جستجوی مقالات مرتبط با کلیدواژه "رشد ترک" در نشریات گروه "عمران"

Rocks normally contain defects such as joints, cracks, and pre-existing flaws from small to large scales. The behavior of these defects is highly dependent on their geometrical characteristics and locations in the rock mass. In many cases, these cracks are located at the interface of two materials. Most of the investigations on interfacial cracks have been analytical and there are a few experimental studies available. In this study, crack initiation, growth, and coalescence of a pre-existing flaw in a rock-like material are investigated under uniaxial compressive loading. Specimens are made of two types of rock-like materials with different compressive and tensile strength. The flaw is placed at the center of specimens. Flaw width is equally distributed in both materials. To investigate the effect of geometrical parameters of the flaw on crack growth processes, rock-like specimens with different flaw angles were made. To show the repeatability of results, three identical specimens of each geometry were made. Digital image correlation technique was used for a detailed investigation of crack growth processes from initiation to failure. Nikon D7100 camera was used to take pictures of the specimen during loading. Ncorr software was used to analyze the pictures. The results showed that crack growth, coalescence, and failure pattern depended on pre-existing flaw geometry. By varying the angle of inclination from zero to 45°, the compressive strength decreased and then, increased with increasing the angle of inclination from 45 to 90°. Specimen with 45° flaw had the least compressive strength because theoretically maximum shear happens at this angle. For flaws with angles of 0 to 30 degrees, failures are caused by the growth of wing cracks. However, from 45 to 75 degrees, failures often occur with the propagation of the shear crack along the interface. At the angle of 90 degrees, a tensile crack is formed at the center of the sample along the interface. The results showed that crack growth and failure pattern depended on pre-existing flaw geometry.

The current research seeks to present a novel method for numerical modeling of concrete behavior at meso-scale. In this method, as the stress distribution at the macro scale can be a suitable indicator for critical areas (crack propagate and growth), the areas under stress (critical areas) are identified through numerical modeling at the macro scale (coarse mesh) and using the extended finite element method and topology optimization. Macro-scale critical regions are considered cumulatively using topology optimization. Therefore, critical regions are modeled at the meso-scale, and the rest are modeled on the macro scale. At the meso-scale, the three phases including aggregate, mortar matrix, and ITZ were considered. The aggregate phase was distributed in the mortar matrix using a random algorithm and a Fuller curve. The accuracy of modeling concrete at the meso-scale using topology optimization was investigated by two examples, the results of which show the appropriate accuracy of the method. Furthermore, the method can reduce the computational cost and analysis time compared to modeling the whole sample at the meso-scale. In this research, the piecewise meshing method was used for numerical modeling. Using this method, traction-separation crack growth and expansion are modeled with appropriate accuracy.

Initiation and progression of cracks in a saturated porous media is an important topic which has attracted considerable attention from researchers in the recent years. Extended finite element method (EFEM) is a contemporary technique removing the necessity of consecutive meshing of the problem in the analysis process. In the EFEM by enriching the elements whose discontinuity there exists, there is no need for re-meshing at each step of the analysis. .In this paper, EFEM is used to evaluate progression of cohesive crack in a two phase saturated porous media. To analyze the saturated porous media, at the first, the equations of mass conservation, momentum conservation, and energy conservation are established to consider simultaneous effects of displacement, pressure, and temperature on the crack progression. The cohesive model is used to simulate crack progression. Heavy-side functions are used to enrich finite elements and the resulting system of equations are solved by Newton Raphson method. Finally, the two numerical models were analyzed by other researchers is considered to evaluate the derived relationships. Numerical result show that maximum variation by other researchers is 5%.

In this study, the effect of polyethylene wax on improving the performance of asphaltic mixtures was investigated through laboratory testing. Experimental samples were taken from asphalt compacting in place containing polymer additives, especially polyethylene waxes. To investigate the role of polyethylene wax in reducing pavement fatigue cracks, samples with dimensions of 20 cm* 5 cm * 5 cm in two conditions without and with the amount of polyethylene wax and constant amount Asphalt ratio of 4.5 in all samples at different temperatures and load frequencies for fatigue were tested. Fatigue test results showed that for all three mixtures arranged with polyethylene wax in all samples, the pavement thickness after the test, for 0.09% wax, it was higher than that of the other asphalt mixtures. That is, the drop in the thickness of the pavement under the fatigue test is minimal, indicating an optimal weight percentage of the polyethylene wax executed in the pavement layer. So that the increase in the percentage of asphalt mixing with polyethylene wax from 0.05 to 0.09 percent, the decrease in the thickness of the pavement under the fatigue test decreases, but from 0.09 to 0.15 percent, the thickness drop increases. Thus, for artillery mixes with 0.09% polyethylene wax, the drop in the thickness of the asphalt pavement layer is less under fatigue testing, that is, the rate of cracking of asphalt armed with 0.09% of the polyethylene wax is minimal.

