Micromechanical modelling of hybrid unidirectional composite materials under fibre tensile loading
- Autores: José Manuel Guerrero García
- Directores de la Tesis: Joan Andreu Mayugo Majó (dir. tes.)
- Lectura: En la Universitat de Girona ( España ) en 2020
- Idioma: español
- Tribunal Calificador de la Tesis: Yentl Swolfs (presid.), Daniel Trias Mansilla (secret.), Soraia Pimenta (voc.)
- Programa de doctorado: Programa de Doctorado en Tecnología por la Universidad de Girona
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Fibre Reinforced Polymers (FRP) are widely employed in lightweight applications mainly thanks to their excellent specific strength and stiffness. Nonetheless, FRP suffer from a lack of ductility and toughness which leads to fibre tensile failure with nearly any damage symptom. Consequently, composite materials are often overdesigned, which potentially limits the costs and weight savings. This also makes them unsuitable for applications where ductility is mandatory.
Fibre hybridisation is a promising strategy which can overcome the quasi-brittle behaviour and low toughness of FRP. When a Low Elongation (LE) fibre is combined with a High Elongation (HE) fibre in a single matrix, a hybrid material is obtained. Within a proper hybridisation design, the failure of the LE fibre in the hybrid can be delayed compared to that of the baseline non-hybrid composite, something known as the hybrid effect. Moreover, an increase of ductility can occur, getting rid of the catastrophic brittle failure of composite materials. Currently, there are three main reasons to explain this phenomenon: dynamic effects, thermal residual stresses and changes in the failure development.
Despite the great benefits of hybrid composites, their fibre tensile failure process is not yet entirely understood. Moreover, derived from this insufficient understanding, there is a lack of efficient tools to predict such failure. Currently, different models exist in the literature that attempt to simulate the failure and damage development of hybrid composite materials. Nonetheless, most models omit dynamic effects and assume simplistic elastic matrix behaviours. In addition, models usually consider the fibres to be placed within a square or hexagonal packing, instead of being randomly distributed. Such a simplification does not allow to properly simulate hybrid composites with fibres of different radius and does not permit to accurately capture the stress redistribution around broken fibres. Last but not least, some models cannot compute the entire stress-strain curve of the material, which does not allow to explicitly predict ductile effects. Thus, the development of new models, able to characterise the tensile failure process of hybrid composites more accurately, is necessary.
Departing from the literature need of developing more accurate models, as well as the requirement of designing and further understanding composite materials exhibiting a ductile behaviour, the main purpose of this thesis is to develop an efficient tool for predicting the tensile failure process in hybrid unidirectional FRP under fibre tensile loading. The model proposed must consider the main mechanisms involved in the tensile failure of hybrid composites, and should also be capable of studying non-hybrid materials. Derived from this, the second main purpose of this work is to establish the influence that different parameters have on the tensile response and failure development in hybrid composites.
On the first contribution originated from this work, a micromechanical progressive failure model which can simulate the fibre tensile failure and damage development in hybrid unidirectional FRP is formulated. The model is validated by comparing with literature results exhibiting a good agreement.
On the second article derived from this thesis, the model is further improved by including a more realistic stress redistribution around broken fibres. The influence the matrix behaviour (plastic or elastic) has on the modelling results is investigated. Results suggest that an elastic matrix cannot realistically capture either the formation of clusters or the damage development in hybrid composites.
On the third article of this work, dynamic effects and thermal residual stresses are added into the model. The influence these features have on the failure of hybrid composites is studied. Results show that, while the dynamic effects have a large influence on the formation of clusters, the thermal residual stresses do not. Their effect in final failure is very reduced for both. Moreover, hybrid composites exhibiting large ductile response are shown.
On the fourth contribution derived from this work, the influence that the specimen size has on the failure and damage development in hybrid composites is investigated. Results prove that scaling up the composite length causes an earlier failure, marginally decreases the ductility of the material, but considerably increases the hybrid effect. Contrary to this, scaling up the composite cross-section decreases the hybrid effect, may delay final failure and may increase ductility. Moreover, the maximum cluster size always increases by increasing the composite volume and the presence of ductility does not alter the size effects.
On the final contribution of this thesis, the influence the hybrid configuration has on the tensile response and damage development in hybrid composites is explored. Furthermore, the model is validated by comparing it against experimental results of hybrid composites. Results show that the intrayarn hybridisation leads to higher hybrid effects, yield stress and strength than the interlayer and intralayer configurations. However, the interlayer and intralayer lead to greater failure and ductile strains than the intrayarn. The dispersion of the fibres is seen to be a key parameter. Finally, the micromechanical model shows qualitatively a good agreement compared with experimental data.
