Fibre metal laminate (FML) is an advanced hybrid material system that consists of layers of thin and light metallic sheets 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. In comparison with monolithic metals and their alloys, FML’s besides offering good specific strength exhibit excellent fatigue and fracture resistance, reasonably good impact strength and high fire resistance that make them a good substitute for metallic structures especially in aerospace sector. In comparison with conventional fibre-resin composites or FRP’s, they offer simpler production and maintenance methods, easy inspection procedures during service and reduced environmental degradation. Glare, a type of FML, comprising thin 2014-T6 aerospace aluminium alloy sheets and E-glass fibre and epoxy resin based composite prepregs, is investigated in this book. Two types of cracked Glare can exist, Type I in which the Mode I crack in aluminum layers grows normally towards the prepreg. interfaces and Type II in which the Mode I crack grows across the aluminum layers.
Hitherto, fatigue and fracture analysis has commonly been reported over Type II FMLs. But there also lies the possibility of development of small, Mode I, surface cracks, in thin aluminum layers that grow normally toward the interfaces of pre-pregs in the laminate (Type I). In such a case, the redistribution of stress state occurs in material layers during crack growth which causes the near crack tip stress fields and energy release rates to differ from that can exist in similarly stressed and cracked homogenous aluminum (without interfaces). Therefore, energy state at the crack tip in aluminum layer of the laminate, as it approaches the interface of pre-preg, is not same as that at the crack tip in homogenous aluminum due to the difference in elasticity properties between parent aluminum and influencing layers on other side of the interface in the laminate. Stresses are further re-distributed with the onset of the effect of plasticity mismatch between materials as the crack reaches the close vicinity of the interface. Crack growth rate finally drops sharply when the crack touches the interface of resin in pre-preg due to the reduction in stress field at the stiff-compliant interface. The crack upon further growth in resin finally deflects along the interface of fiber since the crack can not penetrate the fiber and continues to grow as Mode II crack at the interface. In Type II Glare, Mode I cracks nucleate and grow in weak aluminium layers whereas strong fibres in prepregs remain intact when the laminate is pressed into service. The cracks are shielded due to load bridging over them, the phenomenon commonly known as fibre bridging. Bridging diverts or transfers load away from the cracks towards fibres that in turn reduces the intensity of stress fields around crack tips in aluminium layers. This results in enhancement of fracture toughness of Glare vis-à-vis plain aerospace aluminium alloy specimen in which the load transfer effects do not take place.
The primary purpose of the book is to present the models, both theoretical and numerical, for investigation of the mechanisms of crack tip stress amplification, shielding and fiber bridging in Type I and Type II fiber metal laminates under the influence of residual stresses that are generated due to curing during fabrication of the laminates. The models are well validated by experiments.
The book comprises four chapters – Chapter A to Chapter D. Chapter A covers fundamental details of Glare that includes constitution of Glare, basic formula applicable to laminates, stress-strain constitutive equations and exposure to investigated Mode I cracked laminates. Chapter B presents theoretical and finite element models of Glare that includes assumed dimensional details of Glare, laminate data, load and stress-strain data, theoretical fracture models of laminates, finite element analysis, results from the models and salient findings. Chapter C covers experimental work with actual dimensional details of fabricated Glare, laminate data, actual load and stress-strain data, laminate fabrication procedure, property evaluation of laminate, fatigue and fracture tests and their results, comparison of laminate performance with plain aluminum alloy specimens and salient findings. Finally, Chapter D deals with validation of theoretical and finite element models that includes approach, modification of models, results, comparative analysis and salient findings.
