Study On Fatigue Failure Characteristics And Energy Fracture Criterion Of Fractured Limestone

With the large-scale development of infrastructure projects,rocks are often subjected to repeated loads during the construction of many underground projects.As a complex material,the stability of rock is very important for construction.Study the mechanical properties of fractured rocks under cyclic loading.Has important theoretical value.The work and main content of this thesis are as follows:(1)Carry out static conventional splitting tests to study the failure modes and strength characteristics of various Brazilian discs under different angles.The force-displacement curve of the Brazilian disc with cracks can be divided into three stages:elastic deformation,stable crack expansion and instability,while the deflection double-cracked Brazilian disc has an additional crack compaction stage.All three Brazilian discs are brittle failure;The fracture toughness of a single fracture is calculated,and the relationship between the fracture toughness of KI and KII and the loading angle is studied.(2)Based on the above Brazilian disc specimens,the loading process of several discs can be divided into three stages:initial deformation stage,constant velocity deformation stage and accelerated failure stage.The hysteresis loops of several stages show a trend of'sparse-dense-sparse';The dynamic elastic modulus of each 200 cycles is analyzed.The dynamic elastic modulus is strengthened in the second stage of loading,and the dynamic elastic modulus of rock decreases after entering the third stage.The increase of irreversible deformation is the cause of rock fatigue failure.Therefore,according to the curve of axial strain and cycle number,the failure formula of axial strain and cycle number is deduced to predict the fatigue life.(3)Analyzing the energy evolution law of three kinds of Brazilian disc specimens under the action of graded cyclic loading,the total absorbed energy,dissipated energy and elastic energy all show an upward trend.The total absorbed energy grows slowly,the growth rate gradually slows down with the number of cycles,the dissipated energy rises rapidly after entering the destruction stage,and the elastic energy changes trend is not obvious.Fissure inclination has a certain influence on energy.Single fissure 45~°and double fissure 60~°consume more energy.(4)With the help of non-contact video measurement,analyze the crack propagation failure mode during the hierarchical cyclic loading process.During the failure process,the three Brazilian discs will produce wing cracks and inclined secondary cracks.The cracks interact and merge to produce continuous shaping.Strain accumulates to form crushed small particles,which is different from the brittle failure mechanism under static load.(5)The essence of rock failure is energy dissipation.Based on the principle of minimum dissipated energy proposed by the predecessors,the damage evolution equation under cyclic loading is derived,the number of cycles N is introduced as a variable,and a fracture failure criterion suitable for cyclic loading is proposed.The fatigue life prediction of engineering provides a theoretical basis.