Crack Propagation Mode Research Articles

Characterization of the Crack and Recrystallization of W/Cu Monoblocks of the Upper Divertor in EAST

The microstructure of and damage to the upper divertor components in EAST were characterized by using metallography, EBSD, and SEM. Under the synergistic effect of heat load and plasma irradiation, cracking, recrystallization, and interface debonding were found in the components of the upper divertor target. The crack propagates downward from the heat loading surface along the heat flux direction, and the crack propagation mode is an intergranular fracture. The thermal loads deposited on the edge of monoblocks raise the temperature higher than the recrystallization temperature of pure tungsten, and the microstructure changes from being in a rolled state to being recrystallized. Additionally, cracks exist in both recrystallized and rolled areas. EBSD boundary maps show that the range of the recrystallization area is determined via the heat flux distribution. The Cu/CuCrZr interface of the cooling components near the thermal loading area is debonded, and the structural integrity is destroyed.

Effects of the microstructure on the fatigue fracture of friction stir lap welded Al-clad Al and Al-clad steel sheets

The microstructural characteristics and mechanical properties of friction stir lap welding (FSLW) of thin Al-clad Al (1.5 mm) and ultra-thin Al-clad mild steel (0.7 mm) sheets are experimentally investigated. Optical microscopic observation confirms that the FSLW joints are successfully fabricated without noticeable joining defects, such as cracks and hooks. The material flow is correlated with the silicon (Si) element distribution in the stir zone (SZ). The distribution of Si indicates that material flow is asymmetrical between the advancing side (AS) and the retreating side (RS) of the SZ. The tensile test result shows that all the FSLW joints fracture from the Al-clad Al side of the joints. During the fatigue tests, two different crack propagation modes (vertical crack in the Al-clad Al loading side; horizontal crack in the Al-clad mild steel loading side) are observed simultaneously due to the combined effect of the asymmetric geometry of the FSLW joint and the different loading configurations. The results of fatigue tests suggest that the asymmetric microstructure of the joint retards the propagation of a vertical crack in the Al-clad Al loading side for the RS-loaded FSLW joint. As a result, the fatigue life of the RS-loaded FSLW joint becomes clearly higher than that of the AS-loaded FSLW joint.

Maximum prime vertical strain criterion to predict rupture of core-shell microspheres

Characterization of the rupture behavior is of great significance to evaluate the service life for microcapsules. Due to the small dimension, complex core-shell structure and curved surface of microcapsules, it is challenging to quantitatively determine the rupture position and rupture strength. In the present work, a microsphere-compression model for characterizing the rupture behavior of microcapsules was established. Through the quantitative investigation of the compression rupture behaviors of microcapsules from the extended finite element models, the cracks were found to initiate at the same parallel circle (latitude) of microcapsule where the curvature change of the deformed shell (i.e., prime vertical strain) is the largest. With the increase of compression displacement, microcapsules show three crack propagation modes. Based on such findings, a maximum prime vertical strain criterion, i.e., cracks initiate when the maximum prime vertical strain exceeds a critical value, for estimating the rupture initiation of microcapsules under compression was proposed. The proposed criterion was validated by both numerical simulations and experiments. It demonstrates that the critical prime vertical strain of rupture is independent of the microcapsule dimensions and is an intrinsic parameter to judge the rupture of microcapsules. The critical prime vertical strain of rupture for n-hexadecane@PMMA microcapsules is 0.023±0.003. The proposed model avoids complicated stress analysis and provides an intrinsic criterion.

Fatigue life enhancement mechanism and lifetime prediction of AA6061-T6 open-holed sheet treated by electromagnetic driving dynamic cold expansion

In this study, mechanisms of fatigue life enhancements of electromagnetic driving dynamic cold expansion (DCE) open holed sheets were investigated by testing fatigue life and obtaining residual stress fields experimentally and numerically. Furthermore, a fatigue life prediction method based on three-dimensional stress distributions was proposed. Results showed that DCE method extended higher fatigue life and produced more uniform residual stress along hole axial direction than static cold expansion (SCE) method. The differences in stress distribution led to different fatigue crack initiation and propagation modes of DCE and SCE specimens. Moreover, the predicted fatigue life results were in accordance with experimental data. • Dynamic cold expansion extended larger fatigue life for holed-sheets. • More uniform residual stress was induced by dynamic cold expansion. • Fatigue cracks propagated horizontally in dynamic cold expanded specimens. • Proposed method to predict fatigue life had good agreement with experiments.

