EMA 611 Advanced Mechanical Testing
University of Wisconsin
Rod Lakes

Optical methods for deformation analysis or stress analysis


Holographic interferometry
    We use this method in our lab.
    A laser is used to make a hologram (three dimensional image) of an object. The object is then moved or deformed and a second holographic exposure is made on the same film. For most holography the film must have resolution sufficient to capture the interference fringes, which are spaced 0.3 to about 5 microns apart (one micron is 10^-6 m). Digital cameras do not have enough resolution. Holographic fringes represent the three-dimensional displacement field of the object surface.

Classical interferometry
    We use this method in our lab; measurements of deformation of a piezoelectric actuator are conducted.
    The Michelson interferometer is used to conduct measurements of motion on a scale from microns to nanometers. The interferometer consists of several mirrors and a beam splitter. Laser light is input. The interference fringes may be projected on a screen or converted into an electrical signal via a silicon sensor.

Photography / video
    This is suitable to determine displacement fields for relatively large motion. If done under a microscope, the resolution is about 0.5 microns. Data processing tools, e.g. digital correlation analysis, may be used to extract displacement or strain.

Photoelasticity
    This method makes use of the effect of stress on polarized light. A model of the object is made out of a transparent polymer which exhibits a stress-optic effect. For three dimensional stress fields, the stress can be frozen into a temperature sensitive polymer, which is then cut into sections with a saw, and analyzed. Isochromatic fringes represent principal stress difference.

Photoelastic coatings
    This method makes also use of the effect of stress on polarized light. A coating of polymer is applied to the actual object, and stress is inferred from the fringe pattern. Some commercial systems (see http://www.stressphotonics.com/ in Madison, Wisconsin) make use of digital image processing techniques to obtain a map of stress.

Geometric moire
    A grid of parallel lines or dots is cemented or etched on the object. In the moire effect, displacement fringes are generated as the grid on the object is moved, rotated, or strained in comparison with a reference grid. Resolution depends on the spacing of lines in the grid. Standard moire is sensitive to the in-plane displacement field (parallel to the surface). Shadow moire is sensitive to out of plane displacement; also it does not involve placing anything on the surface of the specimen object.   Shadow moire: map deformation
  moire fringes, block

Speckle photography
    Laser light generates speckles due to interference between waves reflected from irregularities in the object. Rather grainy photographs can be taken, showing fringes associated with the deformation field. Electronic speckle pattern interferometry (ESPI) is a variant which makes use of a digital camera and analysis software.

Differential thermography (SPATE, Delta Therm and related)
    This method makes use of the converse of the thermal expansion of materials. When a material is stressed, regions become warmer or cooler depending on the sign of the stress. An infrared camera (cooled to cryogenic temperature) can detect these slight temperature variations. The infrared emitted by objects near ambient temperature has a wavelength near ten microns, much longer than visible wavelengths and near infrared. The method depends on dynamic deformation of the specimen since temperature variations will otherwise tend to equilibrium. Some commercial systems (see http://www.stressphotonics.com/ in Madison, Wisconsin) make use of digital image processing techniques to obtain a map of stress.

Digital image correlation
    This method uses a computer to compare two images, before and after deformation. Displacement and strain fields are calculated from a comparison of the surface before and after deformation. The software uses a correlation approach. The object is painted with a speckle pattern to provide contrast if the surface does not already have a granular appearance. One digital camera suffices for in plane motion orthogonal to the line of sight. Two cameras are required to evaluate out of plane motion. The software conducts an average over a selected area or region of interest. Use of the software requires a substantial learning curve; controls and procedures are not intuitive.
    Digital image correlation method text, image example.

Laser Doppler interferometry
    Laser light is scattered from heterogeneities in a solid or fluid, and the wavelength shifts due to the Doppler effect associated with motion. This approach is used for vibration or fluid flow.