Toshimichi Ogisu

Delamination detection in composite laminates using dispersion change based on mode conversion of Lamb waves

A new ultrasonic propagation system has been constructed using macrofiber composite (MFC) actuators and fiber Bragg grating (FBG) sensors. The MFCs and FBGs can be integrated into composite laminates because of their small size and high fracture strain. The developed system can send and receive broadband Lamb waves. In this research, this system was used to detect delamination damage in composite laminates. First, the multiple modes of Lamb waves in a carbon-fiber-reinforced plastic (CFRP) quasi-isotropic laminate were identified by transmitting and receiving the symmetric and antisymmetric modes separately. Then, the mode conversions at both tips of a delamination were investigated through an experiment and a two-dimensional finite element analysis (FEA). A new delamination detection method was proposed on the basis of the mode conversions, and experiments were carried out on laminates with an artificial delamination. When antisymmetric modes were excited, the frequency dispersion of the received A1 mode changed, depending on the delamination length owing to the mode conversion between the A1 mode and the S0 mode. This phenomenon was confirmed through the FEA and these results prove that this new method is effective in detecting a delamination in CFRP laminates.

Evaluation of debonding progress in composite bonded structures using ultrasonic waves received in fiber Bragg grating sensors

We have developed a damage detection system that generates ultrasonic waves with a piezo-ceramic actuator and receives them in a fiber Bragg grating (FBG) sensor. In this research, this system was applied to evaluate the debonding progress in carbon fiber reinforced plastic (CFRP) bonded structures. First, small-diameter FBG sensors were embedded in adhesive layers of a double-lap-type coupon specimen consisting of CFRP quasi-isotropic laminates bonded with epoxy adhesive films. Then, an ultrasonic wave at 300 kHz was propagated through the debonded region, and the wavelet transform was applied to the received waveform. The obtained results showed clear differences depending on the debonding length. Hence, a new damage index was proposed using the difference in the distribution of the wavelet transform coefficient. The damage index increased with an increase in the debonded area. Furthermore, this system was applied to a skin/stringer structural element of airplanes made of CFRP laminates. In this case, a correlation coefficient was also calculated from the results of the wavelet transform. As a result, the damage index increased and the correlation coefficient decreased with an increase in the debonded area. Hence the length of the debonding between the skin and the stringer could be easily evaluated.

Development of damage monitoring system for aircraft structure using a PZT actuator/FBG sensor hybrid system

This paper presents a part of the research results on a damage monitoring system using PZT actuators/FBG sensors for advanced composite material structures of new-generation aircrafts. To achieve weight reduction of the aircraft structure, these advanced composite materials have gradually been employed for the primary structure. It is expected that when these materials are extensively employed, an efficient bonded structure such as a hat-shaped stringer will be utilized for the aircraft structure. However, these bonded structures have critical problems such as debonding and delamination at the interfaces of the laminate. Further, a single-step molding process of the structure elements is necessary in order to ensure low cost and thus affordability. However, this low-cost process results in an increase in the non-destructive inspection (NDI) cost. Therefore, an innovative damage monitoring system is required for structural health management. In the present study, the authors have developed a hybrid sensor system that can detect the elastic waves launched from the piezo transducer (PZT) actuator using a high-speed and high-accuracy fiber Bragg grating (FBG) sensor to resolve the issues mentioned above. In this study, the conceptual design of an aircraft that can employ this damage monitoring system was carried out. Subsequently, the application area was selected based on cases of certain kinds of damage. Further, the validity of the damage monitoring system for the verification of the structural integrity of the aircraft was discussed. Next, in order to verify the elastic wave detectability of the FBG sensor, it was confirmed that an elastic wave of 300 kHz is detectable at a distance of 5 cm between the PZT actuator and FBG sensor using an aluminum sheet and CFRP cross-ply laminate and also by considering the relationship between sensor length and sensitivity. Through the present research results, the possibility of applying the damage monitoring system to the composite material bonding structure in an aircraft is presented.