Research on fatigue crack propagation life prediction method for subsea pipelines

Subsea pipelines are subject to complex loading environments, and fatigue damage is one of the main failure modes. Due to manufacturing or environmental factors, crack defects are inevitable in submarine pipelines. For subsea pipelines in service, it is of practical importance to predict the residual fatigue life. On the premise of the conservative estimation method, the MTS criterion is used to analyze the crack propagation mode of subsea pipelines. In the numerical prediction method, the crack stress intensity factor is solved by the finite element method combined with the 1/4 node displacement method, and then the fatigue life of the subsea pipeline is predicted by the da/dN method based on the Paris formula. The results show that when the crack size increment Δa is set reasonably, the predicted results of crack propagation life of subsea pipeline are close to the experiment results, which verifies the accuracy of the numerical prediction method. The choice of the crack size increment has a large impact on the life prediction and needs to be considered in terms of both calculation accuracy and time consumption. The results obtained provide a reference for the safety assessment of pipelines in service.

Shear strength of steel fiber reinforced lightweight self-consolidating concrete joints under monotonic and cyclic loading

The shear behavior of precast concrete segmental bridges depends on the type of joints between the segments. However, there are few studies evaluating the shear strength of steel fiber reinforced lightweight self-consolidating concrete for application to precast concrete bridges. The purpose of this study is to compare the shear behavior of joints made of general concrete with developed concrete that includes lightweight aggregate and steel fiber under monotonic and cyclic loading. In this study, the push-off shear test was performed in accordance with experimental parameters of different concrete mixtures, key type, confining stress, and application of epoxy between the segments. The tests were carried out to assess the shear strength, shear behavior, crack propagation and failure mode of joints. The test results represented shear capacity of joints decreased 15.2% by replacing normal coarse aggregates with lightweight aggregates but could be enhanced through the incorporation of hooked-end steel fibers. The shear capacities of joints fabricated using developed mixture tended to be underestimated compared to analytically calculated strengths. Furthermore, the normalized stiffness of the joint was decreased with an increased number of load cycles under high-amplitude low-cycle load. The shear resistance through aggregate interlock may gradually decrease with increasing number of cycles resulting in failure at 85% of the failure load of the monotonically tested specimen. An appropriate level of confining stress and application of epoxy shall be considered to prevent sudden failure of joints at high amplitude cyclical loading conditions.

Mechanical and cracking behavior of porous rock models containing random circular defects under uniaxial compression

Circular defects are widely distributed in porous rock materials, and the defects greatly affect the mechanical behavior and crack evolution of rock masses. In this paper, numerical models containing random circular defects are constructed based on discrete element method. Then, the uniaxial compressions are numerically performed to reveal the influence of the porosity or size homogeneity of the defects on the mechanical behavior, crack evolution, and acoustic emission (AE) events of the models. The results suggest that a univariant increase in porosity leads to a nonlinear decrease in the peak strength and a linear decrease in the elastic modulus. The number of cracks and AE events decrease with increasing porosity. As the size homogeneity coefficient increases, the peak strengths show a slight linear rise, while the elastic modulus values show a minimal linear downward trend, and the number of cracks and AE events show wave-like increases. The cracks first appear at the location with dense defects, and the cracks initiate from the top and bottom of the circular holes. The crack propagation and intersection modes between two adjacent defects are affected by their positions. These findings provide a reference for the fracture mechanism of rock with random circular defects.

The Failure Mechanism of the 316 SS Heat Exchanger Tube in the Geothermal Water Environment.

