Speaker
Description
The development of new generation of scintillating crystals for High-Energy Physics, Medical Imaging and Security, asks for reliable and fast inspection techniques. Since the crystal production cost is a large part of instrumentation final expenses, it is mandatory an improvement of the production process in terms of production efficiency, i.e. reduced number of defected crystals. The residual stress is a signature of the crystal structural state and mechanical defects may be present, due to production process deviations, that negatively affect performances and compromise mechanical reliability. Inspection methods based on photoelasticty [1] allows to evaluate, in not invasive manner, the internal stress by means of the shape of the observed interference fringe patterns.
The traditional photoelasticity methodology works well along some specific directions of observation, but it lacks spatial resolution: the laser conoscopy [2] overcomes such a limit, but needs a great number of measurements and it is easy to apply only if the sample is cut in particular crystallographic directions. Polariscopes using conoscopic observation along the optical axis are devices used to evaluate crystals residual stresses in a precise, but time-consuming manner.
In this work, we present a methodology for the photoelastic analysis of birefringent crystals, which is based on a modified polariscope. In order to reduce the time for crystal analysis, the light beam shape, that impinges on the crystal surface, has been changed from a solid cone (conoscopy) to a wedge (sphenoscopy [3]). Since the polarized and coherent light is focused on a line rather than on a spot, this allows for a faster analysis leading to the observation of stress distribution along a line. In conoscopy observations, the interference fringes can be modelled as quartic curves. The modelling is quite complex if the observation is performed out of the optic axis. In the sphenoscopy the modelling is quite easier and leads to a straight interpretation, independent on the observation direction. Instead of the punctual conoscopic observation, the sphenoscopy analyses a “stripe”, reducing the observations number. For instance, to evaluate a crystal with a given resolution, the sphenoscopy needs at the most 2n observation instead of the n^2 observation necessary in conoscopy.
In case of absence of stress or defects along a “stripe” the resulting sphenoscopic fringe pattern is composed by straight lines. Curvature or distance variations between the fringes are due to internal stress or defects. In this work, we show the application and validation of this innovative technique by means of a simple interpretation model.
References:
[1] Frocht, M.M., Photoelasticity. J. Wiley and Sons, London, 1965
[2] L. Montalto, N. Paone, L. Scalise, and D. Rinaldi
A PHOTOELASTIC MEASUREMENT SYSTEM FOR RESIDUAL STRESS ANALYSIS IN SCINTILLATING CRYSTALS BY CONOSCOPIC IMAGING
Review of Scientific Instruments 86, 063102 (2015); doi: 10.1063/1.4921870
[3]L. Montalto , D. Rinaldi, N. Paone, L. Scalise and F. Davì.
PHOTOELASTIC SPHENOSCOPIC ANALYSIS OF CRYSTALS
Review of Scientific Instruments, 87(1), 015113, 2016.
