Study On Process Technology And Effect Of Low Velocity Impact On The Tensile Performance Of Unidirectional Braided Laminated Composites

Unidirectional braiding is an improvement of the traditional two-dimensional braiding(2D braiding).It uses auxiliary yarns to replace a set of carbon fiber tow in 2D braiding structure,and the auxiliary yarns is much finer than the carbon fiber tow,which effectively reduce the carbon fiber crimp in 2D braiding structure.The auxiliary yarn used in this paper is nylon yarn with low melting point.During the precuring process,the nylon yarn will be melt to fix the structure.Composites may suffer from low-velocity impact in practical applications,which often leads to visually undetectable internal damage to the composites,with no or only slight indentation on the composites' surface.In order to research the effect of low velocity impact on the tensile properties and tensile failure modes of unidirectional braided laminated composites,the unidirectional braided laminated composites were taken as the main research object in this study,low-velocity impact test,tensile test before and after impact were carried out.At the same time,it was compared with 2D braided laminated composites.In addition,in order to investigate the effect of small variation of braiding angle on the tensile properties,and tensile properties after impact of unidirectional braided laminated composites,three groups of unidirectional braided laminated composites with different braiding angle were designed and prepared.The research results of this paper are as follows:(1)The small change of the braiding angle has a significant effect on tensile performance of unidirectional braided laminated composites.The tensile strength and modulus of unidirectional braided laminated composites will decrease when the braiding angle increases slightly.Tensile strength is relatively affected more by the change of the braiding angle,compared to tensile modulus.(2)Low-velocity impact can impair the tensile capacity of the unidirectional braided laminated composites,and the small change of braiding angle has certain influence on the impact damage resistance of the composites.(3)The tensile properties of 2D braided laminated composites are worse than that of unidirectional braided laminated composites,and the tensile strength and modulus loss rate after impact are also higher than that of the unidirectional braided structure.It is indicated that the reduction of fiber crimp in the braided structure has great effect on the improvement of the tensile properties of the composites,and unidirectional braided laminated composite has better impact damage resistance under low-velocity impact.(4)Different from static tensile test,the tensile fracture position of the specimen subjected to impact is located in the impacted area,indicating that the material is damaged by the impact at the impact location,which leads to the weakening of the tensile properties in the impact area.The tensile failure of the static tensile specimen began with the cracking of the matrix at the edge of the specimen,and the tensile failure of the specimen subjected to the impact damage began with crack propagation near the impact point.(5)Unidirectional braided and 2D braided laminated composites have fiber breakage and resin matrix cracking during the tensile process before and after impact,but the damage area in unidirectional braided laminated composites was bigger than that in 2D braided laminated composites,and the shape of the fracture in Unidirectional braided structure is irregular;During tensile test,the necking phenomenon on sample width direction in 2D braided structure is more obvious than that in unidirectional braided structure,and the fracture presents regular V shape in 2D braided structure.(6)In both unidirectional braided and 2D braided laminated composite tensile tests,the damage on specimen's surface near the braiding mold(the first layer)is more serious than that on the other side of the specimen which is away from the mold(the last layer),and the braiding angle of the first layer is smaller than that of the last layer.The tensile failure on specimen's surface on impact side of both structure composites were mainly characterized by fiber breakage,matrix cracking.The damage of the specimen's back surface was a little different,except for the matrix cracks,the back surface of the unidirectional braided specimens also appeared mutual extrusion among fiber bundles,and fiber cocking;2D braided specimens showed the split of the fiber bundle in sample thickness direction.(7)The damage modes inside the tensile test specimens before and after impact of both unidirectional braided and 2D braided laminated composites appeared interlayer cracks and intralayer cracks.The intra-layer cracks mainly occurred in the resin zone among fiber bundles and among fibers in fiber bundle,and extended along the thickness direction of the test sample.The cracks inside the fiber bundle were discontinuous between adjacent fiber layers,but the resin zone cracks between fiber bundles will expand along the resin zone between adjacent fiber layers.The interlayer cracks of the unidirectional braided laminated composites occurred between the fiber layers,which were parallel to the fiber layer,and the interlayer cracks of the 2D braided laminated composites were inclined.(8)The extent of tensile failure in unidirectional braided and 2D braided tensile specimens is gradually weakened from side surface to middle of the specimen in specimen width direction.However,the extent of damage at the 1/2 width position of the tensile specimen pre-impacted of the two braiding structure composites was more obvious.Unidirectional braided specimen appears a lot of intra-layer cracks,and some resins are detached among fibers in fiber bundle.There are many inter-laminar cracks at the 1/2 width position of the 2D braided sample,and the phenomenon of resin peeling among fiber tows can be observed.