There has been an increasing demand for composite materials in the automotive, defense, and aerospace industries over the past decades due to their distinguished mechanical properties, particularly their optimal weight-to-strength and weight-to-stiffness ratio. The main criticality of composite materials is their vulnerability to out-of-plane loads such as low-velocity impacts which in many cases limits their application and can lead to catastrophic failure. In fact, during the service life, a component made of composites is subjected to various loading conditions (including impacts) which might create damage that remain hidden if specific non-destructive techniques are not applied. This thesis aimed to assess the low-velocity impact of composites through experimental and modeling approaches focusing on characterizing the impact response and failure mechanisms. Also, providing design guidelines and optimizations to improve the low-velocity impact performance. This was achieved by high-fidelity numerical simulations of composite subjected to impacts to improve the accuracy of the models and computational efficiency. Then using the numerical models, together with experimental approaches, to study more practical engineering cases regarding the low-velocity impact performance such as hybridization and repeated impacts. Accordingly, the contributions of this thesis can be categorized into three different tasks; (i) developing Finite Element (FE) models to simulate low-velocity impact considering material non-linear behavior and initiation and propagation of multi-mode failure, (ii) investigating the effect of hybridization of different reinforcements (aramid and S2-glass fibers) to improve low-velocity impact response of composite laminates, and (iii) evaluation of repeated low-velocity impact response of composites. The modeling part [1–4] was aimed at bringing new methodological approaches and simulation algorithms to increase the accuracy and decrease the computational effort needed for the simulation of failure in composite materials with a focus on the micromechanical and macro-homogenous approaches. In each case, the developed models were verified by doing benchmark simulations and comparisons with experimental tests. The research that was performed in this area started from the implementation of different failure criteria such as Puck and Cuntze to improve the prediction of damage caused by low-velocity impacts, and the development of a new search algorithm for finding the fracture angle of Puck failure criteria aiming to increase the computational efficiency of the models. And work in this area continued by developing a hybrid micro-macro mechanical approach for scale bridging and considering the micromechanical uncertainties at macros scale simulation and during explicit simulations, and development of a novel approach to estimate the characteristic element length for FE simulations of composite materials. Subsequently, the hybridization of different fibers (aramid and S2-glass) for the production of superior composite laminates has been a focus of the studies performed during this work [5–7]. Due to their distinctive properties and cost-effectiveness, hybrid composites have found potential use in many engineering fields, which urged the need to extensively evaluate their performance. Experimental and numerical studies on the low-velocity impact behavior of inter-ply S2-glass/aramid woven fabric hybrid laminates were performed to fully characterize the mechanical behavior of these materials. A comprehensive study was performed to assess different interacting parameters (such as stacking sequence and hybridization ratio) in the low-velocity impact response of hybrid laminates over a broad range of impact energies to identify the most desired hybrid laminate in terms of impact resistance. Response surface methodology and multi-criteria decision-making methods were used to evaluate the hybridization effect and propose design improvements. Also, the developed numerical models have been used to evaluate the impact performance in the case of hybrid composites to study the effect of strain rate sensitivity and material on the impacting side. The results showed that significant improvement can be achieved by hybridization which provided a better balance between energy absorption and deformation compared to single fiber laminates. Moreover, during the service life, composite laminates are subjected to various impact loads which, in most realistic cases, do not consist of a single impact case but a multiple-hit situation. It has been shown that a single impact may not lead to any serious damage, while the damage accumulation caused by the repetition of impacts may significantly reduce the load-bearing capability reduction and increase the probability of unexpected failure. Therefore, this issue was addressed by numerically and experimentally investigating the response of composite laminates subjected to low-velocity repeated impacts [8–11]. To cover a broad range of composite materials, laminates made of unidirectional carbon fiber-reinforced polymer and woven aramid/S2-glass epoxy composites were numerically and experimentally studied at different energies. The immense conclusions from this task included the influence of impact energy, impact location, pre-existing impact damage, and hybridization on the repeated low-velocity impact response. In conclusion, the main objective of this was to investigate the mechanical behavior of composite materials subjected to low-velocity impact loads to provide better insight into the durability, mechanical performance, and failure of these structures. In addition, the comprehensive experimental and modeling work that was carried out paves the way for more practical application of this work for the improvement of the design of composite subjected to extreme loads.
