Crack growth under three-point beam bending and fracture energy estimation for snow in relation to acoustic emission responses

Abstract

The non-invasive monitoring of crack initiation and growth processes within snow, a poro-composite matrix, could be useful to predict the structural failure of a composite material. Present work is a new approach utilizing the non-invasive technique of acoustic emission (AE) to study the ongoing cracking and failures processes within snow beams which is difficult to visualize in real time. The progressive crack growth within the beams of snow followed by their failures is studied under three point beam bending in relation to AE. In this work, AE detection platform is developed and implemented for non-contact detection of acoustic emissions generated within a beam with the help of specialized needle-type AE waveguides in conjunction to wideband AE acquisition system. The AEs are detected for entire course of bending, crack nucleation, crack growth and rupture of the beams, both under static and dynamic load conditions. The crack initiation and growth in snow beams were generated without any predefined notch on its surface in a controlled and perceptible manner, particularly at low deformation rates (<0.2 mm/min). The ongoing crack growth, in terms of crack lengths and openings, were also studied from high resolution images, regularly collected during the experimental course. The prominent AE characteristics such as counts, amplitude, absolute energy, etc. were analyzed in context of bending, cracking and rupture of the snow beams for static as well as dynamic load conditions. The total strain energy, estimated in terms of the strain energy density, was imparted to the snow beam which contributed in the crack initiation and growth processes. Further, a fraction of the crack surface energy was manifested in the form of AE energy; therefore, a correlation was established between the crack surface energy and the liberated AE energy for static as well as dynamic loading of snow. The Griffiths model for global energy balance was applied to estimate the crack surface energy. The relation between crack surface energy and corresponding AE energy (released) could be very useful to estimate the fracture energy of the materials directly in terms of AE, non-invasively.