| In the process of mine mining,the dynamic phenomena of rock crack propagation and failure are common.It is equally important to master the rock mechanical properties of dynamic load and static load.Rock fracture toughness is an important parameter to describe the ability of rock materials to resist crack propagation.Its accurate measurement has important guiding significance for the understanding of rock failure mechanism and the prevention of engineering structure failure.The test value of rock fracture toughness changes with the specimen size.In order to further reveal the reason for the size effect of rock fracture toughness,and rock fracture toughness was taken as the research object under the guidance of linear elastic fracture mechanics and elastic-plastic fracture mechanics.The size effect of static and dynamic fracture toughness of specimens with different sizes was studied,using GCTS rock mechanics comprehensive test system,split Hopkinson pressure bar SHPB impact system,digital image correlation method DICM,acoustic emission AE technology,fictitious crack model FCM,and finite element.The causes of the size effect of fracture toughness were explained from the perspective of fracture process zone inocubation,and the effects of impact velocity and configuration difference on the size effect of dynamic fracture toughness were studied.The research shows that the main conclusions are as follows:(1)The length of the fracture process zone of specimen under static load determined theoretically is larger than that without considering the the second-order term and the third-order term of Williams series expansion,and is close to the length of the fracture process zone determined by the finite element combined with maximum principal stress criterion;The length of fracture process zone determined by DICM and AE technology is larger than the theoretical value;The length of fracture process zone and fracture toughness increase linearly with the specimen size,and there is obvious size effect within the range of the test;The length of fracture process zone and the fracture toughness do not always increase linearly with the size,but gradually stabilize by linear regression and extrapolation analysis of Bazant size effect theory.(2)The length of the fracture process zone of specimen under dynamic load increases with time,and goes through the initial,stable and accelerated incubation stages;the variation trend of dynamic fracture toughness is increasing,considering the influence of inocubation of fracture process zone;when the inocubation of fracture process zone is considered by inoculating the fracture process zone to the same length,the dynamic fracture toughness fluctuates slightly with the specimen size in the range of length of fracture process zone(1 mm～5 mm),and the dynamic fracture toughness shows a variation trend of "large in the middle and small at both ends" with the specimen size in the range of length of fracture process zone(7mm～15mm),and the size effect of dynamic fracture toughness can be better reduced;When the fracture process zone is inoculated to the upper limit value and the influence of inocubation of fracture process zone is considered,the size effect of fracture toughness of larger size specimens is reduced to a certain extent;The crack propagation direction of the specimen is from the crack tip to the loading end,and a wedge-shaped failure zone is formed at the loading end.(3)At high impact velocity,the dynamic fracture toughness,fracture energy and the length of fracture process zone are relatively larger,and the increase trend is "first decreasing and then increasing" with the specimen size;The incubation time of the fracture process zone decreases with the increase of impact velocity;Considering the dynamic fracture toughness under the same inoculation length of the fracture process zone,the size effect is reduced at low impact speed and high impact speed,and the dynamic fracture toughness at high impact speed is larger than that at low impact speed.(4)The incident energy,reflected energy,transmitted energy and dissipated energy all show obvious size effect;The size effect of dynamic fracture toughness is also indirectly explained from the size effect of dissipated energy,and the energy consumed in the failure process of large-size specimens is greater;Incident energy,reflection energy,transmission energy and dissipation energy all show obvious rate effect,but the increasing trend is different;The size effect and rate effect of energy dissipation rate are not obvious.(5)For dynamic impact test of HCFBD specimen,considering the influence of inocubation of fracture process zone,the change trend of dynamic fracture toughness increases first and then decreases,and the change range is small,while CSTBD specimen shows a continuous increase trend;When the influence of inocubation of fracture process zone is considered by inoculating the fracture process zone to the same length,the dynamic fracture toughness fluctuates slightly with the specimen size in the range of length of fracture process zone(1mm～7mm),which is similar to that of CSTBD specimen;the dynamic fracture toughness shows a variation trend of " small in the middle and large at both ends" with the specimen size in the range of length of fracture process zone(10mm～15mm),Which is different from the change trend of "large in the middle and small at both ends" of CSTBD specimen;In general,for HCFBD and CSTBD specimens,the size effect of dynamic fracture toughness is reduced to a certain extent by considering the influence of inocubation of fracture process zone when the fracture process zone is inoculated to the same length.There are 111 pictures,12 tables,and 184 references. |
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