This thesis work investigates the Low-Velocity Impact (LVI) and Compression After Impact (CAI) response of epoxy composites reinforced with carbon fibers. Thirty specimens, manufactured using the card-sliding technique with non-crimp fabrics and adopting a Single-Double (SD) stacking sequence, were tested to investigate their susceptibility to impact events. In particular, two groups of specimens feature a constant thickness, while the other groups are characterized by a tapered geometry. The laminates were produced at University of Southern California and subsequently tested at Politecnico di Milano. Initially, low-velocity impact tests were performed using a drop-weight impact tower at various energy levels: 16 J, 22 J, 33 J, and 42 J. To investigate the impact resistance, force-displacement curves along with absorbed energy charts are reported and analyzed. The side opposite to the impact was monitored by an infrared camera to acquire a sequence of images able to describe the evolution of the thermal transient related to the impact damage progression. Infrared thermography was also used for non-destructive evaluation of all the specimens before and after the impact. The obtained results are useful in understanding the impact damaging mechanisms and performing a first estimation of the damaged area. Furthermore, the damage mechanisms were analyzed through front, rear, and cross-section views of the specimens. Subsequently, Compression After Impact tests were conducted on the impacted specimens to investigate their residual compressive strength. Force-displacement curves are obtained from the tests and reported. Thermographic monitoring during these test was again performed on the same side considered in the previous assessment, offering a deeper understanding of the damage mechanisms. Moreover, Digital Image Correlation (DIC) was employed to measure the surface strain evolution during the analysis, allowing to monitor the damage and fracture phenomena involved. Finally, a Finite Element model was developed to investigate the mechanical behavior of Single-Double composites under the same experimental conditions. Force-displacement curves and damage state variables are obtained and compared to the experimental results, revealing the need for a more sophisticated material model to accurately represent the damage progression during an impact event. The results evidence a promising impact resistance of Single-Double materials and, in particular, the potential of tapered geometries to enhance impact resistance. However, it is important to underline that the quality of the manufacturing highly influences the performance of tapered specimens.
Questo lavoro di tesi analizza la risposta all'impatto a bassa velocità e la successiva compressione di compositi a matrice epossidica rinforzati con fibre di carbonio. Nello specifico sono stati testati trenta provini prodotti con la tecnica del card-sliding a partire da tessuti non-crimp e caratterizzati da una sequenza di laminazione denominata Single-Double. I provini presentano diverse geometrie, e in particolare due gruppi sono caratterizzati da un sezione a spessore costante mentre i rimanenti presentano degli assottigliamenti alle estremità. I laminati sono stati prodotti presso l'University of Southern California e successivamente testati al Politecnico di Milano. Inizialmente, gli impatti a bassa velocità sono stati eseguiti utilizzando una torre di caduta a diversi livelli di energia: 16 J, 22 J, 33 J e 42 J. Dai test sono poi state ricavate la curve forza-spostamento e i grafici rappresentanti l'energia assorbita durante l'impatto. Il lato del provini opposto all'impatto è stato monitorato con una termocamera, in modo da ottenere una sequenza di immagini in grado di descrivere la risposta termica dovuta all'evoluzione del danno. La termografia è stata eseguita anche prima e dopo l'impatto per effettuare un'indagine preliminare non-distruttiva sui provini. I risultati ottenuti in questa fase sono utili per analizzare i meccanismi di progressione del danno e anche per effettuare una prima stima qualitativa dell'area danneggiata all'interno del provino. Inoltre, i meccanismi di danno sono stati analizzati anche tramite fotografie della vista frontale, posteriore e trasversale dei campioni. Successivamente, le prove di compressione dopo l'impatto sono state effettuate sui provini danneggiati per stimare la resistenza a compressione residua. Anche per questa seconda fase sono riportate le curve forza-spostamento e le immagini termiche ottenute tramite l'analisi termografica. Inoltre è stata utilizzata la Digital Image Correlation per visualizzare la distribuzione della deformazione principale sulla superficie frontale del provino durante il test, evidenziando i fenomeni di danno e frattura coinvolti. Infine, è stato sviluppato un modello agli elementi finiti per investigare numericamente il comportamento dei compositi Single-Double. Per valutare l'accuratezza dei risultati, le curve forza-spostamento e le variabili di danno sono state confrontate con quelle ottenute sperimentalmente, evidenziando una certa discrepanza. Infatti, un modello del materiale più sofisticato è necessario per descrivere accuratamente tutti i meccanismi di danno coinvolti e soprattutto la loro interazione. I risultati evidenziano una promettente resistenza all'impatto per i compositi Single-Double, e in particolare il potenziale delle geometrie a spessore variabile. Tuttavia, è importante sottolineare che la qualità della produzione influisce notevolmente sulle prestazioni finali dei campioni.
Low-velocity impact and compression after impact investigation of non-crimp fabric single-double CFRP
Malverti, Cecilia
2022/2023
Abstract
This thesis work investigates the Low-Velocity Impact (LVI) and Compression After Impact (CAI) response of epoxy composites reinforced with carbon fibers. Thirty specimens, manufactured using the card-sliding technique with non-crimp fabrics and adopting a Single-Double (SD) stacking sequence, were tested to investigate their susceptibility to impact events. In particular, two groups of specimens feature a constant thickness, while the other groups are characterized by a tapered geometry. The laminates were produced at University of Southern California and subsequently tested at Politecnico di Milano. Initially, low-velocity impact tests were performed using a drop-weight impact tower at various energy levels: 16 J, 22 J, 33 J, and 42 J. To investigate the impact resistance, force-displacement curves along with absorbed energy charts are reported and analyzed. The side opposite to the impact was monitored by an infrared camera to acquire a sequence of images able to describe the evolution of the thermal transient related to the impact damage progression. Infrared thermography was also used for non-destructive evaluation of all the specimens before and after the impact. The obtained results are useful in understanding the impact damaging mechanisms and performing a first estimation of the damaged area. Furthermore, the damage mechanisms were analyzed through front, rear, and cross-section views of the specimens. Subsequently, Compression After Impact tests were conducted on the impacted specimens to investigate their residual compressive strength. Force-displacement curves are obtained from the tests and reported. Thermographic monitoring during these test was again performed on the same side considered in the previous assessment, offering a deeper understanding of the damage mechanisms. Moreover, Digital Image Correlation (DIC) was employed to measure the surface strain evolution during the analysis, allowing to monitor the damage and fracture phenomena involved. Finally, a Finite Element model was developed to investigate the mechanical behavior of Single-Double composites under the same experimental conditions. Force-displacement curves and damage state variables are obtained and compared to the experimental results, revealing the need for a more sophisticated material model to accurately represent the damage progression during an impact event. The results evidence a promising impact resistance of Single-Double materials and, in particular, the potential of tapered geometries to enhance impact resistance. However, it is important to underline that the quality of the manufacturing highly influences the performance of tapered specimens.File | Dimensione | Formato | |
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2024_04_Malverti_Tesi_01.pdf
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2024_04_Malverti_ExecutiveSummary_02.pdf
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https://hdl.handle.net/10589/219706