Investigation On The Influence Of Low-velocity Impact Loading On The Mechanical Properties Of Composite T-joints

Composite T-joint,as a typical composite integral structural component,is widely used in aircraft,boats,automobiles and other structures.Because of its complex damage mechanism and failure mode,and the complexity of molding process;Moreover,the composite structure is vulnerable to low-velocity impact in the process of manufacturing and use,which will significantly affect the residual strength of the structure.The strength of T-joints under impact load is the key problem of its whole structure during its service,so it is very necessary to study the performance strength of composite T-joint under impact load in order to ensure the safety of the whole structure.In this investigation,the composite T-joints(with or without doped barbed wire)and pure web strips were prepared by vacuum-assisted resin infusion molding(VARI)process.In order to investigate the effect of low-velocity impact loading on the mechanical properties of the T-joints,tests such as low-velocity impact test,static compression test and tensile test after low-velocity impact(LVI)were carried out.The specific studies are as follows:(1)The mechanical properties of composite T-joints under low-speed impact were experimentally studied.By referring to ASTM-D 7136 standard,the process of low-velocity impact(LVI)is simulated by using a drop hammer impact test.The T-joints and pure webs are impacted by five energy levels,namely,15 j,20j,25 j,30j and 35 j,respectively,and observed by camera and scanned by SEM.The results show that there are obvious differences in structural performance between T-joint and pure web strip during low-speed impact,and the damage forms of the two are also different.The damage of the triangular filling area in the middle of T-joint and the impact contact area in the middle of pure web strip is mainly caused by the fracture of glass fiber and the cracking of resin matrix,while the damage of the extruded edge of fixture is mainly caused by the transverse fracture of glass fiber around it caused by the mutual extrusion between the fixture edge and specimen during the impact.In addition,the comparison between T-joint(TS0)with undoped wire mesh and T-joint(TS40)with doped wire mesh is analyzed,and it is concluded that doping wire mesh in T-joint can not improve the impact resistance of the structure,but can improve the ductility of the structure itself.(2)The damage evolution law of glass fiber T-joint under static compression is studied.According to GB/T1024.4-2006 standard,the static compression test is to better explain the failure mechanism of low-velocity impact damage,and the restrictive conditions adopted in the static compression process are basically consistent with the low-speed impact test to ensure the authenticity and accuracy of the test.By analyzing the static compression damage results of T-joint,it is concluded that with the development of compression process,vertical cracks and horizontal cracks propagate along the middle interface of web,and the crack length is positively correlated with compression time until the specimen is completely extruded and even the whole structure breaks.The failure evolution and curve changes of TS0 and TS40 during static compression are analyzed and recorded,and it is concluded that the compression resistance of the structure can be greatly improved by adding barbed wire between T-joints,which is about 36.6%.(3)In order to further study the mechanical properties of composite T-joint under low-velocity impact load,the tensile test after LVI loading was carried out,and the influence on the tensile residual strength after LVI loading was analyzed.The damage of composite T-joint during the tensile process after impact and the changes of tensile mechanical curves of the specimen under five energy levels were observed by two-dimensional digital image(VIC-2D)technology.The results show that with the increase of impact energy,the tensile strength of T-joint structure decreases gradually,and the structural cohesion may be reduced by doping barbed wire in T-joint specimens.However,the tensile strength of the whole structure is obviously improved,and the maximum increase is 24.33% when the impact energy is 20 J.From the tensile strain nephogram displayed in the two-dimensional digital image(VIC-2D),it is found that the damage of the filling area only reduces the bearing capacity of the T-joint,and the real reason for the failure of the joint is the peeling of the adhesive layer.Then,in order to improve the strength of GFRP-T joint,a layer of stainless steel wire mesh(SSWN)is inserted between the layers of T joint structure.The test results show that the addition of SSWN reduces the delamination failure area of the web and connects the adjacent GFRP layers,thus avoiding the sudden failure of the structure and improving its tensile properties after LVI.