Abstract:

Objective The mechanical properties of braided composites affect the scope of their applications. However, due to the complexity of the meso-structure of braided composites, the experimental research alone can no longer meet the further exploration of the mechanical properties of braided composites. To further investigate the mechanical response of braided composite tube under torsion loading, it is necessary to establish a finite element model of braided composite tube, which can well reflect the mechanical response state and has low calculation cost.
Method On the basis of previous research, a carbon fiber/resin braided composite tube with a 45° braiding angle was selected as a representative research object. Taking into account the meso-structure and full-scale simulation calculation cost, a finite element model of the braided composite tube based on real size was constructed using a micro-meso-macro multi-scale method, and its torsional loading process was systematically simulated. The torsion loading process of the braided composite tube was systematically simulated, and the shear progressive damage of the unit cell and the torsion progressive damage of the tube were discussed separately. The effectiveness of the established model was verified by comparing simulation results with experimental results.
Results The simulation results indicated that under XY shear loading, the damage of a braided composite tube unit cell generated first at the weak point where the braided fiber bundles interweave around the unit cell before the damage failure of the unit cell. The damage area was symmetrically distributed (Fig. 4). When shear failure generated in the unit cell, the fiber bundle showed significant delamination, which was consistent with the observed phenomenon of fiber separation towards both sides in the fractured fiber bundle in the experimental SEM image (Fig. 5). The torsion loading with an angular velocity of 30(°)/min was applied to the finite element model. The torque-twist angle curves and macroscopic failure morphologies obtained from experiments and simulations showed high consistency. These two cases showed that the model was accurate and effective (Fig. 7). The braided composite tube exhibited brittle fracture characteristics under torsion loading. The overall bearing capacity was stronger in the early stage, and damage elements only generated in the middle of the tube at the end of loading. The tube took only about 3.2 s from the appearance of damage to its structural failure. At the beginning, the composite tube was subjected to torsional force. At this point, the composite tube structure undergone load distribution again as a whole. Until 36.202 s, the middle area of the braided tube reaches the bearing limit due to a small amount of fibers and matrix, forming a damage unit. As the loading progressed, the damage diffused around the tube wall towards both ends of the tube at approximately 45° to the axial direction, which was basically consistent with the fiber bundle space braiding path. At 39.418 s, the braided composite material tube reached its load-bearing limit and the braided tube structure failed. At the same time, finally formed a clear space spiral shear band damage morphology on the surface of the tube (Fig. 8).
Conclusion The finite element model of braided composite tubes constructed based on multi-scale methods can effectively reflect the torsional mechanical response state of the tubes. The spatial braiding path of fiber bundles with impact on the torsional damage propagation of braided composite tubes, which means that the mechanical properties of braided composite tubes can be further optimized by adjusting the braiding path.

Key words: braided composite tube, multi-scale model, finite element simulation, torsion, progressive damage