In this work, the intrinsic reason for the premature failure of a 316 stainless steel heat exchanger tube in geothermal water environment is disclosed. The chemical composition of the tube was tested, and the microstructure was examined for material inspection. Fracture morphology and secondary cracks were analyzed, and electron backscattered diffraction was applied to explore the crack propagation mode. The corrosion morphology was observed. The electrochemical behavior was studied with cyclic polarization and double-loop electrochemical potentiokinetic reactivation. It is found that the main failure cause was stress corrosion cracking (SCC). Attacked by chloride ions, the tube is susceptible to SCC under the residual stress as a result of the substandard Mo and Ni content. The SCC mechanism is localized anodic dissolution, and the propagation mode is a mixture of transgranular SCC and intergranular SCC.

Quantitative fatigue limit prediction of mechanically long crack under mixed modes based on fracture mechanics and fatigue mechanism

To predict the fatigue limit with a mechanically long crack under mixed modes based on the fracture mechanics, fatigue tests were conducted with an inclined notch. As a result, damage accumulation (DA) mode of fatigue crack propagation was found to occur from the crack tip. The DA mode is caused by the formation and coalescence of micro-voids. Therefore, the method of modifying the initial crack length by considering the DA mode crack propagation length has shown potential application in fatigue limit prediction. Moreover, a classification method of DA and normal modes is proposed.

Residual stress distribution and fatigue performance investigations of electromagnetic force-based dynamic cold expansion open-holed sheet

In this study, residual stress distributions and fatigue performances of electromagnetic driving dynamic cold expansion (DCE) open-holed sheets were investigated. Fatigue behaviors were tested, and residual stress fields were obtained experimentally and numerically. Furthermore, a fatigue life prediction method based on three-dimensional stress distribution was proposed. Results showed that DCE method extended higher fatigue life and produced more uniform residual stress along hole axial direction than static cold expansion (SCE) method. The stress distribution differences led to different fatigue crack initiation and propagation modes of DCE and SCE specimens. Uniform stress distributions along axial direction in DCE specimens contributed to random initiations and horizontal propagations of fatigue cracks, while the fatigue cracks in SCE specimens usually initiated at the mandrel entrance layer and propagated to the mandrel exit layer. Moreover, the predicted fatigue life results agreed well with experimental data.

Evolution law of crack propagation and crack mode in coral aggregate concrete under compression: Experimental study and 3D mesoscopic analysis

Coral aggregate concrete (CAC) is one of the typical quasi-brittle materials, which is of great significance to explore the damage process of CAC under load from the perspective of fracture mechanics. Therefore, the Cohesive Zone Model (CZM) was introduced to simulate the physical mechanism of crack propagation in CAC materials at the mesoscale adopting the discrete crack method. The main work included test and simulation parts. The tests included CT scanning and mechanical tests, which were used to calibrate the model parameters of CZM and verify the rationality of the meso-mechanical model. Numerical simulation was used to reveal the internal damage and fracture mechanism of CAC. According to the defined damage variables, it was found that the crack growth rate inside the CAC material evolves regularly with the axial strain during compression failure. The crack of CAC with higher strength grade showed oblique sliding shear failure and was more likely to form fracture surfaces, and the aggregate damage was more significant. The revealing of the cracking modes in each strain stage of CAC provides a theoretical basis for guiding the corresponding crack prevention measures. For example, in the stage of significant shear failure, targeted improvement of shear strength and toughness will bring more significant gains.