Understanding initiation and growth of cracks leads to better evaluation of rock masses behavior. Investigation of crack growth and propagation is noteworthy in geotechnical engineering, petroleum engineering, geology, seismology and many other sciences which encounter with rock fracture mechanics problems. Evaluation of rock slopes stability, design of retaining structures, design of tunnels, and prediction of flow path through rock masses, are some example of application of crack growth study in geotechnical engineering. By evaluating cracks growth, propagation and coalescence, faults seismic behavior can be understood. In general faults are considered as quasi-static cracks, so study of crack propagation under various loading conditions brings important information about faults. Many researchers studied crack growth in Brazilian disk shape specimen under diametrically compression loading. Previous research focused on open crack propagation and closed crack were infrequently included. In this paper using molded gypsum, as a model material, crack growth in Brazilian disk shape specimens with pre-existing open and closed flaws are investigated. The dimension of specimens is 100 mm in diameter and 50 mm in thickness. Inserted open and closed flaws length is 30 mm. Open and closed flaws are inserted using 1 mm thickness steel shims and 0.05 mm thickness silicon tapes, respectively. In this study, the specimens engineering properties were determined by conducting uniaxial compression test and indirect tensile test on Brazilian disks. The experiments were conducted according to ISRM and ASTM standards. Uniaxial compressive strength, Poisson ratio, elastic modulus, and tensile strength of specimens are, respectively, qu=38 MPa, υ=0.25, E=8.5 GPa, and σt=7 MPa. Diametrical compressive loading was applied to the specimens in various inclination angles of flaws with respect to the loading direction. In this paper, influence of open and closed cracks on strength of the specimen is investigated. Fracture toughness of mode I, II and mixed mode I-II, are also determined using analytical solutions. Throughout the tests, slippage between closed crack surfaces is continually monitored by digital microscope. According to the curves of slip distribution along the crack length derived from laboratory data, there is a good agreement between slip distribution along closed flaws and that recorded from real faults by other researchers. Also, results of this study show that open and closed flaws reduce disks strength, significantly (i.e., more than 50%). Inclination angles of the flaw with respect to the loading direction affects crack propagation patterns. Generally, new wing cracks initiate form the flaw tips and then propagate toward the loading direction. Increase in the inclination angle leads to a reduction in the effect of open flaw on crack growth and for angles more than 27.2˚ (i.e., pure shear, mode II) new crack does not initiate from flaw tips. At inclination angle of 90˚, flaw has no effect on crack propagation pattern and the disk fails under tensile splitting condition. In disks with closed flaws, new cracks initiate from flaw tips expect for the case of 90˚ which is the same as open flaws. For the material used in the study, mode I and II fracture toughness are calculated form experimental data and found to be equal to 0.250 and 0.258, respectively. The obtained values for fracture toughness indicate that used material is brittle. Also, the values of mode I fracture toughness for angels above 27.2˚ are negative which point to the fact that flaw tips in these angles are in compression and not in tension.

Fatigue cracking is perceived as one of the primary failures in asphalt pavements. This type of failure originates from the bottom of asphalt layer and then spreads upward as a result of repeated loading. Emersion of this type of cracks is common in asphalt sections with less thickness. In this research, for a better understanding of the mechanisms and parameters of fatigue cracks propagation, the fracture mechanics approach was used. Subsequently, the crack phenomenon and its extension in pavements were theoretically analyzed under three temperatures: low, medium, and high. The results showed that the stress intensity factor is directly correlated with the distance of loading from the center of the crack. Moreover, stress intensity factor was observed to increase more rapidly when the load is applied near the crack in case of having granular base compared to concrete base. This highlights the high impact of type of base, especially granular base, when it comes to changes in stress intensity factor.

Researchers have used different shapes of specimens to investigate fracture toughness of asphalt mixtures. Among these, specimens with circular shape are usual because of ease in preparation from conventional gyratory and marshal compactors. To investigate the crack growth in asphalt accurately, mode I (opening mode) and mode II (shear mode) must be considered simultaneously. In this paper the specimens with two types of crack configuration are suggested for mixed mode fracture studies of asphalt mixtures. These proposed specimens are: semi-circular bend (SCB) specimens with an inclined edge crack subjected to symmetric bending load and semi-circular bend (SCB) specimens with a vertical edge crack subjected to asymmetric bending load.