Experiment and failure mechanism of composite fuselage curved panel under circumferential bending load
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摘要: 对复合材料机身曲板进行了环向弯曲加载试验,采用四点弯加载方式对考核段进行纯弯加载,设计一种加强连接方式避免加载段提前破坏,通过试验对机身曲板的环向稳定性和破坏模式进行了分析。同时,建立了基于内聚力单元的考虑长桁与蒙皮粘接界面损伤的有限元模型,分别使用Quads准则和Hashin准则作为界面和层合板的失效判据分析曲板结构的失效机制,计算结果与试验结果吻合较好。试验及有限元分析结果表明,长桁帽底蒙皮的局部屈曲引起长桁与蒙皮粘接的R区出现初始开裂,并最终扩展为长桁脱粘。随着蒙皮屈曲及长桁脱粘的扩大,蒙皮由局部屈曲变为整体失稳而失去承载能力,最终导致隔框承载过大而发生断裂。根据初始损伤模式,采取了长桁帽内全包工艺改进设计,改进后的曲板结构稳定性和承载能力分别提高了21.9%和16.8%。Abstract: The failure mechanism of the composite fuselage curved panel under circumferential bending load was studied by four-point bending test and finite element analysis (FEA). A set of strengthening and connecting fixture was designed to avoid undesirable failure. A finite element model was also established, in which cohesive element was used to simulate the interface between the hat stringer and skin. Quads criterion and Hashin criterion were used as failure criteria of the interface and the laminate respectively. The results obtained by experiment and FEA are in good agreement. It can be concluded the crack initiates from the R-zone of the bonding area between the hat stringer and skin, due to local buckling of the skin at the bottom of the hat stringer. Then it extends and causes debonding of the hat stringers. With the extension of skin buckling and stringer debonding, the skin shows global instability and loses its bearing capacity, which eventually leads to the failure of the frame due to excessive load. According to the initial damage mode, the full around bonding process between the hat stringer and skin is adopted to improve the circumferential stability of the curved composite panel. The bending test result shows that the bending loads of the initial buckling and overall failure increase 21.9%and 16.8%, respectively.
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图 8 Cohesive内聚力单元本构关系
Figure 8. Constitutive relation of cohesive element
KNN —Normal stiffness; KSS—Longitudinal shear stiff-ness; KTT—Transvers shear stiffness; Nmax—Tensile strength; Smax— Longitudinal shear strength; Tmax—Transvers shear strength; $\delta _{\rm{n}}^{\rm{0}}$—Original relative displacement; $\delta _{\rm{n}}^{\rm{f}} $—Relative displacement at failure; Gc— Equivalent fracture toughness
表 1 M21E碳纤维/IMA环氧树脂复合材料性能
Table 1. Material properties of M21E carbon fiber/IMA epoxy resin composite
Elastic property Value Strength property Value E11/GPa 154 XT/MPa 2 610 E22/GPa 8.5 XC/MPa 1 450 G12/GPa 4.2 YT/MPa 55 ν12 0.35 YC/MPa 285 S12/MPa 105 Notes: E11—Longitudinal tensile modulus; E22—Transverse tensile modulus; G12—Shear modulus; ν12—Poisson’s ratio; XT—Longitudinal tensile strength; XC—Longitudinal compressive strength; YT—Transverse tensile strength; YC—Transverse compressive strength; S12—Shear strength. 表 2 M21E碳纤维/IMA环氧树脂复合材料试件各部件铺层顺序
Table 2. Layup orientation of M21E carbon fiber/IMA epoxy resin composite parts
Part Patch layup Skin [45/−45/−45/90/45/0]s Shear-clip [45/−45/0/90/−45/45/0/0/45/−45/90/0/−45/45] Stringer [45/0/0/−45/90/−45/0/0/45] Frame [45/−45/0/90/45/−45/90/0/−45/45] 表 3 M21E碳纤维/IMA环氧树脂复合材料机身曲板界面性能参数
Table 3. Material properties of adhesive of M21E carbon fiber/IMA epoxy resin composite fuselage curved panel
Parameter Value Interface strength /MPa Nmax Smax Tmax 40 65 65 Fracture Toughness /(kJ·mm−2) GⅠC GⅡC GⅢC 0.37 0.6 0.6 Elastic constant /(GPa·mm−1) KNN KSS KTT 4 324 7 027 7 027 -
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