Calibration of Optical Methods

The optical/infrared stress analysis methods described in the applications pages require calibration in order to turn qualitative stress/strain distribution maps into quantitative stress magnitudes. An example technique is shown below: photoelastic calibration for a transmission four point bend specimen.

Photoelastic Calibration, Four Point Bend

In order to calibrate a particular material for use in transmission photoelasticity, it is perhaps simplest to manufacture a test coupon of known geometry and apply loading in a carefully controlled laboratory test. The optical response of the coupon can be used to generate calibration information for the material which is then valid for other birefringent material manufactured from the same raw materials in the same manner. In this illustration, a prismatic beam of polymeric material has been manufactured for this purpose, and loaded in a four point bend test rig so that the central portion of the specimen is subjected to a well understood stress field.

Photoelastic Calibration, 4 point bend, 0 degrees Photoelastic Calibration, 4 point bend, 45 degrees Photoelastic Calibration, 4 point bend, 90 degrees Photoelastic Calibration, 4 point bend, 135 degrees

(Photoelastic fringe patterns for the central portion of the four point bend specimen, showing: (far left) dark field, where the input and output polariser elements of the polariscope are crossed); (left) grey field, where the output polariser has been rotated through 45 degrees; (right) light field, where the input and output polariser elements are aligned; (far right) grey field, where the output polariser has been rotated through a further 45 degrees)

The above images show the response of the four point bend specimen in a circular polariscope, showing four separate images taken with different output polariser orientations. In the far left image, the zero order fringe is easy to identify (the black band in the upper section of the beam), as is the first order fringe (the purple/blue transition band in the lower section). But for the subsequent images, and even for the dark field image, it is simpler to determine the fringe locations by using quasi-monochromatic light. Below are shown the green channel only from the above Red-Green-Blue (RGB) images, which reveals the fringe locations more clearly.

Photoelastic Calibration, 4 point bend, 0 degrees mono Photoelastic Calibration, 4 point bend, 45 degrees mono Photoelastic Calibration, 4 point bend, 90 degrees mono Photoelastic Calibration, 4 point bend, 135 degrees mono

(Monochromatic photoelastic fringe patterns using only the green channel of the available RGB image information - the fringe locations are now clearly defined for both the zero and non-zero fringes)

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