Loading Rate Dependence Of Strength Research Articles

Experimental Study on the Accelerating Creep of Rock Under Dry and Wet Conditions Based on a New Test Method

AbstractCreep tests under constant stress are among the most important tests for investigating the time-dependent behavior of rock. In previous studies, creep tests were conducted under various loading and environmental conditions; however, it was very difficult to demonstrate accelerating creep at failure at stresses less than 50% of strength in conventional creep tests. In this study, a new test method is proposed that combines an alternating loading-rate test and a creep test in the post-failure region. The accelerating creep as well as the loading-rate dependence of strength were successfully obtained simultaneously from a single specimen. This test method was applied to three rock types under dry and wet conditions. The results of Sanjome andesite under both dry and wet conditions, Kimachi sandstone under dry conditions, and Tage tuff under dry conditions showed a similar relationship between creep strain, creep strain rate, and residual time (i.e., creep lifetime minus elapsed time), from high to low creep stress levels, and a close relationship between accelerating creep and the loading-rate dependence of strength. Further, the mechanism responsible for the time dependence was found not to change significantly from high to low stress levels. In contrast, different results were obtained when high and low creep stress levels were compared in Kimachi sandstone and Tage tuff under wet conditions. The larger increase in creep strain with increasing creep strain rate at lower creep stress levels was related to the shape of the stress–strain curve in the post-failure region.

Effects of water saturation on the strength and loading-rate dependence of andesite

Water saturation conditions in the rock masses around underground openings vary with place and time. For the long-term stability assessment of underground structures, it is essential to understand the effect of water on the time-dependent behavior of rocks. In this study, the effects of water saturation on the loading-rate dependence of strength, which is one of the time-dependent characteristics of rocks, were investigated both theoretically and via experiments. First, the effects of loading rate and water saturation on rock strength and its variability were clarified by means of a theory that is based on both stochastic and rate processes. Next, alternating loading-rate tests were conducted on Sanjome andesite using uniaxial compression testing under various water saturation conditions. The test results showed that the rock strength increases with a decrease in water saturation and that an increase in strength accompanied by a tenfold increase in loading rate is almost constant under various water saturation conditions. All of the test results were consistently explained by our theory of the dependence of rock strength and its variability under different loading rates and water saturation conditions. It was also found that the dependence of strength on the loading rate is equivalent to that on the water saturation, and that the strength obtained from a test could be converted to one under an arbitrary loading rate and water saturation condition. In addition, we proposed a constitutive equation based on these theoretical and experimental results that could reproduce the loading-rate dependence of stress-strain curves under various water saturation conditions using a single set of constants.

New Multi-Stage Triaxial Compression Test to Investigate the Loading-Rate Dependence of Rock Strength

Abstract Compression tests for investigating the time-dependence of rock under confining pressures are time consuming and require more specimens than do uniaxial compression tests. In this study, a new testing method combining multi-stage confining pressure and alternating loading rate is proposed to investigate the loading-rate dependence of triaxial compressive strength from a small amount of rock sample. In the test, a small-size rock specimen 10 mm in diameter and 20 mm in height was loaded under a strain rate alternating between slow and fast and under a confining pressure increased once just after reaching the peak strength. Strengths corresponding to two strain rates and two confining pressures could be obtained from one specimen. The cohesion, angle of internal friction, and loading-rate dependence of strength from the data were consistent with those from the commonly used triaxial compression tests under a constant strain rate or constant confining pressure. The proposed test with a small amount of rock sample can reduce not only the time and cost but also the environmental load of sampling in situ.

岩盤 つくばでの最近の研究 （２） 稲田花こう岩の高圧下の強度とクリープ寿命の分布に関する検討

In designing the long-term stability of deep underground structures in a rock mass, it is necessary to estimate the creep failure time of rocks under confining pressure. In the case of nuclear waste disposal tunnel, the creep failure time should exceed 10,000 years. In this paper, experimental results of tri-axial and multi-stage creep experiments are presented. Both kinds of experiments were conducted under confining pressures up to 40 MPa, including uni-axial stress state. In order to eliminate the loading rate dependency of strength in analysis, the concept of standardized strength is introduced. The Janach's failure criterion was successfully applied to explain the relation between standardized strengths and confining pressures. We introduce a new concept of normalized failure time that is thought to be most substantial value in determining the strength and / or creep failure time of a specific rock specimen. It is shown that the dispersion of normalized failure times, which was estimated from the experimental results, exhibited approximately a Weibull distribution. A conversion formula from normalized failure time to strength and / or creep failure time is presented. Using this conversion formula, we showed that creep stress level should be less than 79 % for survival of 99 % of test pieces after 10,000 years under the confining pressure of 40 MPa. It is also shown that not only the distribution of creep failure times, but also the strength distribution under any loading rate and confining pressure can be calculated from the normalized failure time.

岩盤の破壊基準と長期強度に関する一考察

At first, study on loading-rate dependency of strength was briefly summarized. It is well known that uniaxial (unconfined) compression strength increases with loading rate. Though only limited data are available, some authors reported that confined strength increases with loading rate. Also, tensile strength obtained by uniaxial tension, indirect tension and bending tests increases with loading rate. Based on the summarized experimental results, a failure criterion in which loading-rate dependency of strength is considered was proposed；high criterion for high loading rate and low criterion for low loading rate. As an extreme case, long-term strength or failure criterion at extremely low loading rate can be obtained. The simplest form of the long-term strength is σ1 = a·σ3 where σ1, σ3 are major and minor principal stresses, and constant `a' varies from 2 to 6 depending on rock. For validation of the proposed failure criterion, rock stresses were plotted on σ1 vs. σ3 diagram. It was found that all data except one were plotted under the line σ1 = 4σ3. It is reasonable to consider that, in relatively stable rock mass, rock stress is maintained at a certain value or condition lower than a long-term strength. If so, this result supports the validity of the proposed failure criterion.

強度のばらつきを考慮したー軸引張試験の計算機シミュレーション

In the previous studies, authors carried out the computer simulation by FEM program and discussed the failure process and the size effect of rock specimen model subjected to compressive stress.In this study, behaviour of rock specimen under uniaxial tensile stress is examined by a similar computer simulation and some experimental work. For the computer simulation, FEM program implemented with a non-linear visco-elastic constitutive equation is used. In the experimental work, two kinds of rocks are tested under uniaxial tension to obtain the complete stress-strain curves. Surface of a rock specimen is carefully examined with the aid of CCD camera.The calculated results indicate that failure process of rock specimen is considerably complex, that is, cracks initiate and propagate at many separated locations in the specimen and a macro crack which leads to final failure is not a straight line but having many sharp turns. This calculated result conforms to the experimental result.The computer simulation also indicates that strength of specimen decreases with increase of the number of elements in FEM mesh. However, number of elements exceed 90, then strength dose not change apparently. It is also found that the loading rate dependency of strength decreases with number of elements. These results quantitatively coincide with the results obtained in the previous study for compression stress.