| Damage and fracture damage of rock material is an evolution of initial imperfection, macroscopic cracks and extension, resulting in material macroscopic fracture failure process of continuous change. Based on cement mortar specimens simulated rock material as the research object, the specimen is divided into a fissure and fractures(complete) two, experiment of uniaxial compression is carried out respectively, specimen stress when the relevant mechanical parameters are obtained. In the process of loading, the ultrasonic instrument measurement specimen under stress wave and velocity, the waveform and wave velocity changing with the loading process, systematically studied the damage and fracture damage characteristics of rock material, this paper obtains the research achievements are as follows:(1) mortar fractured specimens have been simulated as fissured rock to do the uniaxial compression experiment, different crack length, the initial propagation angle of specimens and initial crack strength have been obtained by setting 9 experimental conditions, such as 3 angles 45o、60o、75o and 3 crack length 45mm、60mm、75mm, the compression failure mechanism are also analyzed. The results show: when the crack length is the same, the initial crack strength is increased by the increase of angles and when the angle is increased from 60° to 75°, the initial crack strength is increased faster. When the crack angle stays at a certain level, the initial crack strength is decreased by the increase of the crack length. By employing the theory of maximum circumferential stress, failure mechanism of fractured specimens under compression is interpreted, theoretical expression of crack propagation angle is acquired; By comparison, it is know that that measured values and theoretical values of crack propagation angle are in good agreement.(2) By including fracture specimen under uniaxial compression loading experiments, the load at each load level, 6 points to make use of ultrasonic instrument acquisition of ultrasonic wave and wave velocity, each side point of waveform and wave velocity value.Measured results show that the prefabricated crack extending crack tip did not appear before, only arrangement in the prefabricated crack two measuring points 1# and 4# continue to abnormal changes, the site of the waveform is almost does not occur variation;Continue to load reaches the prefabricated crack extension crack tip appeared, each measuring point of waveform in the irregular change;In the crack propagation stage, crack increases with load applied to continue rapid deformation, each measuring point of waveform will have obvious difference with the propagation of the crack;When cracks through the specimen failure occurs, the waveform changes a lot of each measuring point, poor continuity, the integrity of the waveform is destroyed.The change of wave velocity can be divided into three sections, prefabricated crack extending area not:(a) each measuring point velocity with the increase of the strain had no obvious change;(b) prefabricated crack extension transition zone: each measuring point velocity obviously decreased with the increase of the strain;(c) prefabricated crack extension area: each measuring point velocity along with the expansion of the crack gradually reduce, continue to load to the specimen, the measuring point velocity decreased rapidly.(3) Considering rock materials as isotropic material, will contain the site of fracture specimens wave velocity along with the change of strain can be divided into two cases.Arrangement of measuring points in one is considered in the sides of the prefabricated crack specimen loading process, the prefabricated crack produces because of the Angle closure phenomenon, and the specimen within the original micro cracks extend development, lead to measuring point velocity- strain curve disorder, appear unable to judge the change of the relationship between wave velocity and strain.Another kind is to consider measuring point arrangement on both sides of the prefabricated crack, selection of each specimen of the same point(both selected point 6#) measured data is analyzed, are based on the wave velocity changes in experimental models of crack extension of evolution, the wave velocity and the expression of stress-strain relationship.(4) Based on a complete specimen under uniaxial compression loading experiments, get initial crack strength of the specimens and the uniaxial compressive strength.The experimental results show that the full scope of initial crack strength of the specimens in 5.96 ~ 6.77 MPa, with a mean of 6.46 MPa;Uniaxial compressive strength limit range in 11.43 ~ 16.70 MPa, averaged 14.73 MPa.Overall, the initial crack strength of the specimens floating range is small, the uniaxial compressive strength value larger discreteness.(5) Based on complete specimen under uniaxial compression loading experiments, the load at each load level, seven points to make use of ultrasonic apparatus of ultrasonic wave and wave velocity of the acquisition, each side point of waveform and wave velocity value.Waveform measured results showed that 0 MPa, the measuring point 4 # and 5 # waveform abnormal measuring point, around these two points are the initial damage;In 3.0 ~ 7.5 MPa, the measuring point 4# waveform is still abnormal, and the measuring point of the 1#, 2#, 3# waveform abnormal phenomenon, at the end of specimen local micro cracks began to appear.Continue to load, when the specimen surface in the initial crack, seven points waveform showed irregular phenomenon.In the crack propagation stage, initial crack continue to spread rapidly development, ultrasonic wave may also be along with the expansion of crack and irregular change.When cracks through the whole specimen, specimen has lost its carrying capacity, the waveform changes a lot of each measuring point, poor continuity, waveform irregular obviously.Wave velocity test results show that the measuring point 1#, 2#, 3# initial wave velocity value is close to the point of 5#, 6#, 7# initial wave velocity value is close to.Before did not appear in the specimen surface crack, wave velocity with the increase of the strain had no obvious change.After the cracks on the surface of the specimens, wave velocity obviously decreased with the increase of the strain, specimen internal defects extends to the surface of specimen, specimen damage degree is serious.With further extension of damage on arriving at the compressive strength of the specimens, the specimens of internal integrity suffered severe damage, ultrasonic wave velocity of the specimen value reduced a lot.(6) Through uniaxial compression tests of complete specimen, using the ultrasonic instrument records the relative changes in the process of pulse wave velocity in the experiment, the damage variable is defined by wave velocity changes.Will rock damage as the isotropic, adopting the principle of isotropic damage constitutive model and damage evolution is established and the experimental parameter values from generation into the constitutive model were analyzed.Based on complete specimen the analysis of the measuring point velocity- strain relationship curve, the wave velocity and the expression of stress-strain relationship, to study rock material injury to the fracture extension mechanism to provide theoretical reference. |
PDF Full Text Request