Induction Heating and Health Monitoring Solutions for Smart Aircraft Maintenance using Adapted Composite Patches - INDUCERINDUCERINDUCERINDUCER

Induction Heating and Health Monitoring

Solutions for Smart Aircraft Maintenance

using Adapted Composite Patches

INDUCERINDUCERINDUCERINDUCER

Project No: SP1-JTI-CS-2009-01-255770-GRA-01-010

Topic Manager: Alenia Aeronautica

Bonded composite repaired areas of aircraft structures frequently suffer from a variety of factors that

may influence their structural integrity, on either a short or long term basis. In order to overcome

these drawbacks, an advanced aircraft repair and maintenance concept of “smart patches” has been

developed, both for composite to composite and composite to metal repairs, combining “smart curing”

of the repair by induction heating and “smart sensing” using magnetostrictive sensors. One of the main

innovations of this project is that the same element which will be used within the repair for induction

heating (i.e. magnetostrictive sensors array), will be consequently used for lifetime structural health

monitoring of the repaired area, providing accurate mapping of strain development and evolution.

Smart Curing:Smart Curing:Smart Curing:Smart Curing: Induction heating occurs in ferromagnetic materials when they are exposed to a varying magnetic field. This

is the result of the development of “eddy currents” in the material, which means that heat generation is

mainly the consequence of the Joule effect. Heat is no more produced remotely and then transferred to the

repair by conduction (as in the case of “heating blankets”), but is internally generated by means of induced

electrical eddy currents, through a coil placed on top of the patch, that creates a rapidly alternating

electromagnetic field within conductive elements of the repair (susceptors).

Induction heating principle Different susceptor placement options Generation of heat in the repair

The developed induction system within INDUCER consists of a low power generator adapted to the process

needs, a specially designed induction coil and a susceptor in the form of a magnetostrictive materials mesh .

Initial prototype equipment used for

development of the induction principle

“Butterfly” coil configuration

used for heat generation

Final portable induction heating equipment

for in field applications

The induction heating methodology achieves better temperature homogeneity and process efficiency

compared to conventional conduction heating equipment and methods, resulting in direct improvement of the

repair quality, due to the minimization of thermal losses, as heat is generated within or extremely close to the

repair area to be heated. Moreover, the area to be heated is much smaller compared to conventional heating

using thermal blankets, leading to a significant improvement in power consumption compared to current

technology. Finally, being a non-contact process, equipment and consumables requirements are minimized

(cables exiting vacuum bagging etc.), making induction heating more robust, reliable, faster, cheaper and

easier to apply, compared to conventional conduction heating techniques, leading to an environmentally

friendly, low-energy, low-cost and easily interfaced solution for operational field bonding.

Aluminium

Composite

Induction Coil

Susceptor

Induction Heating and Health Monitoring Solutions for Smart Aircraft Maintenance using Adapted Composite Patches - INDUCERINDUCERINDUCERINDUCER

Smart sensingSmart sensingSmart sensingSmart sensing When a magnetic material carrying a low intensity & high frequency alternating current is subjected to an

external magnetic field, it exhibits a sharp change in its electrical impedance, called Magneto-Impedance

(MI) effect. Magnetostrictive Materials (MGSM) change shape when subjected to a magnetic field, due to the

fact that magnetic domains in the material align with the magnetic field, while the magnetic energy changes

when the material is strained (stretched or compressed). This phenomenon is reversible and is simplified into

two reversible energy conversion steps: Electric ⇔ Magnetic & Magnetic ⇔ Mechanical. It was

demonstrated that tensile and torsion stresses modify this domain structure, by introducing circular magnetic

anisotropy and/or helically magnetized interface between both areas, therefore changing the spatial

magnetization distribution close to the surface. Within INDUCER, the change of the MI response under the

effect of stresses was measured using appropriate magnetic flux sensors (i.e. co-centric induction coils) in

order to produce an image of the strains developed in repaired areas, for structural health monitoring

purposes.

Specimens with incorporated

MGSM used for

characterization of properties

Typical diagrams of ∆V

versus strain retrieved for

MGSM (Fe77.5Si7.5B15)

MGSM wires mesh produced

within INDUCER for smart

sensing and heating

Achieved performances

Strain Sensitivity: 0.02%

Spatial Sensor Resolution: 10µm

Overall method resolution: 2mm (depending on the MGSM mesh pitch)

Very good repeatability of

measurements.

Very good stability in time.

MGSM like Fe77.5Si7.5B15 and Co68.25Fe4.5Si12.25B15 were used in the form of wire of approximate diameter

0.125 mm (i.e. equivalent to the thickness of UD prepregs), produced according to the INDUCER

specifications in order to comply with composite repair health monitoring requirements.

Co-centric magketic fluc

sensors and MGSM wire

MGSM and magnetic

flux model setup

MGSM and magnetic flux

simulation results

Magnetic sensor equipped with

IR LED to record scanning

Apart from the hardware design and manufacturing, the appropriate algorithms and software have been

developed to support the MGSM sensing. Main achievements included specially designed software to (a)

Support the magnetic flux scanning by recording the sensor position and indicating successful area scanning

(b) Convert magnetic flux to strain measurements, (c) Visualize results using chromatic codes and (d)

Compare current to previous strain readings, to enable identification of areas where damage may exist.

Scanning progress

supporting software

Visualization of measurements

along MGSM wire

Measurements of

crossing MGSM

Overview of experimental set up for structural

health monitoring technology development

The combination of “smart heating” with “smart sensing” using magnetostrictive wires meshes

developed within INDUCER is expected to result in reduction of overall aircraft maintenance costs,

through (a) increase of repair and maintenance process reliability, (b) achievement of better repair

quality, (c) reduction of aircraft downtime during inspection intervals and (d) enabling certification of

larger and more complex bonded composite repairs, both to composite and to metallic structures.