Concrete-rock Interface Research Articles

Research on compression failure criteria and characteristics of rock-concrete assemblies with rough interfaces

The combination of rock and concrete lining structures is a typical composite structure in the field of engineering. This study is based on the concept of equivalent strain energy and establishes a mechanical equivalent model for rock-concrete assemblies (RCA). Assuming that both rock and concrete satisfy the Mohr-Coulomb criterion, we derive the shear failure criterion of the equivalent model considering the roughness of the rock-concrete interface. The applicability of the model was verified through uniaxial and triaxial tests on eight different types of RCA structures. The research results indicate that an increase in confining pressure enhances the strength of the RCA. When the confining pressure reaches a certain value, concrete only experiences shear failure, and no macroscopic cracks appear in the rock. The structure of the RCA tends towards isotropy. As the height ratio of the RCA increases, its strength decreases. At minimal concrete height ratios, the strength of the RCA gradually approaches that of concrete. This study can provide valuable insights for designing and evaluating stability in engineering rock bodies within diverse geological environments.

Seismic Vulnerability Assessment of Non-Overflow Concrete Gravity Dam Section

Abstract As the economy advances, there is a growing need for energy. Harnessing hydroelectric energy has emerged as a crucial strategy for addressing the energy crisis. To promote hydropower development, numerous tall concrete dams are either under construction or in the planning stages. These dams are frequently situated in areas with elevated water levels and face the challenge of being located in regions with high seismic intensity. An accident resulting from the failure of concrete dams can pose significant threats to both economic development and public safety. Therefore, it is of crucial importance to investigate the stability of such dams, particularly under conditions of high water levels and seismic activity. The finite element program Geostudio is being undertaken to evaluate a complete numerical analysis of the dam. In this paper, Longtan Dam has been selected as the case study for the research. This study is applied in two cases: seismic and static conditions. A two-dimensional seismic numerical analysis was carried out using the Koyna earthquake acceleration time records in 1976. The results of the FEM model concluded that the dam is unsafe under sesmic conditions. The concrete-rock interface, particularly susceptible to sliding, was identified as the most critical part of the dam, presenting the most likely failure mode. The study highlights the significance of its methodology in investigating earthquake effects on dams, prioritizing it over the achieved results. The aim is for the conclusions and recommendations to provide valuable guidance for future studies and initiatives focused on enhancing safety at hydropower dams.

Experimental and numerical investigations on rate-dependent fracture behavior of concrete-rock interface

To ensure the safety and stability of the concrete gravity dams built in the seismic regions, this study investigated the rate-dependent fracture behavior of the concrete-rock interfaces with different roughness degrees. Firstly, direct tensile tests and three-point bending tests were conducted to measure the mechanical and fracture properties of the concrete-rock interface with three roughness degrees, i.e. the 4 × 4 interface, natural interface, and 7 × 7 interface, and under four strain rates, i.e. 10-5/s, 10-4/s, 10-3/s, and 10-2/s. By employing the fictitious crack model and initial fracture toughness-based crack propagation criterion, numerical simulations were conducted to analyze the crack propagation processes of the concrete-rock interfaces under different strain rates. The results indicated that the tensile strength and initial fracture toughness exhibited an obvious increasing tendency with the increase of the strain rates, and they showed a nearly linear relationship with the logarithms of the strain rates. Prediction models were proposed to calculate the tensile strength and initial fracture toughness of the concrete-rock interface under different strain rates. In addition, the initial fracture toughness-based crack propagation criterion was proved to be applicable to analyze the crack propagation processes of the concrete-rock interfaces under both quasi-static and seismic strain rates. It was found that the fully-formed fracture process zone lengths and the corresponding crack length decreased obviously with the increase of the strain rates, indicating that the boundary effect of the concrete became stronger under the higher strain rates, which approached to the fracture behaviors of the large-size specimens.

