The use of composite materials in aerospace engineering is ever increasing. Properties such as low weight, high performance, high stiffness and the ability for it to be tailored specifically for different structural u...The use of composite materials in aerospace engineering is ever increasing. Properties such as low weight, high performance, high stiffness and the ability for it to be tailored specifically for different structural uses has increased its importance in recent years. Specific experimental/numerical activities were developed in the last decade at the aerospace department of Polito and are under consideration in recent years regarding the analysis of fatigue behavior of composite/metallic structural samples under cyclic loading condition. A review of the main significant results connected to three case-studies is presented in the paper. The first is related to the definition and analysis of innovative joint for composite high aspect ratio aircraft structures. The modular design oriented to limit component dimensions for transportability requires the presence of a removable joints able to overcome the typical low bearing failure level of composite pin-loaded joint under static and fatigue loading distribution according to airworthiness requirements. The static experiments demonstrate the consistency of the proposed advanced joint configuration that postpones the joint failure to high load level according to the structural requirements. The cyclic loading phase takes as reference point the failure load of the initial configuration and defines a loading cycle with a maximum higher than the failure load of the initial configuration. The fatigue behavior of the advanced joint also demonstrates a substantial consistency with expected duration of 1 e6 cycles as required by the structural design for operating reasons. Specific indication on fatigue limit with respect to critical amplitude is reported. The second is related to the presence of specific damaged situation in thin-walled fiat/stiffened composite plate. Stiffened configurations are frequently used to increase buckling load level. Unexpected events on solid/stiffened composite panels can introduce a certain level of damage, typically delamination, that can cause reduction in buckling design level and reduction in global strength. The presence of cyclic load and fatigue effect can have an important consequence on damage propagation and structural integrity. The damaged area determined by the skin-stiffener de-bonding of a certain dimension is investigated under static compression and cyclic compression. Local buckling of damaged area is determined and pointed out by tests in uniaxial compression. The experimental static behavior points out the presence of a snap-effect during loading and un-loading. Fatigue loading configuration is applied in the range of post-critical local configuration considered as the most effective situation. Preliminary fatigue results are presented and discussed. The third is related to preliminary investigation on the effect of fatigue life reduction of 2024 AI alloy in corrosive （exfoliation） environment. The effect of corrosion is taken into consideration introducing specific concentration factors into the life estimation relationship. Differences between fatigue in prior corroded specimens and fatigue in presence of corrosive environment are emphasized. No crack propagation is considered. Related concentration factors are derived and compared by the few experimental results in order to define some guidelines for design process and to improve aircraft better evaluation of component structural integrity in operative situations. A preliminary approach is presented in the paper in order to correctly identify the contribution of corrosive environment in coupled fatigue loading phase. The results are discussed and future improvements are suggested.展开更多
文摘The use of composite materials in aerospace engineering is ever increasing. Properties such as low weight, high performance, high stiffness and the ability for it to be tailored specifically for different structural uses has increased its importance in recent years. Specific experimental/numerical activities were developed in the last decade at the aerospace department of Polito and are under consideration in recent years regarding the analysis of fatigue behavior of composite/metallic structural samples under cyclic loading condition. A review of the main significant results connected to three case-studies is presented in the paper. The first is related to the definition and analysis of innovative joint for composite high aspect ratio aircraft structures. The modular design oriented to limit component dimensions for transportability requires the presence of a removable joints able to overcome the typical low bearing failure level of composite pin-loaded joint under static and fatigue loading distribution according to airworthiness requirements. The static experiments demonstrate the consistency of the proposed advanced joint configuration that postpones the joint failure to high load level according to the structural requirements. The cyclic loading phase takes as reference point the failure load of the initial configuration and defines a loading cycle with a maximum higher than the failure load of the initial configuration. The fatigue behavior of the advanced joint also demonstrates a substantial consistency with expected duration of 1 e6 cycles as required by the structural design for operating reasons. Specific indication on fatigue limit with respect to critical amplitude is reported. The second is related to the presence of specific damaged situation in thin-walled fiat/stiffened composite plate. Stiffened configurations are frequently used to increase buckling load level. Unexpected events on solid/stiffened composite panels can introduce a certain level of damage, typically delamination, that can cause reduction in buckling design level and reduction in global strength. The presence of cyclic load and fatigue effect can have an important consequence on damage propagation and structural integrity. The damaged area determined by the skin-stiffener de-bonding of a certain dimension is investigated under static compression and cyclic compression. Local buckling of damaged area is determined and pointed out by tests in uniaxial compression. The experimental static behavior points out the presence of a snap-effect during loading and un-loading. Fatigue loading configuration is applied in the range of post-critical local configuration considered as the most effective situation. Preliminary fatigue results are presented and discussed. The third is related to preliminary investigation on the effect of fatigue life reduction of 2024 AI alloy in corrosive （exfoliation） environment. The effect of corrosion is taken into consideration introducing specific concentration factors into the life estimation relationship. Differences between fatigue in prior corroded specimens and fatigue in presence of corrosive environment are emphasized. No crack propagation is considered. Related concentration factors are derived and compared by the few experimental results in order to define some guidelines for design process and to improve aircraft better evaluation of component structural integrity in operative situations. A preliminary approach is presented in the paper in order to correctly identify the contribution of corrosive environment in coupled fatigue loading phase. The results are discussed and future improvements are suggested.
