The dynamic failure evolution of textile composites, which were subjected to impact velocities up to 1100 m/s, was investigated. Specialized machines were used to fabricate composites from combinations of Spectra®, Kevlara®, and Twaron® fibers and two- and three-dimensionally woven, braided, and needle-punched nonwoven fabrics. This control of fabrication and processing enabled us to characterize response as a function of areal density, fabric finish, and consolidation techniques. Failure was categorized in terms of material layers, debris mass, matrix cracking, fiber failure, and shear-plugging. Results indicate that shear-plugging occurs at velocities corresponding to decreases in debris mass.
In Part I of this series of papers, structures and geometries of the four-step preforms were studied and analysed. In this part, an account is given of similar work conducted on the two-step preforms. Theoretical models for both regular and tubular two-step preforms are established with a few assumptions. Structu.ral geometries of the preforms are analysed aud discussed according to the theoretical models developed. Mathematical relations between the structural parameters, such as the fibre orientation, yam-volume ft-action, and prefoirm contour sizes, as well as their dependence on operating conditions, are derived. It is found that the preform structures are determined hy the constitucint yams, the braiding arrangements, and the process operating conditions. The extreme values ofthe parameters in the jamming conditions are also discussed. To verify the validity of the analytical models, experimental investigatic>ns were also carried out. The experimental results strongly support the theoretical predictions.
It is pointed out that little attention has I>een devoted to the mechanics of blended spun yarns. Most of the work done in this area is limited to strength and elongation, An experimental investigation of viscose/polyester blended yarns was undertaken. The eRect of twist and h)end levels on the initial modulus, dynamic modulus, tenacity, and elongation of these yarns is analyzed.It is shown that both the dynamic and initial moclulii of these yarns increases as polyester percentage increases and then decreases to reach its lowest level at 100% polyester. It is also shown that the dynamic modulus is appreciabty greater than the static modulus and that the former increases as the testing tension increases.
An exploratory study of the fracture behavior and notch sensitivity of a 4-step, 3-D braid-reinforced graphite/epoxy composite has been made. Test methods based on the Mode I compact tension specimen were developed and lower bounds for the damage initiating force and the work of fracture were determined for certain notch-to-braid axis orientations. These values are higher than for laminate composites but showed severe anisotropy. Complementary in-situ and post-mortem optical and scanning electron microscopy were used to identify microstructural failure controlling features and to develop a volume-to-surface structural mapping strategy useful in accounting for some of the observed features of the failure process.
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