Crack Initiation Toughness Research Articles

Fracture performance of concrete-epoxy mortar interface subjected to bending and shear: Experimental and numerical study

To investigate the fracture performance of concrete-epoxy mortar interfaces, experimental and numerical simulation methods were employed to obtain the failure patterns and applied load P versus opening displacement curves of specimens. A special emphasis is placed on the discussions of crack initiation load (Pini), maximum load (Pmax), corresponding deformation (CTODini and CTODmax, CTSDini and CTSDmax), KR and GR curves of I-Type and stress intensity factors of I-II-Type cracks (KIini and KIIini) and facture energy (GF). Test results show that increasing both concrete compressive strength and interface roughness enhances the load carrying capacity and crack initiation toughness, as well as crack extension resistance. Higher concrete compressive strength improves the moment capacity of the interface and increases the proportion of KIini. An increase in the interface roughness enhances the mechanical interlocking interaction at the concrete-epoxy mortar interface, thereby decreasing the CTSD/CTOD ratio and increasing the proportion of KIIini for the I-II Type crack. As the mode ratio (KIIini/KIini) increases, the proportion of shear and bending moment acting on the interface changes continuously, with the shear effect becoming more dominant. Basically, the increase in both the crack-height ratio (a0/D) and initiated crack offset ratio (C1/C0) decreases the fracture performances of specimen. The observations provide a deeper understanding to the design and practice of concrete-epoxy mortar interface.

Uncovering the Intrinsic High Fracture Toughness of Titanium via Lowered Oxygen Impurity Content.

Titanium (Ti) and its alloys are known to exhibit room-temperature fracture toughness below 130 MPa m1/2, only about one half of the best austenitic stainless steels. It is purported that this is not the best possible fracture resistance of Ti, but a result of oxygen impurities that sensitively retard the activities of plasticity carriers in this hexagonal close-packed metal. By a reduction of oxygen content from the 0.14wt% in commercial purity Ti to 0.02wt%, the mode-Ι fracture toughness of the low-oxygen Ti is measured to be as high as KJ Ic ≈ 255 MPa m1/2, corresponding to J-integral-based crack-initiation toughness of up to JIc ≈ 537kJ m-2. This extraordinary toughness, reported here for the first time for pure Ti, places Ti among the toughest known materials. The intrinsic high fracture resistance is attributed to the profuse plastic deformation in a significantly enlarged plastic zone, rendered by the pronounced deformation twinning ahead of the crack tip along with ample twin-stimulated 〈c+a〉 dislocation activities, in the absence of impeding oxygen. Controlling the content of a property-controlling impurity thus holds the promise to be a readily applicable strategy to reach for unprecedented damage tolerance in some other structural alloys.

Insights from tensile fracture properties and full-field strain evolution of deep coral reef limestone under dynamic loads

Coral reef limestone (CRL) commonly undergoes dynamic tension when underground structures of island reefs encounter impacts, explosions, or seismic activities. Given the complexity of biological pores, the dynamic tensile fracture characteristics of CRL are poorly understood. Therefore, the dynamic tensile fracture behaviors of deep CRL were systematically observed by Split Hopkinson Pressure Bar tests and digital image techniques. Comparing to traditional rocks, the macro-pores near failure band would significantly change cracking path. The failure patterns are dominated by loading rate. The strongly dependence of dynamic tensile strength and dynamic crack initiation toughness on loading rate suggests the two indices overcome the effect of CRL macro-pores under dynamic impacts. At low loading rates, tensile fractures predominantly follow intergranular cracks, whereas transgranular cracks dominate at higher rates. The fractal dimension of fracture surface decreases with increasing crack propagation velocity, loading rate, and dynamic crack initiation toughness. Due to the unique marine sedimentary environment, the mechanical heterogeneity in multiple scales distinguishes CRL from terrestrial rock materials. The insights into underlying mechanisms of dynamic tension provide support to optimization of blasting scheme and stability assessments for island underground engineering.

