Stable Crack Growth Stage Research Articles

Research on failure mechanism of landslide with retaining-wall-like locked segment and instability prediction by inverse velocity method

The locked segment is critical for determining the stability of locked segment-type landslides. Research indicates that the volume expansion point marks the transition from the secondary creep stage to the tertiary creep stage in a landslide’s evolution, and also separates the stable crack growth stage from the unstable crack growth stage in the locked segment. Identifying the volume expansion point is essential for early warning and predicting locked segment-type landslides. A series of instruments (resistance strain gauges, acoustic emission system, piezoelectric acceleration sensors, etc.) were used to conduct physical model tests of the landslide with retaining-wall-like locked segment under external load on the landslide’s trailing edge. The evolution process of this landslide was analyzed through changes in slope shape and stress response characteristics. The experimental results reveal the failure mechanism of the landslide with retaining-wall-like locked segment: the upper part of the landslide thrusts and slides, the middle part squeezes and uplifts, the retaining-wall-like locked segment produces a locking effect, and compression-shear fracture of the retaining-wall-like locked segment leads to landslide failure. Based on the deformation and acoustic emission characteristics of the locked segment, a method for identifying the volume expansion point was established. This point was used as the onset of acceleration point in the inverse velocity method to predict the failure time of the locked segment-type landslides, incorporating the three-stage creep model and Fukumoto’s theory.

Predicting fatigue crack growth rate of cement-based and asphalt paving materials based on indirect tensile cyclic loading tests

Fatigue crack growth rate (FCGR) is a critical indicator that reflects the rate at which material deterioration occurs under cyclic loads. However, there is currently a lack of a mechanistic quantitative analysis and prediction of the FCGR of paving materials under external indirect tensile (IDT) cyclic loads. To address this issue, this study aims to develop a mechanistic model for quantitative analysis and prediction of the FCGR in paving materials subjected to external IDT cyclic loads based on the J-integral based Paris’ law. Two types of paving materials, namely cement-treated aggregates and asphalt mixtures, were selected to demonstrate the results and model of the FCGR. The results indicate that the derived J-integral serves as the driving force for crack growth and is influenced by horizontal tensile stress, initial resilient modulus, damage density, specimen size, and surface energy of the paving materials. The fatigue life of cement-treated aggregates is primarily influenced by the crack initiation stage, while for asphalt mixtures, the crack growth stage plays a more significant role. The application of Paris’ law is found to be effective in predicting the FCGR at stable crack growth stage. The coefficients of Paris’ law exhibit minimal variation across different stress levels and loading frequencies, and can be utilized to evaluate the fatigue crack resistance of different paving materials.

Experimental and Numerical Investigation of Rock Failure Process under Hydromechanical Coupling Action

Abstract In order to study the initiation mechanism of rocks under hydromechanical coupling, hydromechanical coupling triaxial tests and acoustic emission tests were carried out on basalt in the Xiluodu hydropower station dam site area in southwestern China. The test results indicate that the basalt displays typical hard brittle behavior, and its peak strength increases as confining pressure rises. Conversely, the peak strength decreases gradually as the initial water pressure increases, which leads to decreased hardness. Meanwhile, tensile failure is the main crack initiation mode under hydromechanical coupling action. During the stable crack growth stage, tensile failure is predominant, complemented by shear failure, with failures mainly occurring in the rock middle position. Contrary to this, during the unstable stage, the rock failure is mainly due to shear failure. The critical pore water pressure failure criterion of rock crack initiation under hydromechanical coupling conditions is derived based on the test results and introduced into the numerical simulation. The hydromechanical coupling failure process and pore water pressure distribution law of basalt are analyzed, and the rationality of the critical pore water pressure failure criterion is verified. These findings are significant for understanding the rock failure process under hydromechanical coupling action and provide a valuable reference for future research.

