Portable Hole-Drilling ESPI Measurements  

Supervisor:  Gary S. Schajer

Renewable Resources Laboratory
Dept. Mechanical Engineering
University of British Columbia
Vancouver BC, Canada V6T 1Z4



Electronic Speckle Pattern Interferometry (ESPI) provides a sensitive, full-field optical method for measuring surface displacements. The technique can be combined with the hole-drilling method to provide an attractive experimental procedure for measuring residual stresses.

The excellent sensitivity of ESPI measurements is a very attractive feature. However, it also presents a serious challenge because it also leads to large responses due to mechanical or thermal drifts. Thus, the measurement equipment must be arranged very rigidly, vibrations minimized and the ambient temperature must be closely controlled to provide a sufficiently stable measurement environment. These requirements typically demand the use of a precision optical table within a controlled laboratory environment.

The need for strictly controlled measurement conditions has impeded the use of the ESPI technique for field applications. This is a serious limitation for hole-drilling residual stress measurements because so many practical specimens are large or immovable, and cannot be brought into a lab. Thus, measurements must be made in-situ, under significantly non-ideal conditions.

The objective of this project is to design and construct a practical ESPI hole-drilling apparatus for field use. The design concept involves creating a highly compact optical system where all parts are rigidly connected with short, direct mechanical connections. Vibration effects are thereby reduced because the device will tend to vibrate as a rigid unit. In addition, the optical path lengths are minimized to reduce the effects of temperature fluctuations and of any residual vibrations. Finally, a two-axis measurement system is incorporated so that a substantial redundancy is available within the measured data.


The design of the portable ESPI hole-drilling device follows the model shown above. To promote portability, the device has been kept compact so that it fits within a cylinder 250mm in diameter. The design incorporates a motorized feed for the drilling head, and automated actuation for the image measurements. The main machining of the device has been completed and the optics are being arranged. In keeping with the concept of having a device for field use, the ordinary laser diodes are used for the illumination. These are run with precise current and temperature controllers to maintain stable operation of the diodes so that they give consistent interferometry patterns. Work is underway to integrate the functionality of the various optical and mechanical components so that the portable ESPI hole-drilling device can work autonomously and complete hole-drilling residual stress measurements in a compact time and with a minimum of supervision.


When compete, the portable ESPI hole-drilling device will be useful for field measurements of residual stress. Because of the two-axis design, the stress measurements will be isotropic, without directional bias, and will have superior stability because of the increased data from the ESPI images made in the two directions.