Feasibility studies on active damage detection for CFRP aircraft bonding structures

This paper presents a part of the feasibility study for employing a damage monitoring system using a PZT actuator/FBG sensor. The goal of this research is to improve the present safety level of aircrafts by ensuring structural integrity and reducing maintenance/operation costs of advanced composite materials that will be employed for the primary structures of the new generation aircraft. Our novel sensor system employing a PZT actuator and an FBG optical fiber sensor can detect several types of damage, such as delamination and debonding, which are expected to occur in the highly efficient bonding structures of an aircraft wing box when the main/tail wing section is designed using composite materials. In this system, elastic waves will be transmitted into the structure by PZT actuators and received by small-diameter FBG optical fiber sensors that are embedded in the critical section of the primary structure of an aircraft. The onset and growth of damage can be detected with a very high accuracy through the change in the received elastic waves. In this study, a conceptual design was implemented in order to employ the novel system. We also selected the optimum application candidate area in order to verify the effects of the system. Further, the investigation and experiment on the novel sensor system that uses a bonded PZT actuator and an FBG sensor was carried out, and the basic damage detection technique was established based on the experimental results of the received elastic wave. Furthermore, compressive tests were carried out using the coupon specimen with an embedded small-diameter or standard-diameter optical fiber sensor. It was verified that the coupon specimen with an embedded small-diameter optical fiber suffered no degradation of its material properties.

Demonstration of detectability of SHM system with FBG/PZT hybrid system in composite wing box structure

We have developed a novel damage monitoring system that can monitor the integrity of composite structures in aircrafts. In this system, fiber Bragg grating (FBG) sensors are used as sensors and piezoelectric transducers (PZT) are used as the generators of elastic waves that propagate in the structure to be inspected. The damage monitoring system can detect the structural integrity by the change in elastic waves that are detected by the FBG sensor and arrayed waveguide grating (AWG)-type filter. We confirmed that the structure health monitoring (SHM) system was able to monitor the damage initiation and propagation by a change in the waveform of the elastic waves in coupon specimens and structural element specimens. In this study, we demonstrate the detectability of the damage monitoring system by using a subcomponent test specimen that simulates an actual aircraft wing box structure composed of carbon fiber reinforced plastics (CFRPs). The FBG sensors and PZTs are bonded to the surfaces of hat-shaped stringers by an adhesive. Damages such as de-bonding and delamination are introduced in the bonded sections of the skin and stringers by impact. Damage monitoring and diagnosis are carried out by the SHM system under ambient conditions. We successfully verify the detectability of our system.

Damage growth detection of composite laminate using embedded FBG sensor/PZT actuator hybrid system

This paper presents a part of the study conducted for developing a damage diagnostic system for an advanced composite material that can be utilized in next-generation aircraft structure. The authors have been working on a detection of elastic wave which can be launched from the PZT actuators, using small- and normal-diameter FBG optical fiber sensors that are bonded to the surface of the CFRP laminate under different conditions. Based on the results, it was verified that it is possible to achieve a high-accuracy detection of elastic wave by using FBG sensors bonded to the surface of the CFRP laminate. It was also verified that the damages generated on the inside of the composite material may be detected by the waveform analysis of the received elastic wave. In this study, the authors succeeded in the embedment of small-diameter FBG optical fiber sensors into the bonding surface of the double-lap type coupon specimen, which simulates the bonding structure of the CFRP composite structure. In this study, we also clarified several issues pertaining to the conditions, methods, and techniques involved in fiber embedding. An optical loss was observed during the embedment process, which may result in the loss of both accuracy and reliability. Based on these observations, the authors developed embedding techniques for optical fiber sensors that can reduce this optical loss. Additionally, the possibility of detecting an elastic wave, which was launched from the PZT actuators bonded to the surface of the coupon and directed to the host material, was verified using double-lap type coupon specimen having embedded small-diameter FBG optical fiber sensors at the bonding surface. Therefore, this specimen has provided an artificial defect that simulates the delamination generated at the bonding interface. Based on the measurements of the elastic wave, it was verified that the change in the elastic wave depends on the damage length, which is caused by the artificial defect. Moreover, based on the analysis of the received elastic wave, the possibility of damage detection was confirmed. The successful development of this damage monitoring system would ease the implementation of structural health monitoring system in aircraft structures in the near future.