Negli ultimi decenni c'è stata una crescente domanda di materiali compositi nei settori automobilistico, della difesa e aerospaziale grazie alle loro proprietà meccaniche di rilievo; in particolare il loro favorevole rapporto peso-resistenza e peso-rigidità. La principale criticità dei materiali compositi è la loro vulnerabilità ai carichi fuori dal piano come gli impatti a bassa velocità che in molti casi ne limitano l'applicazione e possono portare a guasti catastrofici. Infatti, durante la vita utile, un componente in composito è sottoposto a diverse condizioni di carico (compresi gli urti) che possono creare danni che rimangono nascosti se non vengono applicate specifiche tecniche non distruttive.Questa tesi mira a valutare l'impatto a bassa velocità dei compositi attraverso approcci sperimentali e di modellazione incentrati sulla caratterizzazione della risposta all'impatto e dei meccanismi di rottura. Inoltre, fornisce linee guida di progettazione e ottimizzazioni per migliorare le prestazioni di impatto a bassa velocità. Ciò è stato ottenuto mediante simulazioni numeriche ad alta fedeltà di compositi sottoposti a impatti; la definizione di tali simulazioni è stat voltaa migliorare l'accuratezza dei modelli e l'efficienza computazionale. I modelli numerici sono quindi stati investigati insieme ad approcci sperimentali, per studiare casi ingegneristici riguardanti le prestazioni di impatto a bassa velocità; in particolare si sono affrontati temi quali quelli legati all'ibridazione e agli gli impatti ripetuti. Di conseguenza, i contributi di questa tesi possono essere classificati in tre diversi compiti; (i) sviluppo di modelli agli elementi finiti (FE) per simulare l'impatto a bassa velocità considerando il comportamento non lineare del materiale e l'inizio e la propagazione di un danno di vario tipo, (ii) studio dell'effetto dell'ibridazione di diversi rinforzi (fibre aramidiche e dibre S2-vetro ) per migliorare la risposta all'impatto a bassa velocità dei laminati compositi e (iii) la valutazione della risposta ripetuta all'impatto a bassa velocità dei compositi. La parte di modellazione [1–4] mirava a introdurre nuovi approcci metodologici e algoritmi di simulazione per aumentare l'accuratezza e ridurre lo sforzo computazionale necessario per la simulazione del cedimento nei materiali compositi con particolare attenzione agli approcci micromeccanici e macro-omogenei. I modelli sviluppati sono stati verificati effettuando simulazioni di benchmark e confronti con test sperimentali. La ricerca che è stata condotta in quest'area è partita dall'implementazione di diversi criteri di rottura come Puck e Cuntze per migliorare la previsione del danno causato da impatti a bassa velocità; è stato inoltre sviluppato un nuovo algoritmo di ricerca per trovare l'angolo di frattura del criterio di rottura di Puck al fine di aumentare l'efficienza computazionale dei modelli. Il lavoro in quest'area è proseguito sviluppando un approccio meccanico micro-macro ibrido multi scala. Tale lavoro ha permesso di considerare le incertezze micromeccaniche anche su simulazioni su scala macro; in questa attività è stata inoltre sviluppato un nuovo approccio per stimare la lunghezza dell'elemento caratteristico per le simulazioni FE di materiali compositi .Successivamente, l'ibridazione di diverse fibre (aramidiche e S2-vetro) per la produzione di laminati compositi superiori è stata al centro degli studi condotti durante questa parte [5–7]. Grazie alle loro proprietà distintive e all'economicità, i compositi ibridi hanno trovato un potenziale utilizzo in molti campi dell'ingegneria, il che ha reso necessario un approfondimento in merito alle loro prestazioni. Per caratterizzare completamente il comportamento meccanico di questi materiali sono stati eseguiti studi sperimentali e numerici sul comportamento all'impatto a bassa velocità di tali laminati ibridi. È stato condotto uno studio completo per valutare i diversi parametri che interagiscono (come la sequenza di laminazione e il rapporto di ibridazione) nella risposta all'impatto a bassa velocità su un'ampia gamma di energie di impatto per identificare il laminato ibrido migliore in termini di resistenza all'impatto. Metodi decisionali multicriterio sono stati utilizzati per valutare l’effetto dell’ibridazione e proporre miglioramenti di progettazione. Inoltre, i modelli numerici sviluppati sono stati utilizzati per valutare le prestazioni all’impatto nel caso di compositi ibridi per studiare l’effetto della sensibilità alla velocità di deformazione e del tipo materiale sul lato dell'impatto. I risultati hanno mostrato che si può ottenere un miglioramento significativo mediante l'ibridazion; questa ha fornito un migliore equilibrio tra assorbimento di energia e deformazione rispetto ai laminati a fibra singola. Inoltre, durante la vita utile, i laminati compositi sono soggetti a vari carichi d'urto che, nella maggior parte dei casi, non consistono in un singolo caso d'urto ma in una situazione di colpi multipli. È stato dimostrato che un singolo urto può non causare danni gravi, mentre l'accumulo di danni causato dalla ripetizione degli impatti può ridurre significativamente la capacità di carico residua e aumentare la probabilità di guasti imprevisti. Pertanto, questo problema è stato affrontato studiando numericamente e sperimentalmente la risposta dei laminati compositi sottoposti a impatti ripetuti a bassa velocità [8-11]. Per coprire un'ampia gamma di materiali compositi, sono stati studiati numericamente e sperimentalmente a diverse energie sia laminati realizzati in polimero unidirezionale rinforzato con fibra di carbonio che compositi epossidici intrecciati in aramide/vetro S2. Le conclusioni legate a questa attività includono l'influenza dell'energia di impatto, la posizione dell'impatto, i danni da impatto preesistenti e l'ibridazione sulla risposta ripetuta all'impatto a bassa velocità. In conclusione, l'obiettivo principale di questo lavoro è quello di studiare il comportamento meccanico dei materiali compositi sottoposti a carichi di impatto a bassa velocità per fornire una migliore comprensione della durabilità, delle prestazioni meccaniche e delle modalità di cedimento di queste strutture. Inoltre, l'ampio lavoro sperimentale e di modellazione svolto apre la strada ad applicazioni di questo lavoro per il miglioramento della progettazione di compositi sottoposti a carichi estremi.
