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The laser ultrasonics using guided waves excited by laser irradiation has attracted attention as an efficient non-destructive inspection method for carbon fiber reinforced plastic (CFRP) composite laminates. In this paper, to clarify the properties of laser-excited Lamb waves, we investigate the power flow of Lamb waves in an anisotropic CFRP laminate. The temperature rising caused by laser absorption is analytically calculated, and the derived thermal force is input to a finite element model to simulate the generation of Lamb waves. It is found that the power flow has an obvious directivity in a quasi-isotropic CFRP laminate. The power flow is smaller in the direction parallel to the carbon fibers of the surface layer and is larger in the perpendicular direction, but the maximum is in slightly different direction; this result was validated by an experiment. Such directivity pattern is determined by two factors: the distribution of thermal force and the stacking sequence of CFRP laminates. Moreover, we investigate the different simulation models (metal-coated CFRP and cross-ply CFRP) to discuss the influence of each factor on the distribution of power flow separately.

For the development of lightweight biomimetic materials, the compressive properties of the beetle elytron plate (BEP, a type of biomimetic sandwich plate inspired from beetle elytra) and the underlying mechanism thereof were investigated. With the following results: (1) The shared mechanism of trabeculae was revealed by using structural analysis. It is further predicted that a BEP with hollow trabeculae should possess enhanced compressive properties. (2) When the trabecular number (N) in a hexagonal unit of the honeycomb is less than three, the compressive strength of the BEP is rapidly increased with the increment of N. When Nis over four, the deformation capacity is significantly improved because of the arising of S-type buckling deformation in the core structure of the BEP. Furthermore, the definition of the BEP is proposed combined with the biological structure of the beetle elytra. (3) When N=6 and the external diameter of trabeculae is equal to the length of honeycomb walls, the synergistic mechanism between the trabeculae and the honeycomb walls in BEPs is fully exerted. Namely, the trabecula restricts the deformation of the honeycomb walls; in turn, the honeycomb walls provide lateral support for the trabecula. This mechanism leads the core in the BEP to generate an S-type global buckling deformation producing the best compressive properties. The results will greatly impact the biomimetic field of beetle elytra and many industries in which honeycomb structure also serves as a key component.

Origami has shown the potential to design unique mechanical properties and complex three-dimensional shapes by folding through designed crease patterns on flat materials. The authors investigated a new honeycomb-based origami metamaterial called “kirigami honeycomb.” Resembling origami, kirigami honeycomb allows a single flat sheet of material with periodic slits to be folded into a honeycomb shape. Previous studies have reported successful use of this method to create various honeycomb shapes, changing only the folding line diagrams (FLDs). These previous studies have, however, considered only one-directional cross-sectional modifications; the core thickness and curvature changed only in the W-direction. This study proposes a new method that will support complex 3D honeycomb designs made from single flat sheets. A newly defined crease pattern conversion method provides arbitrary scaling of the honeycomb shape in the L-direction. The combined FLD and pattern conversion design methods encourage the cost-effective manufacture of 3D shaped honeycombs from single flat paper sheets. The proposed method is implemented to the design software, enabling to design arbitrary cross-sectional honeycomb cores with optional L-directional scaling.

This paper presents the results of elastic constant determination of Ceramic Matrix Composites (CMC) using two types of ultrasonic measurement systems, and shows the possibility of the elastic constants determination for transversely isotropic materials not only in a macroscopic area but also in a microscopic one at the order of 1 mm. Macroscopic elastic constants were determined from the transmitted wave velocities measured by a double-through-transmission method. Microscopic elastic constants were inverted from the reflection coefficients at a small spot on the surface, which were measured using an ultrasonic micro-spectrometer. The inversion from the reflection coefficients was tried by two ways of analysis: (1) fitting to phase curves of reflection coefficients; (2) fitting to surface wave velocity dispersion. By comparison between these values and those measured by tensile tests, it was conformed that all elastic constants of a unidirectional CMC could be determined by ultrasonic waves appropriately not only in a macroscopic area but also in a microscopic one.

An ultrasonic sensor based on two cascaded phase-shifted fiber Bragg gratings (PS-FBGs) is proposed and demonstrated. In place of an external cavity laser, a broadband amplified spontaneous emission light source is used to demonstrate multiplexing ability suitable for sensor networks. The system has a high sensitivity to ultrasonic waves generated by a PZT actuator placed 7.5 cm away from the PS-FBG, because of the steep slope in the center of the PS-FBG spectrum. A second advantage of the phase shift is to reduce the effective sensor length, leading to the achievement of broadband characteristics. A pencil lead break test was performed and all results are compared to a traditional PZT sensor.

For the non-destructive inspection of carbon fiber-reinforced plastic (CFRP), lasers can be used to generate ultrasonic waves. It is important to optimize the wavelength of the laser to ensure the intense excitation of a usable propagating mode. Real CFRP components used in the construction of airplanes and automobiles are often coated with several types of resin to protect against weathering. These resin layers change the excitation of the ultrasonic waves. Thus, the optimum laser wavelength may be changed by the coating resin. In this paper, we investigated the excitation of ultrasonic waves in a resin-coated CFRP plate using different laser wavelengths. We conducted experiments to convert the laser wavelength using periodically poled LiNbO3 (PPLN) devices. By injecting mid-infrared laser to a coated sample, we observed excited ultrasonic waves using a laser Doppler vibrometer. We found that transparent resins significantly increase the amplitude of the first-arriving longitudinal wave. Furthermore, when the laser was strongly absorbed in the surface layer, the excitation of longitudinal waves was suppressed. These results were clarified by a one-dimensional model of the thermal generation of ultrasonic waves. We concluded that a laser passing through a resin layer is a viable candidate for the effective inspection of coated CFRP by laser ultrasonic waves.

Heat-resistant composites, such as ceramic matrix composites and heat-resistant carbon fiber reinforced plastics (CFRPs), are expected to be used for aircraft engine parts. The development of reliable heat-resistant composite materials requires the use of nondestructive test techniques for evaluating the progression of damage during material testing at elevated temperatures. Furthermore, structural health monitoring (SHM) technologies that operate under harsh environments are expected to be realized for monitoring heat-resistant composite structures. To provide potential solutions for the establishment of such technologies, this research developed a heat-resistant ultrasonic sensor based on a regenerated fiber-optic Bragg grating (RFBG). First, we fabricated an RFBG by annealing a normal fiber-optic Bragg gratings (FBG) sensor. Because the RFBG exhibits high heat resistance at temperatures of 1000 °C, the sensor achieved stable ultrasonic detection at an elevated temperature. In addition, we attempted to use a π-phase-shifted FBG (PSFBG) as the seed grating to construct an ultrasonic sensor with enhanced performance. As a result, the regenerated phase-shifted fiber-optic Bragg grating (R(PS)FBG) sensor possessed a very short effective gauge length and achieved a broad frequency response to ultrasonic waves with frequencies greater than 1.5 MHz. The broadband detectability enables the R(PS)FBG sensor to acquire an accurate response to ultrasonic waves. Hence, we believe the regenerated Bragg grating-based ultrasonic sensors can contribute to establishing an effective nondestructive evaluation method for composite materials, thereby enabling a structural health monitoring system for a composite-made structure operating under extreme high-temperature environments.