| In practical engineering,brittle materials such as concrete are often affected by various factors such as impact load,nonstandard construction,temperature and so on,which will produce various forms of crack defects.These crack defects will seriously affect the mechanical properties and failure mode of brittle materials,and bring great security risks.Therefore,in order to improve the mechanical properties and crack resistance of materials,scholars at home and abroad have carried out a lot of experimental analysis and mechanism research on crack propagation and failure mode of brittle materials such as rock and gypsum with existing cracks.However,there are few studies on concrete structure with existing cracks,and further experimental research and analysis are needed,it provides some theoretical help for practical engineering.A series of uniaxial compression tests were carried out on the concrete prism with cracks at different angles and positions.The crack propagation and failure modes of the specimens were analyzed.On this basis,in order to study the inhibition effect of fiber doping on the existing crack propagation,the uniaxial compression test of fiber reinforced concrete prism with existing cracks was carried out.At the same time,the acoustic emission technology was used to monitor the test process of each group,and the relevant data of the axial compression cracking process of the specimens were obtained.Finally,GDEM software was used to simulate the typical test conditions,and the results were compared with the test results to further explain the test results.The main research results are as follows.(1)The samples are with inclination angle of 30° and 45°.The uniaxial compression test was carried out in the laboratory.The results showed that when the existing crack was located at the edge,the tip of the existing crack near the boundary started to crack first,and then the other tip started to crack.As the crack tip near the boundary had a higher strain energy release rate,the crack was easier to expand.(2)The fiber can inhibit the crack initiation and propagation of the tension wing at the crack tip.With the increase of fiber content,the symmetrical tension wing crack at the crack tip gradually shortens.When the fiber content exceeded 2 kg/m3,the wing crack at the tip of the existing crack basically disappeared,it can effectively restrain tension failure,prevent brittle failure and improve ductility.(3)With the increase of fiber content(0?4kg/m3),the failure mode of concrete prism with cracks gradually changed from tension failure to tension shear failure,and finally to shear failure.(4)With the increase of fiber content,the crack initiation strength and peak strength of prism with fractures increased first and then decreased.The crack initiation strength reached the maximum at 2kg/m3,and the maximum increase was 7%;the peak strength reached the maximum at 1kg/m3,and the maximum increase was 7%.The results showed that the proper amount of fiber could effectively inhibit the expansion of existing cracks,while excessive fiber would reduce the effective pressure bearing area of the specimen,thus weakening its compressive performance.(5)Combined with the relationship between AE characteristics and stress variation with time,the increase of fiber content prolonged the crack initiation time,the time from AE signal peak to signal disappearance and the total time of specimen failure process.The results showed that the fiber increased the crack initiation load at the crack tip,and improved the plasticity and ductility of the specimen.(6)When the existing crack was located at the edge,the tensile stress area near the crack tip of the specimen boundary was larger and expands first;with the increase of polypropylene fiber content,the tensile stress area at the crack tip decreased gradually.When the crack started,the fiber at the crack tip played a full role in transmitting stress,thus delaying the crack tip expansion. |
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