Ahmadreza Hedayat

Precursors to the shear failure of rock discontinuities

Active geophysical monitoring of potential failure along mechanical discontinuities in rock requires identification of precursory signatures to failure in geophysical signals. Active ultrasonic monitoring of shear failure along frictional discontinuities was performed to determine the signatures of potential failure. An instrumented direct shear apparatus was used to apply a constant shearing rate to a discontinuity that was held under a constant normal stress. Transmitted and reflected compressional and shear waves were recorded during the shearing process. Ultrasonic precursors were identified as distinct maxima in the amplitude of transmitted shear waves as well as minima in the amplitude of reflected shear waves that occurred well before the peak shear strength of a frictional discontinuity. The precursors are linked to changes in the local shear specific stiffness along the discontinuity, while the discontinuitys macroscopic shear strength continues to increase prior to failure.

Multi-Modal Monitoring of Slip Along Frictional Discontinuities

Seismic wave transmission and digital image correlation (DIC) are employed to study slip processes along frictional discontinuities. A series of biaxial compression experiments are performed on gypsum specimens with non-homogeneous contact surfaces. The specimens are composed of two blocks with perfectly mated contact surfaces with a smooth surface with low frictional strength on the upper half and a rough surface with high frictional strength on the lower half. Compressional, P, and shear, S, wave pulses were transmitted through the discontinuity while digital images of the specimen surface were acquired during the test. A distinct peak in the amplitude of transmitted wave occurs prior to the peak shear strength and is considered a “precursor” to the failure. Precursors indicate that slip initiates from the smooth surface and extends to the rough surface as the shear load is increased. From the DIC data, slip is identified as a jump in the displacement field along the fracture that initiates from the smooth surface and propagates to the rough surface. Precursors are associated with an increase in the rate of slip across the discontinuity and are a measure of the reduction in the fracture shear stiffness.

Detection and Quantification of Slip Along Non-Uniform Frictional Discontinuities Using Digital Image Correlation

A deformation measurement system based on the principles of digital image correlation (DIC) has been developed to evaluate the process of slip along frictional discontinuities. A biaxial compression apparatus is used to impose shear failure on perfectly mated gypsum specimens with nonhomogeneous contact surfaces. The contact surfaces are made by casting gypsum against flat surfaces with different frictional characteristics and consisted of a smooth surface with low frictional strength on the upper half and a rough surface with high frictional strength on the lower half. Design, implementation, and verification of the DIC measurement system are presented in this paper. DIC successfully identified slip as a jump in the displacement field across the discontinuity. Slip is observed to initiate from the smooth surface with minimum frictional resistance and as the shear load is increased, propagates to the rough surface that has higher frictional resistance. DIC clearly exhibits a reduction in fracture’s shear stiffness based on an increase in the rate of relative vertical displacement across the discontinuity, which initiates from the smooth surface and propagates to the rough surface.

Laboratory Determination of Rock Fracture Shear Stiffness Using Seismic Wave Propagation and Digital Image Correlation

Seismic wave propagation and digital image correlation were used during direct shear experiments on Indiana limestone specimens to investigate the stiffness of rock discontinuities (fractures) approaching shear failure. An instrumented direct shear apparatus was used to apply shear stress to the discontinuity. Compressional and shear wave pulses were transmitted through and reflected from the discontinuity, whereas digital images of the specimen surface were acquired during the test. To measure the dynamic shear stiffness of the rock discontinuities, the displacement discontinuity theory was used and the stiffness was calculated based on the ratio of transmitted to reflected wave amplitudes. The static shear stiffness was calculated based on the ratio of an increment in the applied shear stress to the corresponding increment of relative shear displacement (slip) along the discontinuity. The dynamic shear stiffness measured by seismic wave propagation showed roughly five to ten times greater magnitude than the static values measured by digital image correlation technique. This observation is found to be in agreement with available studies indicating that the frequency-dependent fracture stiffness arises from probabilistic and spatial distributions of stiffness and that dynamic moduli are typically greater than the static values.

