What it shows:
The strength of a material in tension or compression will be affected by discontinuities in its surface structure. This can be demonstrated for glass using microscope slides, and the comparison of failure stress before and after the removal of surface scratches.
How it works:
The slide rests between two custom built test beds (figure 1), the upper bed supporting the load. We use slotted 1kg and 0.5kg masses placed carefully in their holder, and allowing a short time between additions. We find the breaking stress for an untreated slide to be around 4kg. The treated slides survive from 7.5kg up to 25kg.
figure 1 cross section of test bed
Setting it up:
Two or three of the treated slides should be prepared by immersion in 5% hydrofluoric acid for fifteen minutes. Remove the slide by grasping the edges (wearing gloves) and rinse briefly with distilled water. Any rough handling weakens it. The apparatus must be surrounded by a safety screen and the demonstrator should wear safety goggles. A camera is needed for viewing, with 25mm wide angle lens lens with white backdrop.
Can be a little unpredictable, but the explosive failure of the treated slide is spectacular. A good one to keep the audience holding its breath. Thanks to Prof. Frans Spaepen for help in developing this one.