Projection moire measurement of glass specimens retrofitted with safety film

Protection of buildings and critical public infrastructure against blast load has been recently improved by retrofitting glass windows with a safety film. As the exact physical mechanisms of the interaction between glass and safety film are not quite well understood, intensive research is conducted on the properties of this assembly. The loadings on the glass/film assembly are typically dynamic (blast, wind pressure, impact), so the lab tests are done on a drop weight set-up, where a mass is falling on a retrofitted glass plate. In this work, the drop weight setup was combined with pattern projection (moire) technique to study the time history of the out-of-plane deformations of the glass/film assembly. The fringe pattern, projected on the back side of the specimen, was generated by means of a sinusoidal phase grating under divergent high intensity infrared illumination. The whole process was recorded with a high speed camera. Local routines based on fast Fourier transform were used to process the captured images, and to extract the phase. The exact out-of-plane displacements were calculated by means of calibration based on previous shape measurements of several different objects with known dimensions

Study of mechanical characteristics of window security films by phase-stepping photoelasticity

The goal of this study is to apply photoelasticity for analysis of the mechanical behavior of thermoplastic polyester window security films. More specifically, the change of the photoelastic pattern for film samples with mechanical stress concentrators (holes and cracks) under tensile load is observed. For the purpose, the samples are covered with photoelastic birefringent PhotoStress (R) coatings. Being subjected to a tensile external load, the film transfers the strains over its surface to the coating. The latter are observed as interference fringes - isochromatic fringes, which exhibit the difference of the principal strains and isoclinic fringes, which characterize principal strains orientation. The photoelastic measurements were performed with a circular reflection polariscope with a white light and monochromatic illumination (575 nm). In the second case, two-loads phase-shifting technique is used for digital retrieval of isochromatics and isoclinics by successive acquisition of two pairs of four fringe patterns obtained at four different configurations of the optical elements of the polariscope. Since the specifics of the studied thermoplastic materials requires simultaneous recording of the phase-shifted patterns in the non-linear part of their loading curve, we propose an optical arrangement for real-time recording as a future development for solution of non-linear dynamic tasks

Projection moiré measurement of glass specimens retrofitted with safety film

Since the terroristic attacks in 2001, a lot of important government buildings, embassies and critical public infrastructure (railway stations, airports) are protected against blast load. Most of these buildings have standard glazing with float glass, and replacing all float glass with tempered or laminated glass would be too expensive. Therefore the glass windows are retrofitted with safety film, a thermoplastic polyester film that sticks to the glass and holds all glass fragments together when the glass is breaking. These safety films are also applied as protection against hurricanes and burglary. As the exact physical mechanisms of the interaction between glass and safety film are not understood quite well, research is conducted on the assembly of float glass retrofitted with safety film. Also the adhesive between glass and safety film plays a big role, as well as the tin and air side of the glass. Because the loadings on the glass/film assembly are typically dynamic (blast, wind pressure, impact), the lab tests are done on a drop weight set-up, where a mass is falling on the glass plate, retrofitted with safety film. In this paper, the instrumentation of the drop weight setup with projection moire measurement is presented, so that the time history of the out-of-plane deformations of the glass/film assembly can be investigated. On the back side of the specimen a high intensity infrared grating was projected and the whole process was recorded by a high speed camera. For the processing of the projected moire fringes, dedicated phase wrapping algorithms were needed because the fracturing of the glass causes a lot of discontinuities in the phase. Local routines based on Fast Fourier Transform were used to process the captured images, and to extract the phase. The exact out-of-plane displacements were calculated by means of calibration based on previous shape measurements of several different objects with known dimensions