Abstract:

Objective The production costs of most metal variable thickness plates are high and some composite plates with variable thickness are generally based on laminated composites, which are prone to interfacial delamination and property degradation. This study was designed to solve the problem of delamination of laminated composites, aiming to improve the overall performance against impact.

Method Basalt yarn is a green and environmentally friendly materials. And, it has high strength and excellent durability. Three-dimensional(3-D)variable-thickness fabrics with 3-layered angle interlocking structure change from 9 to 3 layers were designed and woven on a conventional loom using basalt yarns as warp and weft yarns. Resin-impregnated variable-thickness 3-D woven composites were prepared using a vacuum-assisted resin transfer molding (VARTM) process. The variable thickness laminated composites were subjected to low velocity impact tests at 25, 50, 75 and 100 J according to American Society of Testing Materials (ASTM) standards. The damage mechanism of the material was analysed by the damage area at 100 J impact.

Results The 3-D woven variable thickness composites exhibited high consistency in stiffness and stiffness degradation under impact energy of 25, 50 and 75 J compared to the same weft lay-up structure. This was based on the fact that the layers of yarn are interconnected in the 3-D woven variable thickness composites, whereas there was little adhesive action between the the same weft lay-up structure. At the same time, the damage load of the composite with the 3-D woven variable thickness structure was greater than that of the two lay-up composites. This indicated that the composite with the 3-D woven variable thickness structure had a higher damage tolerance and carries more load before damage occurs. At an impact energy of 100 J compared to 25-75 J, the damage load was reduced in a 3-D woven variable thickness structure with co-layered layers. This was due to the large amount of kinetic energy that was instantaneously released by the impact head when a high energy impact occurs. These kinetic energies caused the surface bearing yarns in the material to quickly break and lost their load-bearing capacity. The damage load on the material was reduced because the yarn on the bottom layer of the material had not yet reached a responsive state. When the material was impacted in the forward direction, the damage showed a cross-shaped or rectangular distribution with a greater extension along the weft direction than the material along the warp direction. As the thickness variation of the composite was determined by the number of yarn layers, the energy consumed decreased with the thickness, so that the stress could propagate through the material for a longer period of time. Along the warp direction of the material, the number of layers of material within the same thickness layer remained the same as the thickness, and the stress waves were consumed rapidly, creating a short damage extension range. Damage to the material in the dorsal direction also showed a cross-shaped or rectangular distribution with greater extension along the weft direction than the material along the warp direction. But the damage of the composite material with the same warp-layered structure had slightly less damage along the weft direction than the warp direction. This was because the material encountered a strong impact under the action of 100 J impact energy, the energy of the impact was enough to quickly break through the fabric of all layers of the same layered structure. It completely destroyed the integrity of the material and made the damage to the material more concentrated.

Conclusion Compared with laminar composites, 3-D woven composites of variable thickness have higher damage tolerance and better impact resistance because of the internal warp connection. Compared with the metal variable thickness plate, the variable thickness 3-D woven composite material has the characteristics of low production costs, low manufacturing and maintenance cost of production equipment and low energy consumption. It has reference significance for future application research of variable thickness plate.

Key words: three-dimensional fabric with variable thickness, laminated composite, low speed impact, impact failure mode, damage mechanism