Study On Acoustic Emission Identification Of Interface Slip Failure Of Concrete Filled Steel Tube Based On Improved Variational Modal Decomposition Method

Geopolymer materials have the characteristics of low energy consumption,low carbon emissions and high strength in the production process,and can be developed as an alternative to Portland cement-based materials.However,geopolymer materials still have the characteristics of poor toughness and easy cracking,which limits the further application of geopolymer materials in engineering.At present,there are many studies on adding fibers to concrete to enhance the toughness of concrete materials,but relatively few studies on the fracture properties of fiber-reinforced geopolymers,and the toughening law and mechanism of fibers on geopolymer materials have not yet been explored.In this paper,polyethylene fiber and polypropylene fiber are mainly introduced into the geopolymer material,and the fiber factor is defined as a parameter to measure the comprehensive effect of fiber,and the displacement field and strain field in the three-point bending test of the geopolymer specimen are monitored by digital image correlation(DIC)technology and acoustic emission technology,so as to study the influence of fiber factor,water-adhesive ratio,silica content,nano-silica,freeze-thaw cycle times and other factors on fiber reinforced geopolymer.To further improve the mechanical properties of fiber reinforced geopolymer materials,the specific conclusions are as follows:(1)The three-point bending test study of polypropylene and polyethylene fiber reinforced geopolymer showed that the geopolymer specimen could be divided into five typical stages during the fracture process: pre-cracking stage,cracking time,crack stable propagation stage,crack instability moment,and post-crack instability failure stage;When the fiber factor was less than 600,the fracture toughness of the geopolymer increased with the increase of the fiber factor,and when the fiber factor exceeded 600,the fracture toughness decreased with the increase of the fiber factor;under normal circumstances,the fracture toughness of the geopolymer with low water-binder ratios was stronger than that of high water-binder ratios;the unstable fracture toughness gain ratio was greater than the initial cracking toughness gain ratio,indicating that the cracking resistance of the two fibers was fully exerted after the cracking of the specimen;the toughness enhancement of the two fibers in this paper was much higher than that of brucite fiber,steel fiber and PVA fiber.And the reinforcement effect of polyethylene fiber was better,the use of two fibers can make fiber reinforced geopolymer with better fracture performance;Using regression analysis,a fracture toughness calculation model based on water-adhesive ratio and fiber factor was established,and the expressions in this paper were used to fit the data in other relevant literature,which also showed a good fitting effect.(2)On the basis of the above experiments,the fracture performance test of polyethylene fiber geopolymer mortar with variable silica ash was carried out,and the internal damage of the polymer was explored by ultrasonic detection and acoustic emission technology monitoring.The results showed that with the growth of fiber factors,the ultrasonic velocity of the geopolymer decreased,indicating that the incorporation of fibers introduced defects in the geopolymer and leaded to the decrease of the compactness of the geopolymer,so the compressive strength,fracture toughness,fracture energy and ductility index will increase first and then decrease with the increase of the fiber factor,and the values of each index were maximized when the fiber factor was 829.The increase of silica ash content increased the compactness of the geopolymer,which had a positive effect on reducing the negative effects caused by the fiber,so it effectively enhanced the fracture performance of the geopolymer.RA and AF values were introduced by acoustic emission technology to analyze the failure mode of the geopolymer specimen,and the proportion of tensile failure of the geopolymer with high fiber factor and high silica ash content increased.(3)The effects of two influencing factors on the fracture performance of polyethylene fiber reinforced geopolymer were studied by changing the number of freeze-thaw cycles and the amount of nano-silica.The results showed that nano-silica could effectively improve the fracture performance of geopolymers,and its content was positively correlated with the fracture performance.The freeze-thaw significantly reduced the fracture toughness and fracture energy of the specimen,and the number of freeze-thaw cycles was negatively correlated with the two indexes.When the number of freeze-thaw cycles was 0-75 times,the freeze-thaw conditions adjusted the fiber interface bonding,which enhanced the ductility of the geopolymer to a certain extent,and when the number of freeze-thaw cycles reached 75 times,the further increase of the number of freeze-thaw cycles will deteriorate the ductility of the geopolymer mortar;Nano silica can reduce the adverse effects of freeze-thaw on geopolymers,and nano silica can be considered when applying fiber geopolymers in cold zone engineering to improve frost resistance.