

Complex-phase (CP) steels, with a multiphase microstructure, offer an excellent combination of high strength, ductility, and formability, making them an attractive alternative to conventional high-strength low-alloy (HSLA) steels in the automotive industry. However, the microstructure and fatigue property relation in CP steels is complex. This limits the full exploitation of CP steels in applications, such as heavy-vehicles, where excellent fatigue performance of thick-plates after punching holes is the critical parameter. In this work, we initiate the study of the relation between microstructure and fatigue properties of a commercial CP steel (800CP) and compare it with a conventional HSLA (500MC) steel. Fatigue property, tensile property, and fatigue crack growth rate (FCGR) testing are conducted and the performance of the two steels is rationalized using detailed microstructure characterization, before and after fatigue testing. FCGR testing shows that, despite a higher yield strength of the 800CP, both steels have a similar propagation rate due to a more tortuous crack propagation path and a higher quantity of secondary crack formation in the 800CP microstructure. The high cycle fatigue (HCF) testing shows that the fatigue limit in the 800CP is 25% higher. This increase in fatigue limit is attributed to the improved resistance to fatigue crack initiation in the 800CP due to its larger fraction of bainite.

Effect of aging treatment on the microstructure, fracture toughness and fracture behavior of the extruded Mg-7Gd-2Y–1Zn-0.5Zr alloy

A commercial scale Mg-7Gd-2Y–1Zn-0.5Zr alloy extrusion rod with synergetic high yield strength of 350 MPa and fracture toughness KIC of 22.4 MPa m1/2 was fabricated by direct-chill casting, hot extrusion and subsequent aging treatment at 200 °C for 108 h. The microstructures and mechanical properties of the as-extruded and peak-aged alloys were investigated, and the fracture toughness and fracture mechanisms before and after aging treatment were also analyzed. The results showed that the as-extruded alloy consisted of long period stacking ordered (LPSO) phase and bimodal matrix microstructure composed of fine dynamic recrystallized (DRXed) grains and coarse deformed grains. Microstructural analysis near fracture surfaces indicted that twinning was activated in the coarse deformed grains during fracture toughness test, which may be conducive to the improvement of the fracture toughness of the alloys. The lamellar-shaped LPSO phases enhanced the fracture toughness since the formation of secondary cracks parallel to the lamellar LPSO can cause significant energy dissipation and deflection of main crack. The micro-cracks were initiated and propagated in the bulk LPSO phases, which may deteriorate the fracture toughness. Large amount of nano β' phase precipitated in the T5 peak-aged alloy significantly improved the strength of the alloy. Fractography analysis indicated that fracture mechanism changed from ductile fracture of the as-extruded alloy to quasi-cleavage fracture of the peak-aged alloy. The precipitation of β' would suppress twin growth and induce the microcracks initiated at the DRXed grain boundaries and in the deformed grains, which led to slight decrease of KIC.

Formation and suppression of secondary cracks in deposits of colloidal ellipsoids

Primary and secondary cracks often coexist in a dried colloidal deposit. While, primary cracks have been the focus of investigation in most of the theoretical and experimental studies, understanding the formation and time evolution of secondary cracks is scarce. In addition, secondary cracks at times are undesirable especially when used as a template in the nano-lithography. Therefore, a control over their formation and suppression is of high importance. We consider the drying of sessile drops containing colloidal ellipsoids on a solid substrate. The resulting coffee-ring-like deposit of ellipsoids is found to accompany primary cracks in the circumferential direction and secondary cracks in the radial direction. A systematic study is performed to understand the nucleation and propagation of cracks. Our experiments explicitly showed that the primary cracks always nucleate at the top surface of the deposit and then penetrate the interior of the deposit. However, in contrast, the secondary cracks always form at the bottom surface of the deposit, which is in contact with the solid substrate. The secondary cracks are found to form only when the deposit thickness exceeds a critical value. In addition, we discuss methods to completely suppress the formation of secondary cracks by (i) tuning the aspect ratio of the ellipsoidal particles, and (ii) incorporating a small fraction of spherical particles in the drying drop of ellipsoids prior to the evaporation experiments.

