Detection of transverse cracks in CFRP composites using embedded fiber Bragg grating sensors

Abstract

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 an 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. In order to confirm that the change in the spectrum was caused by transverse cracks, the spectra were calculated theoretically. The calculated result reproduced the change in the measured spectrum 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 that the spectrum width at the half maximum is a good indicator for the quantitative evaluation of the transverse crack density in real time.

Mechanism of Subordinate Peak Skewing of FBG Sensor during Cracks Propagation Monitoring on Aluminum Alloy Structure

“…Meanwhile, the FWHM also displayed an obvious correlation with the crack length until a saturation crack length is reached. The spectrum bandwidth is also directly related to strain distribution and hence has been previously applied [14,15]. A shift of the measured central wavelength of less than 0.1 nm is observed during the process.…”

Use of FBG sensors in SHM of aerospace structures

“…This phenomenon can be used to detect damage in the composite structures [11,12]. But it was reported that the chirp of the FBG spectrum was not limited to stress concentrations [13].…”

“…Peters et al (2001) [22] measured reflection spectra in a compact tension specimen with an embedded FBG sensor and simulated the change in the spectrum shape resulting from the large strain gradients. Okabe et al (2000) [11] and Takeda et al (2002) [12] first utilized this feature to detect internal damage in the carbon fiber reinforced polymer (CFRP) laminates. Yashiro et al (2005) [20] also demonstrated that the reflection spectrum of an embedded FBG sensor was useful for identifying damage patterns within the gauge section for notched FRP laminates [23].…”

THE FORTY-EIGHTH HONDA MEMORIAL LECTURE Nondestructive Evaluation and Smart Materials

“…This report shows examples of application to composite materials. 4.2.1 Development of health monitoring technology 22) There is a need to monitor such anomalies as defects and vibrations at all times. It is therefore hoped that optical fibers, piezoelectric elements and electrical resistors and other measuring methods are established.…”