Knitted structures occupy a special position in composite preforming due to their inimitable characteristics. An insight into the knitted structures with respect to their composite preforming characteristics is presented in this article. Directionality of knitted structures and requirements of high performance fibers for knitting have been discussed. Contourability, net-shape preforming, high dynamic mechanical properties along with easy and rapid manufacturability are the important features of knitted structures to match the composite preform requirements. In this article, the work done in the above areas of research have been critically reviewed.

Low-velocity (≅ 1.2-2.4 m/s) repeated drop weight impact tests were carried out on advanced composites (glass, carbon, kevlar in epoxy matrix - LY 556, HT 972) using an instrumented impact test machine (DYNATUP 8250). The incident energies ranged from 3.5 to 15 J (constant impactor mass 5.42 kg). Number of drops to failure (Nf) data were obtained for the above with simultaneous recording of the load-time and energy-time traces. The extent of impact damage for glass and kevlar composites was interpreted using final delamination area maps (tracings). Results for all the three composites showed that with an increase in drop numbers, the peak load (PL) steadily decreased while the total energy (Et) increased, until failure. Also, as the incident energy (Ein) was varied in arithmetic progression, the number of drops to failure (Nf) varied in harmonic progression. Finally it was concluded that repeated drop tests with final delamination area maps assist in understanding the impact damage tolerance of polymer composites.

This paper details about weaving of single-layer 3D ‘T’ profile with fillet for use as insert in composite ‘T’ joints and ‘T’ stiffeners. The ‘T’ insert with fillet was woven using 3 K carbon tows on a narrow width multi-beam automatic loom. Weaving was carried out based on the double-cloth weaving principle. Novelty of the work lied in the approach adopted for designing of the weave architecture in developing 3D ‘T’ profile with fillet portion, arriving at the pick cycle diagram for weave design development which has been detailed in this paper. Test results of composite ‘T’ joints fabricated incorporating the insert showed strength improvement as well as change in crack propagation mode as compared to conventional ‘T’ joint. The continuous insert with fillet acted as a bridging member among the three sections of the ‘T’ joint, thus contributing to performance improvement.

Experimental studies were carried out to understand the impactor mass effects on glass–epoxy composites using an instrumented impact machine (DYNATUP 8250) under low-velocity test conditions. Repeated drop tests were carried out at pre-determined energy levels for selective mass height combinations. Number of drops to failure (Nf) was chosen as the damage criterion. Analysis of test results clearly showed that, at low incident energies, heavier impactors caused more damage to the laminates as reflected by their lower number of drops to failure. This effect diminished gradually at relatively higher incident energies.

This paper details about the manufacturability of integrally woven ‘T’ stiffeners using orthogonal weaving technology based on the pleat formation concept. The development was carried out on a customized automatic take-up-at-will rapier weaving machine equipped with multi-beam warp let-off and electronic dobby. Bridging the gap between the base portions of the flange sections and locking the web sections together by using equally spaced intermittent extra warp yarns were the added innovations in the work apart from integral weaving of the ‘T’ stiffener. These innovations were achieved by suitably modifying the weave design. The development comprised a 5-part weave design for the ‘T’ stiffener and was carried out using 6K carbon tows. Calculations for X, Y, Z contents and overall fibre content by volume, from first principles of cloth construction approaches, for the flange and web sections of the ‘T’ profile, have also been included.