Voids in fiber-reinforced polymer composites: A review on their formation, characteristics, and effects on mechanical performance

Abstract

Voids, the most studied type of manufacturing defects, form very often in processing of fiber-reinforced composites. Due to their considerable influence on physical and thermomechanical properties of composites, they have been extensively studied, with the focus on three research tracks: void formation, characteristics, and mechanical effects. Investigation of voids in composites started around half a century ago and is still an active research field in composites community. This is because of remaining unknowns and uncertainties about voids as well as difficulties in their suppression in modern manufacturing techniques like out-of-autoclave curing and parts with high complexity, further complicated by increased viscosity of modified resins. Finally, this is because of the increasing interest in realization of more accurate void rejection limits that would tolerate some voidage. The current study reviews the research on formation, characterization, and mechanical effects of voids, which has been conducted over the past five decades. Investigation and control of void formation, using experimental and modeling approaches, in liquid composite molding as well as in prepreg composite processing are surveyed. Techniques for void characterization with their advantages and disadvantages are described. Finally, the effect of voids on a broad range of mechanical properties, including inter-laminar shear, tensile, compressive, and flexural strength as well as fracture toughness and fatigue life, is appraised. Both experimental and simulation approaches and results, concerning voids' effects, are reviewed.

Hybrid Manufacturing and Experimental Testing of Glass Fiber Enhanced Thermoplastic Composites

“…These voids have been highlighted in Figure 13. Voids are empty areas between layers where there is no connection and a higher percentage of voids could lead to significant degradation of mechanical properties [27]. GFRP-1 shown in Figure 13b exhibit a neatly packed single layer of glass fiber to resist the tensile force with the brittle PLA and voids highlighted in its background.…”

Surface topographical studies of glass fiber reinforced epoxy-ZnO nanocomposites

“…High void fraction in the neat resin composite is mainly attributed to entrapped air in manufacturing route (pressure, temperature variation) and presence of volatile organic compounds (alcohol, amine and ether groups) which become vapours or gases during curing stage in the matrix. These result in the formation of empty spaces (air pockets or voids) in the matrix [36]. Further increase in the loading of nanofiller content from 1 to 3 wt% , it is perceived that void fraction gradually reduced from 2.760 to 1.641% in composites.…”

Compression Molded Thermoplastic Composites Entirely Made of Recycled Materials

“…The strength of the particulate filled composite depend strongly on the stress transfer between the filler and the matrix [39]. The void content and their shape, size, and location effect on the mechanical properties and to the mechanisms leading to mechanical failure stress/strain concentration effects [40]. This phenomenon is visible in the strain curves of composite SS60 which had relatively high porosity (25.3%) see Figure 12.…”