Abstract:

Objective Three-dimensional (3-D) woven composite materials have broad application prospects in multiple fields due to their excellent mechanical properties, strong designability and the ability to be formed as a whole. Among them, 3-D shallow angle-interlock woven structures have become a research hotspot due to their low cost and excellent performance. However, most of the existing research focuses on the independent analysis of the tightness or static weaving parameters in a single direction, lacking a systematic exploration of the coordinated regulation mechanism in the warp and weft directions and its correlation with mechanical properties.

Method In this paper, under the condition of controlling the total yarn consumption of the fabric to be similar, two three-dimensional shallow interlaced fabric structures with five layers of warp yarns and six layers of weft yarns, and three layers of warp yarns and four layers of weft yarns (denoted as 5J6W and 3J4W respectively) were selected for weaving. To study the influence of the warp and weft tightness ratio on the mechanical properties of 3-D shallow angle-interlock woven composite materials, aramid 1414 fibers were used as raw materials to weave aramid 3-D shallow angle-interlock fabrics with two warp and weft tightness ratios, i.e. 2∶1 5J6W fabric and 1∶1 3J4W fabric. Aramid/epoxy composites were prepared by vacuum-assisted resin transfer molding process. Tensile tests were conducted according to standardized protocols. Meanwhile, combined with the tensile test results and damage morphology, analyze the influence of the ratio of warp and weft tightness on the tensile properties of 3-D shallow angle-interlock woven composites.

Results When the warp and weft tightness ratio decreased from 2∶1 to 1∶1, the warp properties of the 3-D shallow angle-interlock aramid/epoxy woven composites decreased significantly (tensile strength dropped from 364 MPa to 145 MPa, a 60% decrease; modulus decreased from 628 MPa to 279 MPa, a 60% decrease), while the weft properties improved substantially (tensile strength increased from 186 MPa to 545 MPa, a 190% increase; modulus rose from 545 MPa to 1 057 MPa, a 94% increase). The failure modes showed that all warp tensile tests exhibited transverse failure, but the yarn fracture morphologies varied. At a 2∶1 warp and weft tightness ratio, warp yarn fractured layer by layer in a "V" shape, whereas at 1∶1, sudden "|" shaped fracture occurred due to stress concentration from buckling. The weft failure mode shifted from transverse failure at 2∶1 to diagonal oblique crack failure at 1∶1, induced by warp yarn sliding. Structurally, decreasing the warp and weft tightness ratio increased the warp yarn weaving angle from 15°to 38°and intensified buckling triggered stress concentration. Meanwhile, the weft yarn cross-section transformed from a flat strip (roll curvature R=0.1) to an elliptical shape (R=0.4), with the denser structure enhancing load-bearing efficiency. Although the 1∶1 tightness ratio strengthened resin-fiber interfacial bonding, it also became weak sources for crack initiation.

Conclusion The improvement in weft performance directly resulted from the increase in load-bearing yarn content, while warp mechanical properties were compromised by buckling and high stress concentration in resin-rich zones. The tensile failure modes of aramid/epoxy 3-D shallow angle-interlock woven composites were found to have three types, namely, fiber fracture, resin damage and interfacial delamination. Warp tensile fracture occurred in the resin-rich areas of oblique warp yarn segments, whereas weft tensile fracture originated at the edges of warp yarns at interlacing points. The warp and weft tightness ratio regulated yarn interlacing states, fiber volume content, and interfacial bonding, thereby forming differentiated load transfer paths and failure modes.

Key words: warp and weft tightness ratio, 3-D shallow angle-interlock woven composite, tensile property, morphology damage, failure mechanism, aramid/epoxy resin composite