A prediction model of brittle crack arrest toughness in TMCP heavy plates

Micro-alloyed low carbon MnCrNiCuMo steels were proposed to produce heavy plates with a maximum thickness of 100 mm. An integrated industrial route including a thermo-mechanical control process (TMCP) based on a study on continuous cooling transformation was applied in industrial production. Comprehensive mechanical properties tests including the measurement on resistance against crack initiation using crack tip opening displacement (CTOD) approach were conducted. Large scale wide-plate temperature gradient double tension (DT) technique was employed to evaluate the crack arrest toughness Kca in the heavy plates under different conditions. Nil-ductility transition temperature (NDTT) was also tested across the entire thickness section of the plates. The results showed that heavy steel plates were qualified being brittle crack arrest (BCA) products at EH47 grade with yield strength high than 460 MPa. High crack initiation toughness was exhibited by both Charpy-V notch impact property and CTOD values. Crack arrest toughness at −10 °C greater than 253 MPa √m was achieved in a plate even with thickness of 100 mm. The mean value of crack arrest toughness at different temperature measured by the DT approach can be indexed by the highest NDTT in the interior section of the heavy plates leading to an empirical prediction model. Further, fracture mechanics based master curve approach was used to statistically predict crack arrest toughness distribution over a wide range of temperatures showing improved predictability over the empirical model and an existing model in the literature.

Interlaminar fracture behaviour of emerging laminated-pultruded CFRP plates for wind turbine blades

Laminated pultruded composite plates are gaining interest for use in wind turbine blades due to their excellent structural performance with affordable cost. However, there is limited understanding of their fracture properties. The present work explores the interlaminar fracture behaviour of pultruded composite plates, bonded through resin infusion, to form thick CFRP structures. Mode-I, −II, and mixed-mode (I/II) tests were performed to obtain fracture properties at different mixed-mode ratios. Mode I crack propagation exhibits stick–slip behaviour, resulting in brittle failure in a few steps, while mode II provides more stable crack propagation along with cohesive failure. The mixed-mode fracture patterns follow the trend of the mode-mix ratios, in which higher mode-mix ratios (more mode II) induce more stable crack propagation. Benzeggagh-Kenane and power law criteria were compared regarding their prediction of crack initiation toughness given a mode mix ratio, and a linear relation between the mixed-mode I/II fracture toughness components could exist at interfaces of laminated pultruded plates. Meanwhile, applicability of testing standards and the effect of manufacturing-induced defects on fracture properties are thoroughly discussed. The results show that existing standards provide sufficient support for characterising fracture properties of bonded pultruded plates; and that manufacturing-induced defects can be detrimental to crack propagation and cause more brittle behaviour in mode I dominant cases, while beneficial effect of defects by toughening the interface was exhibited in mode II dominant cases.

Effects of nano-silica on fracture properties and mechanism analysis of basalt fiber reinforced concrete

The reinforcing effect of basalt fibers (BFs) on the performance of concrete mainly relies on the bridging effect of fibers, while recently, it has been shown that nano-silica (NS) may further enhance this effect. In this paper, three-point bending fracture tests were carried out on BF reinforced concrete (BFRC) to determine the optimal BF condition (i.e. volume content and length). Based on this optimal condition, the effect of NS content on the fracture performance of BFRC was examined from macroscopic and microscopic perspectives. The results indicate that the optimal BF condition for yielding the best concrete performance was 0.2 % volume content and 6 mm fiber length. Based on this condition, NS at 1 % mass content could improve its fracture performance to the best extent. Specifically, the crack initiation toughness, unstable fracture toughness and fracture energy were 1.83 MPa·m0.5, 3.35 MPa·m0.5 and 364.42 N/m, respectively, which were improved by 5.58 %, 15.93 % and 4.97 % compared to BFRC. NS mainly affects the crack propagation mode of BFRC by enhancing the fiber bridging effect or altering the way BFs play a bridging role. NS at an appropriate mass content can delay the occurrence of cracks and improve the deformation ability of BFRC.

Influence of deformation, microstructure, and temperature on the fracture resistance of technically pure tungsten

The fracture resistance of technically pure W with pre-deformed and recrystallized microstructures was tested in a temperature range of ambient to 700 °C. To account for the impact of microstructure anisotropy, samples with different specimen orientations were investigated. Room temperature tests were carried out in a scanning electron microscope and at elevated temperatures in a vacuum furnace. Both, microstructure and testing temperature significantly influence the fracture resistance of W and a strong dependence of the crack propagation resistance on the crack plane orientation was found. Pre-deformation of the testing material increases the fracture resistance in terms of the crack initiation toughness but shortens the measurable crack resistance curve (R-curve). Testing at elevated temperatures results in higher initiation toughness for both states, pre-deformed and recrystallized; however, only the recrystallized structure shows an R-curve behavior for the used sample geometry.