Experimental investigation and phenomenological modeling of fatigue crack growth in X80 pipeline steel under random loading

Ocean structures are subjected to random environmental loads during their service life. Under the action of random loads, fatigue crack growth within pipeline steel exhibits transient behaviors such as retardation or acceleration. This poses significant challenges in predicting the fatigue life of ocean structures under random loadings. To establish a predictive model for fatigue crack growth in X80 pipeline steel under random loadings, this study first conducts fatigue crack growth experiments under constant amplitude and random loading conditions. Then, based on deep learning methods and the fracture yield zone theory, a phenomenological model is developed to predict fatigue crack growth in X80 pipeline steel under random loading conditions. The results show that under random loading conditions, the fatigue crack growth curve of X80 pipeline steel maintains a relatively clear logarithmic-linear feature during the stable crack growth stage. However, due to the continuous variation of stress ratio with loading sequence, the fatigue crack growth curve exhibits strong fluctuation characteristics; The phenomenological model established in this study for predicting fatigue crack growth under random loading conditions can relatively accurately reflect the overall trend of fatigue crack growth under random loadings, with a maximum prediction error of less than 10%.

Plastic and damage energy dissipation characteristics and damage evolution of Beishan granite under triaxial cyclic loading

The damage and failure of rock under triaxial stress is an irreversible process of energy dissipation, which mainly includes plastic energy dissipation and damage energy dissipation. However, it is difficult and rare to separate plastic energy dissipation and damage energy dissipation and to conduct relevant research on this basis. In this work, a loading control method with a combination of load (0.5 kN/s) and circumferential deformation (0.01 mm/min) was used, and triaxial cyclic loading and unloading tests under five different confining pressures were performed on granite from the Beishan site area of China's high-level radioactive waste repository. The results indicate that the angle between the main fracture surface and the longitudinal axis increases with the increase of confining pressure. According to the characteristics of the stress–strain curve and energy dissipation, the test process can be divided into five stages: the initial compaction and elastic stage, the stable crack growth stage, the unstable crack growth stage, the postpeak unstable fracture stage and the residual stress stage. Importantly, a separate determination method of plastic energy dissipation and damage energy dissipation was proposed, and the evolution characteristics of plastic energy dissipation and damage energy dissipation were revealed. The ratio of plastic energy dissipation to input energy decreases slightly in the initial compaction and elastic stage, stabilizes at a low level in the stable and unstable crack growth stages, increases significantly in the postpeak unstable fracture stage and remains at a high level in the residual stress stage. Meanwhile, the ratio of damage energy dissipation to input energy increases slightly before the unstable crack growth stage, increases abruptly at the end of the unstable crack growth stage and the initial part of the postpeak unstable fracture stage, then declines rapidly, and finally remains at a low level. Furthermore, the qualitative and quantitative research results of damage evolution show that the plastic shear strain and damage variable established by damage energy dissipation can qualitatively and quantitatively describe the damage evolution of Beishan granite respectively. Finally, the verification results of Beishan granite and two other rock types show that the separation method for calculating plastic energy dissipation and damage energy dissipation proposed in this work has good adaptability, and the qualitative damage development analysis based on plastic shear strain can be verified and supplemented with the quantitative damage development analysis based on energy theory.

Fatigue crack growth behavior for gas tungsten arc welded joint of TC2 titanium alloys sheets

Gas tungsten arc welding (GTAW) is a most preferred welding method for titanium alloys. Due to severe inhomogeneity of welded joints resulting from welding heat input and stress concentration, the joints are prone to fatigue crack initiation, and the fatigue crack growth rate (FCGR) and direction have great variability. Thus, it is of great significance to master the fatigue crack growth (FCG) behavior for the evaluation of remaining life. The FCGR tests were performed in different zones of the TC2 titanium alloy joints welded by the GTAW, and the acceleration behavior of the FCG was investigated. The results show that the crack growth is accelerated prematurely at stable crack growth stage (stage II). The precracks in the weld zone (WZ) and heat affected zone (HAZ) present cross-regional growth during the acceleration process. The regions before and after the critical acceleration point correspond to stage IIa and stage IIb, respectively. The FCG life at the stage IIa accounts for 90% of the total life, and the stress intensity factor range at the critical acceleration point is much lower than that at the instability point. It is proposed that the critical acceleration point is taken as the remaining life evaluation point of the TC2 titanium alloy gas tungsten arc welded joint, and the crack should be limited to the stage IIa, where the FCGR decreases according to the base metal, WZ and HAZ. In addition, the control parameters and the critical criterion of the accelerated growth were proposed, which laid a foundation for the remaining life prediction of the TC2 titanium alloy structures welded by the GTAW.

