ESPI non-destructive testing of GRP composite materials containing impact damage

Recent developments in electronic speckle pattern interferometry (ESPI) pioneered by the authors have highlighted the potential for applying the technique to the study of mechanical damage in composite materials especially where non-contact, whole-field, real-time measurements are required. Intensity fringe patterns have been used as qualitative assessment tools for over 10 years. In this case, however, computer based fringe pattern analysis techniques are used in a novel way to extract autonomously both amplitude and phase data. Thus this paper demonstrates how intensity fringes and phase maps from phase stepped ESPI may be used to evaluate delaminations, cracks and other defects in glass-fibre reinforced polyester (GRP) pultruded panels in which the damage originated from instrumented falling weight impact tests. The experimental results show that both intensity fringe patterns and phase maps can be used to detect the internal damage. The intensity fringe approach enables the damage to be examined in real-time but with the disadvantage of limited visibility and readability. In contrast, the phase map approach is capable of showing the fine features of internal damage with higher visibility and readability providing post-processing is applied. In the current work the damaged areas are calculated and compared with those derived from conventional ultrasonic C-scan and sectioning techniques and show promising correlation. It is also demonstrated that there is a consistent correlation between impact event parameters and the extent of revealed damage. Finally of particular significance, in the broader scientific sense, is the fact that in the case of the phase mapping there is the potential to extract information from the computer which could quantify the level of damage and which could be used to alert, say a quality control operator, to the need to make further decisions. The paper therefore signals the real possibility of being able to produce an automated NDT analysis tool.