Evaluation of bonding quality in CFRP composite laminates by measurements of local vibration nonlinearity

Solodov, Igor; University of Stuttgart; Germany

Solodov, I.; University of Stuttgart; Germany
Ségur, D.; CEA, LIST; France
Kreutzbruck, M.; University of Stuttgart; Germany

ID: ECNDT-0279-2018
Download: PDF
Session: Nonlinear Ultrasonics 3
Room: G2
Date: 2018-06-14
Time: 09:20 - 09:40

Conventional ultrasonic NDT instruments make use of the linear elastic response of materials which results in the amplitude and phase variations of the input signal due to its interaction with defects. The nonlinear approach to ultrasonic testing is concerned with nonlinear material response related to the frequency changes of the input signal. In many cases, monitoring material nonlinearity reveals directly the vulnerable areas within material or a product with sensitivity far superior to traditional ultrasonic inspection.
A direct way to evaluate nonlinearity in composite laminate plates is based on the second harmonic measurements for propagating Lamb modes. This technique is believed to be prospective for plate-like specimens and received considerable attention in literature. To generate the so-called cumulative second harmonic of a Lamb wave mode, however, the two crucial conditions are required: phase velocities of the fundamental and the second harmonic modes must be equal and the power flow between them must be non-zero. These strict conditions for selection of the experimental parameters make the nonlinear plate wave technique barely applicable in industrial environment.
In this paper, a new approach based on measurements of a local nonlinear response of the laminate is suggested and applied to characterizing contaminations of adhesive bonding in carbon fibre-reinforced polymer (CFRP). It is shown that a contaminated boundary layer of the adhesive contributes to an overall nonlinear response of the laminate that enables to evaluate and quantify bonding quality caused by various types and levels of single contaminations. All kinds of single contaminations studied in the context of aviation applications result in enhancement of the nonlinear response of the CFRP laminate which is an indication of deterioration of bonding quality. The enhancement of nonlinearity in weaker bonds was also applied to imaging of the contaminated adhesion areas in realistic airplane components.