Characteristics Of Fracture Process Zone Research Articles

Statistics of acoustic emission waveforms in characterizing the fracture process zone in fibre-reinforced cementitious materials under mode I fracture

This article reports on the characteristics of fracture process zone in steel fibre-reinforced concrete (SFRC) under the mode I fracture process using acoustic emission (AE) testing. The generated AE waveforms during mode I fracture process in SFRC were recorded in the laboratory. Using a statistical analysis of AE waveforms, it was observed that as the loading increases, a damage zone consisting of numerous microcracks develops ahead of the predefined notch tip. The location of the generated AE events related to the numerous microcracks were classified into three zones namely (i) major damage, (ii) moderate damage and (iii) low damage. The areas of these regions were evaluated from the distribution of the AE events around the pre-notch. The number of AE events reduced with the increase in the steel fibre content under the same experimental conditions. The major damage zone was located ahead of the notch tip very closely and it comprised of AE events with (i) high peak amplitude, (ii) low information entropy and (iii) longer AE waveform duration.

A novel methodology for characterizing fracture process zone evolution in Barre granite specimens under mode I loading

Fracturing in quasi-brittle such as rock involves the development of an inelastic zone around the pre-crack tip, also known as the fracture process zone (FPZ). The evolution of FPZ, while influencing the fracture behavior of rocks, is difficult to characterize due to the localized nature of material behavior and microstructural heterogeneities. This study presents a digital image correlation technique (DIC)-based method that consistently describes the FPZ evolution in mode I fracture of Barre granite specimens. The FPZ characteristics were also evaluated using acoustic emission (AE) monitoring. The evolution of the crack tip opening displacement (CTOD) was analyzed to estimate the various fracture parameters, such as threshold values of the elastic and critical opening displacements and the size of the fully developed FPZ. The comparison of DIC-based measurements with the AE-based microcracking activities demonstrated that the displacement approach of the former could be used independently to identify the transition among the three stages of FPZ evolution, namely, (1) elastic stage, (2) formation of FPZ, and (3) macro-crack initiation. Experimental results also indicate a linear relationship between the FPZ length and the inelastic component of the CTOD, along with the quadratic relationship between the local dissipated energy and the FPZ length, which is consistent with the linear softening law for material inside the FPZ.

Characteristics of the Fracture Process Zone for Reservoir Rock with Various Heterogeneity

Hydraulic fracturing for oil-gas and geothermal reservoir stimulation is closely related to the propagation of Mode I crack. Nonlinear deformation due to rock heterogeneity occurs at such crack tips, which causes the fracture process zone (FPZ) to form before the crack propagates unsteadily. However, the relationship between the FPZ characteristics and rock heterogeneity still remains elusive. We used three rock types common in reservoirs for experimental investigation, and each of them includes two subtypes with different heterogeneity due to grain size or microstructural characteristics. Drawing on the experiment results, we calculated the FPZ size (represented by the radius of an assumed circular FPZ) in each cracked chevron-notched Brazilian disk, and we reproduced the formation process of the FPZ in marble using the discrete element method. We showed that strong heterogeneity is favorable to large FPZ size, can enhance the ability of crack generation and make crack morphology complex. Coupling the Weibull distribution with fracture mechanics, the dependence of the FPZ size on heterogeneity degree can be theoretically explained, which suggests that the inherent heterogeneity of rocks sets the physical foundation for formation of FPZs. These findings can improve our recognition of propagation mechanisms of Mode I cracking and provide useful guidelines for evaluating reservoir fracability.

Investigation on the characteristics of fracture process zone under cyclic loading: Insights from macro-mesoscopic analysis

Cyclic disturbance is common in engineering which accounts for the main reason for fractures and plays an important role in the fracture process zone (FPZ), leading to unclear characteristics at the macro-mesoscopic level. It is thus essential to focus on the macro-mesoscopic analysis of fracture characteristics in FPZ subjected to cyclic loading. In this paper, a series of tests are conducted on sandstone to quantify the macro-mesoscopic features of FPZ. The experimental results indicate that the cyclic loading has an important effect on fracture toughness, the correction of nonlinear fracture toughness based on FPZ is reduced at different amplitude levels. Detailed scanning electron microscope (SEM) observation revealed the scenario of the damage zone indicating the connection between macroscopic fracture properties and mesoscopic mechanism. Additionally, it is found that more intergranular cracks were produced on the cycling failed surface, which is opposite to static rupture. A cohesive zone model (CZM) was introduced to characterize the constitutive relationship of tensile fracture, combined with digital image correlation (DIC) method, the evolution law of FPZ can be divided into three stages. Moreover, 3D laser scanning technology was applied to calculate and analyze the parameters of failed surfaces, namely, fractal dimension D, asperity height, and slope angle which were positively correlated with surface roughness. Furthermore, the joint roughness coefficient (JRC) increases almost linearly with decreasing amplitudes.

Stochastic evolution characteristics of fracture process zone in concrete

In this study, the stochastic characteristics of the fracture process zone (FPZ) of concrete are analysed. The impacts of specimen size on the critical crack opening displacement, the distribution of cohesive stress, and the critical length of FPZ are studied, and calculations are performed for the stochastic energy consumption curves of concrete. The dispersion of the variance of critical crack tip opening displacement CTODc and the critical length of FPZ are obtained. Combined with the existing experiments, comparative verification is performed. The results show that the critical crack tip opening displacement CTODc and critical length of FPZ increased with the specimen size, whereas the critical cohesive stress at the tip of the initial crack decreased with the specimen size. The dispersion of the variance of critical crack tip opening displacement CTODc and the critical length of FPZ increased with the specimen size.

