Experimental Study On Loading Rate And Size Effect Of Limestone Fracture Characteristics

Rock is regarded as a common and geological material,which is widely used the engineering rock mass.There is a certain difference in fracture characteristics of rock with different sizes,and there are obvious size effects.The effect of loading rate is also important factor that affects the mechanical phenomena of rock materials..In this paper,the fracture characteristics of limestone in different loading rates(the range of 0.002~10 kN/s)and different sizes(the range of 30~150 mm)were studied by using RMT-150 B rock mechanics test system,which get the following preliminary understanding:(1)When the condition of a certain size,the test values of the fracture toughness of the specimen increase logarithmically with the increase of the loading rate.When the loading rate is certain,there is a critical size,which disk diameter is 50 mm.When it is less than this critical size,the fracture toughness test value increases greatly.when it greater than this critical size,the test value increases by a small amount and is basically linearly increased.The reasons caused the loading rate and size effect are discussed,which is helpful to analyze deeply the fracture characteristics of rock and determine the fracture toughness accurately.(2)The true fracture toughness and effective fracture zone length of limestone NSCB samples were obtained by RILEM method,on this basis,the Bazant scale law of limestone fracture toughness under different loading rates was determined.The prediction of fracture toughness has obvious size effect,which is shown to increase with the increase of specimen size,which is consistent with the regularity of fracture toughness test value and size.(3)The true fracture toughness value and the fracture toughness prediction value show a logarithmic relationship with the increase of loading rate,which is consistent with the relationship between the test value and the loading rate.The relative error between the average value of fracture toughness predicted by Bazant scale formula and the average value of fracture toughness test values is less than 5%.It shows that the limestone Bazant size effect formula obtained by RILEM method is more reliable,the fracture toughness value of the sample prediction with high accuracy.(4)The analysis of the deformation and energy of limestone NSCB specimens shows that under the condition of certain size,the average vertical displacement rate of NSCB sample increases with the increase of loading rate and increases linearly.Under the condition of certain loading rate,the average vertical displacement rate decreases with the increase of specimen size,and decreases with the power function trend.Under the condition of certain size,the energy accumulated in the NSCB sample before peak load and the energy rate increases with the increase of the loading rate,and increases logarithmically.the average energy speed increases with the increase of loading rate and increases linearly.Under the condition of a certain loading rate,the energy accumulated in the NSCB sample before peak load increased with increasing size and showed an increase in the power function trend,The energy rate shows the trend of increasing first with the increase of the size,then decreasing and then increasing.the average energy speed increases with the increase of size,and increases with the quadratic function.