Prediction of crack initiation for notched concrete beam from FPZ at maximum load

The initial cracking load (Pi) of a notched concrete beam, difficult to measure, has been predicted from the maximum fracture load (Pmax) together with the notch-tip fracture process zone (FPZ) at Pmax. A simple analytical model is used to link Pi and Pmax together through the FPZ. The corresponding initiation and unstable fracture toughness, Ki and Kun, have also been determined. The model assumes that the lower limit of Pmax from a group of identical notched concrete beams corresponds to the lower limit of the FPZ length FPZL, so that the lowest limit Pmax = Pi is established for FPZL = 0. Detailed FPZ measurements before and at Pmax using a digital image correlation technique were analysed and used to predict the initial cracking load Pi. Six different specimens with various sizes (W = 40, 60 and 80 mm), initial notches (a0 = 12 to 48 mm) and FPZL (7 to 13 mm) at Pmax showed the average ratio of FPZL/(W – a0) was around 0.25, indicating the Pi/Pmax ratio was around 0.67 based on the analytical model. The crack growth resistance KR-curve between the crack initiation toughness Ki at Pi and the unstable fracture toughness Kun at Pmax was also established approximately by the simple model. Estimated intrinsic fracture toughness KIC was compared with Ki and Kun. The influence of average aggregate size dav on Pi and FPZL at Pmax was also discussed.

Impact of heavy alcohol consumption on cortical bone mechanical properties in male rhesus macaques

Chronic heavy alcohol consumption may influence the skeleton by suppressing intracortical bone remodeling which may impact the quality of bone and its mechanical properties. However, this aspect has not been thoroughly assessed in either humans or animal models whose cortical bone microstructure resembles the microstructure of human cortical bone. The current study is the first to investigate the effects of chronic heavy alcohol consumption on various mechanical properties of bone in a non-human primate model with intracortical remodeling. Male rhesus macaques (5.3 years old at the initiation of treatment) were induced to drink alcohol and then given the choice to voluntarily self-administer water or ethanol (4 % w/v) for approximately 14 months, followed by three abstinence phases (lasting 34, 41, and 39–46 days) with approximately 3 months of ethanol access in between. During the initial 14 months of open-access, monkeys in the alcohol group consumed an average of 2.9 ± 0.8 g/kg/d ethanol (mean ± SD) resulting in a blood ethanol concentration of 89 ± 47 mg/dl in longitudinal samples taken at 7 h after the daily sessions began. To understand the impact of alcohol consumption on material properties, various mechanical tests were conducted on the distal tibia diaphysis of 2–5 monkeys per test group, including dynamic mechanical analysis (DMA) testing, nano-indentation, microhardness testing, compression testing, and fracture resistance curve (R-curve) testing. Additionally, compositional analyses were performed using Fourier-transform infrared (FTIR) spectroscopy. Significant differences in microhardness, compressive stress-strain response, and composition were not observed with alcohol consumption, and only minor differences were detected in hardness and elastic modulus of the matrix and osteons from nanoindentation. Furthermore, the R-curves of both groups overlapped, with similar crack initiation toughness, despite a significant decrease in crack growth toughness (p = 0.032) with alcohol consumption. However, storage modulus (p = 0.029) and loss factor (p = 0.015) from DMA testing were significantly increased in the alcohol group compared to the control group, while loss modulus remained unchanged. These results indicate that heavy alcohol consumption may have only a minor influence on the material properties and the composition of cortical bone in young adult male rhesus macaques.

Evaluation of fracture resistance behavior of Alloy 690 material upto 1000°C

Alloy 690 is a nickel-based alloy with a nominal composition of 30% Cr and 10% Fe (wt%). It is denoted as Ni-30Cr-10Fe. This alloy is currently being investigated by various researchers for its use in nuclear industry as a possible replacement of alloy 600 steam generator tubes because of its excellent resistance to inter-granular stress corrosion cracking in water reactor environment. Alloy 690 is mainly used in vitrification melters for immobilization of high level waste in nuclear recycle plant. The temperature of water reactor environment is considerably lower compared to that of vitrification application, where temperature can reach an order of 1000 to 1100˚C. The mechanical and fracture properties of alloy 690 at the elevated temperature is important from the point of view of design and safety analysis of vitrification melter components and it is not available in literature. For this purpose, single edged notched tensile specimens have been fabricated from alloy 690 plates of 10 mm thickness. The fracture specimens have been tested in the temperature range of 25˚C to 1000˚C, which is the operating range of vitrification melters. The crack initiation toughness was obtained using modified blunting line equation in order to account for material strain hardening effect at a given temperature. From the fracture test results, it was observed that the initiation toughness is lowest at 700˚C. To understand this behavior, scanning electron microscopy was carried out on the fracture surface of the tested specimens. The fracture surface of the specimen tested at 700˚C showed presence of brittle phases in the microstructure, which are responsible for lower value of fracture toughness at this condition. The results of this work shall be useful for design and analysis of vitrification melter components.

