Composite materials are finding their way into aerospace applications thanks to their high stiffness-to-mass ratio. Nevertheless, composite components require frequent inspections because of their sensitivity to critical damage. Damage as small as barely visible impact damage (BVID) can grow as result of structural loading, with component failure as possible outcome. Optical fibre sensors (OFS) are considered excellent candidates for permanently installed structural health monitoring (SHM) systems, owing to their many advantages over electrical sensors. Current state-of-the-art BVID detection with OFS has so far however been limited to proof-of-concept demonstrations at low technology readiness levels. In this work, we equipped a total of 16 coupons, made of 5 different state-of-the-art composite materials, with aerospace compatible embedded or surface mounted optical fibre Bragg gratings (FBGs). We impacted each coupon at two locations and acquired the FBG reflection spectra before and after each impact. We first demonstrate how changes in the Bragg wavelength and in the Bragg peak shape can be quantified when the FBGs are exposed to the (non-)uniform strain field of BVIDs. Second, we show that this method was able to successfully detect the BVID in all considered scenarios and that in most cases, it was able to also locate the damage within an uncertainty of ±1 FBG location. Finally, we show the reliability of this method in terms of repeatability and considering the effects of temperature changes and on-ground airplane vibration. To the best of our knowledge, we are the first to use in-flight-compatible embedded and surface mounted FBG-sensors for the detection and location of BVIDs on aerospace-grade composite materials. These results motivate the use of FBG sensors as a permanent sensor network for cost-efficient damage detection in composite aerospace components for locally monitoring damage-prone locations.
An innovative wireless passive system for impact detection on large-scale composite airframe structures is presented. The wireless system is designed to operate with a sensor network for onboard of aircraft for structural health monitoring, of composite airframe. The wireless systems efficient design allows for low power consumption, wireless communication capability, system robustness and large sensing area. The system is evaluated on a large-scale stiffened composite fuselage under different operational conditions. It is demonstrated that it is possible to detect impact events with different impact energy levels and impact locations over a large monitoring area. This work provides a potential solution for aircraft on-board structural health monitoring with no human intervention. This sensing system can be also adapted to other Internet of Things and structural health monitoring applications.
KEYWORDS: Composites, Transducers, Ultrasonics, Wave propagation, Laser Doppler velocimetry, Waveguides, Structural health monitoring, Signal detection, Signal to noise ratio, Visualization
This paper explores the feasibility of using ultrasonic guided Lamb waves to characterise the type and through thickness severity of damage present in composite plate-like structures. Two cases were considered, the first compared isolated subsurface delaminations between plies whilst the second case looked at more complicated barely visible impact damages caused by a low velocity impactor. In this study, the ultrasonic guided Lamb waves were generated by a surface mounted piezoelectric transducer and were sensed by a Laser Doppler Vibrometer. This allowed full wavefield imaging of the Lamb wave interaction with damage without the need for a previously acquired damage free baseline signal. In order to save time and improve the signal to noise ratio, the narrowband toneburst signals are reconstructed from a singular chirp response and a post-processing algorithm. Both cases showed similar results in that the first symmetric mode, S0, which is dominant at higher frequencies, caused mode conversions when interacting with the defects whilst the first anti-symmetric mode, A0, dominant at lower frequencies, mainly caused a change in phase and amplitude across the defects. Both cases also showed that as the damaged area got more severe, the effects of the damage on both modes became more pronounced.
Typical airliners operate in a range of conditions, hence airborne structural health monitoring (SHM) components, must withstand the relevant environmental conditions. Additional to the integrity of the components, the SHM performance (diagnosis and prognosis) must be robust and reliable under environmental and vibration profiles during operation. This work investigates the influence of the operational condition (including temperature, humidity and vibration loads) on the integrity of a piezoelectric based SHM system in terms of integrity of the system and robustness of the diagnosis for detecting barely visible impact damage (BVID) on a CFRP panel. Consequently, compensation techniques are proposed to remove the effect of the environmental loading on the decision-making algorithm. The validity of the proposed algorithm is demonstrated on a composite plate for the operational profile defined in MIL-STD 180G standard for airborne components of a regional aircraft.
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