Crack propagation process and lifetime prediction method of concrete-rock interface under constant loading

This study investigated the crack propagation processes and established the lifetime prediction methods of the concrete-rock interfaces under constant loading. Firstly, the constant loading tests were performed under three-point bending loading on the composite concrete-rock beams with three kinds of interfaces, i.e. natural, 4 × 4, and 7 × 7 interfaces, and under three constant load levels, i.e. initial cracking load, 80 % and 97 % of the peak loads. Then, the clip gauge method and digital image correlation method were employed to detect the crack lengths under constant loading. Finally, the effects of constant load levels on fracture properties of the concrete-rock interfaces were analyzed and the crack propagation processes under constant loading were discussed. The results indicated that the clip gauge method was an available and accurate method to detect the crack lengths of the concrete-rock interfaces under constant loading. Crack propagation processes and crack mouth opening processes under constant loading exhibited the three-stage feature, i.e. deceleration stage, uniform stage and acceleration stage. In addition, the logarithms of the constant loading lifetimes showed an approximate linear relationship with the logarithms of the crack mouth uniform opening rates, and a quantitative relationship was established by using the linear regression analysis to establish the lifetime prediction methods. This study proposed a practical method to predict the service life of concrete structure-bedrock interface. When the crack mouth uniform opening stage was recognized, the lifetime of concrete structure-bedrock interface in service can be predicted by substituting the crack mouth uniform opening rate into the pre-established prediction model. This study can provide the theoretical support for the safety assessment of the concrete structure-bedrock interface in service.

Investigation on the fatigue crack propagation of rock-concrete interface under fatigue loading below the initial cracking load

The fatigue crack propagation of the composite rock-concrete interface under constant-amplitude flexural fatigue loading below the initial cracking load was investigated in the present study. Fatigue pre-crack initiation and propagation tests were first conducted on three types of rock-concrete interfaces under three-point bend loading. It was shown that for the rock-concrete interface, the damage evolution throughout the fatigue loading process can be divided into deceleration and acceleration stages according to the rate curves of flexural stiffness, deformation and crack propagation. The crack propagation rate exhibited two distinct regions when plotted on a log scale, namely the small-crack growth region and the long-crack growth region. The small-crack growth region for the studied rock-concrete interfaces was not particularly pronounced. The long-crack growth region was the Paris’ law region and included both the acceleration and part of the deceleration stages. Based on the experimental results, the fatigue crack propagation rate in the Paris’ law region was then described using the empirical Paris’ law. The exponent m1 in the Paris’ law was found to be approximately constant, with a value of m1 ≈ 21 for the rock-concrete interface. The pre-factor C1 was found to be dependent on both the load ratio R and the type of the rock-concrete interface (represented by an elastic mismatch parameter α). By introducing R and α into the empirical Paris’ law, an adjusted Paris’ law was proposed. The adjusted Paris’ law can describe the fatigue crack propagation rate in a unified way for the same rock-concrete interface of different R and different interfaces in the scope of this research. Based on the adjusted Paris’ law, the fatigue cycles during the crack propagation and fatigue lifetimes of all tested rock-concrete interface specimens were reasonably predicted. For practical applications, an approach was suggested involving a combination of the adjusted Paris’ law and a material coefficient β (approximately β ≈ 0.143 in this study) to estimate the fatigue lifetimes of structures or specimens with existing pre-cracks at the rock-concrete interface.

Failure mechanism analysis and tensile constitutive model of basalt fiber concrete-rock interface based on fiber surface modification

This study investigates the tensile failure mechanism at the concrete-rock interface and proposes a concrete-rock interface tensile mechanical constitutive model considering the degree of rock surface indentation and surface morphology. The Brazilian disc splitting test results demonstrate that the depth and width of rock surface indentations collectively characterize the indentation angle θ, thereby influencing the tensile strength of the interface. Specifically, when 0° ≤ θ < 45°, the concrete-rock interface exhibits a tension-shear stress state, where cracks propagate along the interface leading to the debonding of the rock and concrete. When 45° ≤ θ ≤ 90°, the concrete-rock interface adopts a compression-shear state, with cracks gradually transitioning from the interface to the concrete interior. The latter scenario enables the higher tensile performance of the concrete-rock interface compared to interface fracture. Excessive indentation depth compromises the tensile strength of the interface by inducing bending moments. A mechanical model for the tensile behaviour of the concrete-rock interface is proposed, which effectively predicts the tensile strength of the interface and its evolutionary process.