Microstructure evolution and mechanical properties of bioinspired web-liked (TiB + TiC + Ti3Si)/TC4 composites

In this study, bioinspired web-liked multiphase (TiB + TiC+Ti3Si)/TC4 composites were successfully constructed by employing the boron-modified polysilazane polymer as a source of B/C/Si. The bioinspired architecture consisted of long TiB nanowires with ultra-high aspect ratios (up to 48.2) acting as ‘web’, and hybrid spheroidal TiC and rod-like Ti3Si particles serving as ‘nodes’. The multiphase reinforcements were grown through the unique SLS (solid-liquid-solid) mechanism based on multiple liquid phases at the core-shell TC4@BmPSZ interface. The enhanced strength of bioinspired web-liked composites was attributed to the synergistic load transfer of hybrid reinforcements. Notably, the flexural strength reached up to 1863 MPa with a high crack-initiation toughness (KIc) of 17.8 MPa·m1/2. The insights gained from this study provided a promising design approach for developing of high-performance TMCs with high reinforcement content, utilizing polymer instead of traditional ceramic powder.

The anisotropy of fracture toughness of an α+β titanium alloy by β forging

This study aims to investigate the anisotropic fracture toughness of an α+β titanium alloy processed by β forging. A typical basket-weave microstructure is observed, including the primary α (αp) phase, micron-scaled secondary α (αs) phase and residual β phase. The tensile test results show that the anisotropic strength results from the fine grain strengthening as well as the texture strengthening, and the plasticity difference is determined by the normal stress acting on the main crack in the fracture process. The orientation of the specimen also has a significant impact on fracture toughness. The corresponding toughening models of underlying damage mechanism with different orientations are proposed and discussed. It is found that the internal toughening mechanism is dominant, and external toughening mechanism also plays an important role in fracture toughness, including delamination and crack deflection caused by elongated β grains arrangement. Finally, the differences of J-integral resistance curves with different orientations are investigated in detail. The strength and fracture toughness values, KJIC (crack-initiation toughness) and KSS (growth toughness), are both highest when loading along radial direction (RD), and the positive effects of β grain morphology and two-phase texture on anisotropy of KJIC are summarized.

Study of crack extension and branching behavior of layered materials under impact loading

This study utilized a digital laser dynamic caustics experimental system to conduct three-point bending impact experiments on layered epoxy specimens with different values of angle α between the bedding and the prefabricated crack, as well as the epoxy specimen without bedding. A high speed camera was employed to record the dynamic caustic spot at the crack tip during specimen extension and branching, analyze the dynamic stress intensity factor for crack tip for Mode I and II, crack extension speed, and exploration of the influence of angle α on crack extension and branching behavior. Numerical simulations were performed using the cohesive element method. The findings indicate an increase in crack initiation toughness as the angle α increases. After the initiation of the main crack, the failure mode was Mode I-dominated damage. When it extended near the bedding, the failure mode of the main crack changed to Mode II-dominated damage due to the influence of the bedding, resulting in the formation of branching cracks and microcracks. The main crack again exhibited Mode I-dominated damage after branching. Notably, the branching cracks primarily exhibited Mode II-dominated damage. The occurrence of branching cracks and microcracks intensifies with higher α. The variation of α significantly influences the speed of crack extension.

