Micromechanical cracking processes in rocks directly control macro mechanical responses under compressive stresses. Understanding these micro-scale observations has paramount importance in predicting macro-field problems encountered in rock engineering. Here, our study aims to investigate the development of precursory damage zones resulting from microcracking pertinent to macro-scale rock failure. A series of laboratory tests and three-dimensional (3D) numerical experiments are conducted on andesite samples to reveal the characteristics of damage zones in the form of strain fields. Our results from discrete element methodology (DEM) predict that the crack damage threshold (σcd) values are 61.50% and 67.44% of relevant peak stress under two different confining stresses (σ3 = 0.1 MPa and σ3 = 2 MPa), respectively. Our work evaluates the strain fields within the range of the σcd to the peak stress through discrete analysis for both confining stresses. We note that the representative strain field zones of failure are not observed as soon as the σcd is reached. Such localized zones develop approximately 88% of peak stress levels although the confinement value changes the precursory strain localization that appears at similar stress levels. Our results also show that the distinct strain field patterns developed prior to failure control the final size of the macro-damage zone as well as their orientation with respect to the loading direction (e.g 17° and 39°) at the post-failure stage. These findings help to account for many important aspects of precursory strain field analysis in rock mechanics where the damage was rarely quantified subtly.
Since each rock type represents different deformation characteristics, prediction of the damage beforehand is one of the most fundamental problems of industrial activities and rock engineering studies. Previous studies have predicted the stress–strain behaviors preceding rock failure; however, quantitative analyses of the progressive damage in different rocks under stress have not been accurately presented. This study aims to quantify pre-failure rock damage by investigating the stress-induced microscale cracking process in three different rock types, including diabase, ignimbrite, and marble, representing strong, medium-hard, and weak rock types, respectively. We demonstrate crack intensity at critical stress levels where cracking initiates (σci), propagates (σcd), and where failure occurs (σpeak) based on scanning electron microscope (SEM) images. Furthermore, the progression of rock damage was quantified for each rock type through the fractal analyses of crack patterns on these images. Our results show that the patterns in diabase have the highest fractal dimensions (DB) for all three stress levels. While marble produces the lowest DB value up to σci stress level, it presents greater DB values than those of ignimbrite, starting from the σcd level. This is because rock damage in ignimbrite is controlled by the groundmass, proceeding from such stress level. Rock texture controls the rock stiffness and, hence, the DB values of cracking. The mineral composition is effective on the rock strength, but the textural pattern of the minerals has a first-order control on the rock deformation behavior. Overall, our results provide a better understanding of progressive damage in different rock types, which is crucial in the design of engineering structures.
Mühendislik yapılarının içinde veya üzerinde inşa edilen kayalarla nasıl bir etkileşim içinde olacağını anlamada sayısal modelleme yöntemleri oldukça etkileyeceği bilgiler sunmaktadır. Ancak bu bilgilerin mevcut kaya ortamını ne denli temsil edici olduğu oluşturulan sayısal modelin güvenilirliğine bağlıdır. Bu nedenle bir modelin oluşturulmasında etkili olan mikro parametrelerin en doğru şekilde kalibre edilmesi ve model sonuçlarının, modelin çözünürlüğünden ve/veya boyutlarından bağımsız olması gerekmektedir. Son yıllarda kaya gibi karmaşık katı yapıların mekanik davranışlarının belirlenmesinde yaygın olarak ayrık elemanlar yöntemi (DEM) kullanılmaktadır. Söz konusu çalışmanın amacı, bu yönteme dayanan sayısal bir modelin oluşturulmasında gerekli mikro parametrelerin, bir kayanın makro mekanik özellikleri ve deformasyon davranışı üzerindeki etkisini araştırmaktır. Her bir mikro parametrenin ayrı ayrı ele alındığı çalışmada, Yade açık kaynaklı DEM kodu kullanılarak çok sayıda tek eksenli sıkışma, tek eksenli çekme ve üç eksenli sıkışma dayanım deneyi simülasyonları yapılmıştır. Elde edilen sonuçlar, kayaların tek eksenli sıkışma dayanımının (UCS) öncelikle mikro-kohezyona, tek eksenli çekme dayanımının (UTS) ise birincil olarak partiküller arası çekme dayanımına bağlı olduğunu göstermiştir. Ayrıca bu iki dayanım özelliği mikro-elastisite modülü ve rijitlik oranıyla da güçlü bir etkileşim içindedir. Kayaların deformasyon özelliklerinden olan Young modülü (E) ve Poisson oranı (ν) ise doğrudan mikro-elastisite modülü ve rijitlik oranı ile denetlenmektedir. Mikro-içsel sürtünme açısındaki artış kayanın yenilme zarfının eğimini artırırken, dayanım oranının (UCS/UTS) saptanması deney simülasyonları başlamadan önce atanan koordinasyon sayısıyla belirlenmiştir. Bu çalışma sayısal model parametrelerinin birbirleriyle olan etkileşimlerine göre bir kayanın dayanım ve deformasyon özelliklerinin bağlı olduğu koşulları göstermektedir. Elde edilen sonuçlar, mühendislik yapılarının inşasında karşılaşılacak kaya davranışlarını önceden kestirebilen sayısal modellerin geliştirilmesinde uygulanabilir, pratik ve yol gösterici bilgiler sunmaktadır.
<p>The deformation and failure processes of rocks under stress are primarily induced by microcracking. Detecting this micro-interaction phenomenon before the ultimate failure has paramount importance for predicting the post-failure rock damage characteristics. In this study, we aim to quantify the evolution of microcracking through fractal analyses of scanning electron microscope (SEM) images, captured from three different rock types subjected to uniaxial loading at various stress levels. In terms of uniaxial compressive (UCS) and tensile strength (UTS) values, the rocks range from the strongest to the weakest as being diabase, ignimbrite, and marble, respectively.&#160; All rock samples are uniaxially loaded up to critical stress thresholds as crack initiation (&#963;<sub>ci</sub>), crack damage (&#963;<sub>cd</sub>), and peak stress (&#963;<sub>p</sub>) levels, considering their pre-defined characteristic stress-strain curves. Using the box-counting technique, the fractal dimension values (D<sub>B</sub>) of cracking intensity, induced by loading are determined for all these three stages. Here, it should be noted that higher fractal dimensions represent more intense microcracking according to the fractal theory. The results show that the D<sub>B</sub> values are increasing with the increasing amount of microcracks and the greatest D<sub>B</sub> values are calculated for Diabase due to its highest strength ratio (UCS/UTS). Although the marble has the weakest strength values, it presents a higher D<sub>B</sub> value than that of ignimbrite (D<sub>Bmarble</sub> = 1.215 and D<sub>Bignimbrite</sub> = 1.133) once the &#963;<sub>cd</sub> stress threshold is reached. Furthermore, the D<sub>Bmarble</sub> value is also greater than the D<sub>Bignimbrite</sub> value for the &#963;<sub>p</sub> stress level. It is because marble has a higher UCS/UTS ratio than the ratio of ignimbrite. Our results highlight the important role of rock texture on brittleness which exerts a primary control on fractal dimensions (D<sub>B</sub>). A decrease in volumetric rigidity is more dramatic in marble than in ignimbrite with incremental loading. The insights provide a better understanding of the microcracking process that leads to macro-scale deformations in rock engineering.</p>
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