Investigation of stress-wave-induced crack penetration behavior at the rock-Ultra-High-Performance Concrete (UHPC) interface

The propagation behaviour of initial cracks in concrete penetrating the rock-concrete interface is still unclear. This study investigated the behaviour of stress wave-induced crack penetration through the Ultra-High-Performance Concrete (UHPC)-granite interface and crack propagation speed (CPS) using a combination of the crack propagation gauge (CPG) system and a split Hopkinson pressure bar. The influence of loading rate and UHPC strength grade on the dynamic fracture toughness of the penetration crack entire process was analyzed by experimental–numerical method. The results indicate that the Rock-UHPC interface can suppress the crack opening displacement in UHPC, thereby limiting the CPS. The loading rate significantly enhances the dynamic fracture toughness of the penetration crack's entire process. The interface is the boundary point, and the dynamic fracture toughness at the crack tip shows a trend of first increasing and then decreasing. The increase in concrete strength grade can improve the interface dynamic fracture toughness. Compared to the interface roughness, the enhancement effect of concrete grade on the interface dynamic fracture toughness is one order of magnitude higher. For concrete engineering on rock foundations, increasing the concrete strength grade can effectively prevent the inherent crack defects inside the concrete from expanding into the interior of the rock foundation.

Study on Coordinated Deformation Failure Mechanism and Strength Prediction Model of Rock-lining Concrete

Shotcrete has been widely used in the excavation of caverns and tunnels as a key load-bearing element in the support system. However, the interfacial stresses and bond strengths within the composite structures formed by the interaction of geologic bodies and tectonic formations are not well understood, which can lead to safety accidents and excessive wastage of support materials in engineering. To address this gap, rock-concrete composite specimens were created using six distinct types of surrounding rocks. The strength law of composite specimens was analyzed, and the uniaxial compressive strength prediction model of composite specimens was derived based on Mohr−Coulomb strength yield criterion. The results show that the uniaxial compressive strength of the composite specimen changes with the change of rock type, which is between the strength of rock and concrete. The uniaxial peak strength and elastic modulus of composite specimens are linearly positively correlated with the ratio (Y) of concrete-rock elastic modulus, which is easily affected by concrete properties. However, the peak strain is linearly and negatively correlated with Y, which is greatly influenced by rock. The rock-concrete interface of the composite specimen plays an important role in the failure of the specimen, which changes the stress transfer state of the composite specimen, produces uncoordinated deformation, and finally causes the failure of the specimen. The theoretical and test value error of the derived composite specimen uniaxial compressive strength prediction model are –1.19%~12.20%, and the theoretical calculation model can be used to predict the uniaxial compressive strength of rock-concrete composite specimens.

Effects of Different Loading Angles on Brazilian Splitting Characteristics of Rock-Concrete Interface

Effects of Different Loading Angles on Brazilian Splitting Characteristics of Rock-Concrete Interface

Prediction on residual flexural fatigue life of rock-concrete interface based on a deformation-controlled fatigue crack propagation criterion

To assess the cracking resistance of rock-concrete interface, three-point bending tests were carried on composite rock-concrete specimens under the monotonic and high flexural fatigue loading. The crack opening displacements along the ligament of the beam were continuously recorded in the whole test process. According to the measured crack mouth opening displacements (CMODs), the correlation between the monotonic and fatigue deformations at different cracking statuses was discussed. It has been shown that the deformation development of composite specimens under flexural fatigue loading was mainly caused by the propagation of interfacial crack. The P-CMOD curve under monotonic loading was the envelope curve of those under the different maximum fatigue loads. There were three CMOD control points in the monotonic deformation response to indicate the fatigue crack propagation states, which corresponded to the initial cracking load, the ultimate load and the applied maximum fatigue load in the post-peak part of the monotonic response, respectively. Correspondingly, they manifested the fatigue crack initiation, the inflection point of the fatigue crack propagation rate from deceleration to acceleration and the fatigue failure of the rock-concrete interface. Based on the three CMOD control points, a deformation-controlled fatigue crack propagation criterion was proposed to assess the cracking state of the rock-concrete interface under fatigue loading. Accordingly, a theoretical prediction model, termed as fatigue deformation evolution law, was established to predict its residual flexural fatigue life. This method was verified by the reasonable agreements between experimental and predicted results in the residual fatigue life.

