Abstract
This paper proposes a hybrid structural health monitoring (SHM) solution for a smart composite patch repair for aircraft structures based on piezoelectric (PZT) and fibre optic (FO) sensors to monitor the integrity of a the bondline and detect any degradation. FO sensors are used to acquire guided waves excited by PZT transducers to allow the advantages of both sensor technologies to be utilised. One of the main challenges of guided wave based detection methodologies is to distinguish the effect of temperature on the propagating waves, from that of an existing damage. In this research, the application of the hybrid SHM system is tested on a composite step sanded repair coupon under operational condition (temperature variation) representative of an aircraft for the first time. The sensitivity of the embedded FO sensor in recording the strain waves is compared to the signals acquired by PZT sensors under varying temperature. A novel compensation algorithm is proposed to correct for the effect of the temperature on the embedded FO sensor spectrum in the hybrid set-up. The repaired specimen is then impacted with a drop mass to cause barely visible impact damage (BVID). The hybrid SHM system is then used to detect the damage, and its diagnosis results are compared to a PZT only based smart repair solution. The results show promising application of the hybrid solution for monitoring bondline integrity as well as highlighting challenges of the embedding of FO sensors for a reliable and repeatable diagnosis.
Highlights
Composite structures are increasingly being used in aircraft structures, bonded composite repairs are still not certified on primary structures [1,2]
A hybrid structural health monitoring (SHM) system based on Fibre Bragg Grating (FBG)-PZT sensors has been developed for a composite patch repair, to address the challenge of monitoring the integrity of its bondline
During the development of the hybrid system, several challenges were highlighted: the change in amplitude of reflectivity was different for each recording due to Bragg peak shift and change of shape, which if not corrected can result in guided wave variations that can be mistaken for damage effect
Summary
Composite structures are increasingly being used in aircraft structures, bonded composite repairs are still not certified on primary structures [1,2]. It is challenging to achieve a consistent bondline quality because it is directly related to the skills of the technician. With the recent advances in manufacturing technologies, large scale composite parts are currently being manufactured in one shot for aircraft structures which means that when a primary part is damaged, if there is no possibility of repair, it has to be replaced which is not practical. The accepted practice for composite repair is bolted repair. Many of the aircraft manufacturers and operators have directed their research to finding a solution for composite bonded repair and addressing challenges related to the bondline integrity
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