Theoretical and Applied Fracture Mechanics最新文献

This paper presents new analytical expressions for $J_k$ integrals in terms of the stress intensity factors (SIF) of an in-plane traction free interface crack in an orthotropic/isotropic bimaterial. Using the sextic formalism of Stroh and Muskhelishvili formulation, a general solution for the stress and displacement fields near the interface crack of this bimaterial are derived. This paper also presents an analytical expression for the SIFs of a finite interface crack in an infinite orthotropic/isotropic bimaterial subject to in-plane remote stresses. For validation of the presented theory, the analytical $J_k$ integrals, crack opening displacements and stresses along the interface of an infinite orthotropic/ aluminum plate with a finite interface crack subject to tension are compared with the corresponding numerical results obtained using the boundary element method. For the orthotropic material, different fibre reinforced polymers are considered. The results show that a metal/fibre composite can be tailored to crack growth resistance. This study provides explicit analytical expressions for fracture parameters of cracks between isotropic and orthotropic materials which are not currently available in the SIF handbooks.

Austenitic stainless steels are typically used in hydrogen environments due to their resistance to hydrogen embrittlement, though their mechanical strength is only medium to low. To explore the use of high-strength materials, their susceptibility to hydrogen embrittlement must be studied. Duplex stainless steels, featuring both austenitic and ferritic phases, are among the promising candidates due to their enhanced mechanical properties. They exhibit good mechanical properties but, due to the presence of the ferritic phase, they are sensitive in hydrogen environment. In this study, the notch sensitivity of 2205 duplex is analyzed, both in air and hydrogen environment (at a pressure of 140 bar). Three different specimens are designed with different notch radii, varying the stress concentration factor from K_t = 2 to K_t = 6. A numerical simulation has been carried out in order to calculate the stress and deformation conditions for each configuration and the failure mechanisms have been analyzed for all the cases, establishing the behavior of this material against different types of notches in air and in a hydrogen environment. Moreover, a coupled diffusion-deformation model has been developed using Comsol finite element software. The model is focused on hydrogen transport and incorporates lattice hydrogen diffusion and a stress-assisted diffusion term to account for the influence of mechanical stress on the diffusion process. The results show a high notch sensitivity of this material in the tensile strength embrittlement index, as a consequence of the high hydrogen susceptibility of the ferritic phase enhanced by the high stress–strain states at the notch tip.

In the present study, the effect of flaws on the mechanical properties of high-strength engineered cementitious composites (HS-ECC) was investigated. Compressive tests, tensile tests, and electron microscope scanning microscopic tests were designed to explore the influence mechanism of flaws on mechanical properties. The results show that the compressive failure of HS-ECC was primarily characterized by a high density of vertical micro-cracks, and the specimen maintained exceptional structural integrity. All tested mix ratios demonstrated tensile strain hardening. Notably, the test groups with an 18 mm fiber length exhibited superior crack width control, with an average crack width of less than 30 μm. A new fiber bridging model was proposed, informed by microstructural analysis and crack opening displacement observations. A theoretical formula for the cracking strength of HS-ECC was developed based on the fracture process zone concept. The calculated initial crack strength closely matches the experimental values, with a discrepancy range of [0, 17 %]. This close agreement provides robust validation for the accuracy and reliability of the enhanced cracking strength calculation theory, thereby supporting its application in the design of ECC.

In this work, we have examined the single strain gage technique proposed by Chakraborty et al. (2014) (CMC1) for the accurate determination of mode I stress intensity factor (SIF) in orthotropic composite materials. Numerical simulations carried out on single edge crack specimen with different values of the height to width ratio ($h/b$) and of the crack length to width ratio ($a/b$) have shown the limits of this technique and that the use of a single strain gage is not always reliable to provide precise measurements of SIF, $K_I$. To avoid such a situation, we have proposed two new techniques based on two strain gages named CMC2 and M_CMC2. The CMC2 technique is a natural extension of the CMC1 technique and the M_CMC2 technique is a modification of the CMC2 technique. Accordingly, general finite element approaches are developed to estimate the extent of the valid region of these two techniques. The results of the numerical simulations show that the two techniques CMC2 and the M_CMC2 can give a very accurate value of $K_I$ when the two strain gages are placed within valid region. Also, these results show that these two techniques provide good solutions to ensure an accurate measurement of $K_I$ when the technique of CMC1 fails to provide the desired precision and accuracy of $K_I$. In addition, the results derived from experimental data (Chakraborty et al., 2017) proved the effectiveness of the CMC2 technique.

Understanding and predicting mechanical properties in large-scale structures such as dams poses a significant challenge, largely due to the inherent size effect in concrete materials. This study presents a mesoscopic investigation into the size effect on fracture behavior in dam concrete under three-point bending, employing both physical experiments and mesoscale simulation. Meso-structures of dam concrete specimens, including prefabricated cracks, are initially generated using a novel method tailored for three-point bending experiments. Subsequently, model parameters and solver settings for the phase field method are calibrated based on laboratory experiments. The fracture behavior and mechanical response of dam concrete under three-point bending are systematically modeled and analyzed, with different parameters including loading positions, maximum aggregate sizes, crack-height ratios, and specimen dimensions. Furthermore, a significant size effect is observed in both fracture resistance and flexural strength, as evidenced by computational outputs. Three classical models proposed by Bažant, Carpinteri, and Kim are adopted, to capture the influence of different maximum aggregate sizes and crack-height ratios on flexural strength, and a favorable consistency is observed across a range of specimen heights from 80 to 500 mm. The presented computational analysis provides valuable insights for the application of experimental and numerical outputs in practical engineering scenarios.

