Mechanical characterization of polyvinylchloride pipes using electronic speckle pattern interferometry

The use of electronic speckle pattern interferometry (ESPI) for non-destructive material characterization of thick and thin unplasticised Polyvinylchloride (uPVC) pipes is presented. Pipes are tested by internal pressurization and ESPI gives a complete mapping of the resulting displacement field over the area imaged by the video camera. The results for the strain derived from ESPI data and from the standard mechanical method using strain gauges agree very well with each other. The interferometric method used is non-contact and gives high-confidence results for Young's modulus of uPVC pipes. The fringe counting method gives the total displacement over the field of view imaged by the CCD camera and is subject to a fringe error of 0.5. This simple approach is valid when the displacement behaviour of the sample is known. When this is not the case then it is necessary to calculate the phase map of the displacement by digitally shifting the phase difference between the two beams in the interferometer. We have implemented this technique by modulating the laser diode drive current to alter the wavelength of the laser very slightly between frames. A linear phase map of the displacement is always obtained in the present case.