Influence of Carbon Nanofiber Content on the Fracture Mechanical Properties of Cement-Based Materials: Insights from Three-Point Bending and Nanoindentation Tests

Incorporating intelligent materials in concrete allows for self-sensing capabilities by reflecting the concrete’s strain and tensile status through the electrical properties. In order to enhance the self-diagnostic abilities of intelligent concrete, we conducted research on how intelligent materials affect the fracture mechanics of concrete. This study examines the fracture mechanical properties of the carbon nanofiber cement mortar and paste based on three-point bending and nanoindentation tests. The results showed that the carbon nanofiber content has negligible influence on the crack initiation toughness of the specimens. However, the instability toughness exhibited an initial increase followed by a decrease as the carbon nanofiber content increased. At a fiber content of 0.2%, the crack initiation toughness improved maximally by 13.1% compared to the control group. At a fiber content of 0.5%, both the ductility index and fracture energy increased with the nanofiber content, improving by 84% and 66.3%, respectively. The incorporation of carbon nanofibers did not alter the composition of cement paste hydration products; the fracture toughness of each hydration product varies from 0.14 to 0.59 MPa·m1/2. However, the fracture toughness of individual hydration products and un-hydrated particles is higher than the macroscopic three-point bending fracture toughness values of the specimens.

Engineering and microstructural properties of carbon-fiber-reinforced fly-ash-based geopolymer composites

Alkali-activated fly ash and slag-based geopolymer composites offer sustainable cement alternatives to conventional cement materials. Incorporating fibers mitigates issues such as brittleness, insufficient toughness, and cracking in geopolymers. However, limited systematic research exists on the influence of carbon fiber characteristics on geopolymer performance. In this study, the effects of carbon fibers having varying lengths (6 mm and 12 mm) and volume fractions (0%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0%) on multiple properties of geopolymers, including workability, water absorption, porosity, mechanical strength, drying shrinkage, uniaxial tensile properties, fracture toughness, and microstructure, are investigated. Results indicate that the incorporation of carbon fibers diminishes the flowability of the geopolymer mix and accelerates its setting time. Parameters such as water absorption, porosity, and compressive and flexural strengths initially decrease and then increase with increasing fiber content. Carbon fibers positively affect drying shrinkage, particularly during the early stages. The fiber bridging effect enhances uniaxial tensile properties, reducing brittleness. Carbon fiber addition improves the fracture toughness of the composite, achieving an optimal volume fraction at 0.6%. At this volume fraction, carbon fibers of 6 mm length exhibit better crack initiation toughness and resistance to instability cracking than fibers of 12 mm length. Moreover, the presence of carbon fibers promotes the formation of hydration products, minimises the occurrence of microcracks, and interacts synergistically with the slurry matrix to enhance overall crack resistance and material toughness. This study provides a valuable reference for the development and application of fiber-reinforced geopolymer materials.

Biomimetic micro-laminated structure endows cermet with excellent strength and toughness using CoCrCuFeNi as infiltration layers

Biomimetic micro-laminated structures are gaining traction for their potential to enhance the mechanical properties of composites. A salient challenge lies in fabricating this intricate layered construct with good interfacial adhesion. In this research, we developed a micro-laminated metal/ceramic composite utilizing CoCrCuFeNi as the infiltration layers. However, after CoCrCuFeNi infiltration, the metal layer phase of cermet transforms into FCC + BCC, which belongs to the category of medium entropy alloy. The mechanical properties displayed a direct relationship with the TiC skeleton's content. Notably, cermet with a TiC volume fraction of 65% exhibited bending strength of 770 MPa, crack-initiation toughness (KIC) of 10.31 MPa•m1/2, crack-propagation toughness (KJC) of 35.1 MPa•m1/2. Mechanisms such as crack deflection and branching acted at the crack tip, significantly reducing peak stress. Contrasting prior studies marked by pronounced interface delamination, the robust adhesion between CoCrCuFeNi and TiC in this study facilitated the congruence of strength and toughness. Our findings introduce a pioneering methodology for designing laminated metal/ceramic composites with CoCrCuFeNi infiltration layers, aiming to achieve superior strength and toughness.

Nacre-inspired Ti (C, N)/Al–Cu composites with high damage tolerance and wear resistance prepared by freeze and squeeze casting

Freeze casting is an attractive method for designing lamellar ceramic-aluminum composites with a unique combination of strength and toughness. However, there are few studies on the detailed mechanical properties, especially wear resistance, of lamellar Ti(C,N)/Al–Cu composites. Herein, nacre-inspired Ti(C,N)/Al–Cu composites are fabricated via freeze and squeeze casting, and their mechanical and frictional wear properties are investigated. The results show that compressive strength, bending strength, crack-initiation toughness (KIC), and crack-growth toughness (KJC) of composites are 769.2–996.8 MPa, 547.8 MPa, 13.9 MPa m1/2, and 31.1 MPa m1/2, respectively. The achieved strength and toughness are higher than those of most nacre-inspired ceramic/Al composites. The high damage tolerance is affected by interface binding strength between Ti(C,N) and Al, as well as multiple mechanisms. Moreover, friction and wear properties of composites exhibit obvious anisotropy and the transverse section, i.e., perpendicular to freeze direction, exhibits the best wear resistance with a wear rate of only 0.98 × 10−4 mm3/Nm. This anisotropy can be attributed to the different arrangements of ceramic layers on the different sections, which affects their bearing capacity.

