Multi-axis fiber grating strain sensors have been used to quantitatively measure axial strain and temperature; transverse strain and transverse strain gradients in composite weave structures. This paper overviews the multi-axis fiber optic grating strain sensors and how they can be applied to measuring multidimensional strain fields interior to composite parts with complex composite weave structures. Experimental results are given for the case of a bi-axially woven composite coupon as well as for an E-glass/epoxy composite sample.
In this effort, long gage fiber Bragg grating sensors are used to provide dynamic strain measurements in conjunction with a damage detection algorithm to provide a real-time assessment of the health of a civil structure such as a bridge or a building.Past research in the area of nondestructive damage detection in structures has shown that modal frequencies and modal damping ratios may not be sensitive enough indicators to detect damage, while changes in mode shapes are significantly more sensitive to structural defects or local damage, with the greatest change occurring in the vicinity of the structural defect. In general, local damage conesponds to a local loss of stiffness of some structural element(s). Therefore, damage identification methods based on changes in vibration mode shapes such as changes in mode shape curvature have encountered some success. However, the changes in mode shape curvature are traditionally obtained through double numerical differentiation of "noisy" mode shapes identified using accelerometer data. Thus, the identified mode shape curvatures may be too noisy to identify (i.e., detect, localize, and quantify) low to intermediate level of local structural damage.On the other hand, the effective modal macro-strain vectors used in this effort, which are equivalent to curvature mode shapes, are identified directly from the long-gage fiber grating sensor measurements. Thus, they are in general much less noisy and therefore significantly more reliable than the curvature mode shapes identified using accelerometer data. This is exactly where lies the advantage of using long-gage fiber optic deformation sensors for structural health monitoring and global damage identification of structures.
Arrays of multi-axis fiber grating strain sensors have been integrated into a composite pressure vessel test article, and are used to monitor changes in the transverse and axial strain fields during curing and pressure cycling near cut tow and Teflon tape defects. These changes in the multi-axis strain due to four pressure cycles and repeated impacts are measured and compared to ultrasonic and eddy current scans. Examples of the remote detection of damage using transverse strain and transverse strain gradients is given as well as data showing the ability of the system to distinguish broken tow and delamination defects.
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