Numerical calculation and stress analysis of crack evolution in coal with a central hole under nonuniform load

Hongbao Zhao; Huan Zhang; Guilin Hu; Feihu Wang; Hongbing Wang

doi:10.3139/120.111071

Article

Numerical calculation and stress analysis of crack evolution in coal with a central hole under nonuniform load

Hongbao Zhao , Huan Zhang , Guilin Hu , Feihu Wang and Hongbing Wang

Published/Copyright: August 28, 2017

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From the journal Materials Testing Volume 59 Issue 9

Abstract

The initiation and evolution of the cracks around the hole in coal specimens with a central hole under different uniform loads are studied by the rock failure process analysis software RFPA2D. The results show that homogeneity not only has an influence on the peak strength of coal specimens, but also on the crack propagation directly. The lower homogeneity, the more extensive the stress concentration areas, the rougher the surface crack, and the more irregular the crack propagation path. However, homogeneity has a certain impact on acoustic emission (AE) number and instantaneous effective coalescence rate of cracks, but the influence degree is significantly lower than that under nonuniform load gradient and original cracks. Nonuniform load can cause the obvious stress concentration on the higher load area of the coal specimens (right side), which mainly reflects the micro-crack initiation in the area, and the greater the nonuniform load gradient, the more significant is this trend. The peak stress of models does not change with the change of load forms (the change from uniform load to nonuniform load), but nonuniform load will accelerate the effective coalescence of the surface crack, and the larger the nonuniform load, the shorter the time to achieve the effective coalescence of the surface crack. The original cracks with a certain inclination weaken the stress concentration around the hole. When the dip angle of original crack reaches a certain range (such as 45°), this trend will be more obvious. But it is not closely related to the homogeneity of the coal and the size of the load gradient, and has a certain universal applicability. Finally, the stress around circular tunnel is analyzed based on the theory of elastic mechanics, and a criterion for judging the crack initiation around the laneway is proposed.

Kurzfassung

Die Initiierung und die Ausbildung von Rissen um ein zentrales Loch in Kohleproben wurden unter gleichmäßigen Belastungen mittels der Software RFPA2D (Rock Failure Process Analysis Software) untersucht. Die Ergebnisse zeigen, dass die Homogenität nicht nur einen Einfluss auf die Spitzenfestigkeit der Kohleproben hat, sondern auch direkt auf das Risswachstum. Je niedriger die Homogenität ist, desto beträchtlicher sind die Spannungskonzentrationsbereiche, desto rauher ist die Rissoberfläche und desto irregulärer ist der Rissfortschrittspfad. Die Homogenität hat auch einen bedeutenden Einfluss auf die Zahl akustischer Emissionen und die sofortige effektive Koaleszenzrate der Risse, aber der Grad dieses Einflusses ist deutlich geringer als der unter einem ungleichmäßigen Belastungsgradienten und an originalen Rissen. Eine ungleichmäßige Belastung kann offensichtlich eine Spannungskonzentration auf der Seite der Kohleproben mit der größerer Belastung verursachen, die sich hauptsächlich durch Initiierung von Mikrorissen in dem Bereich widerspiegelt und je größer der Gradient der ungleichmäßigen Belastung ist, desto signifikanter ist dieser Trend. Die Spitzenspannung der Modelle verändert sich nicht mit der Veränderung der Belastungsform (von gleichmäßiger zu ungleichmäßiger Belastung), aber die ungleichmäßige Belastung beschleunigt die effektive Koaleszenz der Oberflächenrisse und je größer die ungleichmäßige Belastung ist, desto kürzer ist die Zeit, um eine effektive Koaleszenz des Oberflächenrisses zu erreichen. Die originalen Risse mit einer bestimmten Neigung schwächen die Spannungskonzentration um das Loch. Wenn der Neigungswinkel des originalen Risses einen bestimmten Bereich erreicht (wie etwa 45°), wird dieser Trend offensichtlicher. Dieser steht aber nicht im Zusammenhang mit der Homogenität der Kohle und der Größe des Belastungsgradienten, außerdem hat er eine bestimmte universelle Anwendbarkeit. Abschließend wurde die Spannung um den runden Lochtunnel basierend auf der Theorie der Elastomechanik analysiert, und ein Entscheidungskriterium zur Beurteilung der Rissbildung um den Pfad wird vorgeschlagen.

*Correspondence Address, Associate Prof. Dr. Hongbao Zhao, School of Resource and Safety Engineering, China University of Mining and Technology, Beijing 100083, China, E-mail: zhanghuanvip01@163.com

Assoc. Prof. Hongbao Zhao, born 1980, is Associate Professor and doctoral advisor. He is a doctor of Mining Engineering, and currently mainly engaged in teaching and research of coal and rock mechanics and gas control technology, and has made some achievements in this field. He works at the School of Resource and Safety Engineering, China University of Mining and Technology, Beijing.

Huan Zhang, born 1991, is a doctoral student of Mining Engineering at the School of Resource and Safety Engineering, China University of Mining and Technology, (Beijing. He is mainly engaged in the research of coal and rock mechanics, gas control technology, and has published several articles in this field.

Guilin Hu, born 1988, is a master student of Mining Engineering at the School of Resource and Safety Engineering, China University of Mining and Technology, Beijing. His research mainly focuses on coal and rock mechanics, and has published several articles in this field.

Feihu Wang, born 1989, is a master student of Mining Engineering at the School of Resource and Safety Engineering, China University of Mining and Technology, Beijing. He is mainly engaged in the research of coal and rock mechanics.

Hongbing Wang, born in 1992, is a master student of Mining Engineering at the School of Resource and Safety Engineering, China University of Mining and Technology, Beijing. He is mainly engaged in the research of coal and rock mechanics and gas control technology.

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Published Online: 2017-08-28

Published in Print: 2017-09-01

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https://doi.org/10.3139/120.111071