| Abstract | Prisutnost različitih konstituenata koji posjeduju specifična materijalna svojstva u vlaknima ojačanim polimerima rezultira složenim fenomenima oštećenjima. Kod tih materijala je uslijed opterećivanja prisutan velik broj pukotina te različiti mehanizmi oštećenja koji se iniciraju te propagiraju na različitim razinama (mikro-, mezo- i makrorazina). Stoga detekcija oštećenja u ovim materijalima zahtijeva primjenu nerazornih metoda ispitivanja. U novije vrijeme, metoda korelacije digitalnih volumena u sprezi sa računalnom tomografijom se sve češće koristi za detekciju i praćenje rasta oštećenja u kompozitnim materijalima. Na ovaj način moguće je bilježiti i analizirati oštećenja na površini, ali i u unutrašnjosti ispitnih materijala.
U ovom radu analizirano je in-situ cikličko vlačno ispitivanje epoksidne smole ojačane mat staklenim vlaknima. Ispitni uzorak je skeniran u neopterećenom stanju, ali i pri različitim ciklusima opterećivanja. Osim toga, nakon loma ispitnog uzorka zabilježena su dva volumena s različitom parametrima skeniranja. Na skenovima zabilježenim kroz eksperimentalni protokol provedena je analiza oštećenja. Primjenom globalne metode korelacije digitalnih volumena na cijelom volumenu izmjerena su polja pomaka. Iz provedenog mjerenja, izračunata su polja najvećih glavnih deformacija te mape korelacijskih reziduala. Na temelju predložene analize identificirani su mehanizmi oštećenja te je određeno područje uzorka s maksimalnim lokalizacijskim zonama deformacija. Unutar tog područja je odabran manji pod-volumen koji služi za implementaciju mehaničke regularizacije te detaljnu analizu oštećenja. Nadalje, provedena je segmentacija odabranog pod-volumena s obzirom na pripadajuću mikrostrukturu, tj. iz segmentirane slike je izrađena mreža konačnih elemenata koja opisuje heterogenu mikrostrukturu. Izrađena mreža je korištena za provedbu analize oštećenja u promatranom pod-volumenu. Analiza je provedena globalnom metodom korelacije digitalnih volumena sa homogenom i heterogenom mehaničkom regularizacijom. Homogena regularizacija je provedena s jednakim vrijednostima materijalnih parametara različitih konstituenata. Kod analize sa homogenom regularizacijom promatran je utjecaj različitih regularizacijskih duljina te je provedena analiza iniciranja i rasta pukotina. Drugim riječima, iniciranje i rast pukotine određeni su promatranjem mapa korelacijskih reziduala i polja najvećih glavnih deformacija, ali i usporednom početnog (tj. nedeformiranog) skena te skena deformiranog uzorka. Nadalje, heterogena regularizacija implementirana je uvođenjem različitih materijalnih parametara za konstituente promatranog kompozitnog materijala. Osim toga, promatran je utjecaj različitih regularizacijskih duljina te različitih materijalnih parametara na rezultate analize korelacije digitalnih volumena. Nadalje, pri heterogenoj regularizaciji je provedena i analiza pukotina u promatranom pod-volumenu te su prikazana polja deformacija i korelacijskih reziduala. Na kraju je provedena analiza sa heterogenom regularizacijom na skenovima kroz sve cikluse opterećivanja uzorka. Kod ove analize su promatrane promjene korelacijskih reziduala i polja deformacija kroz cijelu povijest opterećivanja ispitnog uzorka. |
| Abstract (english) | The presence of different constituents that possess specific material properties in fiber-reinforced polymers result with complex damage phenomena. When these materials are subjected to mechanical loading, a large number of cracks may occur accompanied with different damage mechanisms that initiate and propagate at different scales (micro-, meso- and macroscale). Therefore, the detection of damage in fiber reinforced polymers demands the use of advanced non-destructive testing methods. In recent years, Digital Volume Correlation combined with computed tomography is increasingly being used to detect and monitor damage initiation and growth in composite materials. In this way, it is possible to detect and analyze damage on the surface and in the bulk of the monitored object.
In this work, the test specimen subjected to in-situ cyclic tensile loading was analysed. The material studied herein is a glass fiber mat reinforced epoxy composite. The test specimen was scanned in the unloaded state, as well as during different loading regimes. Finally, two scans of the specimen after failure were acquired. Damage analysis was performed on scans acquired during the experimental protocol. The analysis was performed using global Digital Volume Correlation method over the entire specimen. The maximum principal strain fields and the correlation residual map were obtained from the correlation analysis. Based on the proposed correlation procedure, damage mechanisms were identified, and the critical region of the specimen was determined. Within the critical region, a small sub-volume was chosen for implementation of heterogenous mechanical regularization and detailed damage analysis. Furthermore, segmentation of the selected sub-volume with respect to the corresponding microstructure was performed. From the segmented image, microscale finite element mesh accounting for morphology of the investigated material was generated. Microscale mesh was used to perform damage analysis in the observed sub-volume. The measurement protocol was performed employing the global Digital Volume Correlation approach with homogeneous and heterogeneous mechanical regularization. Homogeneous regularization was performed by using unique material properties for all constituents. In the correlation analysis with homogeneous regularization, the influence of different regularization lengths was studied. Furthermore, identification of crack initiation and propagation was performed. Such analysis was performed by observing the maps of correlation residuals and the main principal strain fields. Additionally, the microstructure of the initial and deformed scans was compared to identify the nature of the damage mechanism. Heterogeneous regularization was implemented by introducing different material properties for the constituents of the proposed composite material. In addition, the influence of different regularization lengths and different material parameters on the results of correlation analysis was investigated. Furthermore, in the case of heterogeneous regularization, the damage mechanisms identification in the observed sub-volume was performed and the strain fields and correlation residuals were analyzed. Finally, the scans acquired over the entire loading history was analyzed with heterogeneous regularization. In the latter analysis, changes in correlation residuals and strain fields were investigated throughout the sample loading history to monitor damage initiation and growth. |
