Theoretical and Applied Mechanics - AIMETA 2022 Materials Research Forum LLC Materials Research Proceedings 26 (2023) 653-658 https://doi.org/10.21741/9781644902431-105 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 license. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under license by Materials Research Forum LLC. 653 Interface reduction in flexible multibody systems CAMMARATA Alessandro 1,a* , MADDÌO Pietro Davide 1,b and SINATRA Rosario 1,c 1 Dipartimento di Ingegneria Civile e Architettura, Università degli Studi di Catania, Via S. Sofia 64, 95125, Catania, Italia a alessandro.cammarata@unict.it, b pietro.maddio@unict.it, c rosario.sinatra@unict.it Keywords: Floating Frame of Reference, Multipoint Constraints, Reference Conditions Abstract. The problem of imposing the reference conditions in a floating frame of reference formulation is coupled with the necessity to reduce the interfaces to virtual nodes required to define the multibody joints. Two methods are implemented for rigid and interpolation multipoint constraints, and the reference condition matrix is derived employing all the interface dofs. The case study of a slider-crank mechanism is discussed to show how different sets of reference conditions can modify the system’s dynamics. Introduction Interface reduction is a recurring problem in substructuring and model reduction theory [1]. In flexible multibody dynamics, the interfaces are primitive geometric features employed to form a joint. In practice, what is done is to individually reduce each interface to a single virtual node, usually a not collocated node outside the volume of the body. This reduction occurs through two types of multipoint constraints (MPCs): the rigid multipoint constraint, usually referred to as the RBE2 element, and the interpolation multipoint constraint, usually referred to as the RBE3 element [2]. Subsequently, the virtual nodes of the two interfaces are linked through kinematic constraints necessary to define a joint. In [3], the authors raised the problem of the scarce use of RBE3 in multibody simulations and identified the disappearance of the dependent coordinates, operated by FE software after eliminating the multipoint constraints, as one of the possible causes. The method proposed in [3] has practical implications to be used in commercial FE software. Still, it neglects essential aspects related to the presence of MPCs and generic reference conditions (RCs) within the Floating Frame of Reference Formulation (FFRF) [4]. Here, a different approach is presented that is perfectly integrated inside the FFRF working with every RCs. Both types of MPC are treated, and the interpolation MPC exploits all interface DOFs without the need to select dependent nodes or to introduce selection criteria. MPCs are directly connected to the reference conditions necessary to define the floating frame correctly. This issue has only been marginally addressed in the literature without providing a general treatment for any RC. The paper is organized as follows. First, the mathematical background of the FFRF and the role of the RCs are introduced. Then, the method to apply the reference conditions to the virtual nodes of rigid and interpolation MPCs is provided. The reference condition matrix is obtained in explicit form for both cases. The case study of a flexible crank is analyzed, and three different sets of RCs have been applied to the interface virtual nodes of the component. Finally, the crank is assembled with a flexible connecting rod and a rigid piston to simulate a single cylinder of an internal combustion engine.