Determination of SIFs and application of a SIF-based fracture criterion for concrete-rock interface under sustained loading

This study proposes an effective method to determine the stress intensity factors (SIFs) of the interface crack considering the viscoelasticities of the bilateral materials and verifies the feasibility of the initial fracture toughness-based fracture criterion for the concrete-rock interface under sustained loading. Firstly, low-level sustained loading tests were conducted on the composite concrete-rock specimens with respect to two interface roughness degrees and two sustained load levels less than the initial cracking load. Then, an implicit creep equation was employed to characterise the viscoelasticities of bilateral materials and then calculate the mechanical responses of the concrete-rock interface in the low-level sustained loading test. The SIFs of the interface crack under sustained loading were determined by using the elastic strain energy density ahead of the crack tip. By averaging the SIFs over the radius of the core region of the crack tip, the representative SIFs under sustained loading were obtained. The test results indicated that the SIFs showed decreasing tendencies under sustained loading, and the initial fracture toughness can be considered as a material property of the concrete-rock interface. The initial fracture toughness-based fracture criterion was verified to be feasible to determine the initial cracking status of the concrete-rock interface under sustained loading. In addition, due to the reductions of the SIFs under sustained loading, additional loads were required to compensate the losses of the SIFs, leading to the enhancement of the initial cracking loads after experiencing the sustained loading.

Numerical simulation of the effect of interface morphology on the fracture behavior of bi-material Brazilian disks

As the most fragile component in a bi-material structures, the performance of rock-concrete interface will greatly affect the stability and durability of the whole structure. To investigate the influence of interface morphology on the interface bearing capacity and fracture characteristics of bi-material Brazilian disks, the sandstone-concrete bi-material Brazilian disk is taken as the research object, a series of sandstone-concrete bi-material Brazilian disks with interfaces of different groove widths, depths and spacings are simulated based on the cohesive zone model (CZM) method in this paper, and the effects of loading angle and location of groove interfaces on the fracture behavior are also investigated, the loading rate of all models is 0.1 mm/min. The loading angle is the main factor affecting the fracture patterns of the bi-material Brazilian disks. The loading angle increases from 0° to 90°, and the failure patterns can be divided into sawtooth fracture along the groove, mixed mode of sawtooth fracture along the groove and tensile fracture of the matrix, and tensile fracture of the matrix. The interface is the main bearing part at small loading angles, and when the loading angle exceeds 30°, the matrix is the main bearing part, and at this time, the morphology of the interface has little influence on the failure patterns and bearing performances of the specimens. The groove interface significantly improves the bearing capacity of the interface, in which the groove width has the greatest influence on the bearing capacity of the interface, and the groove spacing affects the location of crack propagation along the sawtooth (rock sawtooth or concrete sawtooth). The location of the groove interface affects the crack propagation mode at the interface, and this study mainly presents three cracking modes: failure along the rock sawtooth, failure along the concrete sawtooth and the combined failure of the rock sawtooth and the concrete sawtooth.

Mechanics, damage and energy degradation of rock-concrete interfaces exposed to high temperature during cyclic shear

Shear properties of the rock-concrete interface are critical to the stability of deep underground engineering projects, particularly in high-temperature environments caused by geothermal activity, fire or nuclear waste radiation. A series of cyclic shear tests were performed on the pre-heated sandstone-concrete interface under both CNL and CNS conditions. The study carefully examined the degradation characteristics of shear mechanics, as well as the evolution of interfacial damage and energy, during cyclic shear of rock-concrete interface. The experimental results show that the pre-heating temperature and the number of shear cycles have considerable effects on the adhesion, shear strength, normal deformation, shear stiffness and friction coefficient of the sandstone-concrete interface, especially under CNS conditions. Furthermore, the nonlinear evolution of interfacial damage and energy release during cyclic shear is highly dependent on the pre-heating temperature. These test results provide valuable insights for revealing the mechanical degradation and evaluating the shear performance of rock-concrete interfaces in deep underground engineering projects subjected to high-temperature environments.

Intelligent monitoring of concrete-rock interface debonding via ultrasonic measurement integrated with convolutional neural network

This paper presents an intelligent methodology for monitoring and assessing the interfacial debonding damage process of concrete-rock composites through the piezoelectric-based ultrasonic testing method coordinated with convolutional neural network (CNN). The main novelties of this research lie in the following aspects. Firstly, the ultrasonic sensing method is successfully extended to monitoring the debonding of concrete-rock composites with different interfacial roughness for the first time, and changes in recorded time-domain signals, continuous wavelet packet (CWT) time–frequency spectra and wavelet packet energy are thoroughly analyzed. The results confirm the good consistency of wavelet packet energy with the interfacial debonding damage. The key innovation of this study is that a CNN model is constructed and well trained to identify different debonding phases from image-based datasets of CWT spectra. The results well demonstrate the effectiveness and high accuracy of the CNN model. Moreover, comparative analysis with three machine learning-based models is also performed, which demonstrates the superiority of the CNN model. The proposed methodology could facilitate intelligent monitoring and automatic classification of concrete-rock interfacial debonding without manual feature extraction.

