Next generation aircraft: Composite structures research with optical strain gauges

The Japan Aerospace Exploration Agency (JAXA) has relied on HBM’s data acquisition technology to investigate the structural health of recently developed aircraft made with composite materials. The research looked at the development of small to medium-sized jet passenger planes and was undertaken by JAXA as part of a collaborative project with the Tokyo Metropolitan Government and the Tokyo Metropolitan University (TMU).

JAXA’s research focused on both developing the advanced composite structures as well as the smart technology for this next generation of aircraft. In parallel with JAXA’s research, the TMU undertook research into likely future avionics developments.

The research involved two phases, both of which utilized HBM’s equipment for the testing regime. The first phase, which was undertaken in 2011, developed an Advanced Technology Demonstrator (ATD) as a business jet for up to 8 passengers (Fig.1).

The second phase, which took place during 2012, looked at developing a regional jet for 120 passengers. Following the success of the tests on the ATD’s canard wing (Fig.2) a bigger composite structure for a wing in the region of 5 m long was built and static tests undertaken to examine the integrity of the fuel tanks that are located in the aircraft’s main wing.

Both aircraft feature specially developed advanced carbon fiber reinforced laminates (CFRP). The high impact resistant laminates are reinforced by Vectran fiber through-the-thickness stitching to reduce any loss of structural strength through delamination.

Fig.2 ATD full scale nose section mock-up
Fig.3 FBG sensor set-up on upper panel stitched composite skin of a canard torque box

HBM’s optical strain gauge systems (Fig.3) were first used to undertake the structural testing of the ATD’s canard wing design by applying variable loads to the wing using a hydraulic actuator. One of the aims of the project was to monitor the effects of impact damage on the canard wing using 18-channel optical strain gauges (Fig.4). Researchers are also focusing on the development of a Health Monitoring System (HMS) for new wing structures using optical strain gauges.

Fig.4 Monitoring example of impact level and location

The decision to utilize optical strain gages, as opposed to mechanical devices, was to minimize the possibility of any risk of explosion while ensuring that electromagnetic effect (EME) concerns did not impact on the results. A total of 20 optical strain gages were distributed on the wing to give the coverage needed for the stress calculations (Fig.5).

Fig.5 Monitoring display of strain value and estimated strain distribution picture by pre-FEA results

Researchers working on the project were able to save a significant amount of time in fitting the optical strain gauges with HBM’s patch over the time taken to fit standard mechanical strain gauges. The optical strain gauges are suitable for both dynamic and static tests on the structure.

In addition researchers used HBM's MGCplus in combination with three of HBM’s DI410 optical strain gauge interrogators. These are 4-channel devices with a capacity of up to 1,000 measurements/sec. Two of HBM’s M416 multiplexers were also used for connecting up to 320 optical measuring points to provide fully synchronized measurement in real time. Data capture and analysis was carried out with HBM’s catman Enterprise software connected to all the devices.

The use of HBM’s equipment meant that a comprehensive structural analysis could be easily completed. The ability of the analysis software to perform rapid calculations and graphically display the structural deformation helped researchers to predict premature failure of the aircraft structure.

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