Surface deformation, crack formation, and acoustic velocity changes in pyrophyllite under polyaxial loading

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Abstract
Jacketed cubes of pyrophyllite (31.6 mm on an edge) with variable water content were stressed monotonically to failure under polyaxial compression (σ1 > σ2 = 2σ3). The maximum and minimum principal stresses, σ1, and σ3, were applied with pistons, and the intermediate stress σ2 with a transparent fluid. An optical window in the pressure vessel allowed in situ measurements of the σ2 face deformation by optical holography. In addition, the more conventional techniques of stress, strain, and elastic wave velocity measurements as well as optical and electron microscopy were used to study the formation and propagation of fractures. The strength ( σ1 − σ3)f of the samples increased by about 50% as σ2 was increased from 5 MPa to 100 MPa. Air dry samples were stronger than water‐soaked samples by about ∼20%. Increasing the strain rate from 2 × 10−8 to 10−6 s−1 at σ2 = 5 MPa and 30 MPa increased the strength by about 10%. At σ2 = 100 MPa this trend was reversed, and samples that were deformed at the low strain rate were stronger by approximately 5%. The crack morphology of recovered samples was studied by optical and scanning electron microscopy. Subparallel sets of fractures and en échelon fracture patterns ahead of the macrofracture were easily visible with the unaided eye when σ2 was 5 MPa or 30 MPa. However, at σ2 = 100 MPa these fracture patterns were only visible under the microscope, and the cracks appeared much thinner. The holographic observations of the σ2 face revealed the following: as σ1 was increased, broad bulges formed in a crosslike pattern along the lines of the maximum shear stress. Macrofracture initiation, which occurred in a corner, was preceded by concentrated surface deformation. As the macrofracture propagated across the sample, it deviated from the direction of the maximum shear stress. Ahead of the tip of the macrofracture and migrating with it was a pronounced bulge. In response to monotonically increasing σ1, the displacement in the σ2 direction of a point adjacent to the eventual crack plane went through a local maximum that was followed by a local minimum as the crack passed. The results of elastic wave velocity measurements in the σ2 direction were very sensitive to the spatial relationship of the macrofracture and the elastic wave travel path. However, in general, above ∼50% of (σ1 − σ3)f the velocities decreased. As the migrating bulge approached an elastic wave travel path, the velocity decrease became more pronounced. The velocity increased again as the bulge passed.