Anti-impact performance of bionic tortoiseshell-like composites

Biological stiff-soft phase composite structures, such as tortoise shells and conch shells, have excellent anti-impact performance to withstand various types of high-stress events encountered in nature. A remarkable 3D interdigitating zigzag architecture in the tortoise shell, named occlusal suture structure, makes the tortoise shell simultaneously possess high mechanical strength and toughness, which is also an ideal stiff-soft phase composite prototype to mimic. In this study, four types of bionic anti-impact composite plates that feature the hard-soft phase architecture of tortoiseshell (Bio-T), mollusk shell (Bio-M), beetle exoskeleton (Bio-B) and nacre (Bio-N) and a homogeneous structure plate (HSP) are designed, to systematically examine and compare their roles in impact energy dissipation. Through an integrated approach combining additive manufacturing and drop tower testing, the peak force, residual velocity, and energy absorption of these bionic composite plates are studied and compared. Experimental results indicate that the Bio-T composite plate has the best impact resistance, compared with the other three bionic composites. Furthermore, the influence of the ply angle and the dimension of suture structure on the impact resistance of the Bio-T composite plate is identified. The [0°/30°/0°] arrangement is able to resist higher loads before failure. The 3D Bio-T composite plate provides a significant enhancement of anti-impact performance than the 2D Bio-T composite plate. Finally, the crack propagation mode in the suture structure of the Bio-T composite plates is examined, enhancing our understanding of the underlying mechanisms during impact. Such findings may prove useful for the design of future protective apparatus for soldiers and aircrafts, with improved anti-impact performance.

Dynamic multifractal characteristics of acoustic emission about composite coal-rock samples with different strength rock

In order to investigate the instability mechanism of composite coal-rock samples with different strength rock, several uniaxial compression experiments of composite samples were carried out, and dynamic multifractal characteristics of acoustics emission (AE) of composite samples were analyzed, and the influence of rock strength on the failure mechanism and energy evolution of composite samples was revealed. The results show the variation of dynamic multifractal spectrum of AE reflects the changing of crack propagation mode of composite samples during the deformation process. The increasing of rock strength changes the dominant mechanism of crack propagation in the composite sample before yield deformation. The crack propagation mode in the coal of composite sample with low strength rock changes from shear propagation to tensile-shear coexistence, and the crack in the coal of composite sample with high strength rock expands in the form of tensile-shear coexistence. In the elastic stage of composite samples, Δαm of AE energy in coal increases obviously indicating the crack extension mode is becoming complicated, while Δαm of AE energy and count in rock increases slightly and Δαm of AE count in coal and its variation range are small indicating the crack extension mode is simple. The Δαm ascending range of AE energy in coal decreases gradually and the proportion of Δƒm < 0 is more and more with the increasing of rock strength before yield deformation. In the yield deformation and post-peak stage, Δαm of AE energy of composite sample with higher strength rock is less than lower strength rock, which is resulted from the elastic energy releasing of composite sample with higher strength rock is less than lower strength rock. The research results are of great significance for understanding the deformation and failure mechanism of composite samples and the influence of rock on the failure process of composite specimens.

Effects of Ti microalloying on the microstructure and mechanical properties of acicular ferrite casting steel for high-speed railway brake discs

Owing to the continuous interest on high-speed railway trains, research on the brake disc made of ferritic cast steel with high strength and toughness, and the manufacturability of complicated cooling fins has been conducted. In acicular ferrite, Ti is a major element; however, studies on the effect of Ti composition on the mechanical and impact properties are limited to a Ti content of up to 0.05 wt%. In this study, the microstructure and mechanical properties of high-speed railway brake disc steel material with 0–0.12 wt% Ti were investigated. The specimens were subjected to the same quenching–tempering heat treatment, in which acicular ferrite formation was observed by optical microscopy, scanning electron microscopy, electron backscatter diffraction (EBSD), electron probe microanalyzer, and transmission electron microscopy. Ferrite blocks, such as acicular ferrite, granular bainite, bainitic ferrite, and polygonal ferrite, were characterized in detail and quantitatively analyzed by EBSD. For different Ti contents, ferrites of various morphologies were noted. Moreover, the types of precipitates were determined to be cementite, Cr-rich carbide, V-rich carbide, and Ti-rich carbide. The highest hardness, impact toughness, and strength were measured with the addition of 0.06 wt% Ti. The crack propagation mode in the impact specimen was observed using fracture surface analysis. In addition, the relationship between the microstructure and mechanical properties, such as strength, toughness, and hardness, was discussed based on the alloying Ti content. In this study, the effect of acicular ferrite and precipitates on toughness in ferrite casting steel was confirmed, and the optimal Ti content was confirmed.

