Interlaminar fracture testing of multidirectional laminates: on finite width effect and residual stresses
Topic(s) : Special Sessions
Co-authors :
Torquato GARULLI (SPAIN), Albertino ARTEIRO (PORTUGAL), Norbert BLANCO VILLAVERDE (SPAIN), Jordi RENART (SPAIN)Abstract :
Due to their exceptional stiffness-to-weight and strength-to-weight ratios, Fibre Reinforced Polymers (FRPs) are of major interest for lightweight structures design. For their successful deployment, structural safety must be guaranteed; therefore, a deep understanding of their damage mechanisms is of paramount importance.
Delamination, or interlaminar fracture, is among the most dangerous damage mechanisms occurring in laminated FRPs. Consequently, characterization of interlaminar fracture toughness (IFT), is a major concern and is addressed following international standards. These, however, recommend the use of, and are primarily designed for, unidirectional (UD) specimens, where delamination is propagated along the fibre direction. In real structural applications, multidirectional (MD) laminates are used, and delamination may initiate in interfaces between differently oriented layers and grow in any direction. Under these circumstances, the IFT may be different from that measured in standard tests on UD specimens [1].
Despite this, no agreement has been reached on how to characterize IFT in MD interfaces (i.e. between differently oriented layers). This stems from several problems encountered when using MD specimens, namely: three-dimensional (3D) effects, thermal residual stresses, additional dissipation mechanisms (such as plasticity and undesired damage) in off-axis plies, and delamination migration.
For decades, researchers have been trying to design MD specimens able to avoid/minimize these problems or, in alternative, to understand what their effect on IFT would be. A major recent development was the introduction of Fully-Uncoupled Multi-Directional (FUMD) delamination specimens [2], a special class of MD specimens with unique thermo-elastic uncoupling properties. Preliminary studies have shown the benefits of these specimens [2,3], making them viable candidates for standardisation purposes. Besides, they offer an unprecedented design flexibility, enabling a systematic investigation of the previously mentioned issues.
In this study, we focus on 3D effects and the effect of thermal residual stresses. We firstly highlight the fact, rarely acknowledged in the literature, that these two issues, being both intrinsically related to the layup chosen, are interrelated, and that they cannot, in general, be considered one in isolation from the other when designing MD specimens for IFT testing. We then design a tailored set of FUMD delamination specimens and use them to investigate these effects, in isolation from a potential effect of the orientation of the layers embedding the delamination plane. Specifically, we perform an extensive finite element study on the FUMD configurations designed, using both the Virtual Crack Closure Technique and Cohesive Zone Modelling. Furthermore, the FUMD specimens designed are deemed viable for experimental validation of the results obtained in this study.
Delamination, or interlaminar fracture, is among the most dangerous damage mechanisms occurring in laminated FRPs. Consequently, characterization of interlaminar fracture toughness (IFT), is a major concern and is addressed following international standards. These, however, recommend the use of, and are primarily designed for, unidirectional (UD) specimens, where delamination is propagated along the fibre direction. In real structural applications, multidirectional (MD) laminates are used, and delamination may initiate in interfaces between differently oriented layers and grow in any direction. Under these circumstances, the IFT may be different from that measured in standard tests on UD specimens [1].
Despite this, no agreement has been reached on how to characterize IFT in MD interfaces (i.e. between differently oriented layers). This stems from several problems encountered when using MD specimens, namely: three-dimensional (3D) effects, thermal residual stresses, additional dissipation mechanisms (such as plasticity and undesired damage) in off-axis plies, and delamination migration.
For decades, researchers have been trying to design MD specimens able to avoid/minimize these problems or, in alternative, to understand what their effect on IFT would be. A major recent development was the introduction of Fully-Uncoupled Multi-Directional (FUMD) delamination specimens [2], a special class of MD specimens with unique thermo-elastic uncoupling properties. Preliminary studies have shown the benefits of these specimens [2,3], making them viable candidates for standardisation purposes. Besides, they offer an unprecedented design flexibility, enabling a systematic investigation of the previously mentioned issues.
In this study, we focus on 3D effects and the effect of thermal residual stresses. We firstly highlight the fact, rarely acknowledged in the literature, that these two issues, being both intrinsically related to the layup chosen, are interrelated, and that they cannot, in general, be considered one in isolation from the other when designing MD specimens for IFT testing. We then design a tailored set of FUMD delamination specimens and use them to investigate these effects, in isolation from a potential effect of the orientation of the layers embedding the delamination plane. Specifically, we perform an extensive finite element study on the FUMD configurations designed, using both the Virtual Crack Closure Technique and Cohesive Zone Modelling. Furthermore, the FUMD specimens designed are deemed viable for experimental validation of the results obtained in this study.