Stress-strain state of reinforced concrete structures of the LN-1 and LN-2 retaining walls of Zagorskaya PSPP taking into account the opening of interblock joints and the formation of secondary cracks

Relevance. The lower retaining walls of the water intake of the Zagorskaya PSPP perform the important function of protecting the pressure water conduits from the collapse of the soil massif. Two of them (LN-2 and LN-3) were reinforced with anchor rods. Considering the long period of operation (more than 25 years), certain deviations in the work during examinations and field observations were revealed. So, on the front face of the walls, extended horizontal cracks were recorded (opening of horizontal interblock joints and the emergence of secondary oblique cracks on the front surface of the walls). To carry out computational studies of the stress-strain state of the downstream retaining walls was required. The purpose of the work was to determine the stress-strain state of the lower retaining walls of the water intake of the Zagorskaya PSPP taking into account the opening of interblock joints and the formation of secondary oblique cracks. Methods. Computational studies of the stress-strain state of retaining walls were carried out within the framework of the method of numerical modeling of reinforced concrete structures of hydraulic structures based on finite element models. In finite element models, structural features of retaining walls were reproduced, including anchor rods, horizontal interblock joints, actual reinforcement, secondary oblique cracks. Results. The stress-strain state of the retaining walls was obtained. The stresses in the longitudinal and transverse reinforcement were determined, including when the structure was changed due to anchor rods. In horizontally transverse reinforcement, tensile stresses exceeding the yield point are recorded. It took the development of measures to strengthen the lower retaining walls.

Solid-state additive manufacturing of tantalum using high-pressure cold gas-dynamic spray

Additive manufacturing (AM) of refractory metals, including tantalum (Ta), is highly valued due to the wide application of these materials in different industrial sectors where outstanding mechanical properties at elevated temperatures are required. Among metal AM processes, newly introduced cold gas-dynamic spray or, more commonly, cold spray (CS) offers a unique opportunity for solid-state consolidation of refractory metals. This research presents the CS process as a method for additive manufacturing of Ta. Following the successful manufacturing of free-standing Ta, an extensive mechanical characterization at the macro and nano levels has been carried out to evaluate the material's structural integrity. Anisotropy in the mechanical properties, which is one of the major concerns in the AM-produced materials, was extensively analyzed at both the macro and nano levels. The produced Ta's resistance against the pre-existing crack was studied by evaluating the far-field J-integral versus the crack extension (J-R curve). The influence of microstructural characteristics and process-induced defects such as pores and micro-cracks on the Ta's mechanical and fracture properties was studied to explain the performance-microstructure linkage. The macro and nanomechanical testing results indicated an elastic modulus and ultimate tensile strength in the range of vacuum-arc melted or electron-beam melted Ta ingots following cold-working. The microstructure analysis demonstrated a mixture of ultrafine grains and highly elongated coarse grains, explaining the CS-produced Ta high strength. Moreover, excellent isotropy in the mechanical properties was observed at both the macro and nano levels. This finding distinguishes the CS process from the laser-based AM process in which mechanical properties highly depend on the build direction. On the other hand, the CS-produced Ta exhibited brittle characteristics during uniaxial tensile loading and ductile behavior during the uniaxial compression test. Also, stable crack growth accompanied by crack branching was observed in the CS-produced Ta. The crack branching and formation of secondary cracks have been identified as the mechanisms retard crack extension. Overall, this research revealed that the CS process is a promising AM technique for producing tantalum-based components.

Frost-Heaving Cracking Sensitivity of Single-Flaw Rock Mass Based on a Numerical Experimental Method

A novel numerical experimental method is developed herein to study frost-heaving cracking in fractured rock masses by reproducing the physical experimental frost-heaving process. The failure mode of a preexisting closed and water-saturated single-flaw in a rock mass during the frost-heaving process is affected by the inclination angle, flaw width, crack length, and cooling rate. Additionally, a regression model for predicting secondary crack formation and propagation is established by combining multiple stepwise regressions. Overall, the results indicate that preexisting flaw with various inclination angles, flaw lengths, and cooling rates mainly propagates along the flaw-coplanar direction. The secondary crack failure mode is most affected by the flaw width, and the length of the secondary crack increases with increasing inclination angle, flaw width, and flaw length, but decreases with increasing cooling rate. The contributions of the investigated factors to the secondary crack length follow the order: flaw width > inclination angle > cooling rate > flaw length. The results presented herein provide crucial theoretical guidance for engineering and construction projects in cold regions.