Effect of carbonation treatment on fracture behavior of low-carbon mortar with recycled sand and recycled powder

This paper presents a systematic experimental study on the fracture behavior of low-carbon mortars with various contents of carbonated recycled sand and recycled powder (CRS and CRP). Three-point bending tests on pre-notched beams were performed to explore the load-induced fracture processes. Based on the strain and displacement fields measured using the digital image correlation technique, the evolution of the fracture process zone (FPZ) was studied qualitatively and quantitatively. Results indicated that the initial cracking toughness (KIcini), unstable fracture toughness (KIcun) and fracture energy (Gf) of mortar reduced gradually with the increasing recycled sand (RS) content but kept relatively steady with the rising recycled powder (RP) dosage except for KIcini. Carbonation treatment could effectively improve the KIcini of mortar with low RP content and Gf with a high proportion of RS but had limited effect on KIcun. A combination of the FPZ evolution characteristics and crack growth resistance curves suggested that the use of RS would increase microcracking in the stable crack growth stage and thus facilitate the connectivity of microcracks in the unstable fracture stage, while the use of RP mainly affected the crack initiation stage of cement paste in the mortar. The improvement of mortar fracture by CO2 treatment of RS or RP also corresponded to these stages. Regarding fracture characteristics and carbon footprint, mortars with CRS and CRP could achieve a synergy of safety and low carbon and thus can be considered as low-carbon cementitious materials compared to normal mortar.

Experimental study on uniaxial compressive properties and damage evolution of basalt fiber reinforced concrete after being subjected to high temperature

The study investigated basalt fiber reinforced concrete (BFRC) for failure characteristics after being subjected to high temperatures under uniaxial compression. Twenty-five sets of cylindrical BFRC specimens were carried out under different basalt fiber (BF) content and temperature. Based on 3D digital image correlation (3D-DIC) and acoustic emission (AE), the effects of temperature and volume content of BF on the compressive mechanical properties, damage evolution and failure mode of BFRC are analyzed. The results indicate that the proper volume content (0.26%) of BF can improve the compressive strength of concrete. Generally, the uniaxial compressive strength of BFRC decreases with increasing temperature. The internal and external damage evolution trend of BFRC is basically the same, which can be divided into pore compaction stage, stable crack growth stage and unstable crack growth stage. The proper volume content (0.26%) of BF reduces the degree of BFRC damage. The increase in the volume content of BF leads to an increase shear cracks proportion in BFRC, and the failure mode of the main crack gradually changes from tension-shear failure to shear failure. Conversely, the increase in temperature increased the degree of BFRC damage. The internal and external failure modes of BFRC are basically the same. At 20 °C, the failure of concrete without basalt fiber is mainly tensile crack. With the increase of temperature, the main failure mode is tension-shear failure.

