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Abstract Previous experimental and theoretical studies showed that rock strength was proportional to the 1/(n+1)-th power of loading-rate and creep lifetime was inversely proportional to the n-th power of creep stress, where n was the constant value depending on rock type and testing environment. This paper introduced the n values of various rocks and analyzed the relation of the n value to the loading condition, testing environment and other mechanical properties. The n values were almost constant in unconfined various loading conditions if rock type and testing environment were same. The n value decreased in water-saturated condition and increased under confining pressure in proportion to strength. The effect of time was incorporated into Rock Mass Rating (RMR) with the aid of the n value. A formula representing the reduction of RMR due to time was proposed. 1 Introduction Understanding time-dependency of rock is indispensable for geotechnical applications to estimate long-term deformation and stability of underground structures. In previous studies the authors investigated the time-dependent behaviors of rock such as loading-rate dependency of strength (Hashiba et al. 2006, Hashiba et al. 2011) and creep (Shin et al. 2005, Okubo et al. 2010). The results showed that peak and residual strengthswere proportional to the 1/(n+1)-th power of loading-rate, where n was the constant value depending on rock type and testing environment. It was also found that creep lifetime was inversely proportional to the n-th power of creep stress, where n was the same value as the loading-rate dependency under the same loading condition and testing environment. Therefore the n value can be an index of time-dependency for rock. The smaller the n value, the larger the time-dependency of rock, that is the larger the increase of strength with an increase in loading-rate and the larger the increase of creep lifetime with a decrease in creep stress. Okubo et al. (2013) reported the n values obtained in previous studies for ten Japanese rocks and a Chinese coal. This paper introduces the results and analyzes the relation of the n value to the loading condition, testing environment and other mechanical properties of rock.
- Geology > Rock Type (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Materials (0.55)
- Banking & Finance (0.40)
ABSTRACT: Understanding the dynamic mechanical properties of jointed rocks is beneficial for the rational design and stability analysis of rock engineering projects. This study experimentally investigated the influences of joint geometrical parameters on the dynamic fatigue mechanism of jointed rock models using the synthetic materials with artificially intermittent joints. Our results revealed that the dynamic fatigue stress-strain curve of jointed rock is dominated by its static uniaxial curve; the terminal failure strain in dynamic fatigue curve is equal to the post-peak strain corresponding to the maximum cyclic stress in the static stress-strain curve. The studied four joint geometrical parameters significantly affect the dynamic properties, including the energy evolution, the damage variable and the crack coalescence pattern. The higher the geometrical parameter, the higher the fatigue damage accumulates in the first few cycles, and the lower the fatigue life. Two basic micro-cracks, i.e., tensile wing crack and shear crack, are observed in dynamic cyclic tests, which are controlled principally by joint dip angle and persistency. In general, shear cracks only occur in jointed rocks with high dip angle or high persistency, and these jointed rocks are associated with the low fatigue strength, the large damage variable and the low fatigue life. 1. INTRODUCTION
Effect Of Cyclic Loading On Circular Openings - Results Of A Laboratory Simulation
Cho, Taechin F. (Department of Metallurgical and Mineral Engineering, and Applied Superconductivity Center, University of Wisconsin) | Haimson, Bezalel C. (Department of Metallurgical and Mineral Engineering, and Applied Superconductivity Center, University of Wisconsin)