Effect of environmental relative humidity on hydrogen-induced mechanical degradation in an Al–Zn–Mg–Cu alloy

The influence of relative humidity (RH) on mechanical properties of a 7075 alloy, cold finished aluminum wrought product, was studied by slow strain rate technique in atmospheres with various RHs. Hydrogen was introduced into specimens from the testing environment during deformation and then thermal desorption spectrometry was utilized to determine the content of absorbed hydrogen. By increasing the RH in slow strain rate tensile tests, the ultimate tensile strength remained unchanged, the content of hydrogen increased, and the total tensile elongation reduced, revealing an increase in the hydrogen embrittlement sensitivity (HES). In RHs below 40%, the HES was insignificant. However, by increasing the RH, a sharp increase in HES was observed due to the formation of secondary cracks. The results firstly showed that even a normal humidity (40–60%) of the testing environment at room temperature can lead to a significant HES in a wrought aluminum alloy.

Numerical simulation and experimental verification studies on a unified strength theory-based elastoplastic damage constitutive model of shale

The purpose of this study is to establish an elastic–plastic damage constitutive model of shale and simulate the elastic–plastic damage characteristics of shale under stress. Based on the unified strength theory and the mechanics of shale rock samples characterized in laboratory tests, a new elastic–plastic damage constitutive model for shale is established by introducing compression factors and damage variables. The main considerations include the compressibility of primary fractures and pores in the shale core, and the formation of secondary cracks in the rock matrix under stress. A fully implicit backward Euler regression mapping algorithm has been used to solve the model, and the numerical simulation results are in good agreement with the experimental results. The results show that the model established in this paper can accurately simulate the elastic–plastic damage characteristics of shale under stress, and that it provides a new numerical simulation method for describing the elastic–plastic damage in shale.

Nondestructive 3D-evaluation of human dentin by microtomography using synchrotron radiation

Results of Synchrotron Radiation (SR) microtomography of human dentin specimens subjected to compression tests are presented. A special attention is paid to cracks formed in the samples under compression. In all the samples, the dominant crack paths were observed to deviate and deflect around certain areas at or near to dentinal tubules. It is confirmed that secondary (satellite) cracks play an important role in dominant crack blunting. It is shown that microtomography is an effective method for investigation of the formation of satellite cracks and crack-tubule interactions over surface areas and across the whole depth of a dentin specimen. These processes can be visualized and evaluated using reconstructed 2D slices. A qualitative model for the formation of secondary cracks opening at stress concentrators that are dentinal tubules is suggested. It is shown that on the surface of the tubules, which are the closest to the tip of a mode II crack, the tensile circumferential stress can reach extremely high values that are more than enough for generation of secondary cracks.

Macro- and microscale investigations of hydrogen embrittlement in X70 pipeline steel by in-situ and ex-situ hydrogen charging tensile tests and in-situ electrochemical micro-cantilever bending test

The effect of hydrogen on the mechanical properties of X70 pipeline steel was investigated by a combination of macro- and microscale approaches. Various tensile tests under vacuum, in-situ H-plasma charging (IHPC), and ex-situ electrochemical H-charging (EEHC) conditions were conducted to elucidate the hydrogen effect in the macroscale approach. All tensile tests were performed inside an environmental scanning electron microscopy (ESEM) chamber. The IHPC, as a novel hydrogen charging technique, was compared with conventional EEHC while uncharged tensile specimens were used as a reference. The results demonstrated that the variations in the IHPC condition were negligible compared to the vacuum condition, whereas the differences in the ex-situ condition were more prominent. Furthermore, susceptibility to hydrogen in the vacuum-ex situ regime was manifested in the reduction of yield strength and ultimate tensile strength. These findings were confirmed by the fractographic analysis, where some of the effects of hydrogen (e.g. the formation of secondary cracks by detrimental inclusions (MnS and Al2O3) and the transition of fracture features from ductile dimples to cleavage patterns) were well illustrated. On the other hand, micro-cantilever bending tests were performed in the air to avoid hydrogen effects and applied inside a miniaturized electrochemical cell to promote hydrogen uptake. The bending results and post-mortem analysis of the tested cantilevers indicated that the hydrogen-reduced flow stress and hydrogen-induced cracking occurred for the H-charged bent cantilever, while only increased plastic behavior occurred for the cantilever bent in the air.

