Improving Global Quality of Life

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.
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)
(a)
(b)
(c)
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.
50 Improving Global Quality of Life Through Optimum Use and Innovation of Welding and Joining Technologies