As a form of energy dissipation, electromagnetic radiation (EMR) is gradually becoming a mainstream method prediction method for damage monitoring of coal and rock. To better improve the prediction accuracy of EMR, it is crucial to investigate time-frequency characteristics of EMR and the influence of coal and rock properties on EMR that occurs in the fracture process. In this paper, the deformation and fracture process of raw coal, shaped coal, and cement specimens under compression are observed, and time-frequency evolution characteristics of EMR below ultralow frequency (ULF) under different loading stages are evaluated. Then, the inherent reasons of time-frequency evolution of EMR reflecting the damage performance are analyzed. Meanwhile, the effect of loading condition and composition and structure of materials on EMR is discussed. Results show that there is obvious EMR below ULF in the deformation and fracture process of coal and rock materials, and the combined denoising method of ensemble empirical mode decomposition (EEMD) and wavelet is suitable for extraction of EMR. Besides, the amplitude is approximately inversely proportional to frequency of electromagnetic signals, and the relationship between pulse count of EMR and damage is related to the homogeneity of coal and rock materials. Moreover, the time-frequency characteristic parameter of EMR, such as amplitude, pulse count, and frequency spectrum, can indirectly reflect the damage evolution process of coal and rock. These results can provide a theoretical basis for remote monitoring of coal and rock dynamic disaster using EMR below ULF.

中文翻译：

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反映煤岩材料损伤性能的超低频电磁辐射时频演化特性试验研究

电磁辐射（EMR）作为一种能量耗散形式，正逐渐成为煤岩损伤监测的主流方法预测方法。为了更好地提高 EMR 的预测精度，研究 EMR 的时频特性以及煤岩性质对断裂过程中发生的 EMR 的影响至关重要。本文通过观察原煤、成型煤和水泥试件在压缩作用下的变形和断裂过程，评价了不同加载阶段下超低频（ULF）以下电磁辐射的时频演化特征。然后，分析了反映损伤性能的EMR时频演化的内在原因。同时，讨论了负载条件以及材料的成分和结构对EMR的影响。结果表明，在煤岩材料的变形断裂过程中，ULF以下存在明显的EMR，集合经验模态分解（EEMD）与小波相结合的去噪方法适用于EMR的提取。此外，振幅与电磁信号的频率近似成反比，EMR的脉冲数与损伤的关系与煤岩材料的均质性有关。此外，EMR的时频特征参数，如幅度、脉冲数、频谱等，可以间接反映煤岩的损伤演化过程。这些结果可为利用低于ULF的EMR远程监测煤岩动力灾害提供理论依据。