Modern aerospace structures make increasing use of fibre reinforced plastic composites, due to their high specific mechanical properties. However, due to their brittleness, low velocity impact can cause delaminations beneath the surface, while the surface may appear to be undamaged upon visual inspection. Such damage is called barely visible impact damage (BVID). Such internal damages lead to significant reduction in local strengths and ultimately could lead to catastrophic failures. It is therefore important to detect and monitor damages in high loaded composite components to receive an early warning for a well timed maintenance of the aircraft. Non-linear ultrasonic spectroscopy methods are promising damage detection and material characterization tools. In this paper, two different non-linear elastic wave spectroscopy (NEWS) methods are presented: single mode nonlinear resonance ultrasound (NRUS) and nonlinear wave modulation technique (NWMS). The NEWS methods were applied to detect delamination damage due to low velocity impact (<12 J) on various composite plates. The results showed that the proposed methodology appear to be highly sensitive to the presence of damage with very promising future NDT and structural health monitoring applications.

No abstract

Ultrasonic waves are useful tools to characterize the contact forces between components in non-destructive and non-invasive manners. It has been shown that the transmission and reflection coefficients of the ultrasonic wave are sensitive to the contact pressure or other contact parameters. Theoretically, the normal and tangential stiffnesses of the contact interface govern the transmission/reflection coefficients and can be used as parameters to characterize the contact condition. However, weak and incomplete interfaces, formed by rough surfaces in partial contact, show a highly nonlinear behaviour also when they are excited under free vibrations. In particular, the amplitude of the second harmonic is a relevant index of the contact stiffness, and the nonlinear response is strongly influenced by the nominal contact pressure applied to the boundaries. In this study a new theoretical model of the nonlinear interface stiffness was developed where the stiffness of the contact interface was described as a function of the nominal contact pressure. The developed theoretical contact pressure function of the second harmonic generation at the contact interface was found to agree with good accuracy with the experimental data. Moreover, this paper presents also a theoretical and experimental study aimed at developing an integrity index capable of assessing the stiffness of the contact interface between structures when excited by free vibration or under controlled vibration excitation.

The low-velocity impact response of a range of fiber metal laminate (FML) panels was investigated through testing and finite element simulations. The objective of this study was to understand the impact-damage resistance of these novel composites, so that they can be designed optimally for impact-resistant aircraft applications. The FML panels were made up of aluminum alloy 7475 T761 and unidirectional S2 glass/epoxy oriented in a cross-ply configuration. Experimental tests were performed using a free-fall drop dart testing machine. The plate specimens were constrained on a circular edge by the clamping fixture. The shape and the nature of the damage inflicted by impact were evaluated using both destructive cross-sectional microphotography and nondestructive ultrasonic techniques. The tests showed that FML laminates are capable of absorbing energy through localized plastic deformation and through failure at the interface between the layers. In particular, delaminations occurred in the back face of the aluminum-alloy sheet and its adjacent fiber-reinforced epoxy layer and in between adjacent fiber-reinforced epoxy layer. The finite element code, LS-DYNA3D, was used to perform numerical simulations of low-velocity impact to predict the complex damage propagations. The computed post-impact deformed shapes and damage patterns were found to be fairly close to experimental results.

The past few decades have seen significant growth in the development and application of multifunctional media for the enhancement of material properties, thermo-mechanical and sensing properties. This research work reports a novel approach in which a multifunctional material, herein referred to as SMArt composite, can be employed as a structural health monitoring system for strain sensing and damage detection (SMArt sensing and SMArt thermography), but also as an embedded ice protection tool for structural applications (referred as SMArt de-icing). Such a material, obtained by embedding shape memory alloy (SMA) wires within traditional carbon reinforced plastic composites, relies on the possibility of using the wires both to increase the mechanical properties of composites panels and to exploit their intrinsic electrothermal properties. The electrical resistance variation and the internal power resistive heating source provided by the SMA network, enable a built in and fast assessment of the strain distribution and in situ damage visualization via thermographic imaging. The efficiency of these techniques was experimentally validated on a number of SMArt composite laminates with single and multiple internal defects at various depths. The results showed that strain sensing and damage detection were achieved with high spatial resolution and accuracy, without the need to use large external heaters or complex signal processing techniques.