Observation of significant differences between electromagnetic and acoustic emissions during fracture processes: A study on rocks under compression loading

Abstract

Electromagnetic radiation (known as electromagnetic emissions) related to processes of loading and fracture in different types of materials (from metals to rocks) has been widely reported. The physical mechanisms behind these emissions are still under discussion, however, it is commonly accepted that they are created by some of the micro-cracks that appear in the sample during fracture processes. Nucleation and growing of micro-cracks generate mechanical waves (acoustic emissions), therefore, each electromagnetic emission should be linked with some acoustic event. Furthermore, it is expected that the electromagnetic and acoustic activities (number of emissions per second) have the same general characteristics. Contrary to what is usually reported, we find that there are significant differences between acoustic and electromagnetic emissions in loading processes on rocks. These differences were detected during the compression of a typical laboratory-scale sample of granite when it is compressed at a rate of around 20 kPa/s. We found two important discrepancies: i) There were at least 20 electromagnetic bursts (out of around 200) that were not coincident with any acoustic event. ii) The electromagnetic activity in general shows its maximum value when acoustic activity is very low. Both emissions just coincide at the moment of the final collapse. These results strongly suggest the existence of a non-fracture mechanism related to the origin of electromagnetic emissions. This could have important consequences for the field of non-destructive assessment of materials and even in the study of earthquake precursors and forecasting.