As shown at the Laboratory of Rock Mechanics, electromagnetic radiation (EMR) emitted by propagating fractures provides relatively accurate information on the dimensions of the cracks emitting it. In this paper we demonstrate that this method (i.e. EMR analysis) can also be advantageously used for obtaining the exact time sequence of double or triple pulses when they appear simultaneously and, hence, the sequence of the relevant cracks. The method is first used to analyse the time sequence of a triple fracture during failure and for a double fracture in relaxation of a glass ceramic sample. The analysis is in good agreement with the actual fractography of the sample. A similar procedure applied to fracture of chalk enabled us to show that most large fractures are, in fact, double, and to find the specific time sequences involved.
New time-dependent Benioff strain (TDBS) release diagrams were analyzed for acoustic emission during various loading tests and for electromagnetic (EM) radiation emanating during compression and, tension, which end in failure. TDBS diagrams are Benioff diagrams that are built consecutively, each time using a greater number of events (acoustic or EM emissions) using the last event as if it were associated with the 'actual failure'. An examination of such TDBS diagrams shows that at a certain time point (this time point is denoted by the term 'alarm' time), a comparatively short interval prior to actual collapse, their decreasing part is broken by a positive 'bulge'. This 'bulge' is quantified and an algorithm proposed for its assessment. Using the alarm time and other parameters of the failure process (fall, bulge size and escalation factors, bulge slope and slope fall time), a criterion for estimating the time of the actual collapse is developed and shown to agree well with laboratory experimental results.
In a series of measurements of electromagnetic radiation (EMR) from percussion-drilled Solenhofen limestone, nonoscillating signals (associated with electric depolarization) preceded by short oscillating signals (associated with crack propagation) were obtained. Depolarization is therefore assumed to be caused by stress relaxation around a propagating crack. Using the parameters of EMR signals, the polarization generating displacements of the Ca 2+ ions in the calcite lattice caused by drilling were calculated.
In this article we consider the rise and fall time ͑which were earlier shown experimentally to be the same͒ of electromagnetic radiation ͑EMR͒ from propagating cracks. This feature is shown theoretically to be inversely proportional to the pulse frequency and to the fourth degree of the absolute temperature. It is shown experimentally that in glass and in glass ceramics, which are not porous, and in granite, whose porosity is of the order of 5%, is indeed inversely proportional to. In chalk, whose porosity is as high as 40%, however, this relation is not observed. We argue that the latter result is due to the interaction between the cracks which emit the EMR and the pores of the material and specifically to the spread of ensuing temperatures of the cracks caused by this interaction.
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