Broadband nonlinear elastic wave modulation spectroscopy for damage detection in composites

A non-destructive testing procedure is proposed for damage detection in composites using full wavefield measurement obtained using a scanning laser Doppler vibrometer. Vibrations are excited using two low-power piezoelectric actuators leading to nonlinear elastic wave modulation at the defect. One actuator is supplied with a broadband chirp signal and the other actuator is supplied with a single-frequency sine signal. First, a time–frequency filtering method is proposed to extract specific nonlinear components of interest (e.g. second higher harmonic and first modulation sideband) without the need for multiple excitation sequences. Next, damage maps are constructed using broadband bandpower calculation of the filtered nonlinear components. It is demonstrated that the modulation sidebands provide an exclusive imaging of defect nonlinearity, and are not affected by potential source nonlinearity. The proposed damage map construction procedure is applied for various carbon fiber reinforced polymer test specimens with different damage features: (1) coupon with quasi-static indentation damage, (2) coupon with artificial delaminations, (3) bicycle frame with impact damage, and (4) stiffened aircraft panel with partially debonded stiffener. The obtained results indicate the high performance of the developed procedure for detection of various defect types in curved and/or stiffened carbon fiber reinforced polymer components.

Application of Nonlinear Elastic Wave Spectroscopy in the Field of Nuclear Energy

In the field of nuclear energy, two particular applications are considered—the measurement of biological shielding concrete and measurement of high-temperature high-pressure piping. For the measurement of biological shielding concrete, a special manipulator was developed, which is applied through the ionization channels of VVER-440 power plants. Biological shielding concrete is covered on all sides with an 11 mm thick steel liner preventing its assessment using any conventional test method. For the measurement of high-temperature high-pressure piping, acoustic emission sensors installed in the LEMOP system (NPP Temelin) are used for both excitation and data acquisition. In operation, this piping is covered with glass wool insulation, but installed sensors on the waveguides make it possible to assess pipelines without removing the insulation. To demonstrate the material degradation, a set of concrete test specimens and a set of steel test specimens were made. Nonlinear elastic wave spectroscopy is a test method based on measuring the elastic–plastic response of the material whose plastic component is caused by microcracks. Part of this response may be caused by inhomogeneity of the material itself or the nonlinearity of the test system used, which is to be minimized in the measurement. Due to degradation, the number of microcracks increases, increasing the total nonlinear response of the material. The aim of this approach is to monitor the degradation of concrete and metallic materials based on the response of the test system to ultrasonic excitation.