Tensile Failure Behavior of Glass Fiber Reinforced Aluminum Laminate with Defects
Author:Zhang Ji Peng
Supervisor:zhang jia zhen
The successful application of glass fiber reinforced aluminum laminate(Glare)in the primary structures of A380 aircraft has shown its potential for acting as the prime alternative of next-generation aeronautic materials.It should be aware that defects(or damages)are usually inevitable in aircraft during the design and service periods,including the holes that for mechanical connections and the cracks that induced by foreign object impacts.Thus,in view of the damage tolerance design concept,it will be of great engineering and scientific significances to investigate the failure behavior of Glare laminates with defects.A few investigations have been carried out on this aspect,but due to its complexity,some key issues have not been reasonably solved.For example,problems like the failure mechanism of open-hole Glare laminate under off-axis tensile load and the coupling effects between the off-axis angle and geometry dimensions,as well as the effects of metal thickness and volume fraction on the stable crack extension behavior of Glare laminates with cracks,have not been identified.To this point,experimental and numerical investigations were carried out on these aspects in present thesis,which will provide some reasonable technique supports for the application and damage tolerance design of Glare laminate.In order to identify the failure mechanism of open-hole Glare laminate under off-axis tensile load and to elaborate the variations of its mechanical responses with off-axis angle,tensile tests were carried out on open-hole Glare laminates under different off-axis angles,as well as finite element models with considering the thermal residual stress were built.The experimental results and numerical results based on the damage cloud charts and quantity statics of damaged elements revealed that the subcritical damage in open-hole Glare laminate was intensified by the off-axis tensile load and it was benefit for the retarding the fiber breakage.In particular,the failure of open-hole Glare laminate under off-axis tensile load was still tensile dominated though the tensile-shear coupling stress was imposed,but it was controlled by the aluminum.In addition,the mechanical responses and damage behavior open-hole Glare laminate were illustrated more intuitively from the energy dissipation point of view,which was inspired by the consideration that failure behavior of Glare laminate would be closely associated with the contributions of aluminum and glass fiber reinforced plastic(GFRP)layers.In view of the significant scaling effects in open-hole composite laminates,off-axis tensile tests were performed on the open-hole Glare laminates under constant specimen width and constant hole diameter to specimen width ratio conditions.The effects of hole diameter and specimen dimension on open-hole strength and open-hole sensitivity of Glare laminate were investigated,and particularly the coupling relationships between them and the off-axis angle were emphatically discussed.On basis of the failure mechanism of open-hole Glare laminate under off-axis tensile load,the coupling effect between the hole diameter and off-axis angle under constant width condition was elaborated from the point views of modulus retention and strain increase.The coupling effect between specimen dimension and off-axis angle under constant hole diameter to specimen width ratio condition was also associated with these two factors,moreover numerical analyses on internal energy,elastic(plastic)energy and damage dissipated energy of the constituents provided sufficient evidences for revealing this coupling effect as well.At present,the effects of aluminum sheet thickness and constituent volume fraction on the fracture behavior of Glare laminate with crack have not been clearly identified.Thus,six kinds of Glare laminates with different configurations were designed and tested under quasi-static tensile load in present thesis,based on which the effects of aluminum thickness and constituent volume fraction on the crack resistance and residual strength were specified separately,and particularly their influences on the stable crack extension behavior were investigated.The effect of aluminum thickness mainly lied on its fracture toughness,while the effect of constituent volume fraction was closely related with the delayed fracture characteristics of GFRP layers,which was drawn from the damage process analysis.On basis of the experiments,a method for predicting the residual strength of Glare laminate with crack was proposed,in which the crack opening displacement over an 5mm gauge length at the original crack tip(?)was selected as the fracture parameter.This method was established based on the structural integrity assessment procedure,Tsai-Paris method and the superposition principle,and it was validated to predict well for the experimental results.On basis of the above investigations,a titanium in-situ reinforced Glare laminate(TR-Glare)was proposed by substituting the internal aluminum layer in standard Glare laminate with titanium,which behaved a higher elastic modulus and yield strength.Its purpose was to improve the damage tolerance behavior of standard Glare laminate,and meanwhile to guarantee the light weight demand.Experiments and numerical energy analyses on the open-hole TR-Glare laminates revealed that the reinforcing effect of titanium was closely associated with its load bearing efficiency and its function for retarding the fiber breakage.These two reinforcing mechanisms were also embodied in the TR-Glare laminate with crack.Moreover,its residual strength was also well predicted by improving the method that proposed for the standard Glare laminate.