Mode I fracture properties and energy partitioning of sandstone under coupled static-dynamic loading: Implications for rockburst

A series of mode I fracture tests under coupled static-dynamic loading were performed on notched semi-circular bend (NSCB) sandstone specimens using the split Hopkinson pressure bar (SHPB) to investigate the dynamic fracture properties and energy partitioning, which can help further elucidate the rockburst mechanism and its kinetic energy source. The fracture processes were captured by the high-speed camera and the digital image correlation (DIC) technique was used. Effects of dynamic loading rates and static pre-load were then analyzed and highlighted. As the loading rate increases, the crack initiation toughness, crack propagation velocity, crack propagation toughness and energy dissipation (including fracture energy and kinetic energy) all increase, while the proportion of kinetic energy decreases. Low pre-load increases the above parameters compared to no pre-load, while high pre-load does the opposite. All the above behaviors are then explained from a perspective of micro-fracture. The mismatch between energy release and energy requirement for fracture is the cause of the kinetic energy of rockburst, and it can be characterized by the established realistic energy release rate (RERR) index.

Experimental study on the influence of explosive stress wave on the dynamic crack propagation characteristics under biaxial confining pressure

The effect of biaxial confining pressure on dynamic crack and the interaction between explosive stress wave and dynamic crack are studied based on dynamic photoelastic method in this experiment. The experiment consists of five groups, primarily focusing on the angle at which the stress wave encounters the crack. The experimental results reveal two main fractures: mode I fracture and mixed-mode fracture. The influence of biaxial confining pressure on stress waves and dynamic cracks differs between these two fracture modes. Additionally, the Newton-Raphson iteration method is employed to analyze and calculate the fringe pattern of the crack tip at various stages. Subsequently, the calculation results are used to reconstruct the photoelastic fringe. To validate the accuracy of the calculation method, the reconstructed photoelastic fringe pattern is compared with the experimental results. The results show that the presence of confining biaxial pressure enhances the crack initiation toughness, propagation toughness and propagation speed. It also improves the crack's resistance to the impact of the incident stress wave from all directions. When the stress wave interacts with the crack, the photoelastic fringe patterns at the crack tip can be categorized into two main types. One is primarily influenced by the stress wave, while the other is primarily influenced by the stress field at the crack tip. To extract accurate crack tip information, it is essential to analyze the fringe pattern dominated by the stress field at the crack tip.

Dynamic fracture mechanism and behaviour of V-notch white marble specimens with strain gauge and digital image correlation in three-point bending tests

A three-point bending test was conducted on white marble specimens with V-shaped notches. The fracture mechanism of these specimens, including crack initiation toughness and propagation speed under different opening angles, was studied, and their special fracture behaviour was analysed. In this experiment, a falling weight loading device was used to impact the V-notch white marble specimens with different opening angles at the same impact speed. To measure the crack tip initiation toughness, a strain gauge theory is formulated in this study. The calculation results are in good agreement with the digital image correlation (DIC) calculation results. The error is approximately 10%, and the results obtained by the acute angle patching method are stable. The experimental results also show that when the opening angle is large, the specimens crack, and the energy dissipated by the crack initiation is considerable. Owing to the particularity of the white marble structure, the rock fracture behaviour during the dynamic fracture process is clearly observed in the experiment. All the specimens exhibited spreading crack branches at the crack tip under stress. The foregoing occurs because the original defects at the crack tip deform and grow after sustaining stress. Some of the cracks combine to form the main crack and produce branches. In this study, the microcrack areas on the crack tip and fracture path observed in the experiment are collectively referred to as the crack growth area. The expansion of the crack growth area precedes the expansion of the main crack. The expansion of the strain concentration area is calculated by DIC; it reflects the dynamic formation of cracks on the surface of the specimen. Scanning electron microscopy shows that microcracks mainly occur under the intergranular failure mode.