| The development and manufacturing of large aircraft brings forward the impending needs for high-strength and lightweight advanced composite materials.Glare(Glass Reinforced Aluminum laminates)has been treated as the crucial structural materials for fuselage and wing skin owing to its excellent fatigue,good damage tolerance as well as the outstanding contributions to weight reduction.However,Glare shows complicated failure mechanism and difficulty in forming.Meanwhile,the insufficient stiffness limits its further application.The thesis used the high damage-tolerance aluminum-lithium alloy,replacing the 2024-T3 aluminum alloy always adopted in conventional Glare,to develop the novel glass fiber/ aluminum-lithium super hybrid laminates(NFMLs).The aim was to improve the stiffness of the laminates.Meanwhile,the adhesive content,overall performance as well as the shot peen forming of NFMLs were investigated to reveal the failure mechanism and obtain the desired forming method,which could provide the important theoretical basis and engineering guidance for the use of NFMLs in large aircrafts.Firstly,the novel aluminum-lithium layer(0.3mm)in T3 state was successfully obtained through the investigations on the preparation process and age strengthening.Secondly,the NFMLs were prepared.The optimization of the adhesive content was mainly discussed,which was the most essential issue during the fabrication process.Then,the reinforcement effect of aluminum-lithium alloy on the NFMLs was verified.The influence of high-low temperature,hygrothermal aging and thermal fatigue on the mechanical properties of NFMLs was also revealed.Finally,the thesis focused on the shot peen formability of NFMLs after investigating the self-forming characteristic of its leading edge structure.The forming features and the deformation rule of NFMLs were studied.The main conclusions were as follows:1)The 0.3mm thickness aluminum-lithium-T3 layer,according with requirement of NFMLs preparation,was obtained through the process of “annealing-rolling-solution-quenching-aging”.Then,the single-stage aging and pre-deformation + aging of NFMLs was respectively investigated.The results indicated that the successfully prepared metal layer strengthened by large amounts of dispersed δ’(Al3Li)phases.It possessed a tensile strength of 479.12 MPa and a modulus of 82.50 GPa,exhibiting better performance than the 2024-T3 alloy used in conventional Glare.Aluminum-lithium-T3 layer had a fast aging response,and T1(Al2Cu Li)phases dominated the age strengthening.The preferable match of strength,plasticity and toughness of the alloy could be achieved when amounts of T1 IV precipitated with the diameter less than 100 nm.A mass of dislocation,introduced by the pre-deformation behavior,promoted the nucleation of the secondary phases.The high nucleation rate in turn increased the quantities and avoided the coarsening of the precipitates.A pre-deformation within 5% was beneficial to the strengthening and toughening of the aluminum-lithium alloy.2)The thesis investigated the preparation process of NFMLs.Also,the effect of adhesive contents(0g/m2,20g/m2,40g/m2,60g/m2)on the mechanical properties and failure behavior of the laminates was discussed emphatically.The results demonstrated that the obtained NFMLs after milling had no internal defect.Meanwhile,the increasing adhesive content enhanced the interfacial bonding while also led to the thickening of the laminates.The two contradictory factors above dominated the mechanical properties and the failure behavior of NFMLs.The further adhesive addition was apt to deteriorate the fatigue properties of the laminates.The reasons were as follows: on one hand,the fatigue behavior in the dynamic load is more sensitive to the applied stress level.On the other hand,the thickening adhesive layer had low stiffness and high toughness,which weakened the fiber bridging effect and promoted the delamination growth of the NFMLs.Overall,the adhesive content of 40g/m2 is optimal to achieve the excellent performance of NFMLs.3)During the investigations on the mechanical properties of NFMLs,the reinforcement effect of aluminum-lithium alloy in NFMLs was primarily explored.Meanwhile,the effect of high-low temperature(ranging from-55℃ to 120℃)on the properties of NFMLs was studied.Moreover,the thesis also focused on the performance characteristic of NFMLs respectively treated by 70℃/85%RH hygrothermal aging and-65℃~135℃ thermal fatigue.The results indicated that The small-sized T1 phases(diameter < 25nm)generated in the matrix,leading to the age strengthening of the aluminum-lithium alloy.The NFMLs reinforced by aluminum-lithium layer presented obviously improved fatigue and increasing stiffness(8%~12%),when comparing with the conventional Glare.The mechanical properties of NFMLs maintain well within-55℃~70℃ but deteriorated at 120℃.Moreover,the mechanical properties also decreased with the hygrothermal aging.But the laminates still exhibits excellent properties after 70℃/85%RH,3000 h treatment.The residual stress hardly resulted in the performance degradation of NFMLs even exposed to 1000 cycles during the-65℃~135℃ thermal fatigue.The strength of the FMLs even increased with the thermal cycle owing to the positive age hardening behavior of aluminum-lithium layer.The homogeneous and dispersed T1 phases precipitated in quantity,owing to the cyclic phase transition in “initiation-termination” mode.This special strengthening treatment even presented more desired strengthening and toughening effect than traditional aging.4)Based on the large curvature leading edge component,the self-forming characteristic of NFMLs was studied.The results demonstrated that self-forming technology was feasible to manufacture the large curvature NFMLs component such as the leading edge parts.The investigated component was detected no obvious interlaminar defect,presenting good fittability and thickness uniformity.Meanwhile,the forming process did not result in the generation of large residual stress.Even so,the visible spring-back still occurred within one week after forming.5)The studies including the process selection and optimization,deformation characteristics analysis,and the effect of shot peen on the mechanical properties had been performed to reveal the shot peen formability of NFML.The results indicated that shot peen forming was suitable to manufacture the NFMLs component.Good forming effects could be achieved using the small-sized(diameter≤0.425mm)ceramic ball.Also,fiber break and metal/fiber interfacial failure were easy to occur near the shot peened surface when applied with excessive intensity or oversized ball.The forming curvature of 3/2 cross-plied NFMLs had been reduced to 165.24 mm successfully using AZB425 ball under the intensity of 0.193 A.For the deformation characteristics,NFMLs was inclined to deform perpendicular to fiber direction.Meanwhile,plastic deformation was only produced in the shot peened metal layer,while elastic deformation in other layers.Furthermore,comparing with metal materials,NFMLs possessed similar deformation rules following the variation of shot peen intensity and coverage.Besides,shot peeing significantly changed the residual stress state of NFMLs,and the stress relaxation was negligible without the external force or thermal field.The shot peening slightly increased the the strength of the laminates owing to the work hardening of the metal layer.Also,the residual stress field introduced by shot peening decreased the amplitude of stress intensity,leading to the lower delamination propagation rate of fiber layer and generation of crack closure effect.This internal mechanism greatly improved the fatigue properties of NFMLs. |
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