Thermal effects on tensile behavior and failure characteristics of granite-concrete bi-materials with different loading directions

The mechanical performance of the rock-concrete interface is greatly affected by high temperature and stress states in many geotechnical projects. It is significant to study the failure mode of the bi-materials subjected to different loading directions and temperature conditions to improve the bearing capacity of the rock or concrete engineering projects. The natural rough surface is obtained through a splitting test, and a surface close to the true roughness of the tunnel is selected from it. The 3D engraving technique is used to ensure that each specimen has the same rough surface. Then, the granite-concrete bi-material discs with a natural rough surface are heated to 100℃, 200℃, 300℃ and 400℃, respectively. A series of Brazilian splitting tests were carried out with an inclination angle between 0° and 90° to study the tensile properties and failure mechanism of the specimens. The results showed that high temperature would significantly reduce the nominal tensile strength (NTS) of the granite-concrete bi-material. The high temperature had the most apparent reduction in peak load at θ = 0°, and the reduction range was between 25% and 85%. The improvement was between 15% and 60% at θ = 90°. The NTS increased rapidly at first and then slowly with the boundary of θ = 45°. The digital image correlation (DIC) technology was also used to analyze the failure propagation process at the surface. Finally, the stress state on the interface and acoustic emission (AE) signals are analyzed to study the crack types in the failure process. The proportion of shear cracks increases first and then decreases with the increase of inclination angles, with the lowest proportion of 20% at 0° and the highest proportion of 38% at 15°. With the increase in temperature, the proportion of shear cracks increases.

Modeling description of interface shear deformation: A theoretical study on damage statistical distributions

The shear constitutive model is important for the analysis of the interface shear deformation mechanism. In this study, five statistical distributions, including four that have never been applied in interface shear deformation, are introduced to describe the damage evolution of the interface. The corresponding statistical damage constitutive models are developed, and they have been validated using a series of experimental data (both laboratory and in-situ tests) at the rock-concrete interface. The comparison results with laboratory data show that the Mitscherlich model has the lowest prediction accuracy. By comparing with in-situ data, the Weibull model shows the best-predicted performance. Based on the Akaike information criterion metric, the Morgan-Mercer-Flodin (MMF) model performs much better than the other models in the laboratory tests, which indicates the MMF model can be introduced for a comprehensive analysis of interface shear deformation. In addition, the similarities between the MMF model and the Weibull model are found through the canonical transformation of the MMF model. And the analysis of the two unknown parameters of the MMF model shows that they are related to the yield characteristics and strength characteristics of the interface, respectively. Based on the damage variable evolution, the shear deformation of the interface can be divided into three phases, i.e., damage initiation phase, damage acceleration phase and damage slowing phase. And the parameters of the MMF model have an important influence on the damage variable curves.

Crack extension at the concrete–rock interface damaged by high-pressure water environment: DIC research

As a weak link in hydraulic structures such as arch dams, the fracture property of the concrete–rock interface is impacted by various factors, including the material properties of the individual constituents and the fracture process zone (FPZ) at the crack tip. Because the heel area of an extra-high dam has been exposed to a high-pressure water environment, this paper aims to investigate the whole fracture process of the concrete–rock interface after the water pressure effect. In this test, the water pressure was set at 0, 1, 2, and 4 MPa, and the crack deformation of the concrete–rock composite beam under three-point bending (TPB) was monitored in real-time by the digital image correlation (DIC) technique. DIC has been discovered to aid in understanding the damage caused by a high-pressure water environment to material properties. The stable crack extension of TPB composite beams after the high-pressure water exposure was greatly limited, and the corresponding crack length was decreased by up to 35%. The FPZ characteristic length was reduced by 19% when exposed to a high-pressure water environment, resulting in a drop in cohesive toughness on the fictitious crack. KR resistance curves for the concrete–rock interface fracture were constructed to consider the damage to material properties and FPZ evolution by high-pressure water exposure. For a reference for fracture analysis in the dam heel of extra-high dams, the calculation model of critical crack size in elastic fracture and the prediction equation of FPZ characteristic length in nonlinear fracture were constructed for a TPB composite beam.

