Abstract:

Objective At present, most three-dimensional woven T-shaped prefabricated bodies are prepared by weaving the flat fabric first, splicing, stitching, or unfolding the fabric in layers. The overall forming weaving method of the prefabricated body and the structural optimization design method of the prefabricated body junction are not ideal for studying the tensile properties of the T-joint of a three-dimensional woven composite material. The influence of the yarn interweaving mode and the composite material structure at the connection of the vertical rib and bottom plate on its tensile properties, failure mode, and damage mechanism were analyzed. The macroscopic mechanical properties of T-joints of various three-dimensional woven composites were analyzed by experimental tests, which provided a reference for the structural design of T-joint composites.

Method T-joint composites with different structures and connection modes were designed, and prefabricated composite molding was carried out by resin transfer molding (RTM) process. Tensile experiments were carried out, and the full-field strain was analyzed using a non-contact full-field strain measurement system (DIC). The failure mechanism and the influence of different structures and connection modes on the tensile properties of the T-joint composites were analyzed.

Results For the three-dimensional layer woven composite T-joint, the interweaving mode of the weft yarn at the connection of the vertical rib and the bottom plate had a certain influence on the tensile strength of the composite material, and the average tensile strength of the two was not much different, but the sample using the cross connection mode was slightly higher than the sample using the forked connection mode, when the sample was introduced into the bottom plate by forked connection mode, the tensile strength and failure displacement of the composite material were smaller, and the final damage degree and range of the sample were larger, and the bearing capacity was completely lost, which was a catastrophic failure. When the sample adopts the weft connection mode of left and right crossing, the weft introduced by the cross can resist the shear effect generated by the tensile load at the connection, which can effectively limit the expansion of the damage to the inside of the specimen, the ultimate stress of the sample was increased by more than 10%, the damage range of the sample was small, the tensile capacity was strong, and the cross structure at the connection between the vertical rid and the bottom plate could effectively improve the tensile property of the composite T-joint. For the composite T-joints with different structures, compared with the three-dimensional layer woven structure, the tensile strength of the composite sample with multilayer multiaxial structure was increased by 98.46%, because the yarn directly bearing the tensile load in the multilayer multiaxial woven structure was only the weft, with the increase of tensile displacement, the matrix cracks, the weft yarn was gradually pulled off, the structure loses its bearing capacity, and the sample finally fails, while the multi-layer multi-directional structure rib and bottom plate contain ±45° oblique yarn. The yarns in different directions played a role in dispersing and buffering the load during the load loading process, and the composite T-joint with multilayer multiaxial structure could effectively alleviate the stress concentration phenomenon in the connection area between the vertical rib and the bottom plate, thereby significantly improving the tensile property of the T-joint.

Conclusion The sample with cross-joints showed better tensile properties during the tensile process, and the cross-introduced weft yarns could resist the shear effect of the tensile load at the joints, which could effectively limit the damage extension to the interior of the samples. For the comparison of the tensile properties of the samples with different structures, the tensile strength of the multilayer multiaxial composite T-joint structure was greatly improved compared with that of the layered interlock woven structure, and the introduction of ±45° diagonal yarns improved the stress distribution of the sample at the joints, which greatly improved the tensile properties of the composite T-joints, and provided references for the application of the composite T-joints in the future.

Key words: T-joint, multi-layer multi-directional structure, 3-D woven composites, tensile property, failure mechanism