IIW White Paper

5.5 NDT and structural health monitoring (SHM) of welded and composite structures

The trend of introducing light-weight structures in applications such as aviation, railway cars and the automotive industry has involved new joining technology (Laser welding, friction stir welding, adhesive joints) and new materials (high strength steels, Al-, Mg- and Ti-alloys, fibre reinforced composites). The classical NDT technology for maintenance will be replaced by structural health monitoring where in a short term strategy over the next few years, the aviation industry will monitor the fatigue experience of components such as a wing as a first step. These experiments have to be performed according to the damage tolerance concept in order to detect the early development of fatigue flaws and the speed of the damage accumulation. Possible sensor types for continuous monitoring will be tested in reliability tests. In a second step, the optimised sensors will be applied in the structure to monitor critical places where according to the design, fatigue damage can occur. The last step of the SHM strategy is the integration into the structure of reliable lifetime online integrity monitoring of 40 years. The added value by SHM is given by an extension of the maintenance intervals or - if these intervals will not be changed – by reducing the weight of the structure. Some sensors under development are based on fibre technology. Piezoelectric fibres can be embedded in the fibre composites. The fibre is a local transducer which can passively detect acoustic emission propagating from local damage but can also actively emit ultrasound interacting with flaws and producing reflected or scattered waves for detection. Other sensors under development are based on the use of guided waves and can be set at the surface of the composite structure to be monitored. These new type of sensors could be set on composite pressure vessels aiming to store hydrogen for fuel cell development.

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Figure 5.21 Simulation of propagation and interaction of an ultrasonic guided wave with a defect within a composite medium (reproduced courtesy Institut de Soudure/Georgia Tech university)

5.6 Developments with “local engineering” & SHM towards “ Intelligent welded structures”

Materials engineers have the vision to further develop the SHM concept in combination with adaptive features. One idea is to utilise the SHM sensors to determine the actual state of a component, for instance the local mechanical stress state in an aircraft wing or the local dynamic pressure on the leading edge. By embedding piezoelectric actuators in the structure, the wing profile can be locally adapted in order to enhance the air gliding and to reduce fuel consumption or to damp vibrations. The wing of the plane is becoming intelligent, the structure is smart. In the case of railway cars, development projects in Europe aim for noise reduction by embedding piezoelectric sensors and actuators. A dream is that materials can be developed with a self-healing or self- repair function. In this case, the SHM sensors give early detection of the beginning of damage and the intelligent processing of the structure, initiates the repair by a local engineer.

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Through Optimum Use and Innovation of Welding and Joining Technologies

Improving Global Quality of Life