NDI& E are essential to evaluate the structural health of an advanced composite structure. In critical components or parts where non destructive tests are difficult to perform or whose behavior is not yet entirely known, such as joints or adhesively attached patches, it is of extreme importance to find health monitoring systems that may reduce to acceptable levels the penalizing safety factors. A structural health monitoring strategy for patch repairs of carbon/epoxy structures was studied and implemented. The functioning of Chirped Fiber Bragg Grating sensors was analyzed with local and gradual heat tests, evaluating their capability of sensing the presence of peaks in strain and sensible strain field variations through its physical length. The final objective was being able to obtain from the only information given by the reflection spectrum from the sensor the presence of a patch debond, and estimating its size with acceptable accuracy. No information about the load should be considered as known, although all the output given by the sensor is directly dependent by the strain field. From the calibration charts and tables prepared two methods to achieve this goal were extracted. The same sensors were placed and embedded into patches, and the two methods were tested experimentally. As a support of the tests, some FEA analysis of the problem was carried out, supported in turn by tensile tests for material characterization. The sensor was tested at three levels. First, it was gradually attached to a laminate, the laminate was pulled and the sensor was interrogated. Secondly, the sensor was placed on a real patch, and the patch was attached to a sandwich specimen with a damaged carbon/epoxy skin. Light was injected with different debond sizes and results were carefully analyzed. Finally, a fatigue test was performed to let the cyclic strain produce the debond. Light was injected from the two sizes of the optical fiber, and a different behavior in the two situations was noticed and characterized. However, the two methods designed were applicable only injecting the light from the size recommended by the producer. Good results were obtained, replicating in the realistic tests approximately the same intrinsic errors that were encountered when the methods were designed.
NDI& E sono processi estrememamente importanti per valutare la salute strutturale di strutture avanzate in materiale composito. Tuttavia, in componenti particolarmente delicati dove test non distruttivi sono estremamente difficili da effettuare, oppure il cui comportamento non è ancora completamente noto, sarebbe utile avere a disposizione sistemi di monitoraggio strutturale che permettano di ridurre a livelli accettabili i fattori di sicurezza, in sè molto penalizzanti. Questi i motivi che hanno spinto allo studio e la realizzazione di una strategia SHM per riparazioni con pezze in composito di strutture in fibra di carbonio. Il funzionamento di FBG chirpati è stato analizzato con test di riscaldamento locale e graduale, cercando di verificarne la capacità di avvertire la presenza di picchi di deformazione e cambi del campo di deformazione lungo la loro lunghezza. L’obiettivo finale era essere in grado di riuscire a riconoscere dallo spettro di riflessione proveniente dal sensore la presenza di un distaccamento della pezza, e possibilmente stimarne l’entità con sufficiente accuratezza. Non era possibile fare affidamento su informazioni riguardo al carico sopportato dalla struttura, sebbene l’output del sensore ne fosse estremamente influenzato. Dai grafici e le tabelle di calibrazione sono stati ideati due metodi per raggiungere il traguardo prefissato. I sensori sono stati incollati o inglobati in pezze, e i due metodi sono stati verificati sperimentalmente. A supporto dei test un’analisi agli elementi finiti del problema è stata effettuata, supportata a sua volta da prove sperimentali di caratterizzazione del laminato utilizzato. Tre livelli di test sono stati eseguiti. Inizialmente la fibra è stata parzialmente incollata a un laminato, il quale è stato sottoposto a tensione e sotto tensione il sensore è stato interrogato. Successivamente, il sensore è stato incollato su una vera pezza, a sua volta incollata su un campione di pannello sandwich con una delle pelli in composito danneggiate. Della luce è stata mandata nella fibra con distaccamenti di diverse misure, e i risultati sono stati analizzati. Infine, si è tentato di produrre un distaccamento graduale della pezza attraverso una prova a fatica. La luce è stata inserita nella fibra attraverso entrambi i capi alternatamente, e la riflessione si è rivelata molto differente nei due casi. In particolare, i metodi creati precedentemente si sono rivelati validi solo quando la luce proviene dal capo suggerito dal produttore. In generale sono stati ottenuti buoni risultati, nei test sperimentali gli errori di misurazione replicavano abbastanza fedelmente quelli intrinseci dei due metodi proposti.
SHM of composite patch repairs with the use of CFBG sensors
CASSI, MATTEO
2012/2013
Abstract
NDI& E are essential to evaluate the structural health of an advanced composite structure. In critical components or parts where non destructive tests are difficult to perform or whose behavior is not yet entirely known, such as joints or adhesively attached patches, it is of extreme importance to find health monitoring systems that may reduce to acceptable levels the penalizing safety factors. A structural health monitoring strategy for patch repairs of carbon/epoxy structures was studied and implemented. The functioning of Chirped Fiber Bragg Grating sensors was analyzed with local and gradual heat tests, evaluating their capability of sensing the presence of peaks in strain and sensible strain field variations through its physical length. The final objective was being able to obtain from the only information given by the reflection spectrum from the sensor the presence of a patch debond, and estimating its size with acceptable accuracy. No information about the load should be considered as known, although all the output given by the sensor is directly dependent by the strain field. From the calibration charts and tables prepared two methods to achieve this goal were extracted. The same sensors were placed and embedded into patches, and the two methods were tested experimentally. As a support of the tests, some FEA analysis of the problem was carried out, supported in turn by tensile tests for material characterization. The sensor was tested at three levels. First, it was gradually attached to a laminate, the laminate was pulled and the sensor was interrogated. Secondly, the sensor was placed on a real patch, and the patch was attached to a sandwich specimen with a damaged carbon/epoxy skin. Light was injected with different debond sizes and results were carefully analyzed. Finally, a fatigue test was performed to let the cyclic strain produce the debond. Light was injected from the two sizes of the optical fiber, and a different behavior in the two situations was noticed and characterized. However, the two methods designed were applicable only injecting the light from the size recommended by the producer. Good results were obtained, replicating in the realistic tests approximately the same intrinsic errors that were encountered when the methods were designed.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/87602