Fractal characteristics and damage mechanism of acoustic emission during whole fracture process of ultra-high performance concrete
In order to study the acoustic emission(AE)mechanism and fractal characteristics of ultra-high performance concrete(UHPC)in the whole process of axial tensile failure.Firstly,the failure tests of UHPC axial tensile specimens reinforced with different steel fibers(end hook type,microwave texture type and straight circular type)were carried out,and the AE signals in the process of UHPC axial tensile failure were obtained.Secondly,the dominant frequency of the signal was calculated by fast Fourier transform,and the distribution characteristics of the dominant frequency in the test process were analyzed.Then,based on the fractal theory,the fractal analysis of the signal was carried out by using the fractal box dimension calculation method,and the correlation between the fractal box dimension and the dominant frequency was explored.Finally,the wavelet packet transform method was used to calculate the average frequency band energy during the failure process,and the evolution law of each frequency band energy under different stress states was analyzed.The results show that the end hook and ripple of the deformed fiber produce a higher dominant frequency signal during the sliding process with the matrix,while the straight circular fiber produces a lower dominant frequency signal during the sliding process with the matrix.In the failure stage of the specimen,the signal distribution of the dominant frequency band increases,but it is still dominated by matrix micro-crack and relative slip of steel fiber.The fractal box dimension is concentrated before the cracking of the specimen,and the fractal box dimension increases when the specimen is about to be destroyed,and becomes dispersed after cracking.The fractal box dimension of the signal generated by the new micro-crack is similar to that before cracking,and the fractal box dimension of the signal generated by the macro-crack propagation is reduced.The average fractal box dimension increases with the increase of the dominant frequency,which is approximately a second-order polynomial.The higher the dominant frequency,the higher the signal complexity.The energy of AE signal is concentrated in the 1 to 4 frequency band.The main energy distribution of different fiber reinforced specimens is different in the frequency band,but the energy of all specimens changes from low frequency to high frequency when they are about to be destroyed.It shows that the acoustic emission signal has good fractal characteristics and can well reflect the damage characteristics of UHPC in the process of axial tensile failure.3 tabs,10 figs,36 refs.