Tensile strength and failure behavior of rock-mortar interfaces: Direct and indirect measurements

The tensile strength at the rock-concrete interface is one of the crucial factors controlling the failure mechanisms of structures, such as concrete gravity dams. Despite the critical importance of the failure mechanism and tensile strength of rock-concrete interfaces, understanding of these factors remains very limited. This study investigated the tensile strength and fracturing processes at rock-mortar interfaces subjected to direct and indirect tensile loadings. Digital image correlation (DIC) and acoustic emission (AE) techniques were used to monitor the failure mechanisms of specimens subjected to direct tension and indirect loading (Brazilian tests). The results indicated that the direct tensile strength of the rock-mortar specimens was lower than their indirect tensile strength, with a direct/indirect tensile strength ratio of 65%. DIC strain field data and moment tensor inversions (MTI) of AE events indicated that a significant number of shear microcracks occurred in the specimens subjected to the Brazilian test. The presence of these shear microcracks, which require more energy to break, resulted in a higher tensile strength during the Brazilian tests. In contrast, microcracks were predominantly tensile in specimens subjected to direct tension, leading to a lower tensile strength. Spatiotemporal monitoring of the cracking processes in the rock-mortar interfaces revealed that they show AE precursors before failure under the Brazilian test, whereas they show a minimal number of AE events before failure under direct tension. Due to different microcracking mechanisms, specimens tested under Brazilian tests showed lower roughness with flatter fracture surfaces than those tested under direct tension with jagged and rough fracture surfaces. The results of this study shed light on better understanding the micromechanics of damage in the rock-concrete interfaces for a safer design of engineering structures.

Probabilistic finite element analysis of failures in concrete dams with large asperities in the rock–concrete interface

Common analytical assessment methods for concrete dams are unlikely to predict material fracture in the dam body because of the assumption of rigid body behavior and uniform- or linear stress distribution along a predetermined failure surface. Hence, probabilistic non-linear finite element analysis, calibrated from scale model tests, was implemented in this study to investigate the impact of concrete material parameters (modulus of elasticity, tensile strength, compressive strength, fracture energy) on the ultimate capacity of scaled model dams. The investigated dam section has two types of large asperities, located near the downstream and/or upstream end of the rock–concrete interface. These large-scale asperities significantly increased the interface roughness. Post-processing of the numerical simulations showed interlocking between the buttress and the downstream asperity leading to fracture of the buttress with the capacity being determined mainly by the tensile strength of the buttress material. The capacity of a model with an asperity near the upstream side, with lower inclination, was less dependent on the material parameters of the buttress as failure occurred by sliding along the interface, even with inferior material parameters. Results of this study show that material parameters of the concrete in a dam body can govern the load capacity of the dam granted that significant geometrical variations in the rock–concrete interface exists. The material parameters of the dam body and their impact on the capacity with respect to the failure mechanism that developed for some of the studied models are not commonly considered to be decisive for the load capacity. Also, no analytical assessment method for this type of failure exists. This implies that common assessment methods may misjudge the capacity and important parameters for certain failure types that may develop in dams.

Bonding Fracture and Deformation Behaviors of Sandstone–Concrete Interface Subjected to Different Temperatures under Three-Point Bending

Thermal operations threaten the safety of geological engineering, especially the rock–concrete interface, as the link of in situ stress transmission is crucial to structural stability. In this study, three-point bending experiments were conducted on sandstone–concrete binary specimens to investigate the thermal effect on fracture behaviors of the rock–concrete bonding interface. The deformation evolution characteristics were analyzed by digital image correlation (DIC). The results demonstrated that the temperature has a remarkable degradation for the bonding capacity of sandstone–concrete interfaces, which can be classified into the rapid-damage stage and steady-damage stage with a critical temperature of 300°C. The phased degradation is closely related to the decline in interfacial adhesion and the thermal damage to concrete, instead of sandstone. By determining the strain inflection point of different positions on the crack path, a new idea is proposed to reveal the crack propagation process. During the loading process, the crack propagates slowly to a certain length and then coalescences violently when subjected to a small load increment. In addition, the crack initiation load is weakened by the elevated temperature or lower interface roughness.