The meso-fracturing mechanism of marble under unloading confining pressure paths and constant axial stress

Failure tests on marble during unloading confining-pressure under constant axial stress and simulations with the particle flow code were performed. The influence mechanism of the unloading rate of the confining pressure, initial unloading stress, and confining pressure on the failure characteristics of, and crack propagation in, marble was studied. By using the trial-and-error method, the conversion relationship between the unloading rates of confining pressures in laboratory tests and numerical simulations was ascertained. Micro-cracks formed in the unloading process of confining pressure are dominated by tension cracks, accompanied by shear cracks. The propagation of shear cracks lags that of tension cracks. As the confining pressure is increased, more cracks occur upon failure of the samples. The proportion of shear cracks increases while that of tension cracks decreases. The failure mode of samples undergoes a transition from shear-dominated failure to conjugated shear failure.

Failure Mechanism of Rock Specimens with a Notched Hole under Compression—A Numerical Study

Discontinuities are natural structures that exist in rocks and can affect the stability of rock structures. In this article, the influence of notch presence on the strength and failure evolution around a hole in compressed rock specimens is investigated numerically. Firstly, the uniaxial compressive test on a rock specimen with a circular hole is modeled, and the failure evolution in the specimen is simulated. In a separate model, notches are created at the surface of the hole. Results show that, when the notches are created in the model, a failure zone around the hole is transferred to a distance away from the surface of the hole. In addition, a parametric study is carried out to investigate the influence of the notch length and the confining pressure on the fracturing behavior of the specimen. Numerical results presented in this article indicate that the presence of notches at the surface of the hole and their dimensions can affect the fracturing mechanism of the specimen. In some cases, the failure at the boundary of the hole is prevented when the notches of certain dimensions are added to the hole. The insights gained from this numerical study may be helpful to control the failure around underground excavations.

Failure Mechanism of Rock Specimens with a Notched Hole Under Compression - A Numerical Study

Discontinuities are natural structures that exist in rocks and can affect the stability of rock structures. In this article, the influence of notch presence on the failure evolution around a hole in compressed rock specimens is investigated numerically. Firstly, the uniaxial compressive test on a rock specimen with a circular hole is modeled and the failure evolution in the specimen is simulated. In a separate model, notches are created at the surface of the hole. Results show that when the notches are created in the model, failure zone around the hole is transferred to a distance away from the surface of the hole. In addition, a parametric study is carried out to investigate the influence of the notch length and the confining pressure on the fracturing behavior of the specimen. Numerical results presented in this article indicate that the presence of notches at the surface of the hole and their dimensions can affect the fracturing mechanism of the specimen. In some cases, the failure at the boundary of the hole is prevented when the notches of certain dimensions are added to the hole. The insights gained from this numerical study may be helpful to control the failure around underground excavations.

Piercing and Surface-Crack Defects in Cold Combined Forward-Backward Extrusion

Metal flow tends to be complex and difficult to predict in the combined forward-backward extrusion (CFBE) process. Piercing and surface-crack defects are phenomenal in forming fasteners featuring a forward extruded pin and a backward extruded cup. In this work, a series of the CFBE tests with various combinations of the forward extrusion ratio (FER) and the backward extrusion ratio (BER) were conducted. A forming limit diagram, detailed with the piercing and surface-crack defects on the forward extruded pin or the backward extruded cup, was developed to provide a conception in choosing appropriate extrusion ratios in forming fasteners with such pin-and-cup features. With the aid of the forming load-stroke curves and the finite element analysis of fracture damage, the fracturing mechanism for the CFBE process was provided.

Silica diagenesis-driven fracturing in limestone: an example from the Ordovician of Central Pennsylvania

Fracture patterns, interactions, and crosscutting relationships are tools for interpretation of fractures as paleostress indicators for past tectonic events and as past or present-day fluid-flow networks. In the Appalachian Basin in Central Pennsylvania along Mount Nittany Expressway Route 322 lies a significantly stratified fracture set hosted in Ordovician age limestone. Tectonic strain is a problematic mechanism for these fractures because they are hosted in individual beds lacking apparent mechanical significance relative to other limestone beds in the outcrop. Many of the fractures are layer-parallel, a characteristic commonly observed in shales, due to shales' mechanical anisotropy and tendency to develop fluid overpressures; however, these fracture-hosting limestones lack obvious mechanical anisotropy. Fracture orientations vary, but desiccation, bentonite swelling, and dolomitization are eliminated by an interpreted transgressional paleoenvironment and a deficiency of the hypothesized minerals. X-ray diffraction determined the composition of samples collected, point-count quantification determined fracture intensity, and optical petrography recorded scaled petrographic photographs. Comparison between fracture intensity and host-rock minerals reveal that silica content is consistently depleted in fractured layers relative to unfractured layers. The diagenetic transition of biogenic silica to quartz is suggested to be the driving mechanism based on silica being present as biogenic grains, as well as cement and detrital grains, and fractures being filled with calcite cement. Silica migration explains the volume lost from fractured layers in a proposed horizontal fracturing mechanism whereby the host rock shrinks but is excluded from vertical contraction.