Dynamic Modeling of Structures with Bolted Joints

Dynamic Modeling of Structures with Bolted Joints The primary function of a joint in an engineering structure is to connect, usually stiffly, two (or more) separate substructures. Joints introduce two features to a structure: amplitude dependent stiffness and amplitude dependent damping. The analysis of modeling jointed structures can be briefly explained in three steps: The first step, the nonlinear static solve, is used to determine which nodes in the interface are stuck together, and which nodes are not expected to be in contact. In the second step, the problem is reduced slightly by assuming that nodes that are out of contact will remain out of contact during the analysis. The remaining nodes are considered to be potentially slipping. Though under certain circumstances these could be divided into stuck and slipping, in which case the stuck nodes just need to be attached with linear springs. The third step is the dynamic analysis. Once the contact interface has been prescribed, the next challenge is determining the quantity of interest. For frequency responses, harmonic balance-based simulations are preferable as the frequency response will exhibit nonlinearities dependent up on the excitation amplitude. The studying test case structure is made of two cantilevered beam of Aluminum as substructures which are jointed at their free-ends by four bolts when assembled. Basically, the FRFs are obtained experimentally using hammer at the points labelled on the individual beams and the assembled structure. The response measurement is performed by LASER Doppler Vibrometer (LDV) measuring the out-of-plane deflections. Additionally, the beams and the assembly are modelled in the finite element software, ANSYS APDL v17.2 with actual geometric and updated material properties. The meshing is done with SOLID186 brick element with the mid-side nodes. In this concept is tried to achieve the best converge of the nodes and elements in the contact area of the two cantilever beams. There is a method that could be used to find the energy dissipation and frequency. It is proper to introduce the “Iwan contact model”. The Iwan model consists of an infinite number of spring-slider units, called Jenkins element. Jenkins element is an ideal elasto-plastic element, composed of a single discrete spring in series with a Coulomb damper characterized using a critical slipping force. The Iwan model exhibiting hysteretic characteristics suitable to model transition behavior from stick to slip appears in a joint. When an external force is applied to the model, it is distributed between Jenkins elements and some dampers, with low critical slipping force, saturate and slip. This phenomenon, known as micro-slip, causes a softening effect in the frictional sliders, stiffness and dissipates energy. As the applied force increases, more sliders saturate, and finally there would be an ultimate force at which all dampers would slip and the macro-slip phase begins. The main advantage of the proposed method is that it uses the dissipated energy in the contact interface while other methods involve the complex procedure of identifying the restoring force of the contact.