Strong Glass Needs Stronger Measuring Methods

Automatic scattered light polariscope SCALP developed by Johan Anton in GlasStress Ltd.
Automatic scattered light polariscope SCALP developed by Johan Anton in GlasStress Ltd.

Imagine an elevator full of ballerinas. On the next floor half of ballerinas go out and the same number of sumo wrestlers comes to the elevator. This is basically what happens during the process of chemical strengthening of glass. Instead of rapid cooling, the glass object is put into a bath of molten salt. Some ions on the surface of the glass will be replaced with larger ions from the molten salt bath. For example, this method makes the screens of our smartphones stronger and more scratchproof.

There are, of course, many cases when the strength of the glass is more essential—for example, we do not want windscreens or skyscraper windows to break easily. The most common way to determine the strength of the glass is to put it under stress until it breaks. But this method requires many sheets of glass to break and many working hours. A faster and much more cost-effective way is to use a polariscope—a device that measures the internal residual stress of the glass.

In Estonia, a small group of researchers are working on developing these polariscopes. Senior researcher Johan Anton developed the first version of the scattered light polariscope SCALP in 2003. Since then it has been developed further to give more precise stress measurement results. All polariscopes are mainly assembled by Mart Paemurru in GlasStress Ltd.

Fast Enough to Be Used in Industry

Andreas Valdmann, junior researcher at the Institute of Physics of the University of Tartu, explained that the stresses in chemically strengthened glass do not give exact results because light no longer travels in a straight line.

“The first method capable of direct non-destructive residual stress measurement in chemically strengthened glass is gradient scattered light tomography,” said Siim Hödemann, researcher at the Institute of Physics of the University of Tartu. Valdmann added that this method takes into account the bending of light and thanks to that gives more accurate results when measuring the stresses of the chemically strengthened glass.

The principle of the oblique incidence scattered light method for micron-scale stress profile measurement. A laser beam is passed through the glass surface at an 81.9° angle of incidence via a coupling prism. Scattered light fringe pattern along the laser path length was recorded by a camera from observation direction. From the intensity of scattered light, the stress profile is obtained.

The principle of the oblique incidence scattered light method for micron-scale stress profile measurement. A laser beam is passed through the glass surface at an 81.9° angle of incidence via a coupling prism. Scattered light fringe pattern along the laser path length was recorded by a camera from observation direction. From the intensity of scattered light, the stress profile is obtained.

Hödemann, Valdmann and their colleague Valter Kiisk recently presented the results of numerical experiments that tested an iterative approach which can remove refraction-induced errors from scattered light tomography[1].

Siim Hödemann said that they essentially tested software, especially one algorithm. “If we can add this algorithm to polariscope software, it will be even more dependable. There would be no need for a destructive break test, products can simply be certified by anyone who has SCALP and the user can rely on factory certification,” he explained.

Valdmann added that iterative approach means repeating the process with every recurrence coming closer to an exact result. “The most interesting find was that we only needed 3–5 steps to get the exact result. This means that the method is fast enough to be used in industry,” he noted.

Measuring Complex Shape Glass Objects

Researchers plan to implement the results and new knowledge in the updated version of polariscope SCALP, produced by GlasStress Ltd., for even greater accuracy. The polariscope could be used to measure the strength of the glass in many spheres of activity that use light but strong glass—such as motorsport, space and military industry.

So far, researchers have mainly developed algorithms that can remove the influence of light bending from scattered light tomography, which is mainly intended for stress measurement in flat glass sheets.

The researchers also have at their disposal a polariscope AP-07, which can measure stress in objects with a more complex shape. This polariscope was developed by Johan Anton, Andrei Errapart and Hillar Aben. For example, the polariscope AP-07 can be used to measure ultra-high stresses in famous Prince Rupert’s drops.[2]

The next step is to develop a similar algorithm to remove the influence of light bending from stress profiles that are measured in chemically strengthened hollow cylinders. This can potentially help to increase the accuracy of stress measurement in chemically strengthened glass syringes that are used in healthcare industry. If stresses can be measured more accurately, a stress profile can be engineered so that glass syringes become more durable.

[1] Hödemann, Siim; Valdmann, Andreas; Kiisk, Valter (2017). An iterative approach to remove the influence of light ray bending from micron-scale scattered light tomography. Optics and Lasers in Engineering, 91, 30−40, 10.1016/j.optlaseng.2016.11.003.

[2] Aben, H., Anton, J., Õis, M., Viswanathan, K, Chandrasekar, S., Chaudhri, M.M. On the extraordinary strength of Prince Rupert’s drops. Applied Physics Letters, 109, 231903 (2016); doi: http://dx.doi.org/10.1063/1.4971339

This article was funded by the European Regional Development Fund through Estonian Research Council.