Failure Behavior Of Coupled Force And Environment Of CFRP Composites With Holes

Epoxy-based carbon fiber composite materials are susceptible to external shocks and humid/heat environments during actual use.The common epoxy resin matrix is exposed to the humid/hot environment for a long time,its interface property will change.After the laminate structure is prepared,the surface and internal of the laminate are easy to cause damage,which leads to a reduction in the life of composite materials in different aircraft structures,when the resin reacts with the external environment and undergoes physical/chemical changes.However,the internal damage of composite materials is often difficult to find,and it needs to be characterized with non-destructive testing equipment,which leads to the use of composite materials with damage.Under different external environmental conditions,the impact resistance of T700 epoxy carbon fiber composite materials for aviation,the change of compressive residual strength after impact and the failure mechanism are studied.The research results are of great significance to the post-impact compression performance of composite materials in changing environments like epoxy resins in humid and hot environments and the expansion of the application range of aircraft.In the design of composite material openings,based on international standards and engineering applications,for different fastener hole arrangements,the stress and damage distribution of various layers under low-velocity impacts of different designs are studied.And combined with the FEM software ABAQUS to evaluate the damage degree based on the Hashin criterion,to explore the impact of different double hole design like spacing and arrangement angle on the impact resistance of laminates.The results show that when the impact energy is the same,the stress concentration at the edge of the hole and the depth of the pit are mainly affected by the distance between double holes.The distance is larger,the stress concentration and the depth of the pit are smaller.The two-hole arrangement angle affects the fiber damage of the surface and the bottom layer.When the arrangement angle is the same as the fiber layer angle,damage degree and area are the largest.In terms of single environmental,the effects of temperatures and time on the compressive strength after impact in a water bath and a high-temperature environment are studied.Analyze the surface strain distribution and changes of laminates with impact damage during the postimpact compression experiment through Digital Image Correlation(DIC);combine DIC technology and static strain gauges to specify strain in the post-impact compression process.The results were evaluated.Scanning Electron Microscope(SEM)is used to compare and analyze the morphology characteristics of the laminates after compression failure with environmental damage.The results show that a longer time of water bath has little effect on the properties of the composite material,while the epoxy resin affects the interfacial performance after absorbing water and swelling,and the resin and the fiber are easier to separate at the fracture position.In the high temperature environment,part of the epoxy resin is oxidized and redistributed,meanwhile unoxidized resin part has good interface bonding.However,as the time of processing increases,the mechanical properties of the composite laminates decrease linearly.At the same time,the strain data collected by DIC technology is more stable than static strain gauge and the frequency is more controllable,which means that DIC has the possibility of replacing strain gauges in experiment.In terms of force and environment damage,the effects of different environmental types and orders on the compression performance after impact are studied,and the damage mechanism is analyzed based on the results of non-destructive testing and microscopic morphology.The results show that the microscopic morphology contains multiple characteristics of environmental treatments,and the compressive mechanical properties of laminates after impact are mainly determined by the second treatment method.The order of environmental treatment will affect the mechanical properties and micro-morphology.When treatment time is same,adding high temperature treatment before water bath will reduce the performance compared with pure water bath treatment.Adding water bath before high temperature treatment will improve the performance compared to pure high temperature treatment.