Study On Compression Properties Of Hat-stiffened Panel Of Carbon Fiber Reinforced Resin Matrix Composite After Impact

Carbon fiber reinforced resin matrix(CFRP)composite hat-stiffened panel can greatly increase the stiffness and axial load transfer efficiency of the structure without increasing the wall thickness,which is widely used in aviation,automobile,wind power and other fields.However,the composite hat-stiffened panels will inevitably suffer from low-speed impact in the use and maintenance process,such as runway gravel,maintenance tools falling,etc.This kind of impact often forms a pit on the surface of the test piece which is not easy to check by visual inspection,but the performance of the test piece has been greatly affected,sometimes the performance drops by as much as 50%,which is prone to accidents in the subsequent use and poses a huge threat to the safety of the component.In addition,considering the long-term exposure of composite materials to extreme environments such as damp and heat,it is necessary to find out the influence of different impact energy and impact locations on impact damage,buckling/post buckling mode,buckling load and final failure load of CFRP hatstiffened panels subject to heat-moisture treatment and not,and to clarify the form and mechanism of damage in components,so as to provide damage tolerance design reference to improve the safety of components.This paper mainly studies the following aspects:(1)Five different energy impact tests and post impact compression tests were carried out for a typical hat-stiffened panel.The typical damage characteristics of specimens under different impact energy were obtained by visual inspection and ultrasonic C-scan.The relationship between impact energy,damage characteristics and residual compressive strength were studied,and the failure mode and strain distribution of the hat-stiffened panel under different impact energy and compress were analyzed.The results showed that the failure of stiffened panel started from the delamination at the impact position,and the final compression failure position was random when the impact energy was 0J and 10 J,and the area of delamination area was large;when the impact energy was greater than 10 J,the failure position was the impacted area and the area of delamination area was small.When the impact energy was 40 J and 50 J,the specimen basically lost the buckling capacity,which led to the sharp reduction of the residual compression load.(2)In view of the random impact positions of the hat-stiffened panel in actual use,the typical impact energy was selected as 20 J,and the three typical positions of the skin edge,the joint of the skin stiffened panel and the middle part of the skin were selected to study the influence of different impact positions on the impact/compression damage characteristics of the hat-stiffened panel.The depth of the pit and the area of the damaged area were measured by ultrasonic C-scan.The effects of impact damage at different locations on the buckling and post buckling behavior of stiffened panels were studied by axial compression experiments.The results showed that the depth of the pit and the area of the damage were different because of the thickness of the specimen and the boundary conditions.The compressive strength of the specimens at point A and point C decreased by 19.51% and 25.83% respectively after impact,and the compressive strength of the specimens at point B decreased by 34.27% after impact,because point B was the joint of skin and stiffener,and the impact damage affected the interface bonding performance.However,it is worth noting that the buckling loads of specimens under different impact positions are not significantly different,which are reduced by about 33%.(3)By comparing with the damage characteristics of stiffened panel at room temperature,the influence of heat-moisture environment on the impact damage characteristics and residual compressive strength was studied.Firstly,the moisture absorption experiment of the stiffened panel was carried out,then compressed the specimen after heat-moisture treatment/low-speed impact.Through the compression test,the stiffness degradation and damage propagation law of the stiffened panel were obtained.The results showed that the pit depth and delamination area of the specimens were reduced after heat-moisture treatment,because the treatment improved the ductility of the resin.The C-scan results showed that many "needle like" delaminations were produced in the delamination area.The buckling and post buckling modes of specimens were not significantly different,mainly including fiber tearing,stiffened panel fracture and delamination at the interface.However,the average decrease of buckling load and failure load is about 10% higher than that in Chapter 3.SEM results showed that the main reason for the degradation of the properties of the specimens is the separation of the fibers and the matrix caused by the humid and hot environment.In this paper,the influence of impact energy/impact location,and heat-moisture environment on the impact damage characteristics and compression buckling/post buckling mode of the hat-stiffened panel were systematically investigated,which provided a reference for the damage tolerance design of the composite hat-stiffened panel,provided data for the "building block" experiment of the hat-stiffened panel,and laid a foundation for promoting the engineering application of the stiffened panel.