Pseudo-ductility in CFRP laminates through the ply weakening method
Author(s)
Sun, Jingjing
Type
Thesis
Abstract
High-performance continuous Fibred Reinforced Polymer (FRP) composites have high specific stiffness and strength but lack ductility. This work presents a strategy to promote progressive failure and ‘pseudo-ductility’ in CFRP laminates by introducing periodic ply weakening (continuous cuts or perforations normal to the fibre direction) into selected plies.
An analytical model based on Linear Elastic Fracture Mechanics (LEFM) was devised for unidirectional (UD) laminates. The model considers the evolution of interlaminar stress transfer zones (initiated from the ply weakenings) in a rigid-perfectly-plastic matrix system. The predicted pseudo-ductile stressstrain response consists of an initial approximately-linear stage with stiffness close to that of the continuous composite, an intermediate metal-like plateau stage and a final strain-hardening stage. The tensile response is tailorable by adjusting the geometry and distribution of the ply weakenings and the ratio of the continuous and the weakened plies.
The analytical predictions were validated by tensile tests of weakened UD laminates and the results confirmed that the pseudo-ductility was controlled by delamination or debonding initiated from the ply cuts or perforations, respectively.
A preliminary study showed that pseudo-ductility can be achieved in multi-directional laminates, by embedding weakened 0° plies between Quasi-Isotropic (QI) sub-laminates. For the perforation weakening method, it was found that the fibre orientation in the plies adjacent to the weakened plies has a significant influence on the pseudo-ductility.
An analytical model based on Linear Elastic Fracture Mechanics (LEFM) was devised for unidirectional (UD) laminates. The model considers the evolution of interlaminar stress transfer zones (initiated from the ply weakenings) in a rigid-perfectly-plastic matrix system. The predicted pseudo-ductile stressstrain response consists of an initial approximately-linear stage with stiffness close to that of the continuous composite, an intermediate metal-like plateau stage and a final strain-hardening stage. The tensile response is tailorable by adjusting the geometry and distribution of the ply weakenings and the ratio of the continuous and the weakened plies.
The analytical predictions were validated by tensile tests of weakened UD laminates and the results confirmed that the pseudo-ductility was controlled by delamination or debonding initiated from the ply cuts or perforations, respectively.
A preliminary study showed that pseudo-ductility can be achieved in multi-directional laminates, by embedding weakened 0° plies between Quasi-Isotropic (QI) sub-laminates. For the perforation weakening method, it was found that the fibre orientation in the plies adjacent to the weakened plies has a significant influence on the pseudo-ductility.
Version
Open Access
Date Issued
2019-05
Date Awarded
2019-10
Copyright Statement
Creative Commons Attribution NonCommercial Licence
Advisor
Robinson, Paul
Sponsor
Engineering and Physical Sciences Research Council
Grant Number
EP/I02946X/1
Publisher Department
Aeronautics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)