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2 2024

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Mechanical behaviour of fracture layer failure in secondary mining composite roof strata

Zhao Guozhen, Chang Fengyi, Wang Shuai

Archives of Mining Sciences

2024

Vol. 69, no. 4

655--674

In the extremely close distance of lower coal seam mining, affected by secondary mining, the fractures expand through the layer in the composite roof, and this process is complex and diverse. In this paper, a three-point bending load test of different strength rocks and their combinations is conducted by combining acoustic emission. The results indicate that the overall deflection angle in the lower roof is approximately 8° smaller than that in the upper roof. When the fracture extends from the brittle roof to the plastic roof, the deflection angle in both types of roofs increases. When the plastic roof extends to the brittle roof, the deflection angle of the brittle roof increases while that of the plastic roof decreases. The damage degree of the composite rock mass during the failure process lies between the two single rock masses that make up the composite rock mass. The damage is dense at the initial expansion moment of the fracture in the rock mass, but weak when the interface expands through the layer. The energy evolution of the composite roof mainly depends on the high-strength roof, and the horizontal migration process of the fracture at the interface does not release energy.

A Study of Acoustic Emission Based RA-AF Characteristics of Polypropylene Fiber-Reinforced Recycled Aggregate Concrete Under Uniaxial Compression

Zhou Daowen, Yang Xin, Tang Yu, Miao Yutao

Archives of Acoustics

2024

Vol. 49, nr 4

601--612

In order to research the acoustic emission characteristics of polypropylene fiber-reinforced recycled aggregate concrete under uniaxial load, 20 groups of test specimens with a coarse aggregate substitution rate of 25 % and 50 % are designed and fabricated to conduct the acoustic emission test under uniaxial compression, and the evolution laws of the acoustic emission b-value, the cracking modes and the acoustic emission RA-AF moving averages with time are studied. The laws of influence of the coarse aggregate substitution rate and coarse-fine polypropylene fiber on the acoustic emission b-value of RAC are discussed. The K-means clustering method is adopted for two-dimensional clustering analysis of the shear cracking and tensile cracking, and then the SVM is used to obtain the boundary between the two types of clusters. The time distribution laws of shear cracking and tensile cracking of the polypropylene fiber-reinforced recycled aggregate concrete are analyzed. The changes in the moving averages of RA and AF of RAC test specimens with time are studied, and the research indicates that as the RA value decreases, the shear cracking gradually reduces and the tensile cracking gradually increases and dominates.