13th International Conference on Fracture June 16–21, 2013, Beijing, China -1Fibre Bridging and Crack Tip Shielding in Glare: Numerical and Experimental Validation Sunil Bhat1 1 School of Mechanical and Building Sciences, VIT Univeristy, Vellore, 632014, Tamil Nadu, India * Corresponding author: ss_bht @rediffmail.com Abstract The paper validates the phenomenon of fibre bridging and crack tip shielding in Fibre Metal Laminate (Glare) with the help of numerical and experimental procedures. Laminates, with Mode I cracks of different sizes in all their aluminum layers, are subjected to load-extension test to obtain critical loads for estimation of their fracture toughness. Similarly cracked, plain aerospace aluminum alloy specimens are also tested for fracture. Fracture toughness of Glare laminates is found to be higher than those of plain aluminum alloy specimens. Cracked laminates are finally modeled under critical loads by finite element method for quantification of fibre bridging in them. Crack tip shielding is demonstrated. Keywords Fibre metal laminate, Fibre bridging, Glare, Crack tip shielding 1. Introduction Fibre metal laminate (FML) is an advanced hybrid structure that consists of layers of thin and light metallic sheets which are alternately bonded and cured with composite prepregs by heat and pressure, each prepreg built up of several resin impregnated fibre cloth layers laid in similar or different orientations. Besides offering gain in specific strength, FML exhibits properties like excellent fatigue and fracture resistance, good impact strength and high fire resistance that makes it a good substitute for monolithic metallic structure especially in aerospace and aircraft applications. FML (Glare), comprising several aerospace aluminum alloy layers and glass fibre based composite prepregs, is considered for investigation in the present work. Cracks can nucleate in soft aluminum layers, across the interfaces of prepregs, when Glare is pressed into service. The cracks are however shielded due to fibre bridging. Bridging diverts load towards stronger fibres in prepregs that in turn diminishes the intensity of stress fields around crack tips thereby augmenting fracture toughness of Glare vis-à-vis plain aerospace aluminum alloy. Published work, notably by Guo et al. [1], Alderliesten et al. [2] etc., confirms superior fatigue and fracture properties of FML’s. This paper presents an explicit validation of the phenomenon of fibre bridging in Glare with the help of numerical and experimental procedures. Glare laminates, with Mode I cracks of different sizes in all their aluminum layers, are subjected to load-extension test to obtain break or critical loads for estimation of their fracture toughness. Similarly cracked, plain aerospace aluminum alloy specimens are also tested for fracture. Toughness values of laminates are found to be higher than that of plain aluminum alloy specimens. The laminates are finally modeled under critical loads by finite element method. Crack tip shielding in them is demonstrated and convincingly verified. 2. Construction of Glare Refer Figure 1. Glare laminate consists of three, 0.4 mm thick, 2014-T6 aerospace aluminum alloy sheets, bonded alternately with two prepregs at curing temperature of 160 deg. C, each prepreg built up of three composite layers in the sequence, c0-c90-c0. A composite layer consists of 4 mil or 0.1mm thick unidirectional E-glass fibre cloth that is coated on both the sides with a thin layer of epoxy resin. Composite, c0, has fibres laid in y direction i.e. along the direction of the applied load
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