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Compressive and Tensile Regions
The GFP 1000 by Stress Photonics is capable of measuring the residual strains imparted into automobile windshields during the manufacturing process. The strains oriented along the 0 degree and 45 degree shear planes are automatically measured using a rotating analyzer, and a lock-in algorithm is implemented to calculate the maximum in-plane shear as well as the direction the of the first principal strain.
In this example, shear strains oriented along the 45-degree shear plane show the components of the normal compressive and tensile strains at the edge of a freestanding automobile windshield. In the case of a free edge the shear strains measured by the polariscope are equal to the normal strains in the windshield. Compressive strains are designed into the edge of the windshield to inhibit the formation and growth of edge cracks. The residual tensile region is limited in how far it can extend into the glass (only a few centimeters) and in terms of peak magnitude. The region displayed in the image is roughly a 2"x1.5" area of the windshield.
Figure 1. Image of a section of windshield showing the characteristic compressive and tensile regions at the edge
It is important to note that the two sections of windshield discussed in this document were inspected without altering the windshield in any way. The black backing commonly found on back surfaces along the edges of most automobile windshields was left unchanged, so as to simulate the condition of a windshield immediately after manufacture. This backing only reflects a few percent of the incident light, yet useful data was still acquired in matter of seconds. For applications where using a more reflective backing is possible, a significant improvement in signal to noise ratio will be observed.
Click on any of the following for more information or use the arrows above to proceed through the complete details about the GFP 1000 as a glass inspection tool.
- Crossover from Compression into Tension - Line Plot
- Complex Residual Strain Pattern at a Windshield Corner - Strain Image
- Complex Residual Strain Pattern at a Windshield Corner - Line Plot
For additional information concerning the measurement of residual strains in glass and plastic or the measurement of strains in other materials please contact Stress Photonics Inc.
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