ABSTRACT ABSTRACT: The mechanical behavior of hollow cylinders of Niagara dolomite under cyclic internal pressurization was investigated using a computerized testing system. This loading configuration approximately simulates pressurized boreholes or tunnels, or annular caverns under electromagnetic forces of superconductive magnets, subjected to periodic internal pressure fluctuation. We used loading rates equivalent to 10 to 10,000 sec./cycle, approaching levels representing diurnal cycling. Test results suggest that expanding rock opening size weakens the fatigue resistance. Additionally, fatigue life of openings in Niagara dolomite depends both on the maximum cyclic stress level and the cyclic period. Life is reduced when the opening is subjected to higher cyclic stress or/and to longer cyclic periods. The total hysteresis energy, which represents the amount of energy dissipated during cyclic pressurization, and the total permanent tangential strain, are each linearly related to fatigue life when plotted on a logarithmic scale. A potentially useful method is suggested for estimating the fatigue life of circular openings under cyclic internal loading based on the measurement of the average permanent tangential strain per cycle. 1 INTRODUCTION An understanding of the mechanical behavior of rock under cyclic loading is essential for the rational design and longevity estimation of underground openings subjected to fluctuating forces. The mechanical properties of rock under static loads have been studied by many investigators. But the behavior of rock under cyclic loading, which often causes failure of rock structures below their static strengths, has been largely neglected and is still not fully understood. Information concerning the mechanical response of rock under cyclic load is scarce due in part to the time consuming nature of such laboratory tests, the narrow scope of cases in which cyclic fatigue is of interest and, perhaps, due to a lack of awareness that the cyclic effect can be decisive. The mechanical behavior of rock under uniaxial and triaxial cyclic loading has been investigated at the University of Wisconsin for the last 15 years or so. These and other studies have established that cyclic loading in rock can bring about fatigue (the phenomenon of pre- mature failure at maximum applied stress level lower than the static strength). For any level of maximum applied stress (defined as 'fatigue strength', S, and expressed as the ratio of this maximum stress to the corresponding static strength) above a threshold characteristic of the rock there is a corresponding number of cycles (defined as 'fatigue life', N) that can bring about fatigue failure. Experimental results show that in uniaxial compression rocks can fatigue at S levels as low as 0.7 if loading is repeated hundreds of thousands times (Haimson and Kim, 1971); in uniaxial tension the same fatigue strength requires ten thousands cycles or so (Tharp, 1973); whereas in cyclic uniaxial tension - compression even a relatively low maximum compressive stress (one third of the compressive strength) accelerates the tensile fatigue mechanism, and an S level of 0.7 requires only several hundreds cycles to bring about failure (K. Kim, 1976). On the other hand, cyclic triaxial compression improves fatigue life by several orders of magnitude as compared with uniaxial loading, depending on the confining pressure (Haimson, 1974).
- Research Report > New Finding (0.68)
- Research Report > Experimental Study (0.54)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Dolomite (0.48)
SUMMARY: The effects of cyclic loading on soft saturated porous rock have been investigated. A typical phenomenon is so called cyclic fatigue in which a material fails at a stress level lower than its static strength. Deformation, strength and behaviour of pore pressure under quasi-static and cyclic loading were studied in undrained test conditions. In addition, creep tests were conducted in order to know the long-term strength. The sample used in the experiments is porous soft tuff whose unconfined compressive strength is about 16 MN/m. Laboratory experiments have been performed to know the correlation between creep, cyclic loading and conventional strain-rate constant tests. The concept of complete stress strain curve was evaluated. RESUME: Les effets de chargement cyclique sur roche poreuse saturee molle ont ete examines. Un phenomène typique est la soidisante fatigue cyclique, dans laquelle un materiel se rompt à un niveau de tension plus bas que sa resistance statique. La deformation, la resistance et le comportement de la pression de l'eau interstitielle sous chargement quasi-statique et cyclique ont ete vises dans des conditions d'essai non draine. De plus, des essais de fluage ont ete faits pour savoir la resistance à long terme. Les echantillons des experiences sont du tuf poreux mou, dont la resistance de compression sans frottement lateral est environ 16 MN/m. Des essais de laboratoire ont ete faits pour savoir la correlation entre le fluage, le chargement cyclique et les essais conventionels de vitesse de deformation constante. ZUSAMMENFASSUNG: Die Effekte der zyklischen Belastung auf weichen, gesattigten porösen Felsen sind untersucht worden. Ein typisches Phanomen ist die sogenannte zyklische Ermuedung, wobei ein Material bei einem Druck niedriger als die statische Festigkeit