Bending Performance Study Of Three-dimensional Woven Composite I-beam With Holes

Carbon fiber composite I-beam,as a structural member of aerospace aircraft and unmanned aircraft,inevitably requires the introduction of hole defects in the web of the I-beam in order to facilitate the passage of pipes and electrical circuits.Most of the current members often use mechanical opening of the material,which is prone to delamination and cracking of the composite material,leading to degradation of the material performance and even failure.In this paper,we studied the three-dimensional woven I-beam fabric with one-piece weaving into holes,and prepared composite I-beams to expand the application of I-beams in the field of composite materials.In this paper,prefabricated I-beams with perforated webs were woven in one piece using three-dimensional braiding,and three-dimensional braided composite I-beams with perforations were prepared using vacuum-assisted resin infusion molding technology.The braided I-beam with holes is a pre-woven I-beam fabric containing holes,and then filled with round hole molds to form holes during curing,with good continuity of fibers around the holes.The drilled I-beam is a solid web 3D woven composite I-beam with mechanical openings,and the fibers around the holes are cut.Through four-point bending experiments,the composite I-beams with different number of holes in the web obtained by mechanical drilling and 3D braided one-piece forming process were tested and analyzed,and the bending performance and damage evolution process of both were studied by combining with 3D-DIC image acquisition and analysis system,and the damage morphology was observed.It is found that the overall bending curves of braided and drilled composite I-beams have similar trends.With the increase of the number of holes,the more obvious the effect of braided hole process on the enhancement of the maximum load.Under the single-hole condition,the maximum load of braided I-beam is close to that of drilled hole;under the two-hole condition,the maximum load of braided I-beam is slightly higher than that of drilled hole,with an increase of about 4.1%;under the three-hole condition,the maximum load of braided I-beam is significantly higher than that of drilled hole,with an increase of about 14.5%.For the same number of holes,the maximum load of braided I-beam and its corresponding displacement are greater than that of drilled holes.The stress distribution in the web of the drilled composite I-beam is more concentrated than that of the braided perforated part.The macroscopic cracking direction of the web of the 3D braided composite I-beam with holes is at an angle to the bending loading direction,which is a shear damage mode.In the initial stage,the stresses are concentrated in the area around the web holes;as the loading proceeds,the stresses gradually shift and concentrate above the middle holes until the fibers above the middle holes break and the material fails.There is a clear difference in the damage morphology between the holes in the webs of braided and drilled composite beams.In the former case,severe fiber-matrix separation occurred in the resin-rich triangle around the hole,while in the latter case,the fiber breaks were extracted from the inner wall of the hole by exposure.For the braided-hole composite I-beam,the bending curves of the composite I-beam with different number of holes showed different trends.The curve of two-hole composite I-beam shows a slow decreasing trend after reaching its peak load,and still can bear more than 76% of the load,which is obviously higher than that of single-hole and three-hole.While the single-hole and three-hole composite I-beams drop precipitously after reaching their peak loads and the material fails.The three-dimensional woven composite I-beam with holes under bending load undergoes matrix cracking,fiber-matrix separation,and fiber fracture.The maximum stress point is located in the concave area above and below the hollow web area of the web,and the resin surface around the hole is the first to crack,with cracks extending along the perimeter of the hole and in the direction of the weave angle,causing material failure.