Mechanical behaviour of composite materials under low-velocity impacts
Rezasefat Balasbaneh, Mohammad
2022/2023
Abstract
There has been an increasing demand for composite materials in the automotive, defense, and aerospace industries over the past decades due to their distinguished mechanical properties, particularly their optimal weight-to-strength and weight-to-stiffness ratio. The main criticality of composite materials is their vulnerability to out-of-plane loads such as low-velocity impacts which in many cases limits their application and can lead to catastrophic failure. In fact, during the service life, a component made of composites is subjected to various loading conditions (including impacts) which might create damage that remain hidden if specific non-destructive techniques are not applied. This thesis aimed to assess the low-velocity impact of composites through experimental and modeling approaches focusing on characterizing the impact response and failure mechanisms. Also, providing design guidelines and optimizations to improve the low-velocity impact performance. This was achieved by high-fidelity numerical simulations of composite subjected to impacts to improve the accuracy of the models and computational efficiency. Then using the numerical models, together with experimental approaches, to study more practical engineering cases regarding the low-velocity impact performance such as hybridization and repeated impacts. Accordingly, the contributions of this thesis can be categorized into three different tasks; (i) developing Finite Element (FE) models to simulate low-velocity impact considering material non-linear behavior and initiation and propagation of multi-mode failure, (ii) investigating the effect of hybridization of different reinforcements (aramid and S2-glass fibers) to improve low-velocity impact response of composite laminates, and (iii) evaluation of repeated low-velocity impact response of composites. The modeling part [1–4] was aimed at bringing new methodological approaches and simulation algorithms to increase the accuracy and decrease the computational effort needed for the simulation of failure in composite materials with a focus on the micromechanical and macro-homogenous approaches. In each case, the developed models were verified by doing benchmark simulations and comparisons with experimental tests. The research that was performed in this area started from the implementation of different failure criteria such as Puck and Cuntze to improve the prediction of damage caused by low-velocity impacts, and the development of a new search algorithm for finding the fracture angle of Puck failure criteria aiming to increase the computational efficiency of the models. And work in this area continued by developing a hybrid micro-macro mechanical approach for scale bridging and considering the micromechanical uncertainties at macros scale simulation and during explicit simulations, and development of a novel approach to estimate the characteristic element length for FE simulations of composite materials. Subsequently, the hybridization of different fibers (aramid and S2-glass) for the production of superior composite laminates has been a focus of the studies performed during this work [5–7]. Due to their distinctive properties and cost-effectiveness, hybrid composites have found potential use in many engineering fields, which urged the need to extensively evaluate their performance. Experimental and numerical studies on the low-velocity impact behavior of inter-ply S2-glass/aramid woven fabric hybrid laminates were performed to fully characterize the mechanical behavior of these materials. A comprehensive study was performed to assess different interacting parameters (such as stacking sequence and hybridization ratio) in the low-velocity impact response of hybrid laminates over a broad range of impact energies to identify the most desired hybrid laminate in terms of impact resistance. Response surface methodology and multi-criteria decision-making methods were used to evaluate the hybridization effect and propose design improvements. Also, the developed numerical models have been used to evaluate the impact performance in the case of hybrid composites to study the effect of strain rate sensitivity and material on the impacting side. The results showed that significant improvement can be achieved by hybridization which provided a better balance between energy absorption and deformation compared to single fiber laminates. Moreover, during the service life, composite laminates are subjected to various impact loads which, in most realistic cases, do not consist of a single impact case but a multiple-hit situation. It has been shown that a single impact may not lead to any serious damage, while the damage accumulation caused by the repetition of impacts may significantly reduce the load-bearing capability reduction and increase the probability of unexpected failure. Therefore, this issue was addressed by numerically and experimentally investigating the response of composite laminates subjected to low-velocity repeated impacts [8–11]. To cover a broad range of composite materials, laminates made of unidirectional carbon fiber-reinforced polymer and woven aramid/S2-glass epoxy composites were numerically and experimentally studied at different energies. The immense conclusions from this task included the influence of impact energy, impact location, pre-existing impact damage, and hybridization on the repeated low-velocity impact response. In conclusion, the main objective of this was to investigate the mechanical behavior of composite materials subjected to low-velocity impact loads to provide better insight into the durability, mechanical performance, and failure of these structures. In addition, the comprehensive experimental and modeling work that was carried out paves the way for more practical application of this work for the improvement of the design of composite subjected to extreme loads.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/198395