Abstract:

Objective To meet the requirements of the beam structure on the wing for the main load-bearing components, a bending-shear coupling device which can verify the different ratio of flexural shear coupling loads is designed. The experimental device can verify the actual bearing characteristics of the large-size beam structure. It is benefited to provide reference data for the prediction of wing load performance and structural design. It hopes that the research can optimize the residual bearing strength of large-size C-beams in the field of aerospace.

Method The large-size C-beams were manufactured by medium-mold high-strength carbon fiber T800 and epoxy resin. The nominal thickness of the cured single layer was 0.191 mm. The rib dummy is made of aluminum alloy T7451 and the fastener brand is titanium alloy bolt. The designed bending-shear coupling device consists of an upper wall fixing devices, a loading fixtures as well as an anti-twist device. Anti-twist devices are provided on both sides of the loading fixture to prevent the C-beam twisting during bend loading increment. The finite element models were established by ABAQUS/Explicit to explore the damage evolution during post-buckling failure.

Results The influence of bending-shear coupling conditions of large-size C-beams on mechanical properties and post-bulking behavior were comprehensively investigated for novel applications.The results show that three different design limit load (DLL) could been added to evaluate the bending-shear behaviors of large-size C-beams. The wall fixing devices is mounted on the load-bearing wall, and the two ends of the test specimen are respectively connected with the upper wall fixing devices and the loading fixture. There are two parallel and spaced loading devices under the loading fixture. Through the strain gauges distribution, the deformation and torsion of the large-size C-beams have been monitored. In order to verify the validation of the experiments, the symmetry of the top and bottom flange and web appear excellent as well. Under pure bending load of the large-size C-beams, the higher load is borne by the flange. The beam web becomes the main load-bearing component under the action of bending-shear coupling load. The values of 3.28DLL and 2.3DLL are considered as buckling critical loads of large-size C-beams. In post-buckling stage, the strain gauge data on the specimens was reduced to varying degrees, but the specimens were not destroyed with loading to the maximum load. It illustrates that the remaining structural stiffness and strength are still sufficient to support the continued bearing of the structure. The fluctuation of load-strain curve is formed by matrix cracking, fiber buckling or interlayer failure. The damage started to expand outward along the 45° direction with the Hashin criterion. The damage of large-size C-beams is formed by matrix compress cracking, fiber compress failure. Meanwhile, the failure form was twisted collapse of the top flange with the LaRC05 criterion. The matrix cracking, fiber tensile failure are composed of the main failure modes.

Conclusion A novel bending-shear coupling loading device was proposed and designed to effectively prevent stress concentration and torsion caused by the eccentricity of the C-beam during loading while adjusting the bending-shear ratio. The post-buckling behavior and failure prediction of large-size composite C-beam under bending-shear coupling load was studied by experiment. The large-size C-beam exhibited a high post-buckling load-bearing capacity regardless of its bending resistance or shear resistance. During the experiment, the carbon fiber reinforced composite beam webs reached the first order buckling mode at the loading of 3.28DLL in Bending-shear 1 and 2.3DLL in bending-shear 2 respectively. In addition, the catastrophic failure of composite beam web was not observed until the load percentage reached 4.6DLL. The damage started to expand outward along the 45° direction with the Hashin criterion. Meanwhile, the failure form was twisted collapse of the top flange with the LaRC05 criterion. This study can offer a practical reference for the post-buckling performance of large-size C-beam structures after bending-shear coupling.

Key words: large-size C-beam, bending-shear coupling, strain gauge, post-buckling, failure prediction