bricht. Verformung, Festigkeit und Betrage des Porenwasserdrucks unter quasi-statischen und zyklischen Belastungen unter undrainierten Versuchsumstanden sind erforscht worden. Weiterhin sind Kriechversuche zur Erforschung der langfristigen Festigkeit gemacht worden. Die in den Experimenten verwendeten Proben sind poröse, weiche vulkanische Tuffe, deren Kompressionsfestigkeit bei verhinderter Seitendrehung ungefahr 16 MN/m betragt. Laboratoriumsversuche zur Erforschung der Korrelation zwischen Kriechen, zyklischer Belastung und konventioneller konstanter Verformungsgeschwindigkeit sind angestellt worden. INTRODUCTION Foundations of dams, roads and bridges, underground space like tunnels and chambers are subjected to cyclic loading caused by earthquakes, traffics, blasting, etc. The effects of cyclic loading on several different civil engineering materials such as steel, concrete and soil have been investigated. A typical phenomenon is so called cyclic fatigue in which a material fails at a stress level lower than its static strength. However, 'little work in this subject have been done in the area of rock mechanics. The influence of combined stresses and pore Water pressure have not been investigated. It is known that the fatigue curve in cyclic loading is similar to the static creep curve. The reason why is not well documented theoretically or experimentally. Scholz and Koczynski (1979) tried to explain these rock behaviors under cyclic loads with hard crystalline rocks. Their conclusion was that three types of cracking result in dilatancy stress-induced cracking; stress-corrosion cracking and fatigue' cracking. Rock fracture is sensitive to which type is prevalent. In engineering practice, in relation to cyclic loading on rock, an idea of complete stress-strain curve was presented by Haimson (1974). This idea may be useful to explain phenomenologically the similar behaviors between cyclic and creep loading conditions. The purpose of this research is to examine a number of features of rock deformation and fracture that are not well--,observed in more conventional test. Soft saturated porous sedimentary rocks were selected for undrained triaxial tests. Deformation, strength and behavior of pore water pressure under quasi-static and cyclic loading have been investigated. In addition, creep tests were conducted to know the "long-term" strength. The results of these tests are interpreted in view of the complete stress-strain curve. CONCEPT OF COMPLETE STRESS-STRAIN SURFACE It is known that the accumulated permanent strain for different upper peak cyclic or static stresses is bounded by the complete stress-strain curve (Haimson, 1974). The envelope of gradually increased repeated (GIR) loading curve is also found to be the complete stress-strain curve (Akai, et. al., 1981) as shown in Fig. l(a). It is recognized that the higher the strain rate is, the stronger is the rock. At the very slow rate of strain test, a rock specimen sometimes does not show even a distinguished peak. The confining pressure in triaxial tests also change the shape of stress-strain curve as depicted in Fig. l(a). Therefore, the infinite number of complete stress-strain curves may be defined with different strain rates and confinements. This means that a surface of complete stress-strain which bounds a state is possibly established in an adequately defined stress, strain and time space. If a complete stress-strain curve is a test path on the complete stress-strain surface, it may be used to predict failure of rock as a result of creep and relaxation.
- Geology > Geological Subdiscipline > Geomechanics (0.70)
- Geology > Rock Type > Sedimentary Rock (0.61)
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (0.54)
ABSTRACT To examine the fatigue process of granite, Westerly granite specimens were subjected to a cyclic loading test under uniaxial compression at a maximum of 160MPa and a minimum of 80MPa, and the crack growth patterns were analyzed. As a result, at the primary stage, marked crack growth was identified in quartz grains, but no preferential orientation was found in these cracks. It showed that the degradation in this stage was caused by the growth of pre existing cracks mainly in quartz grains. During the second stage, the crack initiation was more frequently than previous stage. Crack development shifted into feldspar grains from quartz grains. It was estimated that the initiation and growth of these cracks resulted in the slight increasing rate of strain in this stage. At the final stage, many intergranular cracks parallel to the loading direction were formed by linking cracks generated during the former stages.
- Geology > Rock Type > Igneous Rock > Granite (0.89)
- Geology > Mineral > Silicate > Tectosilicate > Quartz (0.69)