A numerical study of the influence from pre‐existing cracks on granite rock fragmentation at percussive drilling

Abstract

SUMMARYThe aim of this study is to investigate the effect of pre-existing, or structural, cracks on dynamic fragmentation of granite. Because of the complex behavior of rock materials, a continuum approach is employed relying upon a plasticity model with yield surface locus as a quadratic function of the mean pressure in the principal stress space coupled with an anisotropic damage model. In particular, Bohus granite rock is investigated, and the material parameters are chosen based on previous experiments. The equation of motion is discretized using a finite element approach, and the explicit time integration method is employed. The pre-existing cracks are introduced in the model by considering sets of elements with negligible tensile strength that leads to their immediate failure when loaded in tension even though they still carry compressive loads as crack closure occurs because of compressive stresses. Previously performed edge-on impact tests are reconsidered here to validate the numerical model. Percussive drilling is simulated, and the influence of the presence of pre-existing cracks is studied. The results from the analysis with different crack lengths and orientations are compared in terms of penetration stiffness and fracture pattern. It is shown that pre-existing cracks in all investigated cases facilitate the drilling process.

Experimental and numerical study of the damage process inRCbeam‐column sub‐assemblages during a progressive collapse scenario

“…The Denoual-Forquin-Hild (DFH) anisotropic damage model [37][38][39] has been employed in numerous studies to simulate the growth of macro-cracks in concrete structures subjected to impact loadings [40][41][42] or the fracturing process in a granite rock under percussive drilling 43 or subjected to spalling tests or edge-on impact tests. [43][44][45] The DFH damage model considers three damage variables (D 1 , D 2 , and D 3 ) that are used to model the loss of stiffness along the three orthogonal directions that define the three principal directions of the stress tensor. The strain tensor ε ii is related to the stress tensor Σ ii according to [37][38][39]…”

Brittle materials at high-loading rates: an open area of research

“…The EOI technique has been widely used to investigate the damage modes produced by impact loadings of ceramic materials [12,46,47], ultra-high-strength concrete [48] and rocks [3,15,49]. An ultra-high-speed camera is used in the so-called open experimental configuration to observe the growth of damage at recording frequencies of approximately 1 Mfps (million frames per second).…”

Application of microtomography and image analysis to the quantification of fragmentation in ceramics after impact loading

“…This confinement system ensures a plane strain compression loading near the impact point to prevent any compressive damage in the target. This type of experiment has been performed on ultra-high-strength concrete [10], limestone [11], microconcrete and common concrete [12,13] and granite [14,15]. Finally, it is concluded that all types of brittle material such as ceramics, rocks, concretes or glass tested in EOI experiments have been shown to give rise to a high density of oriented (micro-)cracks initiated from volume or surface defects.…”