Assessment of predominant microstructural features controlling 3D short crack growth behavior via a surrogate approach in Ti-6Al-4V

Complex 3D microstructure and mechanical properties can be precisely characterized and linked by employing the X-ray micro-tomography combined with EBSD serial sectioning. Global sensitivity and principal component analysis can be used to rank and coarsen the contributing features that control the mechanical response. Afterwards, by creating surrogate models, the relationship between the most contributing features and mechanical properties can be further analyzed via support vector machines. The aforementioned novel approach was applied to study the relationship between 3D microstructural features and short crack behavior in a Ti-6Al-4V alloy. It was revealed that short crack path, growth rate, and its variation are mainly affected by the interaction with neighboring grains; however, as the short crack front becomes larger, there is a decrease and a change in the importance of those interactions. A high number of grains in contact with long cracked α grains in the loading direction may impose a constraint on the crack opening resulting in low crack growth rates. For the larger crack front, the increase in the shear stress field around the cracked grains leads to crack bifurcations and to the formation of secondary cracks, resulting in a decrease in crack driving forces with low crack growth rates. It was concluded that short crack behavior is strongly affected by the shape, size, and crystallographic features of its neighboring grains, which cause variation in the shear and tensile stress fields resulting in crack growth rate variation.

Evolution of rail surface degradation in the tunnel: The role of water on squat growth under service conditions

Squats on open tracks, as opposed to track inside tunnels, have been reported by railway networks across the world. Theoretical research hypothesised that the hydraulic entrapment of water is critical to crack propagation associated with squat growth. The requirement for a fluid to enable crack propagation provides a rational to why squats vanish as soon as the railway enters a tunnel. However, the metallurgical evidence from the ex-service rails damaged by the presence of water at wheel and rail contact interface is lacking. In the current work, a localised section of rail was found to suffer severe surface damage associated with water dropping from an air-conditioning system located in a tunnel roof. This section of rail represented an ideal controlled location for detailed metallurgical investigation to examine the formation of white etching layers (WELs) under both dry and wet conditions. The location allowed to determine the relationship between WELs and squat initiation; and also to investigate mechanics of water on crack propagation and squat growth of ex-service rail.It was found that the presence of water on the rail surface is not an important condition for WEL formation. White etching layer in dry rails was observed to be associated with a distinguished sub-layer; called brown etching layer (BEL). Brown etching layer is thought to be a development stage of white etching layer. Crack initiation on rails inside the tunnel was confirmed to be caused by ratchetting (severe plastic deformation of the surface layer), with the requirement of the rail top surface to contain WEL. Crack propagation was manly influenced by the presence of water. The metallurgical analysis revealed significant amount of oxides existed along the crack faces. The presence of oxides associated with an absence of shear deformation on the cracks faces revealed that crack growth was driven by Mode I loading (tensile opening) associated with the mechanism of hydraulic entrapment of water. Furthermore, both fracture behaviour of some cracks, and confirmation of the presence of corrosive products inside the cracks by EDS analysis, represent strong evidence that stress corrosion cracking was involved in both crack growth and the observed formation of secondary cracks.

Temperature Effect on the Fracture of Laser Welded Joints of Aviation Aluminum Alloys

This paper describes an experimental study of a temperature effect on the fracture of laser welded joints of Mg- and Cu-containing aviation aluminum alloys. The fracture of alloys and their welded joints under a uniaxial loading at temperatures of −60, 20, and 85◦C is under study. It is revealed that the strength and ultimate strain of welded joints of Cu-containing alloys decrease as temperature rises because of the formation of fixed zones of localized plastic shears. Heating and cooling suppress the Portevin–Le Chatelier effect and significantly reduce the ultimate strain of a Mg-containing alloy, even though such reduction is not observed in a welded joint. It is shown that, the maximum limiting elongation of the welded joint of a Mg-containing alloy is achieved at a negative temperature, while the formation of secondary cracks is begins during heating.