Experimental Study on Shear Performance of Post-Tensioning Prestressed Concrete Beams with Locally Corroded Steel Strands

To study the effect of the local corrosion of prestressed steel strands on the shear failure mode and shear bearing capacity of concrete beams, unilateral steel strands in four post-tensioning prestressed concrete (PC) beams are corroded, and the shear test of four PC beams are performed. Moreover, a simplified calculation method for the shear bearing capacity of concrete beams with diagonal steel bars is proposed considering the effect of cross-sectional reduction of steel bars, the degradation of mechanical properties, and the cross-sectional damage of concrete. Results shows that the crack propagation mode and failure mode are unrelated to the corrosion of prestressed steel bars when the shear span ratio of beam is the same. The shear capacity of the beam decreases with the increase of corrosion rate, but the decreasing rate is lower than the increasing rate of the corrosion rate. The growth rate of stirrup stress is much greater than that of load after concrete tension and compression loss cracking, and the yield of stirrup can be used as a sign of the ultimate bearing capacity of the beam. In addition, by comparing the experimental and numerical simulation results, the proposed simplified calculation method for the shear bearing capacity of concrete beams is of high accuracy.

The EIFS-based fatigue life prediction approach of nickel-based single crystals with film cooling holes at elevated temperature

In this study, a new framework for the fatigue life prediction of nickel-base single crystal (SX) superalloys with different drilling film cooling holes (FCHs) at high temperatures (900 ℃ and 980 ℃) is investigated based on surface integrity quantification and fracture mechanics. For all the tested SX superalloys with anisotropy (smooth and FCHs specimens), the initial damage state is regarded as the equivalent initial flaw size (EIFS) that is independent of the specimens and hole geometry in the same drilling, and the rationality of EIFS is verified for the first time by conducting numerous fatigue tests and comprehensive surface integrity analysis. Subsequently, the fatigue crack path and microstructure of different specimens at different temperatures are analyzed to reveal the crack initiation mechanism and propagation modes, and a new equivalent stress intensity factor, ΔKeq, is proposed to describe the crack propagation driving force. The EIFS and ΔKeq are combined, and a fatigue crack growth rate (FCGR) with better fit is obtained to comprehensively reflect the different fracture modes (the mixture of Stage I and Mode I). Finally, based on the experimental observation and FCGR description, the fatigue life of the FCHs structure at room and high temperature is predicted to be 3–5 times the dispersion zone of the total fatigue life, and the ultimate defect length is proposed to guide the engineering practices.

A new method for measuring the adhesion strength of rock-concrete specimens based on the calibration interface of the modified flat-joint model

In tunnel engineering constructed by NATM, the adhesion strength between surrounding rock and lining is an important index to measure the safety of tunnel construction. However, the field measurement and laboratory test inevitably have the problem of low efficiency. In contrast, reasonable numerical simulation can be an alternative way to study rock-concrete adhesion strength. Therefore, in this paper, the influence factors of the adhesion strength of rock-concrete are studied by direct tensile laboratory test combined with particle flow code in three-dimension (PFC3D). The flat-joint model was used to reproduce the behavior of rock and concrete. The concept of effective elastic modulus of bi-material interface was introduced to derive the modified equation of interface normal stiffness. Then the modified flat-joint model was applied to PFC3D based on the Fish language and the interface parameters were calibrated. Compared with the results of laboratory tests, it is found that this method can accurately simulate the adhesion strength between rock and concrete, which can also restore the post-peak response of laboratory tests. Therefore, more parameter studies are considered which show that: the mechanical properties of rock have little influence on the adhesion strength of composite specimens. However, the physical properties of rock, such as porosity, have great influence on that, which can be seen from the crack propagation mode at the interface of composite specimens. Sandstone cemented better with concrete will retain the rate sensitivity of rock and concrete materials to the greatest extent. When the experimental strain rate increases from 0.06 s−1 to 0.9 s−1, the adhesion strength of sandstone-concrete specimens increases by 50 %, while that of granite-concrete specimens hardly changes. The research results can be helpful to better understand the adhesion performance of rock-concrete interface and have certain engineering significance for tunnel construction supported by shotcrete.

