Abstract
We present the first experimental observation and quantification of the evolution of the three-dimensional (3D) deflection of fibres leading up to kink-band formation in notched unidirectional glass fibre-epoxy composites. This has been achieved by in situ synchrotron X-ray computed tomography (CT) quantified by advanced image analysis. The 3D trajectories of individual fibres were extracted to monitor the change in fibre profiles with increasing compressive load. Initially the fibre trajectory varies significantly from fibre to fibre, the waviness of which cannot be represented by a simple idealised profile. However, the change in the fibre shapes during axial compression is similar for all fibres, as the fibres deflect towards the final kinking direction. The precursor to kink-band nucleation has been identified as a local tilting of fibres in the region where the kink bands ultimately form, defining a micro-buckle band. Even at 99.9% of the failure load this micro-buckle band is much narrower (~150 μm (~12 fibre diameters) wide), and the angle of inclination of the band much shallower (~8°), compared to the final 300–400 μm (25–33 fibre diameters) wide, and 25–30° inclined, kink band. It is notable that kink-band boundaries are delineated by fibre fractures, many of which exhibit wedge-shaped multiple fractures.
Highlights
The compressive strength of long fibre reinforced polymers (FRPs) can be as low as 60%–70% of their tensile strength [1,2], rendering them susceptible to failure under compressive stress in practical applications
This sharp instability in the devel opment of kink bands is in accord with that observed in UD carbon fibre reinforced polymers (CFRPs) [5,25,45]
There are a few regions where fibres are highly curved but not fractured, mostly located around the periphery; this effect has been captured for CFRPs in some experiments [8] and some numerical models [46], and can be attributed to less lateral constraint around the periphery
Summary
The compressive strength of long fibre reinforced polymers (FRPs) can be as low as 60%–70% of their tensile strength [1,2], rendering them susceptible to failure under compressive stress in practical applications. Experiments have shown that the formation of kink banding is the key mechanism that gives rise to the premature axial compressive failure of unidirectional (UD) FRPs [3,4,5,6,7,8]. The term, kink band, was first intro duced to describe the deformation of axially compressed cadmium single-crystal wires [9] and such bands have been observed in various materials with layered or anisotropic structures [10,11,12], especially FRPs [4]. It has been found that kink-banding is not limited to UD FRPs, occurring in multi-directional FRPs [13,14]. Deeper insight into the kink-band damage mechanisms is essential in terms of their safer and wider application
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