Effects of elevated temperature and water re-curing on the compression behavior of hybrid fiber reinforced concrete

Over the past decades, the effect of elevated temperatures on the mechanical properties of concrete has received significant attention because of the importance for fireproof design, safety evaluation and repair of fire damaged constructions. However, very few researches have been dedicated to the consequences of water re-curing of a thermally damaged concrete. In this paper, the effect of elevated temperatures and water re-curing on the mechanical response of two hybrid fiber reinforced concretes was investigated. For the first hybrid reinforced mix, long hooked-end and medium hooked-end steel fibers were adopted, as well as high strength short wave shaped steel fibers. In the second mix, polypropylene fibers substituted the short-wave shaped steel fibers. The mechanical behavior in compression was studied by measuring the stress-strain response. The elevated temperatures above 200 °C decreased: Young's modulus, compressive strength and toughness of the hybrid fiber reinforced concretes. The stable crack growth stage was reduced with the increase of the temperature above 200 °C, with an extension of the unstable crack growth stage for both kinds of hybrid fiber reinforced concrete. Water re-curing partially recovered Young's modulus, compressive strength, toughness, and extended the stable crack growth. An existing model was modified for predicting the stress-strain response of thermally damaged hybrid fiber reinforced concrete.

Loading rate dependence of staged damage behaviors of granite under uniaxial compression: Insights from acoustic emission characteristics

A series of laboratory-based uniaxial compression tests combined with acoustic emission (AE) monitoring was conducted to investigate the loading rate dependence of AE characteristics and the damage process of granite. Experimental results showed that the mechanisms of formation and evolution of cracks within the rock during loading can be characterized by AE parameters and real-time spatial AE locations. With increasing loading rate, the cumulative AE count decreased as a negative power function while the cumulative AE energy decreased almost linearly. Decreasing b-values indicate additional large-magnitude AE events and more concentrated distribution modes of fractures under high-speed loading conditions. Moreover, with increasing loading rate, the dominant failure mechanism was observed to transition from shear to tensile. A transition from tensile to shear crack modes during the damage process of each sample was noted to correspond with a decrease in b-values and the onset of crack nucleation, which may serve as a diagnostic precursor to rock failure. Crack nucleation patterns also exhibited rate dependence. Under low-speed loading conditions, nucleation was initiated at the unstable crack growth stage and was characterized by interactions between linked tensile microcrack arrays to form a shear macrorupture zone, whereas crack nucleation advanced to the stable crack growth stage under high-speed loading conditions, resulting in vertical splitting.

Mechanical degradation mechanism of rock under seismic disturbance stress

During the action of periodic seismic disturbance stress, the rock mass surrounding buried structures will show fatigue damage owing to cyclic loading. To explore the fatigue characteristics and mechanical degradation mechanism of this process, fatigue loading and uniaxial compression tests were carried out on gypsum samples. The conclusions are as follows. With increasing seismic wave amplitude, the stress loading and unloading velocity gradually increases, and the opening of the stress–strain hysteresis loop gradually increases. With increasing seismic wave frequency, the dip angle of hysteresis loops gradually inclines to the σ -axis (where σ is stress), and the variation amplitude of hysteresis loops on the ε -axis (where ε is strain) decreases gradually. This shows that the greater the amplitude of the seismic wave and the lower the frequency, the greater the damage to rock. Under seismic wave disturbance, with increasing number of disturbances, the original cracks in the sample gradually spread and infiltrate the rock, and secondary cracks gradually develop, indicating that the physical and mechanical properties of the rock gradually deteriorate. There is a strong correlation between the acoustic emission of samples and the stages of the stress–time curve for uniaxial compression. According to the acoustic emission characteristics, the stress–time curve can be divided into four stages: the initial loading stage, the stable crack growth stage, the unstable crack growth to failure stage and the post-peak stage. Compared with the sample before fatigue, the peak strain decreases significantly and the impact energy index increases significantly after fatigue loading. This indicates that some original cracks in the sample are compressed under seismic disturbance, the brittleness of the sample is enhanced and the rockburst tendency is greatly enhanced.

