Torsion is an important loading mode of tubular shaped composites in engineering applications (e.g. drive shafts). In this respect, a good understanding of the underlying damage mechanisms is crucial for failure prediction and structure optimisation. Until now, limited work has been reported that helps elucidate the real-time damage accumulation mechanisms in three dimensions during the torsional failure process. In this project, the first in-situ time-lapse X-ray computed tomography (CT) study of single-layer braided carbon fibre reinforced polymer (CFRP) tubes (fibre volume fraction ~40-45%) loaded progressively under torsion has been reported, aiming to establish the relations between damage mechanisms and braid architecture. Four braid architectures (braid pattern - diamond (1/1) and regular (2/2); braid angle - 35 degree and 45 degree) have been studied. In general, the 2/2 braided CFRP tubes exhibited ~15%-20% higher shear strength and ~25%-30% higher shear modulus than 1/1 braids. With regards to the effect of braid angle; for a given braid pattern, the 45 degree braided CFRPs have slightly higher shear moduli and a lower shear strength than the 35 degree braids. Under the applied torque (shear stress), one set of the braid tows is approximately in a state of axial tension (AT) and transverse compression, whereas the other set in axial compression (AC) and transverse tension. X-ray CT results show that for the 1/1-45 degree braided CFRP tubes, damage initiates in the form of intra-tow cracks in the -45 degree AC tows and circumferential inter-tow debonding between ±45 degree braid tows, followed by out-of-plane fibre micro-buckling and kink-band formation in the -45 degree AC tows at crimped tow cross-over points. By contrast, for the 2/2-45 degree braided CFRP tube, damage initiates from the radial inter-tow debonds between adjacent -45 degree AC tows. Nevertheless, the subsequent damage sequence resembles that for the 1/1-45 degree braided CFRP tube. However, it is noteworthy that the shear strength for the 2/2 braided CFRP tubes drops much more significantly beyond the peak stress than the 1/1 braids. This is related to the buckling of -45 degree AC tows together with the sudden propagation of circumferential inter-tow debonding along vertical zones. This gives rise to a lower remnant strength than that of 1/1 braids once damage has initiated and also highlights the importance of tow cross-overs in constraining large-scale inter-tow debonding. Moreover, the effect of a pair of open-holes (having a diameter one fifth of the inner diameter of the tube) diametrically opposite one another on the torsional behaviour has been assessed for the 1/1-45 degree braided tubes. The shear stiffness and the shear strength have been degraded by ~11% and 17%, respectively. The variation of strain distribution has been monitored by stereo digital image correlation, but also the local damage evolution around the holes has been tracked by X-ray CT. Compared with the unnotched structure, new damage modes, in-plane fibre micro-buckling in the -45 degree AC tows and radial inter-tow debonding between +/-45 degree braid tows, have occurred local to the hole. In summary, the tow cross-over density and the level of tow crimp are important factors to consider for the design of torsion resistant braid structures. For applications that require high shear strength and/or high shear stiffness, 2/2 (regular) braided CFRP tubes are well suited; whereas for applications that require high structural integrity once damage has started to develop, 1/1 (diamond) structures are a better option because the immediate loss of strength is less marked. When introducing holes into such tubes, the tow cross-over regions of the braid structure should ideally be avoided to minimise damage during torsional loading.
Date of Award | 1 Aug 2021 |
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Original language | English |
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Awarding Institution | - The University of Manchester
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Supervisor | Prasad Potluri (Supervisor) & Philip Withers (Supervisor) |
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- Braided composites
- Damage mechanisms
- Time-lapse
- Synchrotron radiation CT
- Fibre reinfoced polymers
Damage evolution in braided composite tubes under torsion studied by in-situ X-ray computed tomography
Chai, Y. (Author). 1 Aug 2021
Student thesis: Phd