Abstract
Barely visible impact damage (BVID) due to low velocity impact events in composite aircraft structures are becoming prevalent. BVID can have an adverse effect on the strength and safety of the structure. During aircraft inspections it can be extremely difficult to visually detect BVID. Moreover, it is also a challenge to ascertain if the BVID has in-fact caused internal damage to the structure or not. This paper describes a method to ascertain whether or not internal damage happened during the impact event by analyzing the high-frequency information contained in the recorded acoustic emission signal signature. Multiple 2 mm quasi-isotropic carbon fiber reinforced polymer (CFRP) composite coupons were impacted using the ASTM D7136 standard in a drop weight impact testing machine to determine the mass, height and energy parameters to obtain approximately 1” impact damage size in the coupons iteratively. For subsequent impact tests, four piezoelectric wafer active sensors (PWAS) were bonded at specific locations on each coupon to record the acoustic emission (AE) signals during the impact event using the MISTRAS micro-II digital AE system. Impact tests were conducted on these instrumented 2 mm coupons using previously calculated energies that would create either no damage or 1” impact damage in the coupons. The obtained AE waveforms and their frequency spectrums were analyzed to distinguish between different AE signatures. From the analysis of the recorded AE signals, it was verified if the structure had indeed been damaged due to the impact event or not. Using our proposed structural health monitoring technique, it could be possible to rapidly identify impact events that cause damage to the structure in real-time and distinguish them from impact events that do not cause damage to the structure. An invention disclosure describing our acoustic emission structural health monitoring technique has been filed and is in the process of becoming a provisional patent.
Highlights
IntroductionStructures manufactured using composite materials, whether thermosets or thermoplastics, must be made in a nearly perfect state such that they do not introduce any dangerous risks during the operational lifetime of the aerospace structure
The authors have described an acoustic emission (AE) based structural health monitoring method [34] that can analyze the AE signal signatures obtained from an impact event and can ascertain if the impact event has caused an extensive damage inside the composite structure or not
Preliminary impact tests conducted on 2 mm quasi-isotropic coupons were used to estimate the mass, height and energy combinations to obtain approximately 1” impact damage size using incremental energy impacts on various test coupons and post-impact data analysis to estimate force-time histories and energy-time histories
Summary
Structures manufactured using composite materials, whether thermosets or thermoplastics, must be made in a nearly perfect state such that they do not introduce any dangerous risks during the operational lifetime of the aerospace structure. The manufacturing process of composite structures can introduce significant manufacturing flaws and operational damage during its service life. These types of defects may lead to catastrophic failures if they are not detected at the earliest stages of development using efficient structural health monitoring techniques. Visible impact damage (BVID) is a type of damage that occurs most often in composite structures. It can occur during the manufacturing stages or during the
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