Optical fiber strain sensors with light weight, small dimensions and immunity to electromagnetic interference are widely used in structural health monitoring devices. As a sensor, it is expected that the strains between the optical fiber and host structure are the same. However, due to the shear deformation of the protective coating, the optical fiber strain is different from that of host structure. To improve the measurement accuracy, the strain measured by the optical fiber needs to be modified to reflect the influence of the coating. In this investigation, a theoretical model of the strain transferred from the host material to the optical fiber is developed to evaluate the interaction between the host material and coating. The theoretical predictions are validated with a numerical analysis using the finite element method. Experimental tests are performed to reveal the differential strains between the optical fiber strain sensor and test specimen. The Mach-Zehnder interferometric type fiber-optic sensor is adopted to measure the strain. Experimental results show that the strain measured at the optical fiber is lower than the true strain in the test specimen. The percentage of strain in the test specimen actually transferred to the optical fiber is dependent on the bonded length of the optical fiber and the protective coating. The general trend of the strain transformation obtained from both experimental tests and theoretical predictions shows that the longer the bonded length and the stiffer the coating the more strain is transferred to the optical fiber.

The influence of multi-walled carbon nanotubes (MWCNT) on the structural dynamic behavior of MWCNT/epoxy nanocomposites was investigated. Two different types of MWCNTs, pristine MWCNT and functionalized MWCNT, were used in this study. Carboxylic acid-functionalized MWCNTs (MWCNT-COOH) were obtained by oxidation pristine MWCNTs via sonication in sulfuric-nitric acid and characterized by Fourier transform infrared spectroscopy (FTIR). Dynamic behaviors of the MWCNT reinforced nanocomposite including the natural frequency and damping ratio were determined using free vibration test. Experimental results showed that the damping ratio of the nanocomposite decreases with the increase of the MWCNT addition, while the natural frequency is increasing with the increase of the MWCNT addition. Functionalized MWCNTs improved the interfacial bonding between the nanotubes and epoxy resin resulting in the reduction of the interfacial energy dissipation ability and enhancement of the stiffness.

Graphene nanoplatelet (GNP) and multi-walled carbon nanotube (MWCNT) hybrid films were prepared with the aid of surfactant Triton X-100 and sonication through a vacuum filtration process. The influence of GNP content ranging from 0 to 50 wt.% on the mechanical and electrical properties was investigated using the tensile test and Hall effect measurement, respectively. It showed that the tensile strength of the hybrid film is decreasing with the increase of the GNP content while the electrical conductivity exhibits an opposite trend. The effectiveness of the MWCNT/GNP hybrid film as a strain sensor is presented. The specimen is subjected to a flexural loading, and the electrical resistance measured by a two-point probe method is found to be function of applied strain. Experimental results demonstrate that there are two different linear strain-sensing stages (0–0.2% and 0.2–1%) in the resistance of the hybrid film with applied strain. The strain sensitivity is increasing with the increase of the GNP content. In addition, the repeatability and stability of the strain sensitivity of the hybrid film were conformed through the cyclic loading–unloading tests. The MWCNT/GNP hybrid film shows promising application for strain sensing.

Based on the shift of the Bragg wavelength, fiber Bragg grating (FBG) sensors have been employed to measure a variety of physical parameters such as stress, strain, displacement, temperature, vibration and pressure. In this work, a simple and easy way to be implemented FBG sensing methodology was proposed to measure the temperature and strain simultaneously. Half of the FBG was bonded on the host structure, while the other half of the FBG was left free. The host structure was an aluminum test specimen with dimensions of 20 × 3.8 × 0.5 ${\mathrm{cm}}^3$. As the host structure subjected to mechanical and thermal loadings, the Bragg wavelengths reflected from the bonded and unbonded FBGs are different. Theoretical predictions of the Bragg wavelength shifts of the bonded and unbonded FBGs were presented. Utilizing the Bragg wavelength shift of unbonded FBG, the temperature can be determined and is independent of mechanical strain. The Bragg wavelength shift of the bonded FBG allows the determination of the mechanical strain. The temperature measured by FBG sensor was compared with the result from a thermocouple, while the mechanical strain was validated with the theoretical prediction. Good agreement between the experimental measurement and theoretical prediction demonstrates that temperature-strain discrimination can be realized using the proposed method with one single FBG sensor.

A flexible hybrid film consisting of graphene nanoplatelets (GNPs) and multi-walled carbon nanotubes (MWCNTs) was prepared and employed as a multifunctional sensor to monitor temperature and liquid leakage, based on the piezoresistive effect. The influences of the GNP content on the mechanical, thermal, and sensing properties were investigated. Experimental results showed that both the hardness and Young’s modulus of the hybrid film were decreased with an increasing GNP content, while the thermal conductivity exhibited in an opposite trend. The electrical resistance of the hybrid film decreased was linearly with an increase in temperature. The resistance change increased linearly with an increase of the solvent adsorption. These features demonstrated the potential applications of the hybrid film in the detection of temperature, and liquid leakage. The sensitivity of leakage detection is increasing with the increase of the GNP loading, while temperature sensitivity is in the opposite trend.

Ultrasonic is one of the most common uses of a non-destructive evaluation method for crack detection and characterization. The effectiveness of the acoustic-ultrasound Structural Health Monitoring (SHM) technique for the determination of the depth of the surface crack was presented. A method for ultrasonic sizing of surface cracks combined with the time domain and frequency spectrum was adopted. The ultrasonic frequency spectrum was obtained by Fourier transform technique. A series of test specimens with various depths of surface crack ranging from 1 mm to 8 mm was fabricated. The depth of the surface crack was evaluated using the pulse-echo technique. In this work, three different longitudinal waves with frequencies of 2.25 MHz, 5 MHz and 10 MHz were employed to investigate the effect of frequency on the sizing detection of surface cracks. Reasonable accuracies were achieved with measurement errors less than 7%.