Anisotropic Mechanical Behavior of Stratified Sandstone Subjected to Cyclic Loading

Abstract To explore the influence of bedding structure on mechanical properties and failure mechanism of anisotropic rocks, six stratified sandstone specimens with different bedding orientations were tested by uniaxial static and cyclic loading. Acoustic emission (AE) and digital image correlation (DIC) were exploited to analyze the mechanism of crack initiation and propagation. The results show that with the increase of the bedding orientation, the peak strength of the stratified sandstone decreases first and then increases, behaving an obvious anisotropic characteristic. The AE characteristic-stress-strain curve can be divided into four stages: crack closure stage, linear elastic deformation stage, stable crack growth stage, and unstable crack growth stage. Moreover, as the bedding orientation increases from 30° to 90°, the crack closure stage gradually lengthens, while the linear elastic deformation stage first shortens and then lengthens. Under uniaxial static compression, due to the presence of bedding surface, the rock surface has a relative slip trend as a result of the uneven distribution of strain on the left and right parts, and the strain field boundary is almost consistent with the bedding orientation. Furthermore, cyclic loading can increase the rock stiffness, and the stain field boundary of 60° stratified sandstone sample in each cycle is slightly different in the cyclic loading.

Reconstruction of fan blade dynamic loading prior to its fracture

To maintain the functionality of fan blades, it is important to know the distribution of dynamic stresses in the blade, their amplitude and vibration frequency. Understanding of the dynamic loading pattern will allow us to determine under what conditions the engine was operated, to identify and prevent emergencies that could lead to the blade fracture. The purpose of this work is to understand the cause of the fan blade fracture that occurred during the engine ground start. Due to fractographic analysis of blade fragments it was revealed that the fracture occurred due to the initiation of fatigue cracks in blades. The place of crack initiation and parameters of crack growth were established, spectral analysis of the fracture was carried out. To establish the reason of fatigue crack initiation it was necessary to determine the dynamic state of the blades during their destruction. The sections of the second stage of stable crack growth, during which fatigue striations are formed, were determined using the fractographic method. Using the spacing of the fatigue striations and Paris's law, the crack stress intensity range was determined. Modeling of crack propagation in the blade was carried out to define the stress state. The ability to determine the stress intensity factor at each step of crack growth and its comparison with research data made this work possible. The simulation showed under what conditions manifold increase in stresses occurs and made it possible to obtain the expected value of vibration amplitude. Additional modal analysis showed a resonant form that caused the fatigue nature of crack propagation. The demonstrated approach established crack growth conditions and revealed the cause of blade fracture.

Experimental investigation on Mode I crack growth and damage mechanism of sandstone subjected to cyclic loading

To further investigate the Mode I crack growth and damage mechanism, compression tests were performed on notched semi-circular bending (NSCB) specimens at different cycle amplitudes. The crack propagation gauge and digital image correlation (DIC) recorded the entire process of crack growth. The real-time crack growth rate and evolution of the horizontal strain field during crack growth were obtained. Based on the crack growth rate, the crack growth process can be divided into three stages: the cracking latent period (primary stage), stable crack growth stage (secondary stage), and accelerated crack growth stage (tertiary stage). The cracking latent period accounts for a large proportion of the entire crack growth. The stable crack (i.e., steady subcritical crack growth) expanded slowly to a certain extent after initiation, and then the accelerated crack growth led to failure. With the increase in the cyclic amplitude, the cracking latent period is shortened and the crack growth rate is accelerated. The crack growth rate is positively correlated with the stress intensity factor. Moreover, scanning electron microscopy (SEM) was used to analyse the development of the fracture process zone (FPZ) and the subcritical crack damage mechanism. The SEM images showed two damage phenomena: (1) intergranular cracks caused by grain debonding (primary mechanism) and (2) transgranular cracks (secondary mechanism).