Experimental and Numerical Investigation of the Center-Cracked Horseshoe Disk Method for Determining the Mode I Fracture Toughness of Rock-Like Material

This paper presents a new procedure for determining the fracture toughness of rock-like specimens using the diametric compression test with the center-cracked horseshoe disk (CCHD) method. Using finite element analysis, a dimensionless stress intensity factor was obtained and a formula was rendered for determining mode I fracture toughness. To evaluate the accuracy of the measurement results produced by the CCHD method, fracture toughness experiments were conducted on the same rock-like material using the notched Brazilian disk (NBD) method. The CCHD tests were simulated using a two-dimensional particle flow code for validation of the experimental results, and a great agreement between the pattern of crack initiation and propagation between the experimental and numerical simulations was observed. Lower values of fracture toughness were obtained from CCHD experiments than NBD tests due to purely tensile stress distribution at the tip of the existing notch in CCHD method.

Experimental and Numerical Study of Shear Fracture in Brittle Materials with Interference of Initial Double Cracks

A simultaneous experimental and numerical study of shear fracture of concrete-like materials is carried out using Brazilian disc specimens with initial double edge cracks and fourpoint bending beam specimens with double edge-notches. The interference effects of two cracks/notches are investigated through varied ligament angles and crack lengths. It is shown that shear fracturing paths change remarkably with the initial ligament angles and crack lengths. The cracked specimens are numerically simulated by an indirect boundary element method. A comparison between the numerical results and the experimental ones shows good agreement.

Effect of tensile strength of rock on tensile fracture toughness using experimental test and PFC2D simulation

The effect of tensile strength on the tensile fracture toughness of rock like specimen was studied in this paper. Brazilian test was done to determine tensile strength of material. A compression to tensile load transforming (CTT) device was developed for determination of mode I fracture toughness of concrete. Also particle flow code (PFC) was used for validation of the experimental outputs. Three concrete slabs with different tensile strength were prepared for investigation of the effects of tensile strength on the fracture toughness. The samples were made from a mixture of water, fine sand and cement with different ratio. These samples were installed in CTT device. A 30-tons hydraulic load cell applied compressive loading to CTT end plates with a constant pressure of 0.02 MPa per second. Compressive loading was converted to tensile stress on the sample because of the overall test design. The results show Fracture toughness has a close relationship with tensile strength of concrete so it increases with increasing the tensile strength. In constant join length, the angle of crack growth related to normal load was decreased with increasing the grain size. Numerical simulation shows that failure pattern and fracture toughness was nearly similar to experimental results. Finally, it can be concluded that CTT device was capable for determination of fracture toughness of concrete.

Ultrasonic investigation of granular materials subjected to compression and crushing

&NA; Ultrasonic wave propagation measurement has been used as a suitable technique for studying the granular materials and investigating the soil fabric structure, the grain contact stiffness, frictional strength, and inter‐particle contact area. Previous studies have focused on the variations of shear and compressional wave velocities with effective stress and void ratio, and lesser effort has been made in understanding the variation of amplitude and dominant frequency of transmitted compressional waves with deformation of soil packing. In this study, continuous compressional wave transmission measurements during compaction of unconsolidated quartz sand are used to investigate the impact of soil layer deformation on ultrasonic wave properties. The test setup consisted of a loading machine to apply constant loading rate to a sand layer (granular quartz) of 6 mm thickness compressed between two forcing blocks, and an ultrasonic wave measurement system to continuously monitor the soil layer during compression up to 48 MPa normal stress. The variations in compressional wave attributes such as wave velocity, transmitted amplitude, and dominant frequency were studied as a function of the applied normal stress and the measured normal strain as well as void ratio and particle size. An increasing trend was observed for P‐wave velocity, transmitted amplitude and dominant frequency with normal stress. In specimen with the largest particle size (D50 = 0.32 mm), the wave velocity, amplitude and dominant frequency were found to increase about 230%, 4700% and 320% as the normal stress reached the value of 48 MPa. The absolute values of transmitted wave amplitude and dominant frequency were greater for specimens with smaller particle sizes while the normalized values indicate an opposite trend. The changes in the transmitted amplitude were linked to the changes in the true contact area between the particles with a transitional point in the slope of normalized amplitude, coinciding with the yield stress of the granular soil layer. The amount of grain crushing as a result of increase in the normal stress was experimentally measured and a linear correlation was found between the degree of grain crushing and the changes in the normalized dominant frequency of compressional waves. HighlightsDeveloping a novel one‐dimensional compression device equipped with ultrasonic transducers.Finding a general consistency between ultrasonic amplitude and mechanical behavior.Defining a new term, seismic yield stress, corresponding to mechanical yield stress.Finding a relationship between dominant frequency and inter‐particle voids.Finding a linear relationship between normalized dominant frequency and amount of crushed particles.