A new hybrid phase-field model for modeling mixed-mode cracking process in anisotropic plastic rock-like materials

Mixed-mode cracking is widely encountered in rock-like geomaterials under compression-controlled loadings. Some pending issues still need to be addressed. This work focuses on a new hybrid phase-field fracture method for cracking behavior of anisotropic plastic materials. Several new approaches are introduced. Firstly, a unified decomposition method of elastic strain energy driving crack growth is proposed by combining the advantages of classical spectral and volumetric–deviatoric decomposition algorithms. Then a new damage field evolution criterion is established, allowing to considering a nonlinear interaction between two basic crack propagation modes. The contribution of plastic strain energy to cracking process is investigated by considering an anisotropic yield criterion and combined isotropic-kinematic hardening laws. The validation of the established phase-field model is assessed by a series of representative examples, showing different types of cracking patterns. Sensitivity analysis of some key parameters involved in the model is also carried out to clarify the respective impacts on the progressive cracking patterns. Finally, the effectiveness of the proposed approach is validated by comparison with experimental results.

Comparative study of the effects of different nanomaterials on the failure process of saturated concrete in an underground reservoir of a coal mine

The macroscopic failure of concrete used to form an artificial dam in an underground coal mine reservoir is the final manifestation of crack propagation and penetration: the mode of crack propagation is key to the safety evaluation of concrete structures. To improve the performance of concrete in such an artificial dam forming an underground coal mine reservoir, many scholars have investigated nano-modified concrete, but the effects of nanomaterials on the crack propagation mode and failure mode of saturated concrete are not yet fully understood. In the present research, uniaxial compression and acoustic emission (AE) experiments were conducted on saturated concrete specimens containing different types and amounts of nanomaterials, and the crack propagation mode, failure mode, and bearing capacity of different saturated concrete specimens were compared and studied. The results show that: the failure mode, crack propagation mode, and bearing capacity of saturated concrete can be modified by adding nanomaterials, and the type and content thereof can affect the modification effect provided. Considering the strength, bearing capacity, and crack-propagation mode, saturated concrete specimens containing 3 wt% nano-Al2O3 exhibit the best performance, with characteristics of high strength, high bearing capacity, steady crack propagation, and small-scale cracking. The results have guiding significance for damage-warning and safety assessment of hydraulic structures.

Experimental study on crack propagation and failure mode of fissured shale under uniaxial compression

Analyzing the crack propagation evolution mechanism and monitoring the failure mode characteristics are the keys to understanding the hydraulic fracturing of shale reservoirs, especially in shale formations with significant bedding and brittleness characteristics. Apart from bedding, there are many primary fractures in shale. To better study the crack initiation and propagation law and final failure mode of shale with primary fractures of different inclination angles, a series of uniaxial compression tests of shale with double fissures of different inclination angles (0°, 30°, 45°, 60° and 90°) were conducted based on acoustic emission (AE) and digital image correlation (DIC) techniques. The results show that: 1) The peak strength and peak strain of shale with double fissures both increase with the increase of the inclination angle of fissures, while the elastic modulus generally remains stable. 2) Crack initiation stress of shale with double fissures increases with the increase of the inclination angle of fissures, while the initiation angle decreases. 3) Tensile cracks dominate overall, but more shear cracks are generated and developed at the final buckling failure, and the proportion of shear cracks at 30° and 60° is much higher than that of the other three angles. In addition, fractal dimension and connectivity analysis show that the crack network of 45° is the most complex. 4) As the inclination angle of fissures increases, the failure mode of the fissured shale specimen gradually changes from tensile failure to tensile-shear failure. Hence, this study will contribute to the development of shale gas and provide theoretical guidance for exploring fracture propagation laws under shale reservoir stimulation.