Size effects on the characteristics of fracture process zone of plain concrete under three-point bending

This paper studies the size effects on the characteristics of fracture process zone (FPZ) of plain concrete, including its physical features and softening behavior. Three-point bending tests were performed on pre-notched beams with similar geometry and different dimensions. Electronic speckle pattern interferometry (ESPI) technique was employed to monitor the crack evolution and full-field deformation of the beams. The development of FPZ was evaluated qualitatively based on the strain fields and quantitatively based on the displacement fields obtained by the ESPI technique. By adopting the incremental displacement collocation method, the tension softening curves (TSCs) of specimens with different sizes were estimated and approximated as bilinear curves. The features identified from the TSCs were correlated to the physical features in the development of FPZ. Finally, an empirical tension softening model and corresponding size-independent parameters were obtained based on the estimated TSCs of different sizes specimens.

The crack opening displacement of rock fracture process zone

The original displacement value of fracture process zone can be obtained by digital image correlation technology. According to the virtual crack model, the formula to obtain the opening displacement is given in the experiment. Basing on the damage Mechanics theory and the actual deformation characteristics of fracture process zone, the traditional opening displacement distribution function of fracture process zone is modified by defining the wave coefficient and the damage factor of the horizontal elastic modulus. The measured opening displacement is compared with the opening displacement of the traditional theoretical function and modified function, and the results show that the opening displacement is non-linear fluctuation characteristic distribution influenced by damage. The revise distribution function not only reflects the overall growth trend of the opening displacement, but also reflects the local fluctuation characteristics. It has an important theoretical significance for understanding the non-linear characteristics of rock fracture process.

Fracture process zone characteristics and identification of the micro-fracture phases in recycled concrete

Normal and recycled aggregate concrete differ significantly in their fracture characteristics. The cracking in recycled concrete initiates earlier; the fracture length is generally larger and the use of high proportion of recycled aggregate results in higher strain gradients in the fracture process zone. However, in spite of the negative influences, the mechanical responses of normal concrete and concrete with 30% recycled aggregate are similar to each other. This paper shows the influence of recycled aggregate proportion on the characteristics of fracture process. Furthermore, classification of the recorded acoustic signals during the fracture test was carried out; three clusters correspond respectively to aggregate fracture, cracking in cement mortar and cracking at the interfacial transition zone were discriminated. The evolution of each micro-fracture phase in terms of the mechanical state of the material was analyzed. A comparison of the apparition chronology of each cluster for normal concrete and recycled concrete was carried out.

Investigation on fracture parameters of concrete through optical crack profile and size effect studies

In this paper, Digital Image Correlation (DIC) technique has been used to map the full field of displacement and strain for visualizing the fracture growth, fracture propagation and formation of Fracture Process Zone (FPZ) in notched concrete beams under bending. Further, for exact quantification of concrete fracture parameters such as crack opening displacement, characteristics of FPZ and fracture energy, a new scheme called Optical Crack Profile (OCP) originated through DIC experiments, is developed. In addition, a comprehensive analysis of fracture parameters estimated from Bazant size effect laws, Jenq–Shah model and OCP technique have been carried out.

Laboratory evaluation of CCM parameters for mode-I cracks in HMAs

A series of experimental testing, data acquisition and analysis procedure to determine substantial fracture parameters for openning mode (mode-I) cracks in hot mix asphalt (HMA) has been developed. The cohesive crack model (CCM) has been adopted for the identification and evaluation of fracture parameters: Tensile strength; Fracture Energy; Critical Crack Opening Displacement; and Characteristics Crack Opening Displacement. The concept of pseudo fracture energy has been introduced to better account for nonlinear and time dependent fracture in HMAs. Behavioral trend between pseudo fracture energy and reduced strain rate agrees with that of fracture energy determined from LVDT measurements. At low temperatures, the fracture parameters remain constant irrespective of reduced strain rates. This implies that the Linear Elastic Fracture Mechanics (LEFM) could be applied for low temperature cracking in asphalt concrete. Finally, a better understanding of cracking mechanism relating to strain localization in a fracture process zone (FPZ) was achieved. Further study on characteristics of FPZ due to other cracking modes is necessary to bnefit the pavement performance model.

Comparisons of Concrete Fracture Models

Fracture properties for linear elastic fracture mechanics (LEFM), singular-fracture process zone (S-FPZ), and nonsingular-fracture process zone (NS-FPZ) models were determined from the experimental results of three-point bend tests. The responses for each fracture model were numerically calculated and compared. All three fracture models could simulate the measured load and crack mouth opening displacement (CMOD) versus load-point displacement relations. However, for the LEFM model the stress intensity factor needed to increase continuously with crack extension, and for the S-FPZ model the fracture process zone characteristics need to change continuously if the critical stress intensity factor was to remain constant. The LEFM model showed the largest resistance and the slowest crack extension, while the NS-FPZ model showed the smallest resistance and the fastest crack extension. The responses for the S-FPZ model were intermediate between those for the LEFM and NS-FPZ models and the total fracture energy de...

Correlation between characteristics of fracture process zone and tension-softening properties of concrete

The microcracking properties of concrete are studied by means of the three-dimensional acoustic emission technique in order to discusse the phases of the fracture process zone of concrete. It is revealed that the length of the fracture process zone ahead of the notch tip seems to be independent of the maximum aggregate size but the width is obviously influenced by this factor. The tension-softening properties are analysed as an inverse problem. Among the four parameters defining a bilinear tension-softening diagram, the critical crack width is most influenced by the heterogeneity. The numerical results are correlated with the phases of the fracture process zone. The size dependence of the fracture energy of concrete is also discussed on the basis of the characteristics of the fracture process zone and an empirical formula is proposed to describe the size dependence.