Umberto Polimeno

Low-velocity impact behavior of fiber metal laminates

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.

Multifunctional SMArt composite material for in situ NDT/SHM and de-icing

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.

Detecting loosening/tightening of clamped structures using nonlinear vibration techniques

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.

Smart nonlinear acoustic based structural health monitoring system

The objective of this work was to demonstrate the feasibility of nonlinear vibration/acoustic/ultrasonic diagnostic tools to be implemented in a structural health monitoring system for imaging damage. In particular, the sensitivity a second harmonic imaging technique (SHIT) based on material nonlinear elastic effect known as second harmonic generation (SHG) was investigated. Examples of the capability and limitations of the proposed damage detection process to detect and image barely visible impact damage (BVID) due to low velocity impact (<12J) are presented for various composite laminated. The presence of microcracks, debonding, delamination, etc… could induce the material to behave in a nonlinear elastic fashion and it is highlighted by the presence and amplitude of harmonics in the spectrum of the received signal. The results showed that the proposed SNIT methods appear to be highly accurate in assessing the presence and magnitude of damage with very promising future NDT and structural health monitoring applications. Moreover the technique was validated with two conventional NDT techniques: pulse thermography and thermosonic. The first failed in detecting the damage on the impact face, but delamination on back surface was localized. The second technique was capable of localising and quantifying the damage on the impacted surface agreeing well the results obtained using non linear method.

Impact Damage Detection in a Stiffened Composite Wing Panel Using Digital Shearography and Thermosonics

There has been a growing interest in the use of composites especially in structural application ranging from aerospace to automotive and marine sectors. However, their performances under impact loading represent one of the major concerns as impacts may occur during manufacture, normal operations and maintenance. This paper presents two novel NDT techniques, thermosonics and digital shearography (DISH) to detect and assess barely visible impact damage (BVID) produced on a stiffened composite wing panel by unknown low energy impacts. Thermosonics is based on synchronized infrared imaging and ultrasonic excitation. Despite the apparent simplicity of the experimental setup, thermosonics involves a number of factors, e.g. acoustic horn location, horn crack proximity, horn-sample coupling etc., that significantly tend to influence both the degree and the period of the excitation. Then, a numerical-experimental procedure for the assessment of the size and depth of delamination by digital shearography (DISH) is proposed. The flaw detection capabilities of DISH have been evaluated by measuring the dynamic response of the delaminated area to applied stresses. The shearographic methodology is based on the recognition of the (0 1) resonance mode per defect. A simplified model of thin circular plate, idealized above each impacted area, is used to calculate the natural frequency of vibrating delamination. The numerical difference between experimental resonance frequencies and those computationally obtained is minimized using an unconstrained optimization algorithm in order to calculate the delamination depth. The results showed that thermosonics is a quick and effective method to detect and localize BVID damage while the combined shearography and optimization methodology was able to size and localize delamination due to low velocity impacts.

Corrosion identification on an aluminium plate-like structure by monitoring the wave propagation phenomena

In order to reduce the problems related to the detection of corrosion damage in aircraft structures, it is vital to develop new robust, accurate and reliable damage detection methods. A possible answer to this problem is offered by an evolution of the Non linear Elastic Wave Spectroscopy (NEWS), a wave propagation methodology developed for geophysics applications. The NEWS damage detection based technique consists in the analysis of the frequency spectrum generated by a bi-harmonic signal. In pristine condition, the signal spectrum presents two picks at the excitation frequencies. In presence of damage, the material starts to behave non-linearly around the damage location, and this behaviour shows up in the bi-harmonic excited signal spectrum as side bands and harmonics of the excited frequencies. The magnitude and the number of the side bands and harmonics are related to damage size and magnitude. In this study, numerical findings on a welded aluminium plate-like structure are reported in order to understand the sensitivity of the NEWS technique to detect corrosion damages. The results showed that the proposed methodology appear to be highly sensitive to the presence of corrosion damage, but the methodology has yet to be developed and applied to aircraft structures and much work is needed to demonstrate the effectiveness of the methods and the ease-ofimplementation in a structural health monitoring system. 1 Phone: +441234750111 ext.5220 Fax: +441234752149 email: m.meo@cranfield.ac.uk

In situ damage detection in SMA reinforced CFRP

The purpose of this paper is to analyse the possibility to manufacture and verify the self-sensing capability of composite materials plates with an embedded network of NiTi shape memory alloys (SMA) used as transducers for structural integrity. Firstly, the thermo-electrical material properties of SMAs were investigated to assess their capability to sense strain within. The results showed that the electrical resistance variation provided by the shape memory alloys network enables a built in and fast assessment of the stress distribution over the entire structure. Then, by transmitting a low amperage current, results in an electric and thermal flow through the entire SMA network. Using an IR Camera it is possible to capture the emitted thermal waves from the sample and create an image of the thermal field within the material. Consequently, analysing the behaviour of the heating curves on different points of the sample, it is possible to identify potential variation in the apparent temperature of the composite, leading to the identification of damages within the composite structure.