Search

We have developed a generation pulsed-laser scanning method for visualizing the propagation of ultrasonic waves. While scanning a target object with a pulsed-laser beam to generate thermal-exited ultrasonic waves, we detected the propagated waves with a fixed PZT transducer. Although the detected waves were generated from different irradiation points, we were able to produce moving images of the ultrasound generated at the reception-transducer position by reconstructing the measured waveform data. This method has the following features that make it superior to the conventional visualization methods such as photo-elasticity method, reception probe scanning method and computer simulation. (1) it enables us to visualize ultrasonic waves propagating on a complex-shaped object with curved surfaces, steps, and dents. (2) it provides excellent working efficiency by eliminating the need for adjustments to the laser incidence angle and the focal distance. For these reasons, we believe that this new method can be effectively applied to the inspection of defects in the field. In this study, we examined the applicability of this method to CFRP materials, and the results demonstrate the validity of this method for nondestructive flaw inspection in CFRP-structures.

When fiber Bragg (FBG) sensors are embedded in carbon fiber reinforced plastic (CFRP) laminates, the reflection spectrum from the FBG sensors split into two peaks because of the non-axisymmetric thermal residual stress. This deformation of the spectrum will lead to misreading in strain measurements or crack detection in the laminates. In the present research, three types of FBG sensors: uncoated normal, polyimide-coated normal, and polyimide-coated small- diameter FBG sensors were embedded in CFRP laminates, and reflection spectra from the sensors were measured during the fabrication process of the laminates in order to evaluate the effect of thermal residual stress on the reflection spectra. Through the comparison of results obtained for the three FBG sensors, it was found that the effect of thermal residual stress on the reflection spectrum could be decreased when the optical fiber was coated with polyimide in the present laminate configuration and embedment position. Furthermore, these changes in the spectra during the curing process were simulated theoretically. Thermal residual stress components at the core were obtained by FEM analysis, and the spectra were calculated using the couple mode theory and the transfer matrix method. Considering the birefringence effect, the calculated spectra reproduced the measured spectra very well.

Small-diameter fiber Bragg grating (FBG) sensors have been developed by Hitachi Cable Ltd. and the authors. Since the outside diameter of polyimide coating is 52 micrometers , embedding of the sensors into carbon fiber reinforced plastic (CFRP) composites prepregs of 125 micrometers in thickness does not deteriorate the mechanical properties of the composite laminates. In this research, the small-diameter FBG sensor was applied for the detection of transverse cracks in CFRP composites. The FBG sensor was embedded in 0 degree(s) ply of a CFRP cross-ply laminate.

In the present research, fiber Bragg grating (FBG) sensors were applied for the detection of transverse cracks, which cause strain distribution within the gage length, in carbon fiber reinforced plastic (CFRP) cross-ply laminates. An uncoated FBG sensor was embedded in 0° ply on the border of 90° ply in a CFRP cross-ply laminate. The reflection spectra from the FBG sensor were measured at various tensile stresses. As a result, the reflection spectrum became broad and had some peaks with increase of the transverse crack density in the 90° ply. After the crack density was saturated, the spectrum became narrow and had one large peak again. For confirming that the change in the spectrum form was caused by transverse cracks, the spectra were calculated theoretically. The calculated result reproduced the change in the measured spectrum form very well. These results show that the occurrence of transverse cracks can be detected from the change in the form of the reflection spectrum, and the spectrum width at the half-maximum is a good indicator for the quantitative evaluation of the transverse crack density on real-time.