A Basic Study of CFRP Laminates with Embedded Prestrained SMA Foils for Aircraft Structures

This paper presents some basic research results for the application of the smart materials and structural systems to aircraft structure. The authors conducted quasi-static load-unload tests and fatigue tests on several kinds of quasi-isotropic carbon fiber-reinforced plastic (CFRP) laminates with embedded prestrained shape memory alloy (SMA) foils. The damage behavior and the fatigue behavior were evaluated based on the effects of the recovery stress of SMA, and the relationship between the applied strain and the transverse crack density is discussed. It was found that the developed smart materials achieved a maximum improvement of 34% in the onset strain of the transverse cracks and a maximum improvement of 60% in the onset strain of delamination as compared with the standard CFRP laminates in the static condition. However, the fatigue properties of the smart material were not improved due to the material degradation of the 90°-ply at 80°C. The fatigue test by the multistep strain method using a structural element specimen was used to verify the results of coupon tests. The results indicate the possibility of using coupon data as a design database.

Improved Surface Treatment of SMA Foils and Damage Suppression of SMA-Foil Embedded CFRP Laminates

The authors have been conducting research and development studies on some applications of embedded SMA foil actuators in CFRP laminates for weight reduction and improvement of reliability for next generation aircraft. The goal of this research is to develop a method for the suppression of damage growth in CFRP laminates. It is certified that one of the key technologies is bonding properties between SMA foils and CFRP laminates. In this paper, improvement of bonding properties between SMA foils and CFRP and confirmation of suppression effects of damage growth are described. Some surface treatments (sol-gel method, anodic-oxidation method and spattering method) were performed on SMA foils. Then, peel resistance and single lap shear strength tests were performed using surface treated SMA foils. It was found that treatment by 10%NaOH is the most effective treatment for improvement of bonding properties. The treated surface was found porous and rough, which is supposed to provide anchoring effects of SMA/CFRP interfaces. Then, quasi-static load-unload tests were performed using the optimum treatment as mentioned above to investigate the damage behavior of quasi-isotropic CFRP laminates with embedded SMA foils. Micromechanisms of fracture behavior and the correlation between the crack density and the residual strain were discussed. The recovery stress of the SMA foil generated at over Af temperature showed capability of suppression for onset of transverse cracks in quasi-isotropic CFRP laminates with embedded SMA foils.

Modeling of Thermo-Mechanical Behavior of Ti-Ni Shape Memory Alloy Foils Embedded in Carbon Fiber Reinforced Plastic Laminates

Ti-Ni shape memory alloy (SMA) foils are embedded into carbon fiber reinforced plastic (CFRP) cross-ply laminates in order to suppress the occurrence and progress of the transverse cracks in the laminates. When the pre-strained SMA foil is heated, the shape memory effect produces appropriate recovery compressive stress in the direction to suppress transverse cracks. However, Ti-Ni SMA foil has these specific characteristics due to the existence of a rhombohedral phase (R-phase). Thus, in this research, in order to use this SMA foil as an actuator, an extended Brinson model was proposed to clarify complicated behaviors of SMA. Then, for the determination of the parameters in the constitutive equation in this model, experimental tests were performed for the SMA foils. Using this model, several simulations were conducted and compared with experimental results. The agreement indicated that this model could express the thermo-mechanical behavior of the Ti-Ni SMA foil adequately. Finally, the behavior of the SMA foil during the CFRP manufacturing process was calculated on the basis of this model. The calculation results proved that the recovery stress already existed after the manufacturing process and the embedment of 4% pre-strained SMA into CFRP cross-ply laminate had the effect of suppressing the occurrence of transverse cracks.

Development of damage suppression system using embedded SMA foil sensor and actuator

The recent studies suggest possible applications of shape memory alloy (SMA) for a smart health monitoring and suppression of damage growth. The authors have been conducting research and development studies on applications of embedded SMA foil sensors and actuators in CFRP laminates. The goal of this research is suppression of damage growth in CFRP laminates. At first, the authors proposed a concept of damage suppression in CFRP laminates. Then, the development studies are conducted in three phases. The first phase is the improvement of interlaminar shear strength between SMA and CFRP laminates. Some surface treatments were investigated for the improvement of bonding property by peel resistance test and single lap shear strength test. The second phase is the investigation of fabrication technique for producing a CFRP panel with embedded SMA foils. Fixture jigs were devised to introduce tensile loads during the fabrication process. The third phase is the strength demonstration of CFRP laminates with embedded SMA foils. Some strength test were conducted to obtain the design data for aircraft structures. It is confirmed that the shrinking force of pre-strained SMA influences to the strength and the crack density of CFRP panel.