29 Jun, 2013
PRESENTATION: From an industrial point of view, materials are not in themselves interesting. What really makes them useful is the possibility to give them the ideal or correct shape and make products that make everyday life easier. The characteristics of plastics, be they chemical, structural or whatever, determine the corresponding method for giving them shape and the final properties of the finished product. The contours of equal mechanical tension can be analysed in birefringent plastics by using polarising sheets and this means that the injection point can be appreciated, if there is one, or the areas where there is a build up of stress.
- Polarized patterns, Steve Dail, Phys. Teach. 51, 245 (2013)
- Polarization Imaging and Insect Vision, Adam S. Green, Paul R. Ohmann, Nick E. Leininger, and James A. Kavanaugh, Phys. Teach. 48, 17 (2010)
- Polarizing Filters Aren’t Supposed to Do THAT!,Matt Evans, Phys. Teach. 46, 59 (2008)
- A Polarizer Demo Using LCDs, Gerd Kortemeyer, Phys. Teach. 46, 58 (2008)
INTRODUCTION: Photoelasticity is a property that can be used to analyse and record mechanical stresses in components. These objects are subjected to numerous forces that can be analysed by observing the colour bands that appear on them when they are placed between two polarised sheets, thanks to the properties of birefringence and polarised light.
We can interpret the distribution of forces in these materials, that is, the points that have an equal direction of main forces or equal difference of main forces.
OBJECTIVE: To analyse mechanical forces in different materials by using photoelasticity.
MATERIALS: polarised sheets, projector, light source, various everyday plastic objects.
SETUP: Place the object to be analysed on the projector screen, located between two polarised sheets. Then switch on the projector. Finally, the contours of equal mechanical stress can be observed.
EXPLANATION: The polarised sheets are polarising devices that absorb the light vibrating on different planes except for one, which is allowed to pass through. Two perpendicular polarised sheets do not let light through.
When a photoelastic material is placed between two perpendicular sheets, isochromatic bands can be seen that are related to the mechanical stresses in the material.
By using a table that relates fringe order with the colours of the isochromatic bands, the material’s mechanical stresses can be interpreted.
CONCEPTS: polarisation, photoelasticity, birefringence, polariscope. Brewster’s law.
- R. Serway, Física, Mac Graw Hill, 2010.
- P. Tipler, Física para la Ciencia y la tecnología, Reverté, 2012.
- R. Ehrlich, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, Princeton University Press, 1997.
STUDENTS 2012-2013: José Ignacio Iglesias, Ángel López, Isabel López.
LINK pdf STUDENTS (in Spanish):