| At present,when the finite element method is used to deal with the problems such as the strong nonlinearity of single-layer reticulated structure and the failure of joint details,there still exist some disadvantages that this method is not easy to converge,needs manual intervention and depends on experience,which is difficult to get a satisfactory result.In view of this,based on PFC3 D software platform,discrete element models suitable for continuous medium are proposed,which are applied to the numerical simulation of the collapse instability analysis and joint connection failure of the single-layer reticulated structure,so as to make the best of the advantages of the discrete element method and broaden its application in the engineering field.The main contents are as follows:Based on the parallel bond contact model in PFC3 D,a single row regular arrangement discrete element model for structure of bar system is proposed,and the unit of the arrangement is defined as "equivalent beam element".Using the theory of simple beam,the spring stiffness of parallel bond contact between elements is derived.Numerical examples show that the "equivalent beam element" can accurately simulate the structure with arbitrary cross-section.The discrete element model of single-layer reticulated structure is established by using the "equivalent beam element".Based on the PFC3 D platform,the velocity loading method is proposed,which simulates the whole process of structural elastic instability accurately,and can track the details of the structural instability,local instability,overall instability and multiple instabilities.On the basis of the cubic array discrete entity model,the three-dimensional discrete entity models of the Body Centered Cubic arrangement and the Face Centered Cubic arrangement are further constructed.Based on the three-dimensional discrete solid element method for continuous medium and taking the energy equivalence as the principle,the relation between the mesoscopic parameter(contact spring stiffness)and the macro elastic constants(elastic modulus and Poisson’s ratio)is established,which reflects the Poisson ratio effect of the material.It is proved that the applicability and accuracy of Cubic model and BCC model are higher than that of FCC model when simulating the mechanical behavior of structure in the elastic stage.Using the basic quantities of the discrete element method,not only the yield criterion and flow rule are expressed,but also the elastoplastic contact spring constitutive equation of the Cubic model is determined.Besides,the elastoplastic analysis of the structure is set as an independent program module by using the programming of the Fish language in PFC3 D.It is proved that Cubic model can be used to simulate the development of the plastic zone distribution effectively in the elastoplastic stage.Based on the elastoplastic method of bar system structure with parallel bond contact in PFC3 D,a discrete element model of single-layer reticulated structure composed of the rods with actual cross sections is established,which simulates the elastoplastic instability process of single-layer reticulated structure and the plastic region distribution of the rods effectively.Using the "equivalent beam element" and the cubic discrete entity element,a multi-scale PFC3 D discrete element model of single grid structure is established.Taking the welded hollow ball joints and a small segment “s” of the rods connected with them as the micro scale model to simulate by using cubic discrete entity element,and taking the rest of the structure as the macro scale model to simulate by using "equivalent beam element",the contact spring stiffness between the multiscale interfaced elements is derived,the equivalent section inertia method of the multi-scale elements cross section of the rods is given.By using the method of wall loading in PFC3 D,the instability process of single-layer reticulated structure considering the effect of joint failure is effectively simulated. |
PDF Full Text Request