Effect of Tensile or Compressive Overload on the Fatigue Crack Growth of Friction Stir Welded 19501 Aluminum Alloy

Abstract In this paper, crack growth tests on 19501 aluminum alloy friction stir welded butt joints after a single overload cycle in tension or in compression on compact tension (CT) specimens, along with constant amplitude tests, are investigated. The effect of the tensile overload or compressive overload applied just after 20,000 load cycles on the crack growth rate, crack growth path, and growth profile are investigated. The overload cycle exhibits a significant influence on fatigue life endurance. Increases of 39.14 % and 131.58 % in fatigue life with delayed cycles of 13,874 and 46,646 are seen because of the application of 1.78 times tension or compression overload. The crack propagation after overload cycles is investigated by inspection of the fatigue crack propagation rate. Results are discussed with reference to the retardation characteristic parameters, such as retardation ratio, plastic zone size, crack deviation, etc. Compressive overload is found to be more effective in retarding the growth rates compared to tensile overload of the same magnitude in friction stir welded joints. Secondary crack formation is seen after the tensile overload, whereas no such secondary crack formation is observed with compressive overload. The possible reasons for the crack growth behavior due to tensile or compressive overload are further analyzed in terms of scanning electron micrographs and plastic zone size.

Cracks evolution and multifractal of acoustic emission energy during coal loading

Coal samples with different joints morphology were subjected to uniaxial compression experiments, cracks evolution was recorded by Nikon D5300 and acoustic emission (AE) energy signals were collected by AEwin Test for Express-8.0. During loading process, coal samples deformed elastically with no obvious cracks changes, then they expanded gradually along the trace of the original cracks, accompanied by the formation of secondary cracks, and eventually produced a large-scale fracture. It was more interesting that the failure mode of samples were all shear shape, whatever the original cracks morphology was. With cracks and damage evolution, AE energy radiated regularly. At the early loading stage, micro damage and small scale fracture events only induced a few AE events with less energy, while large scale fracture leaded to a number of AE events with more energy at the later stage. Based on the multifractal theory, the multifractal spectrum could explain AE energy signals frequency responses and the causes of AE events with load. Multifractal spectrum width (∆a), could reflect the differences between the large and small AE energy signals. And another parameter (∆f ) could reflect the relationship between the frequency of the least and greatest signals in the AE energy time series. This research is helpful for us to understand cracks evolution and AE energy signals causes.

The effect of surface oxidation behavior on the fracture toughness of Ti–5Al–5Mo–5V–1Cr–1Fe titanium alloy

In this study, the effect of surface oxidation behavior on the fracture toughness of Ti–5Al–5Mo–5V–1Cr–1Fe alloy suffered oxidation treatments at the temperatures of 500 °C, 600 °C, 700 °C and 800 °C is studied by means of OM, SEM and EDS analysis. For comparison, the fracture toughness of this alloy suffered vacuum annealing treatments at the temperatures of 550 °C, 600 °C, 650 °C and 700 °C are also investigated. The results show that at the heating temperatures below 600 °C, the fracture toughness values of Ti–5Al–5Mo–5V–1Cr–1Fe alloy suffered vacuum annealing treatments and oxidation treatments show no obvious difference. However, at the heating temperature of 700 °C, the fracture toughness of Ti–5Al–5Mo–5V–1Cr–1Fe alloy undergone oxidation treatment is almost 50 MPa m1/2 lower than that undergone vacuum annealing treatment. This is due to that at the oxidation temperature of 700 °C, the oxide layer thickness is above 5 μm. In this situation, its influence on the fracture toughness of Ti–5Al–5Mo–5V–1Cr–1Fe alloy cannot be ignored. The existence of oxide layer can decrease the total area of shear lips on the fracture surface, decrease the crack propagation tortuosity and limit the formation of secondary cracks on the side surface of KIC specimen. This can result in a big decrease of fracture resistance, which is detrimental to the fracture toughness of Ti–5Al–5Mo–5V–1Cr–1Fe alloy.