Fiber-reinforced polymer (FRP) composites are widely being used for structural applications due to their attractive properties. Particularly, to fabricate complicated structures by FRP composites, joining composite structure elements through fasteners require drilled holes. More than 60% of the structural FRP components are rejected owing to poor quality of holes. The FRP composites are heterogeneous due to multi-layer architecture, multi-phase material combinations, and hybridization. Hence, while drilling the behavior of FRP composites depends on various machining and tool parameters in addition to workpiece properties. As a result of drilling, the quality of drilled holes deteriorates due to delamination, matrix crack, matrix burning, fiber pull-out, buckling, spalling, and fuzzing in the drilled hole zone. To minimize all these defects researchers tried to design material, tool and machining parameters and reported. In this paper, the authors have made an attempt to summarize up-to-date progress in structure properties and drilling study of unidirectional and bidirectional multi-layered composites, three-dimensional fabric-reinforced composite and hybrid composite and similarly, emphasize the advanced characterization techniques to investigate the drilled hole quality. In addition, cryogenic assisted drilling, lubricated drilling, and micro-drilling literatures have been discussed. Based on the literature, conclusions and future scope have been reported.

In this study, non-hybrid and hybrid (Kevlar, carbon and glass) fabric epoxy composite laminates were fabricated with different stacking sequences by hand lay-up followed by hot-compression molding. Experimental tests were conducted to investigate tensile, flexural, and hardness characteristics. It was found that the stacking sequence did not significantly affect the tensile strength and hardness values of the composites; however, it affected their flexural strength. Damage morphology of the specimens through SEM images showed that the major damage mechanisms in the composites were delamination, fiber breakage, pull-out, and matrix cracking. Based on the static experimental results, the high-velocity impact behavior was investigated through simulation study using LS-DYNA finite element analysis (FEA) software. To study the ballistic impact, a steel projectile with a hemispherical penetrating edge at impact velocities of 100 m.s⁻¹, 250 m.s⁻¹, and 350 m.s⁻¹ was considered. Among non-hybrid fabric epoxy composite specimens, Kevlar/epoxy specimen was found to have the highest impact energy absorption followed by carbon/epoxy and glass/epoxy, respectively. Regarding the hybrid fabric epoxy composite specimens, the ones with Kevlar plies in the rear face exhibited better energy absorption compared to other stacking sequences. The non-hybrid glass/epoxy specimen had the lowest energy absorption and highest post-impact residual velocity of projectile among all specimens. From the FEA results, it was noted that impact resistance of hybrid composites improved when Kevlar fabric was placed in the rear layer. Thus, the stacking sequence was observed to be of substantial importance in the development of fabric-reinforced composite laminates for high-velocity impact applications.

The combination of awareness of harmful industrial processes, environmental concerns, and depleting petroleum-based resources has spurred research in developing sustainable materials from renewable sources. Natural bio-based polymers have replaced synthetic polymers because of growing concern about environmental sustainability. As a result of heating and distilling cashew nutshell liquid (CNSL), cardanol has emerged as a promising bio-retrieved component that can be used to make bio-based epoxy. The current work intends to investigate the mechanical properties of three kinds of cardanol-based bio-based epoxies in anticipation of widespread use. Vickers hardness, tensile and flexural strength are used to characterize mechanical properties. Additionally, a water absorption test is carried out to examine the weight gain properties of all the bio-based epoxy variants selected. FormuLITE 2 (FormuLITE 2501A + FormuLITE 2401B) exhibited the highest Vickers hardness, tensile and flexural strength among the three variants. Moreover, it exhibited a water absorption rate nearly equivalent to that of the conventional LY556/HY951, and thus, FormuLITE 2, the bio-based epoxy resin having 34% of bio-content blended with conventional epoxy, proves to be the best option out of the selected bio-based epoxies to be used further as the matrix material for the fabrication of biocomposites.