Moisture susceptibility and cracking resistance of asphalt pavement made with sulfur additive are two important failures of this type of asphalt mixture that should be considered. Recently, researchers have been trying to improve the mechanical performance of sulfur asphalt mixtures by modifying the asphalt mixture manufacturing method, modifying sulfur manufacturing technology, and applying laboratory conditioning. In this laboratory work, an attempt was made to improve the mechanical properties of an asphalt mixture made with a type of sulfur material called BituSul by different methods. The first method was the moisture susceptibility test and the second method was the fracture test. In the first step, 30 % of PG70-10 bitumen was replaced with BituSul, and a moisture susceptibility test was performed. After estimating the minimum moisture susceptibility of the BituSul-reinforced asphalt mixture, the fracture test was performed on the asphalt mixture without additives, BituSul-reinforced asphalt mixture, and BituSul-reinforced asphalt mixture containing New Jig Coal Tailings (NJCT) (with a replacement ratio of 50 % filler). To compare the fracture behavior of the mixtures over time, two conditions, including one freeze–thaw cycle (FTC) and about fifty-six hours of the aging period, which was equivalent to four years of failure, were applied to the mentioned samples separately. Finally, the samples were tested under mixed mode I/II at −12 °C and + 24 °C. The findings showed that the manufacturing technology used in this research, including how to design and manufacture the samples, the type of bitumen used, and the type of sulfur used (BituSul), caused a 2 % decrease in moisture susceptibility compared to the base sample. The fracture resistance results showed that the application of 0 and one cycle of FTC decreased the fracture indices of the mixture reinforced with BituSul at −12 °C and + 24 °C. Also, the results showed that the fracture stiffness improved for samples containing BituSul at −12 °C and + 24 °C. Finally, the results showed that applying aging to BituSul mixtures and BituSul mixtures reinforced with NJCT improved fracture resistance, fracture flexibility, and fracture stiffness. However, NJCT could not compensate for the improvement of fracture indices of asphalt mixture caused by the presence of BituSul; therefore, the application of aging was reported to be a suitable option to improve the fracture indices of BituSul-reinforced mixtures.

The fracture toughness of rocks is a crucial fracture parameter in unconventional geo-energy exploitation and plays a significant role in the fields of fracture mechanics and hydraulic fracturing. This research first introduces the processing approaches of AE (acoustic emission) signals for further elucidating the fracture mechanisms. To measure the mode-II fracture toughness of Longmaxi shale, the PTS (punch-through shear) specimen is employed because it can accomplish both mode-II loading and mode-II fracturing. Interestingly, the Gaussian function provides a superb description of the distribution of AE amplitudes, and the discrepancies in AE amplitude distribution are conspicuous for PTS shale specimens with typical bedding angles. The conventional RA-AF analysis of AE signals is more appropriate for qualitative analysis of failure mechanisms. When relatively higher RA values are monitored, implying that the partial stress drop and large-scale or more intense cracks take place, which can adequately furnish precursory information for engineering failure diagnosis. Based on the correlation between damage mechanisms (or fracture modes) and the characteristic bands of AE dominant frequency distribution, the dominant frequency-based hierarchical clustering method is developed by this work to quantitatively identify the damage or fracture modes, then the demarcation of AE dominant frequency ranges is determined as low, intermediate, and high, with corresponding tensile, tensile-shear, and shear cracks.

In this paper, the computational performance of alternate minimization (AM) and Broyden–Fletcher–Goldfarb–Shanno (BFGS) to solve the coupled partial differential equations arising within the framework of the phase field method is numerically studied. Both the numerical approaches are employed on an adaptive phase field method. In the case of BFGS, a double-loop recursive algorithm in combination with a line search is implemented. The local refinement is based on a pre-defined threshold on the damage variable and element size. The incompatible nodes due to adaptive refinement are handled by variable-node elements. The performance of the solution algorithm is numerically studied for a few problems with different loading conditions, viz., pure mechanical, pure thermal and combined thermo-mechanical. From the study, it is opined that the BFGS algorithm is 2$\sim$5 times faster than the AM approach, which is typically used for adaptive phase field framework. In addition, the detailed numerical implementation is presented.

Unlike cases of direct or indirect tensile loading, analysis of Mode I fracture in cases of compressive loading must necessarily consider two-dimensional flaw geometries. The consequent analytical complications have resulted in little theoretical evolution towards understanding Mode I fracture in compressive loading. Researchers have recently observed that the linear elastic (LE) stress fields surrounding two-dimensional flaws in compression appear to have indicative value regarding Mode I fracture, but no rational framework has yet been proposed for their interpretation. Herein it is demonstrated that by idealizing void surfaces as fractal, and using what will be called the “small flaw assumption,” LE stress fields can be interpreted to obtain significant insight regarding brittle Mode I compressive fracture. Using fractal voids, a rational and consistent explanation for the satisfaction of the stress and energy criteria at the surface of two-dimensional flaws is presented. The discussion resolves many typical theoretical issues in the literature in a manner consistent with fundamental Griffith theory, and accounts for size and shape effects. The framework further enables the computationally efficient prediction of peak K_I values associated with the propagation of line cracks from two-dimensional flaws from a brittle medium subject to macroscopic uniaxial compression via LE stress fields.

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.