Structure Design And Flexural Properties Of Tapered Three-Dimensional Braided Composites

Three-dimensional (3D) braided composites possess excellent fracture toughness, low delamination tendency, high impact damage tolerance and outstanding fatigue resistance, which makes them strong candidates in the fields of aerospace industry, national defence and biomedical engineering etc. However, the majority of 3D braided composites are not with constant cross-sectional sizes in practical applications. Instead, their sectional sizes usually change along the length direction to form tapered appearances. Traditional braiding technique can only produce uniform composites. Tapered composites are mainly obtained by cutting and polishing their uniform conterparts. Obviously, fiber properties and preform integrities are seriously damaged by cutting, thus mechanical properties of the final composites are greatly deteriorated. Based on the four-step row and column braiding process, net-shape preparation methods of tapered 3D braided composites are proposed. Meanwhile, their microstructures and flexural properties are investigated systematically. The research work mainly includes the following four parts:1. Basic principles, plan design and implementation procedures of the yarn-reduction technique and the yarn-addition techniqueTapered 3D braided preforms can be net-shape prepared by the yarn-reduction technique or the yarn-addition technique, respectively. A yarn-reduction unit or a yarn-addtion unit are the most basic yarn groups to be removed or added. There are three yarns with two of them in one same row or same column for a surface unit and four yarns in two adjacent rows and columns for an interior unit. To ensure that the four-step movement rules are not disturbed after yarn-reduction or yarn-addition, the reduction units or the addition units should exist in all rows of the braiding array to change the sectional width and exist in all columns to change the sectional thickness.Different yarn-reduction plans can be designed according to the distribution of the reduction units. When the sectional width is decreased, reduction units in neighboring rows can be arranged either in two same columns or in different columns, which are called the column yarn-reduction plan and the row unit yarn-reduction plan, respectively. If the sectional thickness is decreased,reduction units in neighboring columns can be located either in two same rows or in different rows by the row yarn-redution plan and the column unit yarn-reductin plan, respectively. Similarly, the yarn-addition technique can be divided into the column yarn-addition, row unit yarn-addition, row yarn-addition and column unit yarn-addition plans, too. Procedures to implement the yarn-reduction technique mainly include reduction units cutting and yarn rearrangment.Procedures to implement the yarn-addition technique mainly include yarn rearrangement, yam addition and cutting of the uninterwined yarns.2. Observation and modelling of the preform microstructures near the yarn-reduction cross-section and the yarn-addition cross-sectionNo obvious free yarn-ends, pores or yarn concentration areas are observed on the surface of the yarn-reduction or yarn-addition preforms. Smoothly trapezoidal shapes are formed near the yarn-reduction or yarn-addition cross-sections. Preform microstructures are special in the tapered regions and then become the same as those of the uniform preforms about four machine cycles after yarn-reduction or yarn-addition.Models of the special yarn trajectories formed after yam-reduction and yarn-addition are established and formulae of the yarn lengths and yarn orientation angles are derived. Results indicate that two special yarn groups appear after a surface or an interior reduction unit are removed, respectively. The length and the orientation degree of one yarn groups both increase,while the other yarn groups only reverse the orientation direction compared with preforms without yarn-reduction. Two special yarn groups are formed due to the insertion of a surface addition unit.Their yarn lengths are longer and their orientation degrees are larger than preforms without yarn-addtion. After an interior addition unit is added, the yarn length and the orientation angle of one yarn groups are both increased. However, the other yarn groups change their orientation direction only.Models of the special yarn interlacing patterns after yarn-reduction or yarn-addition are established and changes in quantity and position of the interlacing points are analyzed. It can be found that one interlacing point disappears after one reduction unit is removed. Interlacing points in the second step after yarn-reduction move inside to the center of the two adjacent points without yarn-reduction. Then the interlacing pattern is the same as regular four-step braiding in subsequent steps. After one addition unit is added, one new interlacing point is formed. Furthermore,interlacing points in the second step after yarn-addition move outside to the center of the two adjacent points without yarn-addtion. Then the interlacing pattern becomes regular in the following steps.3. The influence of different yarn-reduction plans on the flexural properties and failure mechanisms of single-time yarn-reduction compositesConsidering that the yarn-reduction technique and the yam-addition technique are mutually inverse, flexural properties and failure mechanisms of the column yam-reduction composites and two types of row unit yarn-reduction composites are mainly investigated. Meanwhile, the cutcomposites and the uniform composites are also studied to make comparisons. Results indicate that flexural properties of the column yam-reduction composites and the row unit yarn-reduction composites are significantly higher than those of the cut composites and slightly lower than the uniform composites. The row unit yarn-reduction composites have better flexural properties than the column yam-reduction composites, while flexural properties of the two row unit yarn-reduction composites are nearly the same. The increase of braiding angles, the formation of resin pockets and free yarn-ends, the stress concentration due to sectional size decrease are reasons why the yarn-reduction composites have lower flexural properties than the uniform composites.Structural integrity damage and fiber property loss caused by cutting result in the weakest flexural properties of the cut composites. Distribution of the reduction units is the most important factor which leads to property differences between the column yarn-reduction composites and the row unit yarn reduction composites.Damage process of the row unit yarn-reduction composites can be divided into three stages with different failure mechanims. Matrix microcrackings near resin pocket regions and weak interfacial debondings are primary failure mechanisms in the initial damage stage. Then unstable propagation of matrix crackings and fiber pulling-outs prevail in the damage developing stage Finally, yarn breakages become the dominant failure mechanism in the serious damage stage. Yarn breakages, interfacial debondings and matrix crackings tend to happen earlyer and under lower stress in the column yarn-reduction composites and the damage is more severe. The integrate fiber networks are ruined by cutting in the cut composites. Matrix on the surface is even broken into pieces and falls off. Furthermore. fiber pulling-outs and yarn breakages are more prominent than in the yarn-reduction composites.4. The influence of distance between yarn-reduction cross-sections on the fracture morphologies and flexural properties of double-times yarn-reduction compositesWhen the distances between two adjacent yarn-reduction cross-sections are one to three pitches, flexural damage of the composites mainly occurs near the second yarn-reduction cross-section although the crosshead is placed in the center of the two yarn-reduction cross-sections. Composites damage in the center of the two yarn-reduction cross-section are rather slight. When the distances between two adjacent yarn-reduction cross-sections are four or five pitches, obvious flexural damage can be found in the tapered region of the composites right below the crosshead, whereas no significant damage is observed near the second yarn-reduction cross-section. As the distances between two adjacent yarn-reduction cross-sections increase from one pitch to four pitches, flexural strengths and flexural moduli of the tapered composites become higher and higher, too. When the yarn-reduction cross-section distance increases to five pitches,flexural properties of the tapered composites are nearly the same as those of the composites with four pitches yarn-reduction cross-section distance. Furthermore, their flexural properties are close to the properties of single-time yarn-reduction composites, too. From the analysis above, it can be seen that the distance between two adjacent yarn-reduction cross-sections should be longer than four pitches in order to ensure that flexural properties in the tapered region between the two yarn-reduction cross-sections are not greatly influenced by the latter yarn-reduction cross-section.