Considerations for Discrete Element Modeling of Rock Cutting

This study attempts to build a framework within the Discrete Element Method (DEM) to produce a reliable predictive tool in rock cutting applications, such that the cutting forces and fragmentation process are reasonably estimated. The study is limited to shallow depth cutting, often the mode of cutting involved in drilling operations.\ud Rock cutting requires the consideration of tool-rock interaction and the damage or fracture of rocks. With respect to modeling, rock cutting becomes a sequence of difficult problems: A contact problem first arises as a cutter advances and interacts with a target rock. This is followed by the problem of determining the location and nature of the rock failure. In the event of rock failure, a modeler must then consider modeling the initiation of the fragmentation process. The adopted approach utilizes the intrinsic capability of DEM to adequately consider contacts and model fractures. The commercial DEM codes PFC2D and PFC3D from Itasca were used.\ud This modeling effort focuses on the rock cutting that occurs during rock scratching tests. Two primary reasons provide the impetus of this investigation: first, a rock scratching test possesses all essential characteristics of a general rock cutting problem; second, available test data, particularly data obtained by Richard [1], provide a basis for validation. Modeling the scratch test also served another purpose for understanding the mechanics of drilling into rock because the cutting action is very similar to that of a single polycrystalline diamond compact (PDC) bit.\ud The validation of the present modeling effort utilizes an observation made by Richard and Dagrain [2] during shallow cuts that the specific energy obtained in a scratch test is approximately equal to the uniaxial strength of the rock. Rocks were represented as bonded particles [3]. This study first explores the sensitivity of the essential parameters that affect rock behavior and parameter selection necessary to realistically represent a rock. Extensive two-dimensional analyses were first completed and followed by three-dimensional analyses, all of which were conducted under an ambient pressure environment.\ud This study also addressed an important question regarding rock porosity. The current practice often implicitly considers porosity. Essentially, a porosity that is computationally simple and advantageous but ultimately unrealistic is used and other DEM parameters are consequently adjusted until the desired modulus and strength are produced. This sample is then considered mechanically equivalent. The ability to substitute rock materials of low porosities with higher values is extremely beneficial for computational efficiency. Samples with small porosity values were generated by solving the Apollonius’ problem to fill voids with particles, and therefore, the influence of initial sample microstructure could be studied.\ud The Unconfined Compressive Strength (UCS) for most rocks is generally about ten times greater than that of the tensile strength [4]. This ratio, considered to be realistic rock behavior, has been historically difficult to obtain in similar models. In order to achieve this strength ratio, microdefects were also introduced into the sample. This study was able to implicitly model porosity by introducing optimal microdefects percentages in order to create equivalent rock samples with varying porosity values. Moreover, a connection between two-dimensional and three-dimensional samples was also established by finding an appropriate porosity to match the two models.\ud This study presented a validated and simplified framework for modeling rock cutting, and should be useful for general applications for a wide variety of fields. Preliminary work on cutting under high pressure was also initiated and yielded results that would be useful for subsequent studies.\u