Investigation On The Mechanical Behaviour Of Gusset Joints Of Flat Aluminum Alloy Lattice Structures

Gusset joints is one of the most important connection forms of single-layer aluminum alloy lattice structures, and mainly applied to single-layer reticulated shell structures which are based on membrane internal force. Currently, the research of such joints on mechanical behavior and design method is still very limited. In recent years, gusset joints are applied to flat aluminum alloy lattice roof structures based on bending -predominant, which make it urgent to research the mechanical behavior and bearing capacity of gusset-type joints. For gusset joints of flat aluminum alloy lattice roof structures in this dissertation, experimental investigation and numerical analysis combined with practical engineering application are carried out, focusing on investigating the working mechanism, finite element simulation method, mechanical behavior and failure modes of such joints. The main work of this dissertation includes the following several aspects:(1) Mechanical parameters of the 6061-T6 aluminum and the 304HC stainless steel lockbolt are obtained by tensile tests and the pretension in lockbolt is also determined. The failure modes of 12 lockbolted lap connection of aluminum alloy plates divided into 4 groups are obtained by shear tests, including the shearing failure of lockbolt, hole wall bearing failure of plate, end tearing failure of plate and the side tearing failure of plate. There are four characteristic stages in the load-displacement curves of lockbolted lap connection under shear loading, which consists of friction segment, slip segment, bearing segment and strengthening segment. It indicates that the terminal value of the elastic stage among load-displacement curves can be adopted as the design bearing capacity of the connection. In this case, the ratio of the ultimate bearing capacity to the design bearing capacity is about 2.3, and this lap connection is belong to pressure-bearing type connection.(2) By means of the FE software ABAQUS, the method for numerically simulating the lockbolted lap connection of aluminum alloy plates is studied on the basis of shear test. Compared with the test results, the effect of solid element type, meshing density and friction coefficients of contact surface on the accuracy of numeric model is investigated. The coefficient of friction between aluminum alloy plates and that of the aluminum alloy plate and stainless steel lockbolts can be set as 0.25 and 0.15 respectively, which is obtained through curve fitting. Adopting the incompatible solid element is more effective to narrow the gap between the numerical and experimental results than simply increasing the mesh density. Furthermore, experimental tests and numerical calculations are carried out for two double lockbolted lap connections. The load-displacement curve, macroscopic deformation and strain of each measuring point of numeric models fit well with those of specimens. It reveals that the accuracy and validity of the method proposed in this paper for simulating this kind of lap connections.(3) According to the practical engineering application, two full scaled test pieces of gusset joint are tested and analyzed by FE method. During loading processes, the development of deformation and strain as well as their damage form are studied, which shows that the friction between I-shaped aluminum flange and cover plate is overcome prematurely and then a clear slippage appear due to the limitation of the pretightening force in lockbolts. The sliding curve obtained from the test presents three stages the fiction stage, sliding stage and crushing stage, and the gap between rod and hole is equivalent to the sliding distance. Then FE models of gusset joint based on the lockbolted lap connection are established to simulate the loading processes of two test pieces. It is discovered that, no matter for the grand indices such as joints’ vertical displacements or for the micro indices such as sliding curves and strain curves of the test-points, the FE model matches the experimental highly.(4) Based on the well tested FE modeling method of gusset joint, the mechanical behavior and damage form of the standard gusset joint under typical working condition is numerically analyzed. The research indicates that, the gusset joints have three damage forms:the locally buckling of the web, strength failure of the flange section and strength failure of the cover plate section. The relative strength between the shear force and moment are key factors of the bearing capacity of the members connected to joints, and the large shear force is the main reason that causes the buckling of the beam-web. When a single beam connected to the joint mainly bears moment (shear-bending ratio is small), failure easily occurs on the weak section of the I-shaped beam flange. When all the members connected to the joint carry large moment together, failure occurs on the weak section of the cover plate. Moment makes lockbolts both on the up and down side subject to shearing, and most outside circle (the 5th circle) of the lockbolts connected to the stretched flange are in the most unfavorable shearing position and so do the most inner side circle (the 1st circle) of lockbolts connected to the squeezed flange. Increment of the shear force makes the lockbolts on the squeezed flange bearing pulling force, and the most unfavorable loading position locates on the most inner circle.(5) A typical gusset support joint is tested and FE analyzed. According to the mechanics character of the joint that bearing large shear force and moment at the same time, a scheme of two stages is proposed to simulate its stress state efficiently. Research indicates that, weak point of the gusset support joint is the rivet hole section on the stretched flange of the beam connected to the joint, and brittle fracture damage occurs on this point. After loading processes, the stress level of connection area between cover plate and I-shape Aluminum beam is high, stress transfer route is obvious and distributes narrowly. In contrast, the whole stress level of the cover plate connected to the squeezed flange is much lower. The lockbolts on the crack area of the stretched flange are in most unfavorable loading condition the shearing condition, but lockbolt failure does not appear because of the high strength of the stainless steel.