Fracture Behavior Of Quasi-Brittle Material Subjected To Three-point Bending Beam Test

Brittle materials are materials which are destroyed only by slight deformation under the action of external force and the deformation of ideal brittle materials is considered as linear and irreversible.Quasi-brittle materials,such as concrete,ceramics,glass,some rocks and graphite,will appear microcrack generation,propagation and convergence under the action of external load.The stress-strain curve shows that there is a nonlinear section next the back of the linear elastic deformation section.Nonlinear refers to the behavior of the object is not directly proportional to the force exerted on it,so its stress-strain curve reflects the linear and nonlinear fracture mechanics behavior,this kind of material is more accurately called quasi brittle material.The mechanical properties and damage failure of quasi-brittle materials are the basic scientific issues concerned in the fields of instability and failure of materials or structures,disaster prediction and monitoring,construction and maintenance of engineering.The mechanical properties and damage failure of quasi-brittle materials are the basic scientific issues concerned in the fields of instability and failure of materials or structures,disaster prediction and monitoring,construction and maintenance of engineering.The research history of quasi-brittle fracture mechanics is not long,since its appearance,it is a research direction that attracts much attention.Fracture energy,fracture mouth opening displacement(CMOD)and fracture opening displacement curve(COD-y)are important parameters to characterize the fracture behavior of materials and reflect the mechanical properties of materials.In this paper,the relationship between the mid-span deflection and the crack opening displacement of the beam in the three-point bending test is introduced in detail.the experimental requirements and calculation formulas of fracture energy calculation are simplified by the fitting formula,the new calculation method is more general and the calculation accuracy is improved,it is suitable for both concrete and graphite materials.Due to the multiple strength specimens used in concrete beams,the law of related parameters with intensity changes is also discussed.The non-contact and full-field digital holographic technique(electronic speckle pattern interference image)was used to observe the crack propagation during cracking,and the crack open distance displacement(COD)was recorded for the full length of the crack.The precracking process during crack propagation was investigated.The nonlinearity exhibited by the edge is improved based on the formula of the two-parameter model,and the functional relationship between the crack length and the open displacement of the concrete and graphite materials is given.In addition,the difference between the cut-off point of the crack opening displacement(COD)curve and the fitting formula of graphite and concrete materials is compared.Finally,the main work on fracture behavior of quasi-brittle materials is summarized in this paper,and the further experimental and research contents are pointed out.