Fatigue damage numerical simulation of cement-treated base materials by discrete element method

Fatigue failure of the cement-treated base (CTB) is always paid more attention by highway constructors. Meso-scale cracking evolution plays an essential role in the research of fatigue deterioration mechanism, which is hard to be characterized using experimental techniques alone. The objective of this paper is to investigate the fatigue behaviour of CTB materials under cyclic loading through the discrete element method (DEM). A parallel bond evolution model (PBEM) was developed to simulate the time-dependent fatigue damage. Integrated with the PBEM, a fatigue damage evolution model (FDEM) was established and implemented in the Particle Flow Code (PFC5.0) to describe the fatigue decay behaviour under cyclic loading. The fatigue modelling parameters were further calibrated by the notched semicircular bending (NSCB) strength and fatigue test. Then a series of virtual NSCB fatigue tests were conducted to study the decay law of CTB materials. The simulation results indicate that the fatigue damage accumulates in a nonlinear manner under cyclic loading and the steady crack growth is the main stage of fatigue failure. With the increase of stress ratio, the stable crack growth stage is shortened obviously. The established PBEM is capable of capturing the fatigue behaviour and cracking process of CTB materials. It provides a reliable numerical technique for modelling and investigating fatigue damage of cement-treated base materials.

Assessment of Appropriate Experimental Parameters for Studying the Kaiser Effect of Rock

The Kaiser effect of rock has been extensively studied due to its wide application in in-situ stress measurement and rock damage quantification. The uniaxial cyclic loading and unloading (UCLU) test is commonly employed to examine the rock Kaiser effect. However, how the two critical parameters, including prescribed stress in the first loading cycle (σA) and loading strain rate (lsr), affect the appearance of the Kaiser effect lacks thorough understanding. We systematically performed UCLU tests on 75 sandstone specimens under 25 combinations of σA and lsr. σA spans from 0.5σc (σc is the uniaxial compressive strength) to 0.9σc, and lsr ranges from 10−5 s−1 to 10−3 s−1, respectively. The acoustic emission characteristics of all the rock specimens are continuously monitored over the entire tests. We find that the Kaiser effect is unanimously observed in the stable crack growth stage, corresponding to the stress levels of 0.5σc to 0.7σc because under a lower stress, the Kaiser effect is easily covered by the acoustic emissions generated by microcrack friction. The loading strain rate also heavily affects the occurrence of the Kaiser effect. When lsr does not exceed 10−4 s−1, the Felicity ratio (FR) rises quickly as lsr ascends, whereas FR increases less pronouncedly once lsr exceeds 10−4 s−1. Therefore, a relatively high loading strain rate, i.e., lsr higher than 10−4 s−1, is suggested to facilitate the appearance of the Kaiser effect.

The influence of the microtexture and orientation of columnar grains on the fatigue crack growth of directed energy deposited Ti-6.5Al-2Zr-Mo-V alloys

This work aims to investigate the influence of the orientation and microtexture of columnar grains on the fatigue crack growth of a Ti-6.5Al-2Zr-Mo-V titanium alloy fabricated by directed energy deposition. In this paper, the fatigue crack growth rate test in three sampling directions in a directed energy deposited Ti-6.5Al-2Zr-Mo-V titanium alloy using compact specimens was carried out. The crack length was measured visually, and the fatigue crack growth rate of the stable crack growth stage was obtained. During the test, the influence of the microstructure on the crack growth was directly observed. In addition, the complete crack front shape was indicated on the fracture surface by the marker load technique, and the crack growth behavior was obtained. An optical microscopy, a scanning electron microscopy and a laser confocal microscopy were used to observe and clarify the influence of the columnar grain boundary on the crack growth behavior and the interaction between the crack front and microstructure. The results show that the fatigue crack growth rate in the three sampling directions is different in the low ΔK region; the columnar grain boundary has no significant effect on the fatigue crack growth behavior, but the columnar grain itself has an effect on the fatigue crack growth behavior, which is indicated by the irregularity of the crack front shape in different columnar grains. Microhardness testing and electron backscattered diffraction were used to explain the above phenomena based on static and orientation characteristics. It was found that the microtexture and orientation of the columnar grains are responsible for differences in the crack growth rates, and the orientation of the columnar grains also determines the extent of the difference.

