Full field stress analysis of window security films by phase-stepping photoelasticity

The paper presents a two-dimensional interferometric photoelastic phase-stepping measurement in reflection mode with a Mach-Zender interferometer combined with a circular polariscope. As such, the paper continues the study on photoelastic analysis of the mechanical behaviour of thermoplastic polyester window security films by including retrieval of isopachics parameter. A series of six photoelastic fringe patterns are recorded at different preliminary known orientations of the polarization elements in the circular polariscope to build both isochromatic and isoclinic phase maps which give the loci of points with a constant difference of principal stresses and constant principal stress direction respectively. Holographic recording of four fringe patterns is applied for retrieval of isopachic fringes which give the sum of principal stresses. The coherent length of the used DPSS laser VERDI2 exceeded substantially the optical path difference between the reference and object beam which ensured high contrast of the recorded interference patterns. Measurement of both isochromatic and isopachic maps permits to separate the stress components all over the sample. The samples were covered with a photoelastic birefringent PhotoStress coating. The change of the photoelastic pattern for film samples with mechanical stress concentrators (holes and notches) under tensile loading is observed. To study the protective properties of the security film fitted to a glass we made a photoelastic interferometric measurement of an assembly glass/film with a stress concentrator which had been simulated by sticking two precisely cut glass pieces with parallel edges in contact with each other between two films. A two-load phase-shifting technique is used for unambiguous phase retrieval of photoelastic parameters