| The split Hopkinson pressure bar is a commonly used experimental device to test the dynamic mechanical properties of materials.However,in the impact test using cylindrical samples to test the dynamic mechanical properties of rock,whether the sample size and rod diameter match or not,there is still a lack of in-depth research on the reliability of SHPB test and the dynamic mechanical properties of samples.The research in this field is of great significance to promote the wider application of SHPB device in rock dynamic mechanics.When impacting samples of different sizes on a 100 mm SHPB system,based on the experimental requirements of SHPB and the theory of stress wave propagation,the transmission and reflection process of elastic wave and the stress and particle velocity state of each stage were theoretically analyzed in the incident-sample-transmission system,and the variation law of the stress difference at both ends of the sample with the number of transmission and reflection under the impact of SHPB equipment with different rod diameters was compared;The impact compression tests of sandstone samples with three diameters(50 mm,75 mm and 100 mm)and five aspect ratios(0.4,0.5,0.6,0.8 and 1.0)were carried out on Ф100 mm SHPB system,and the impact compression tests of sandstone samples with eight aspect ratios(0.2,0.3,0.4,0.5,0.6,0.8,1.0 and 2.0)were carried out on Ф50 mm SHPB system.The variation law of stress wave on the compression bar was analyzed by strain gauge and high-speed photography The stress-strain curve,strain rate time curve,dynamic failure process and dynamic stress balance law of the sample,and the optimal size range of sandstone sample suitable for SHPB device under two rod diameters were discussed.Six main conclusions were made:(1)Except that the length of the sample affect the propagation distance,the change of the cross-sectional area of the sample will also affect the times of transmission and reflection.The reasonable selection of specimen size is an important method to ensure that the relative stress difference at both ends can meet the negligible axial inertia effect in a short time.(2)Under the impact of Ф100 mm SHPB system,the dynamic stress peak of Ф50mm samples are more affected by the length diameter ratio than that of large diameter samples,and the dynamic stress peak of Ф75 mm and Ф100 mm samples is close;Under the impact of Ф50 mm SHPB system,the peak value of dynamic stress tends to be stable when the sample length increases to a certain value.(3)As the sample size increases,the variation curve of strain rate with time changes from "single peak" to "double peak".The loading of high constant strain rate can be realized by increasing the mismatch between sample diameter and rod diameter.(4)Due to the influence of the superposition of stress waves,the specimen with a diameter of 100 mm produces a large tensile stress in the middle,and the composite failure with radial and axial cracks occurs;Due to the influence of Poisson’s ratio,the rock block outside the sample with a diameter of 100 mm do not participate in the stress wave propagation process due to internal crushing,cracking and separation.(5)Under the condition of similar loading waveform,the increase of rod diameter prolongs the time for the stress at both ends of the sample to tend to uniformity,and the oscillation zone of large-diameter compression rod lasts a long time.Due to more complex damage and crack development in the sample,the stress balance factor curve oscillates obviously in the stable falling zone.(6)The concept of superposition factor for comparing the coincidence degree of waveform alignment is proposed,and the dynamic stress balance research system is constructed together with the balance factor.It is suggested to select Ф75 mm specimens with length diameter ratio of 0.3 ～ 0.4 for Ф100 mm SHPB impact test;And Ф50 mm SHPB impact test with length diameter ratio of 1.0.There are 48 figures,3 tables and 82 references. |
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