Application of embedded fiber-optic distributed strain sensing for detection of matrix cracks in CFRP laminates

Abstract. FBG sensors with a gauge length of 100 mm were embedded in CFRP laminate specimens and monitored by the OFDR-FBG distributed strain measurement system. When a tensile load was applied to the specimen, matrix cracks were generated internally and their number increased. As the cracks developed, a peak appeared in the distributed strain. Soft X-ray inspection and optical microscopy of the specimen edge showed good agreement between the location of the cracks and the peaks. The finite element method was used to calculate the internal strain change during crack initiation, and its effect on the distributed strain was discussed. The peaks were clearly visible when the crack density was low, and when the crack density was high, the peaks were difficult to see due to peak overlap. With the resolution of the system and sensor used in this study, it is practically possible to distinguish areas where cracks have accumulated from areas where they have not accumulated. The proposed matrix crack detection method can also be applied to cases using other types of fiber optic distributed strain measurement systems. Given the widespread use of machine learning and other analytical techniques for measurements, the use of distributed strain is not limited to determining the number of peaks and can have many possibilities.