Experimental Study On Energy-type Size Effect And Strain Rate Effect Of Quasi-brittle Material Type II

The physical properties of quasi-brittle materials(rocks,concrete)are related to the size of the samples and the loading rate.At present,there is no comprehensive experimental tests on and the physical explanation of the type II energy size effect and strain rate effect.In this paper,a notched semicircular bend rock specimen is loaded by the split Hopkinson pressure bar(SHPB)system and the experimental data was record using strain gauges and high-speed camera,The initial fracture toughness of the specimens under different sizes and strain rates is determined,the size of the fracture zone was observed,and the law of type II energy size effect and strain rate effect of rock material was investigated.The main research results include:(1)The Hopkinson pressure bar loading device and high-speed camera triggering device have been improved,and the success rate of Hopkinson bar signal collection has been increased.When the signal is triggered automatically,the signal of the strain gauge and the high-speed camera are synchronized.(2)The test results show that the initiation fracture toughness of rock samples increases linearly with the increase of loading rate and shows significant strain rate effect.Under the same loading rate,the rock initiation fracture toughness increases with the increase of specimen size,the size effect of rock fracture toughness become stronger with the increase of strain rate,i.e,the higher the loading rate is,the more remarkable the type II energy size effect is.With the same radius of the specimen,the experimental results show that the higher the notch ratio of the specimen,the greater the initial fracture toughness.According to the experimental results,the three-dimensional fitting between initial fracture toughness,loading speed and specimen radius was established.(3)According to the shooting results of high-speed video shooting,digital image processing technology was used to process the photos,and the main strain cloud diagram,displacement vector diagrams,and high-strain region images were obtained to evaluate the development trend of the fracture process zone.The results show that higher loading rates will produce a larger fracture process zone and the corresponding specimens will have higher strength.The results are consistent with those of the strain gauge on Hopkinson's bar.