Strains occurring on application of loads to small specimens of dental materials were determined by measuring the changes in the angle of the beams of light diffracted from gratings ruled on the specimens. The method gives a rapid response to stress and is effective over very short gauge lengths.In determining the properties of dental materials, it is desirable, but often difficult, to make measurements on specimens comparable in size to the dental restorations made from these materials. The measurement of stress-strain relationships presents a particular problem, because the dimensions of many commonly used strain gauges are not adaptable to such specimens. One method of measuring strain which will permit the use of short gauge lengths is the diffraction-grating strain gauge.Light striking the grating is diffracted at a specific angle that is dependent on the order observed, the wavelength, and the spacing between the lines. When stress is applied to a specimen on which a grating has been ruled, the change in the angle of diffraction can be used as an indication of the magnitude of the strain.The diffraction grating method offers several advantages. Depending on the quality of the grating, the gauge length can be quite small, even a few tenths of a millimeter. Attachment of strain-measuring devices to the specimen during testing is eliminated. Also, if a photoelectric method is used for measurement of the change in the angle of diffraction, rapid changes in strain can be recorded easily.Probably because of the difficulty of ruling gratings, little use has been made of them for measuring strain. Bell1'2 developed the method in the course of his investigations of dynamic plastic strain. The specimens used in his work were rods approximately 1 inch in diameter and 4 to 7 inches long. A diffraction grating similar to a screw thread was cut around the circumference of the specimen by an extremely accurate lathe. These specimens were then dynamically stressed by impacting them with a known force. The resulting strains were observed as they passed along the rod. Bell was able to measure strains from a few microinches per inch up to 10% with an accuracy of better than 5 %, using gauge lengths ranging from 0.001 inch to 0.030 inch.4'5 Filbey'; has described a method of using photomultiplier tubes for rapidly measuring the change in angle of light diffracted from a strained grating.Materials and Methods The method employed involved (1) ruling a grating on the specimen, (2) directing a beam of monochromatic light perpendicularly onto the grating, (3) loading the specimen in compression, and (4) measuring the changes in angle of diffraction as indicated by changes in signal as the diffracted beams moved along V-shaped slits in front of the photomultiplier tubes.
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