Kazuro Kageyama

Prestandardization study on mode II interlaminar fracture toughness test for cfrp in japan

Abstract This paper summarizes results from a series of interlaboratory round robin tests (RRTs) performed in order to establish a JIS standard for mode I interlaminar fracture toughness test using double cantilever beam (DCB) specimens. For the case of unidirectional laminates, brittle and toughened CF/epoxy, and CF/PEEK systems were used. Only a brittle CF/epoxy system was used for woven laminates. The round robin tests were conducted with two main aims: first, to examine the influence of starter films and the precracking condition on the initial mode I fracture toughness values; and second, to establish the definition of initial fracture toughness. Polyimide starter films stuck to the epoxy matrix, and caused unstable crack growth from starter films. Comparison of the tests with and without mode I precracks from starter films indicated that tests with precracks gave lower values of initial fracture toughness. The definition of initial fracture toughness values was discussed, based on the reproducibility. A 5% offset point was recommended as the initial fracture toughness from the RRT results. The influence of loading apparatus, data reduction methods, etc. was also discussed.

Application of Fiber-Optic Distributed Sensors to Health Monitoring for Full-Scale Composite Structures

The purpose of structural health monitoring (SHM) is to lead a structure to be safer at lower cost. SHM systems capable of assessing structural integrity during manufacture and in-service operation would allow timely maintenance actions to increase safety and lifetime of structures. In such systems, it is important to evaluate the actual state of a structure. Recently, fiber-optic sensors have been actively developed, and one can measure many kinds of the physical measurands by them. Since they also have excellent characteristic, such as immunity of electromagnetic interference, durability and capability to realize distributed sensing, they are supposed to be suitable sensors for SHM systems. We installed fiber-optic sensors into full-scale composite structures to monitor strain or temperature during manufacture or to monitor in-service structural performance, i.e., stiffness. The structures applied with the sensors are International Americas Cup Class (IACC) yachts and a Japanese experimental reentry vehicle, namely, HOPE-X, that are made of carbon fiber reinforced plastic. The fiber-optic sensors used in this study are two kinds of distributed sensors using Brillouin scattering and Raman scattering, respectively. The former can measure strain or temperature and the latter can measure temperature at an arbitrary region along an optical fiber. We could successfully measure strain or temperature of the full-scale composite structures in field and access the structural state. The results of this study demonstrate the great potential of fiber-optic distributed sensors for practical applications to large composite structures.

Guided Wave and Damage Detection in Composite Laminates Using Different Fiber Optic Sensors

Guided wave detection using different fiber optic sensors and their applications in damage detection for composite laminates were systematically investigated and compared in this paper. Two types of fiber optic sensors, namely fiber Bragg gratings (FBG) and Doppler effect-based fiber optic (FOD) sensors, were addressed and guided wave detection systems were constructed for both types. Guided waves generated by a piezoelectric transducer were propagated through a quasi-isotropic carbon fiber reinforced plastic (CFRP) laminate and acquired by these fiber optic sensors. Characteristics of these fiber optic sensors in ultrasonic guided wave detection were systematically compared. Results demonstrated that both the FBG and FOD sensors can be applied in guided wave and damage detection for the CFRP laminates. The signal-to-noise ratio (SNR) of guided wave signal captured by an FOD sensor is relatively high in comparison with that of the FBG sensor because of their different physical principles in ultrasonic detection. Further, the FOD sensor is sensitive to the damage-induced fundamental shear horizontal (SH0) guided wave that, however, cannot be detected by using the FBG sensor, because the FOD sensor is omnidirectional in ultrasound detection and, in contrast, the FBG sensor is severely direction dependent.

Strain monitoring of a single-lap joint with embedded fiber-optic distributed sensors

We have developed a fiber-optic distributed sensor which can measure strain distributions along fiber Bragg grating (FBG) with the high spatial resolution. This sensing system is based on optical frequency domain reflectometry and a long-length FBG whose length is about 100 mm can be used. We can identify the longitudinal strain at an arbitrary position along the FBG using signal processing technology. In this study, long-length FBGs were embedded into the adhesive layers of the two single-lap joints and we could successfully measure the strain distributions inside the adhesives. In one single-lap joint, the adherends were carbon fiber reinforced plastics and in another one, they were aluminum. Theadhesive was epoxy in both cases. The measured results were compared with the calculated ones by nonlinear finite element (FE) analysis in which the large displacement and the elasto–plastic response of the adherend or adhesive material were account for. We found that in most of the applied loads, the agreement between the measured results and the calculated ones obtained from an intact FE model is excellent. While the measured strain distributions inside the adhesive layer of the aluminum single-lap joint were varied at the end of the overlap in the higher applied loads and they were much different from those of the intact model, an FE model with debonding was made and it could represent such variations. We could also monitor the strain distributions inside the adhesive during the manufacturing process and we observed the perturbation in residual strain distributions after curing. Consequently, we can say that the fiber-optic distributed sensor with the high spatial resolution is very useful not only to assess the structural integrity of adhesive joints but also to improve numerical analysis techniques and manufacturing processes for them.

