Nowdays, the study of Carbon-Carbon composite materials for high-technology application is acquiring more and more importance. Carbon-Carbon materials are a class of composites of carbon fiber reinforcement in a matrix of graphite. Their main advantage, compared to conventional polymer matrix composites, consists in the ability to provide high performances up to temperatures of 3000°C. In addition, the high dimensional stability, low thermal expansion and excellent tribological properties make this material employed not only in aerospace but also racing field. This thesis is part of a broader partnership between the Department of Science and Technology Aerospace and Brembo Racing, focusing the attention on the experimental and numerical investigation of C-C materials to assess the structural integrity of components used for production of braking systems. Since the brake is a safety component, it is fundamental to predict the failure mechanism of this material itself. Firstly, the characterization of toughness, so far never computed, is carried out by means of Compact-Tension experimental tests. The CT specimens have been cut out directly from a C-C blank disc, thus, due their limited dimensions, the whole test set-up has been designed ad hoc. Quite satisfactory results are obtained applying advanced experimental method only recently adopted for polymeric matrix composite materials. The techniques applied for the detection of crack lengths during the propagation and for data reduction allowed to obtain reliable data of fracture toughness. In this activity, numerical models of CT specimens have been developed to correlate the experimental results and continuum mechanics approaches, based on the cohesive zone model and the Progressive Failure Analysis, have been considered. A sensitivity analysis on the mesh size has been carried out in both approaches. The obtained results stated that whereas the cohesive zone model is slightly sensitive to element size, PFA is strongly mesh-dependent, showing a limit. In addition, the cohesive zone model has been used to investigate more carefully the intra-laminar damage mechanism, showing that the response is influenced by the material strength and toughness. On the other hand, the Progressive Failure Analysis, altough it not based on energy criterion, gives good results in terms of reproduction of CT damage in the experimental tests.
Lo studio dei materiali compositi Carbon-Carbon per applicazioni ad alta tecnologia sta, oggigiorno, acquistando sempre più importanza. I materiali Carbon-Carbon rappresentano una classe di compositi realizzati con rinforzo in fibra di carbonio e matrice grafitica. Il loro principale vantaggio, rispetto ai materiali compositi a matrice polimerica, risiede nell'avere elevate prestazioni fino a temperature di 3000{^\circ}C . In aggiunta, l'elevata stabilità dimensionale, il basso coefficiente di espansione termica e le eccellenti proprietà tribologiche rendono questo materiale impiegato non solo in campo aerospaziale ma anche automobilistico. Questo lavoro di tesi è parte di una collaborazione fra il Dipartimento di Scienze e Tecnologie Aerospaziali e Brembo Racing S.p.A, ponendo l'attenzione sull'investigazione sia sperimentale che numerica dei compositi C-C per valutare l'integrità strutturale dei componenti usati per realizzare sistemi frenanti. Poichè il freno copre un ruolo primario ai fini della sicurezza, è fondamentale predire i meccanismi di rottura del materiale stesso. Prima di tutto, la caratterizzazione della tenacità, mai effettuata fino ad oggi, è stata realizzata mediante prove sperimentali Compact-Tension. I provini CT sono stati ricavati direttamente da un disco grezzo, pertanto, a causa della loro dimensione limitata, l'allestimento della prova è stato progettato ad hoc. Risultati piuttosto soddisfacenti sono stati ottenuti applicando un avanzato metodo sperimentale, solo recentemente adottato per materiali compositi a matrice polimerica. Le tecniche usate per monitare la lunghezza di cricca e le tecniche di riduzione dati hanno permesso di ottenere valori sensati di tenacità. In questa attività sono stati realizzati modelli numerici dei provini CT per correlare i risultati sperimentali e metodi della meccanica del continuo, basati su modelli a zona coesiva e Progressive Failure Analysis, sono stati considerati. Un' analisi di sensitività alla mesh è stata svolta utilizzando entrambi i metodi. I risultati ottenuti hanno mostrato che mentre il modello a zona coesiva non è molto influenzato dalla dimensione della mesh, la PFA risulta mesh-dipendente, mostrando così un suo limite. In aggiunta, il modello a zona coesiva è stato usato per investigare più nel dettaglio il meccanismo di danno intra-laminare, mostrando come varia la risposta al variare della resistenza e tenacità. D'altro canto, la PFA, sebbene non basata su un criterio energetico, riesce a cogliere bene l'andamento del danno nelle prove sperimentali.
Characterization and modeling of fracture response for carbon-carbon composite laminates
FARINELLI, ROBERTA
2016/2017
Abstract
Nowdays, the study of Carbon-Carbon composite materials for high-technology application is acquiring more and more importance. Carbon-Carbon materials are a class of composites of carbon fiber reinforcement in a matrix of graphite. Their main advantage, compared to conventional polymer matrix composites, consists in the ability to provide high performances up to temperatures of 3000°C. In addition, the high dimensional stability, low thermal expansion and excellent tribological properties make this material employed not only in aerospace but also racing field. This thesis is part of a broader partnership between the Department of Science and Technology Aerospace and Brembo Racing, focusing the attention on the experimental and numerical investigation of C-C materials to assess the structural integrity of components used for production of braking systems. Since the brake is a safety component, it is fundamental to predict the failure mechanism of this material itself. Firstly, the characterization of toughness, so far never computed, is carried out by means of Compact-Tension experimental tests. The CT specimens have been cut out directly from a C-C blank disc, thus, due their limited dimensions, the whole test set-up has been designed ad hoc. Quite satisfactory results are obtained applying advanced experimental method only recently adopted for polymeric matrix composite materials. The techniques applied for the detection of crack lengths during the propagation and for data reduction allowed to obtain reliable data of fracture toughness. In this activity, numerical models of CT specimens have been developed to correlate the experimental results and continuum mechanics approaches, based on the cohesive zone model and the Progressive Failure Analysis, have been considered. A sensitivity analysis on the mesh size has been carried out in both approaches. The obtained results stated that whereas the cohesive zone model is slightly sensitive to element size, PFA is strongly mesh-dependent, showing a limit. In addition, the cohesive zone model has been used to investigate more carefully the intra-laminar damage mechanism, showing that the response is influenced by the material strength and toughness. On the other hand, the Progressive Failure Analysis, altough it not based on energy criterion, gives good results in terms of reproduction of CT damage in the experimental tests.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/137322