Fibre-reinforced plastic

Composite number

Epoxy

Buckling

Materials science

Shear (geology)

Computed tomography

Shear stress

In situ

Radiology

Torsion (gastropod)

Tomography

Volume fraction

Physics

Anatomy

Optics

Composite material

Medicine

Braid

Meteorology

X-ray

Composites Science and Technology

Here we present the first real-time three dimensional (3D) observations of damage evolution in a carbon fibre reinforced polymer (CFRP) composite tube under torsion. An in-situ torsion test of 1/1 45° (diamond) braided carbon fibre-epoxy circular composite tube was performed and the damage process was characterised by synchrotron X-ray computed tomography (CT). A number of damage modes and their damage sequence has been identified and monitored globally and in more detail within a representative region of interest. In particular, intra-tow cracks and inter-tow debonding have been found to occur almost simultaneously at low shear strains (1.5%). It is noteworthy that inter-tow debonding was initially constrained within repeated braid units before propagating and connecting with other damage modes in 3D. The area fraction of inter-tow debonds was quantified at different stages and it was found to dramatically increase with increasing shear strain beyond 1.5%. The total volume fraction of the observed intra-tow cracks of various forms was seen to grow rapidly beyond shear strain of 2.0%. Beyond the peak shear stress (at a shear strain of 2.5%), fibre micro-buckling and kink bands occur in the tows subjected to torsion induced axial compression at crimped regions close to tow crossovers. Tow crossovers control many aspects of damage propagation under torsion, positively by localising inter-tow debonds and negatively by initiating fibre micro-buckling.

Damage evolution in braided composite tubes under torsion studied by in-situ X-ray computed tomography