Dispersion analysis of Lamb waves and damage detection for aluminum structures using ridge in the time-scale domain

In this paper, the dispersion of Lamb waves in aluminum structures was systematically analyzed to differentiate the mode of each package in Lamb wave signals and localize damage. Piezoelectric transducers were bonded on the surfaces of aluminum structures, functioning as actuator and sensor to excite and acquire Lamb waves, respectively. Wavelet transform was applied to the acquired Lamb wave signals, in which the optimal mother wavelet was selected using the concept of Shannon entropy to obtain the most accurate location of each wave package. The ridge and contour of the Lamb wave signals in the time-scale domain were obtained to distinguish the mode of each wave package and pinpoint these packages for estimating the actual group velocities of dispersion curves and localizing damage. The proposed approach could help search the actual dispersion curves in the excitation frequency band by using only one Lamb wave signal. Ridges in the time-scale domain and the actual group velocities were further used to identify damage in the structures. Results demonstrate that the proposed approaches were effective in dispersion analysis, wave mode differentiation and damage localization.

Distributed Strain Sensing from Damaged Composite Materials Based on Shape Variation of the Brillouin Spectrum

Among fiber-optic distributed sensors, Brillouin optical time domain reflectometry (BOTDR) can effectively measure overall strain in a structure. However, since the spatial resolution of BOTDR is generally more than 1 m, it is difficult to detect inhomogeneous strain distributed within 1 m along a fiber. In this paper, we propose a new technique to detect strain changing sharply within the length of the spatial resolution. This technique is based on the fact that the profile of the Brillouin spectrum changes depending on strain distributions. We confirmed the dependency of the Brillouin spectrum on strain distributions theoretically and experimentally.

Smart marine structures: an approach to the monitoring of ship structures with fiber-optic sensors

Prevention of serious damage of marine structures might be achieved by monitoring the loading conditions and by inspecting the structural integrity. The concept of smart structures with a fiber-optic sensor network can be applied to marine applications. There are plenty of marine structures in huge dimensions, for example, the overall length of a very large crude carrier exceeds 200 m. A fiber-optic laser-Doppler velocimeter was developed as a displacement sensor with long gage length. The optical fiber sensor has potential advantages as a structure monitoring sensor: (i) unlimited gage length, (ii) applicability to dynamic measurement and (iii) less affected by temperature. Free vibration behavior of the coupon specimen could be monitored with excellent dynamic resolution. Flexural deformation of the specimen under cyclic bending load was also monitored, and linearity and sensitivity of the developed sensor system was confirmed. An optical time-domain reflectometer (OTDR) was applied to the damage detection of composite materials in which optical fibers were embedded. Onset and location of fiber breaks could be detected by using the OTDR technique, which was applied to tensile tests of resin, and tensile and flexure tests of cross-plied GFRP. The failure strain of the optical fiber had rather wide scatter when it was embedded in the composite, and the average values also depended on the condition of embedment. Location of the failure points could be measured with sufficient accuracy.

Doppler effect in flexible and expandable light waveguide and development of new fiber-optic vibration/acoustic sensor

New principle and a geometrical arrangement of an optical fiber for a vibration/acoustic measurement are proposed in the present paper. The sensor is based on a new finding that a frequency of light wave transmitted through a bent optical fiber is shifted by vibration at the bent region. The phenomenon can be explained as Dopplers effect in flexible and expandable light waveguide. Several configurations of the sensor have been designed, and very high sensitivity is achieved in the extremely wide frequency range. Principle, sensor configuration and theoretical sensitivity, measurement system, and some experimental consideration are described in the present paper. The sensor sensitivity was examined experimentally in the low and middle frequency range, and the detectability was confirmed experimentally in the frequency range of acoustic emission signals.

Optimal Mother Wavelet Selection for Lamb Wave Analyses

Structural health monitoring (SHM) system, usually consisting of a sensor network for collecting the structural response signal and data analysis algorithms for interpreting the signal, plays a significant role in fatigue life and damage accumulation prognostics. Wavelet transform (WT) has gained popularity as an efficient means of signal processing in SHM, in which an optimal mother wavelet-based WT can carry out feature extraction with high precision. This article is to provide criteria of optimal mother wavelet selection in Lamb wave analysis for SHM, motivation of which is that small error in Lamb wave analysis can result in much larger error in damage localization because of very fast propagating velocities of Lamb waves. A concept, Shannon entropy of wavelet coefficients, was established to calibrate the degree of optimization of the selected mother wavelet. As application, various mother wavelets selected using the proposed criteria were applied to Lamb wave signals acquired from CF/EP composite laminates containing delamination. With the optimum mother wavelet, the essential information of the delamination-generated Lamb waves was achieved with high precision. The results demonstrate the excellent capacity of the approach for selecting the most appropriate mother wavelets for Lamb wave analyses and therefore damage localization.

Measurements of strain distributions with a long gauge FBG sensor using optical frequency domain reflectometry

We developed a strain measurement system based on optical frequency domain reflectometry and applied it to measuring strain distributions of a specimen in tensile tests. In the sensing region on the specimen, five FBG sensors of 6 mm gauge length or one of 100 mm gauge length was bonded and strain measurements were implemented with both configurations. By using the former configuration, we could successfully carry out accurate quasi-distributed strain measurements. The later allowed fully-distributed measurements for 100 mm at the high spatial resolution. Such performance of high resolution sensing can be applied to health monitoring of a structure which may have stress concentration. In this paper, we describe the principle of the measurement system and the results in the tensile tests.