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Composite tubes have an asymmetry deformation and loading distribution under transverse bending. This work investigated the effect of the stacking sequence on the critical damage to a hybrid tube with 45° and 60° braided layers. In-situ infrared thermal imaging was adopted to observe the initiation and development of internal damage. As the outermost reinforcing material in the stacking structure, the 60° braided layer with high contact strength inhibited damage to the contact area at the top and improved the bending strength of the specimen at the initial stage of loading. As the inner reinforcing material in the stacking structure, the 45° braided layer with high bending toughness maintained the high stability of the load-carrying capacity during bending and restrained the tensile failure at the bottom of the tube. However, the interlaminar interface of braided layers with different braiding angles reduced the interlaminar fracture toughness. The interlaminar delamination failure developed rapidly as the bending at the bottom of the tube increased. This delamination damage determines the peak load-carrying capacity of the laminated structure. • The mechanical behavior of the stacking structure braided tube with the outer 60° braid and the inner 45° braid be studied. • Infrared thermography is applied to detect and analyze damage distribution degree in the loading process. • The load and damage distribution of stacking structure braided tubes are analyzed. [ABSTRACT FROM AUTHOR]

The effect of axial yarns on progressive bending damage of braided composite tubes is studied in this paper. Four specimens with different number and distribution of axial yarns were experimentally tested by quasi-static three-point bending. At the same time, infrared thermography was used to observe the damage in-situ. Combined with the cross-section morphology of Ultra-Depth 3D Microscope, the damage evolution and distribution characteristics of braided tubes were identified. It is found that axial yarns enhance the mechanical properties of braided composite tube. The detected changes of infrared maximum temperature have a good correlation with the damage initiation and failure mechanism of the specimen. The abrupt changes of the maximum temperature indicate the occurrence of local fiber breakage, especially the axial yarn breakage. If there are axial yarns on the compression side of the tube, the maximum temperature increases by 0.5 °C in the linear elastic stage, and two initial damages of debonding and fiber breakage will occur. The debonding started at the interface of the axial yarn. The axial yarns increased fiber crimp, resulting in obvious wavy delamination between layers of the specimen, and the axial damage range was expanded. In addition, the mixed structure with axial yarns only on the compression side can inhibit the damage from spreading down the braided tube wall. • The progressive bending damage of triaxial braided tube was studied. • The abrupt change of infrared maximum temperature indicates local fiber breakage. • The mixed structure with axial yarns only in the upper half can inhibit damage spreading downward. [ABSTRACT FROM AUTHOR]

Localized deformation and damage mechanism of triaxial braided composite tubes, with different quantities and positions of axial yarns, were experimentally studied under transverse low-velocity impact. A high-speed infrared thermography was applied to analyze the transient thermo-mechanical failure process and afterwards verified by the external and internal damage characterization. It is found that the distribution of axial yarns has a significant effect on the mechanical behavior of braided composite tubes. The bending stiffness and peak force of the tubes were enhanced with the increased quantity of axial yarns, while the impact lasting time and displacement presented an opposite trend. And the more axial yarns assembled at the impact side, the greater force oscillatory of the tubes under low-velocity impact, accompanied by the decreased heat generation in small-angle braided tubes and the alleviated fiber tows fracture, delamination and in-plane shear cracks inside the wall of large-angle braided tubes. On the premise of the same quantity of axial yarns, the position effect plays a key role. Axial yarns concentrated at the impact side could restrain the damages spreading to the non-impact side by changing the damage mode of large-angle braided tubes. If a larger force is needed at the early stage, axial yarns could be inserted at the local position most probably to be impacted; Otherwise, it is highly suggested to braid at least one axial yarn in each layer at the impact side and non-impact side respectively to reinforce localized strength and prevent the impact-side indentation and non-impact-side delamination. • High-speed IR thermography verified by the external and internal damage characterization. • Effect of axial yarn quantity and location on mechanical behavior and damage mechanism. • Force oscillation response caused by dense distribution of axial yarns at the impact side. • Necessity of axial yarns in alleviating the severity of localized damages at the impact side. • Importance to install at least one axial yarn at the non-impact side of large-angle braided tube. [ABSTRACT FROM AUTHOR]