Abstract
In this paper, a research activity, focused on the investigation of new reinforcements able to improve the toughness of composite materials systems, is introduced. The overall aim is to delay the delamination propagation and, consequently, to increase the carrying load capability of composite structures by exploiting the fiber bridging effects. Indeed, the influence of fiber bridging related Mode I fracture toughness (GIc) values on the onset and propagation of delaminations in stiffened composite panels, under three-point bending loading conditions, have been experimentally and numerically studied. The investigated stiffened panels have been manufactured by using epoxy resin/carbon fibers material systems, characterized by different GIc values, which can be associated with the material fiber bridging sensitivity. Experimental data, in terms of load and delaminated area as a function of the out-of-plane displacements, have been obtained for each tested sample. Non-Destructive Inspection (NDI) has been performed to identify the debonding extension and position. To completely understand the evolution of the delamination and its dependence on the material characteristics, experiments have been numerically simulated using a newly developed robust numerical procedure for the delamination growth simulation, able to take into account the influence of the fracture toughness changes, associated with the materials’ fiber bridging sensitivity. The combined use of numerical results and experimental data has allowed introducing interesting considerations of the capability of the fiber bridging to substantially slow down the evolution of the debonding between skin and reinforcements in composite stiffened panels.
Highlights
When composite materials are applied to aerospace, one of the main, safety-driven, design objectives is to increase the capability of the structures to withstand the operating loads when accidental damages occur
Laminated structures often are characterized by inter-laminar failures, arising from pre-existing manufacturing defects, stress concentration, impact with foreign objects, and which can rapidly evolve towards structural collapse due to global or local buckling phenomena [2,3,4,5,6,7]
As already this paper is focused on thepanels assessment of the fiber toughening effectremarked, on skin stringer in composite stiffened manufactured withbridging materialinduced systems toughening effect on skin stringer in composite stiffened panels manufactured with material systems with respectively, low and high sensitivity to the fiber bridging phenomenon
Summary
When composite materials are applied to aerospace, one of the main, safety-driven, design objectives is to increase the capability of the structures to withstand the operating loads when accidental damages occur. The R-curves behavior, due to the bridging, has been experimentally investigated by Sørensen and Jacobsen in [16] for unidirectional carbon fiber/epoxy resin composite specimens. Large scale bridging phenomenon has been found in the specimens with 45◦ //45◦ crack interface, concerning the 0◦ //0◦ interface Such results have been confirmed in [19], where the plateau value of the GIc for the 45◦ //45◦ plies interface has been found 70% higher than the one found for 0◦ //0◦. The experimental tests have been numerically simulated using a robust finite elements procedure able to mimic the delaminations propagation by taking into account the changes in critical energy release rate with the crack length and, the fiber bridging phenomenon [25]. The experimental and numerical results have been found very useful to improve the understanding of the effect of the materials’ fiber bridging sensitivity on the skin-stringer separation phenomenon in stiffened composite panels.
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