Detection of Impact Damage Using a Nonlinear Ultrasound Approach

Abstract

Developments in carbon fibres reinforced plastic (CFRP) materials have allowed a radical advancement in lightweight aerospace applications. However, these components are sensitive to low velocity impact damages that can considerably degrade the structural integrity and, if not detected, they can result in loss of control and catastrophic failures. Literature provides a quantitative number of linear and nonlinear diagnostic imaging methods that can continuously provide a detailed image of the structural damage. This paper presents a nonlinear Structural Health Monitoring (SHM) imaging method, based on nonlinear ultrasonic guided waves (GW), for the detection of the delaminations in a CFRP composite structure caused by low velocity impact loadings. In particular, this research study images the damage by using a second order nonlinear signature. The proposed technique, based on a combination of second order phase symmetry analysis (PSA) with chirp excitation and inverse filtering (IF), is applied to a number of waveforms containing the nonlinear impulse responses of the medium. Phase symmetry analysis was used to characterise the second order nonlinearity of the structure due to delamination, by exploiting its invariant properties with the phase angle of the pulse compressed chirp signals. Then, the IF approach was applied to a number of waveforms stored into a database containing the experimental second order transfer function of the structure. Unlike other ultrasonic imaging methods, the present technique allows achieving the optimal focalization of the nonlinear source in the spatial and time domain, by taking advantage of multiple linear scattering and a small number of receiver sensors. In addition, this methodology does not require any baseline with the undamaged structure as well as iterative algorithms for the detection of the nonlinear source. The proposed method was compared to classical C-scan showing a robust and accurate system to image impact damage in composites structures.