Tensile fatigue tests using acoustic emission (AE) measurements were performed on unidirectional carbon fiber reinforced plastic (CFRP) laminates with a notch to investigate the relationship between the fatigue damage behavior and the associated AE behavior. Immediately subsequent to the initiation of the tests, splitting occurred along the direction of the fiber from the tip of a notch, and rapidly progressed for all investigated stress levels. Thereafter, the rate of development was almost constant after 3×10⁴ cycles and the AE behavior also changed. These results indicate that the AE behavior is indicative of the splitting behavior. We discussed the AE behavior based on the experimental results for the splitting behavior and the relationship between the AE, and the fatigue damage behavior of the unidirectional CFRP laminate was obtained. From this relationship, it is possible to predict the damage type, size, and quantity by comprehensively evaluating the frequency characteristics, and each parameter of the AE waves.

L-shaped carbon fiber reinforced plastic (CFRP) laminates were designed and fabricated for application in lower limb orthosis. Four types of CFRP laminates were constructed to attain a bending moment of 10 Nm at 5° rotation. Bending tests were conducted on the laminates using a newly developed testing fixture. Further, acoustic emission (AE) measurements were conducted during these tests to characterize the microscopic damage behavior in the laminates. It was confirmed that the laminates designed in this study have an appropriate stiffness that supports walking. Moreover, microscopic damage behaviors were well-characterized to monitor the rise time and duration of AE waves.

Carbon fiber reinforced plastic (CFRP) is composed of carbon fiber and a polymer material, which is a matrix resin. Resin-dependent damage initiation behaviors, such as matrix cracking and interfacial debonding, are affected by the viscoelasticity of the resin (effects of temperature and strain rate), and the strength of the entire CFRP is also affected. In situ observation is required to investigate the effects of viscoelasticity on the damage initiation behaviors. Therefore, in this study, the effects of temperature and strain rate on the damage initiation behaviors of unidirectional CFRP were investigated using the acoustic emission (AE) technique. To understand the details of AE information, frequency analysis was performed. In the frequency analysis, the resonant frequency of AE sensors affects the frequency properties of the detected signals, so we attempted to eliminate the effect of the sensors by deconvolution of the AE sensor frequency properties from the detected signals. As a result of the deconvolution, we could distinguish the failure modes as matrix cracking, fiber/matrix interfacial debonding, and fiber breaking. From these results, matrix cracking and fiber/matrix interfacial debonding were affected by temperature and strain rate; however, fiber breaking was not affected. Scanning electron microscope observation of the fracture surface confirmed that the resin around the fiber was affected by temperature and strain rate. From this damage initiation behavior of the matrix resin, which was affected by the viscoelasticity, it became clear that the behavior influenced the strength of the entire composite material.

It is difficult to evaluate the initiation and propagation of fatigue cracks in type-3 composite vessels used at hydrogen stations. Therefore, a smart vessel using an optical fiber acoustic emission sensor (OFAES) was developed. The relationship between the winding length of the OFAES, which was wound over the vessel, and the sensitivity of the signal was first studied, and it was confirmed that AE signals could be detected with high sensitivity by winding the OFAES according to half the resonance frequency wavelength of the radial direction of the vessel. Using this result, we then fabricated a smart pressure vessel in which the OFAES was wound in a striped pattern according to the half wavelength over the vessel. The sensitivity of the developed vessel was then compared with that of a commercially available piezoelectric (PZT) sensor by an artificial AE sensor. It was found that the PZT sensor was more sensitive when the sensor distance was closer to the AE source; however, the signal-to-noise ratio of the waveform detected by the PZT sensor decreased owing to higher attenuation as the propagation distance increased. Furthermore, the developed vessel was able to detect AE signals with constant sensitivity and frequency characteristics without depending on the position of the artificial AE source.