Ultrasonic Spectrum Analysis of Granite Damage Evolution Based on Dry‐Coupled Ultrasonic Monitoring Technology

The self‐developed dry‐coupled rock ultrasonic monitoring system is adopted to set up a multidirectional and multiwaveform ultrasonic monitoring network, which aims to analyse the evolution law of acoustic spectrum parameters in the process of granite loading failure under uniaxial compression, to explore the dominant acoustic spectrum characteristic information at different stages of granite loading, and to verify in situ the damage monitoring of time‐effect deformation. The results show that the wave velocity, amplitude, and amplitude‐frequency of the first wave and the velocity of P‐wave and S‐wave show a significant upward trend in the rock compaction section. After entering the elastic stage, the three spectral parameters become peacefully stable, and the stage transformation is obvious. In the stable crack growth stage, with the initiation of the crack, the dominant frequency of S‐wave shows a significant stage transition compared with the global ultrasonic wave velocity and the first arrived amplitude, and the dominant frequency decreases by 6%. In the unstable crack growth stage, the three acoustic spectrum parameters present obvious downward trend, and the first arrived wave amplitude of S‐wave is found to have a significant decline of 39.1%. On the eve of failure, the amplitude‐frequency of S‐wave shows different feature from the P‐wave; that is, S‐wave transfers from the state of multipeak in wide frequency to the state of single peak in low frequency, which is the failure precursor of the rock sample.

Dynamic Response and Energy Evolution of Sandstone Under Coupled Static–Dynamic Compression: Insights from Experimental Study into Deep Rock Engineering Applications

To deeply understand the rock failure characteristics under actual engineering condition, in which static geo-stress and dynamic disturbance usually act simultaneously, impact tests were conducted on sandstone subjected to axial static pre-stresses varying from 0 to 75 MPa by a modified split Hopkinson pressure bar. The fracturing process of specimens was recorded by a high speed camera. Dynamic parameters of sandstone, such as strain rate, dynamic strength and energy partition were acquired. Fracture mechanisms of pulverized specimens were identified by the method combining the displacement trend line and digital image correlation technique. Moreover, fragments of failed specimens were sieved to obtain the fragment size distribution. Test results revealed that, under the same incident energy, the dynamic compressive strength increases first, then decreases slowly and at last drops rapidly with the increase of pre-stress, and reaches the maximum under 24.4% of uniaxial compressive strength due to the closure of initial defects. Four final patterns were observed, namely intact, axial split, rock burst, and pulverization. The rock burst only occurs when the pre-stress lies in the elastic deformation stage or initial stable crack growth stage and the incident energy is intermediate. For pulverized specimens, the fracture mechanism is transformed into shear/tensile equivalent from tensile-dominated mixed mode as the pre-stress increases. Specimens with 75 MPa pre-stress release strain energy during failure process, contrary to specimens with lower pre-stresses absorbing energy from outside. The crushing degree of pulverized specimens exhibits a positive correlation with the pre-stress as a consequence of higher damage development in rock.

Effect of compressive load and crack closure on fatigue crack growth of commercial pure titanium at negative load ratios

In this paper, the effect of compressive load and crack closure on mode-I fatigue crack growth (FCG) behavior at negative load ratios is studied with compact-tensile-shear (CTS) specimens made from commercial pure titanium (CP-Ti). The FCG behavior at negative load ratios under constant maximum load controlled has been explained by the interaction of compressive load effect and crack closure. The variation of crack closure level at negative load ratios within stable crack growth stage is obtained by the experiment. The crack closure level keeps constant which makes the crack closure effect dominate the FCG behavior in pre-stage. While in post-stage, the crack closure level drops gradually. The crack closure effect can be neglected currently and the compressive load effect plays a dominate role in post-stage. Meanwhile, the influence of crack closure effect on mechanical behavior at crack tip is described by finite element method (FEM) to explain the anomaly in pre-stage. It is confirmed that there exists a maximum reversed plastic zone induced by the crack closure effect during unloading process in pre-stage at negative load ratios.