Application of phase stepping Electronic Speckle Pattern Interferometry (ESPI) to non-destructive testing of GRP composite materials

The non-destructive testing of polymer composite materials is becoming increasingly important due to their wide application as engineering components and structures. Electronic Speckle Pattern Interferometry (ESPI) has promising potential to serve this purpose because it is a noncontact, whole-field and real-time measurement system. This paper represents research results concerning the application of phase stepping ESPI to the evaluation of defective GRP samples in which damage was introduced by impact loading. The defect was detected by correlation fringe patterns but speckle noise, intensity variation and interference of background fringes reduced initial technique effectiveness. The phase stepping was introduced using a piezoelectric transducer mounted mirror to produce three interferograms. The optical phase data were extracted and presented as wrapped and unwrapped phase maps, showing the fine features of internal damage with high visibility, and readability. Comparative data was generated by conventional ultrasonic C-scan and sectioning techniques with the results showing a good correlation with phase stepping ESPI.