Mechanical Performance Analysis On Scarf-patch Repaired Foam Core Sandwich Panel Of SR20 Airplane

The three-dimensional scarf-patch repair finite element theoretical models and engineering models are established based on the composite foam core sandwich structure panel of SR20 airplane. The convergence of the models are analyzed and the effective mesh density is given. The rational finite element meshes are created according to the criterion that the element’s Jacques ratio is greater than 0.7. The logical boundary condition is given.Stress and strength analysis of the theoretical and engineering models of composite foam core sandwich structures with damage reaches to the maximum size limit of the manual regulation is carried out. The material principle direction stresses distributions of the sandwich structure with no defects and repaired structure under uniaxial tensile loading, twin-axial tensile loading and pure shear loading are given. The strengths of the intact and repaired structures are calculated based on maximum stress criterion. It is shown that, the highly stressed areas with highest stiffness gradient are the overlapping regions of the motherboard and the repair patches. The extreme stress values appear on the points near the boundary between the matherboard and repair patches. The secondary damage of the theoretical model with non-penetrating damage occurs on the surface layer which has been repaired. Due to the symmetry of the structure, the external load and the repairing layers, the secondary damage of the theoretical model with penetrating damage may occur on the either side of the surface layers. The surface layers locate right on the boundary of the repair patchs, where grave stress concentration occurs due to grave geometry mutations which causes local stiffness increase of the repaired area. The secondary damage of the engineering models with non-penetrating or penetrating damage both occurs on the surface layers without geometric twist. These surface layers can transmit the tangential load on the boundary well and play a major role in bearing the external load. The structural discontinuity at the border of surface layer causes grave stress concentration. For the repaired engineering model with non-penetrating damage, the strength recovery coefficient is 88.63% under uniaxial tensile loading,96.55% under twin-axial tensile loading, and 92.98% under pure-shear loading. For that with penetrating damage, the strength recovery coefficient is 86.67% under uniaxial tensile loading,96.00% under twin-axial tensile loading, and 91.58% under pure-shear loading.The stress concentration caused by the structural stiffness increasing of the repaired area is the main reason for the strength decline after repairing. The initial damage mode is not the determinant for the strength of the repaired structure. The strength of the repaired structure decreases with the increase of the number of additional surface patches. Additional surface patches can reduce the stiffness gradient and increase the anti-stripping capacity of the repaired area which in return will improve the maintenance quality. One or two additional surface patches are recommended.Stress arid strength analysis of the engineering model of composite foam core sandwich structure with damages exceeding 20% and 50% of the maximum damage limit according to the manual is carried out. The secondary damages of the engineering models with non-penetrating or penetrating damage both occur on the surface layers. The damage mode and the failure position are consistent with that of the engineering model on which maximum damage allowed by the manual regulation occurs. In case of the engineering model with non-penetranting damage exceeding 20% of the maximum damage limit of the manual, the strength recovery coefficient is 89.45%under uniaxial tensile loading,96.55% under twin-axial tensile loading, and 94.43% under pure-shear loading. For the engineering model with penetrating damage exceeding 20% of the maximum damage limit of the manual, the strength recovery coefficient is 88.23% under uniaxial tensile loading,96.27% under twin-axial tensile loading, and 92.04% under pure-shear loading. In case of the engineering model with non-penetrating damage exceeding 50% of the maximum damage limit of the manual, the strength recovery coefficient is 90.28% under uniaxial tensile loading,96.83% under twin-axial tensile loading, and 94.43% under pure-shear loading. For the engineering model with penetrating damage exceeding 50% of the maximum damage limit of the manual, the strength recovery coefficient is 88.23% under uniaxial tensile loading,96.55% under twin-axial tensile loading, and 92.98% under pure-shear loading.Compared with the engineering model with maximum damage allowed by the manual, the engineering model with damage exceeding 50% of the maximum damage limit gains a slightly higher strength recovery coefficient. The expanding of the damage do not cause decline of the repaired structure strength. The main reason for the strength decrease of the repaired structure, the stress distribution without using additional surface patches, and the variation of the stress distribution with the number of the additional surface patches are the same with that of the engineering model with maximum damage allowed by the manual. Additional surface patches can reduce the stiffness gradient of the repair area and improve the repair quality. The composite foam core sandwich structure with 20% or 50% bigger size damage than the maximum damage limit mentioned in the manual can be repaired well. Half-thickness or low elastic module repairing layers can be used to repair the damages of sandwich foam core panel, which can reduce the local structure stiffness of the repaired area and improve the strength after repairing.