Numerical Simulation of the Semi-Rigid Behaviour of Integrally Attached Timber Folded Surface Structures

Anh Chi Nguyen, Aryan Rezaei Rad, Andrea Stitic, Yves Weinand
2019
Article

Résumé

Timber folded surface structures assembled using semi-rigid multiple tab and slot joints (MTSJ) have been shown to form feasible structural systems with high load bearing potential. However, for their further development and use on large building scales, a pertinent model for prediction of their structural behaviour has yet to be developed. This paper focuses on simplified numerical methods for accurately modelling the semi-rigid structural behaviour of bidirectional timber folded surface structures with multiple tab and slot connections. Within this scope, the structure behaviour is considered to be in the elastic stage. Three practical methods of analysis for such structural systems are presented. The first two approaches use the Finite Element Method (FEM), where the theory of plates and shells are applied. In the first method, the MTSJs are modeled using strip element models, while, in the second strategy, spring models are used. The third modeling strategy elaborates on the new macroscopic mechanical models, referred to as macro models. Sets of one-dimensional (1D) elements are used to represent the mechanical behaviour of the entire system. Both linear and geometric nonlinear analysis are performed for all three modeling strategies. The numerical results are then validated against the large scale experiments. Comparison of the strip and spring element model results have shown that the strips represent more accurately the experimentally obtained values. Concerning the macro modelling approach, very good agreement with both detailed FE modelling approaches, as well as experimental results, were obtained. The results indicate that both linear and nonlinear analysis can be used for modelling the displacements within the elastic range. However, it is essential to include geometric nonlinearities in the analysis for accurate modelling of occurring strains as well as for displacements when considering higher load levels. Finally, it is demonstrated that including semi-rigidity in the numerical models is of high importance for analysing the behaviour of timber folded surface structures with MTSJ.

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https://infoscience.epfl.ch/record/264908?ln=fr

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Anh Chi Nguyen, Aryan Rezaei Rad, Andrea Stitic, Yves Weinand
2019
Article

Résumé

Official source

https://infoscience.epfl.ch/record/264908?ln=fr

À propos de ce résultat

Concepts associés (13)

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Publications associées (51)

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High speed milling (HSM) is the most known machining process due to its application in various industries. In milling, a rotating cutting tool removes a large amount of material along a predefined toolpath to manufacture the final part with a desired shape. Milling of prismatic parts1 is very important in automotive, aerospace, mold and die industries. Even complicated parts are machined from a blank first by 2.5D roughing followed by 3D-5D finishing. Modern production floors have adopted high speed CNC2 machine tools to execute part programs, developed by CAD/CAM3 systems, to manufacture the final workpiece. The overall productivity of the milling process depends on the choice of cutting conditions and the toolpath. Current CAD/CAM systems do not provide any guidance to select cutting conditions due to the unavailability of models of the complex physical and dynamic interaction of machine tool and workpiece systems. Moreover, toolpath generation by CAD/CAM packages is purely geometric in nature and results in engagement angle variation along the toolpath. The selection of cutting conditions and toolpath rely solely on the part programmer’s experi- ence, CAD/CAM systems, handbook guidelines or specifications provided in the catalogues of cutting tools and machine tools. Their poor selection often causes chatter, high fluctuation of cutting forces, and/or violation of the available limits of power and torque of the machine tool. These phenomena result in poor surface finish, workpiece damage, high cutting tool wear, violation of tolerance limits, additional cost, unwanted waste and significant reduction in machine tool working life. In order to avoid these problems, part programs need to be verified iteratively using trial and error experiments and often conservative cutting conditions are selected. These practices lead to long preparation time of part programs and lower machining performance, which in a nutshell significantly lower overall productivity. Moreover, machine tool capabilities are not fully utilized due to the conservative selection of cutting conditions. In order to address these challenges, a genetic algorithm (GA) based optimal milling (OptMill) system is developed for optimal selection of cutting conditions and/or toolpath for a given set of inputs of machine tool/spindle/tool holder/cutting tool and workpiece system. Operational constraints of the machine tool, such as spindle speed and feed limits, available spindle power and torque, chatter vibration4 limits due to the dynamic interaction between cutting tool and workpiece, permissible limits of bending stress and deflection of the cutting tool and clamping load limits of the workpiece system are embedded. The developed system is applied to different industrial use cases: (i) Minimization of pocket milling time considering one-way toolpath (ii) minimization of machining time for multi-feature prismatic parts with the imple- mentation of pre-processing modules: extraction of toolpath and workpiece boundary from APT5 and STEP6 files respectively and calculation of engagement angle along the toolpath (iii) optimal selection of cutting conditions and corresponding smooth and constant engagement toolpath for pocket milling. The selected cutting conditions and/or toolpath are also validated using dedicated experiments conducted during the course of the research work. The present research work is inspired from an ongoing CTI project7. Following enhanced methodologies the identification of important inputs to mathematical models for prediction of cutting forces and chatter free limits have also been developed to expand the scope of the developed OptMill system. • Tangential force coefficients, an important input for prediction of cutting forces and chatter free limits, are identified experimentally with the use of a cutting force dy- namometer. This experimental setup is quite costly and not practical for industrial implementation. An enhanced methodology is presented for the indirect identifica- tion of tangential force coefficients from the spindle motor current. The methodology includes the development of an empirical model for cutting torque prediction from spindle motor current with the implementation of a spindle power model that accounts for all mechanical and electrical power losses. The cutting torque predicted by the developed model is then used for tangential force coefficient identification, and is also validated experimentally with direct measurement using a cutting torque dynamometer. • Dynamicresponseofeachvariantofmachinetool/spindle/toolholder/cuttingtool,in terms of FRF8, is required to predict chatter free limits accurately. FRF is often measured with hammer testing experiments. In order to avoid these tedious tests, an enhanced procedure using the receptance coupling technique is implemented to predict the FRF of a machine tool/spindle/tool holder/cutting tool system for different cutting tools. The predicted FRFs via numerical simulation are also validated with experimental measurement. Though the existing mathematical models predict accurately the chatter free limits, their use in small production floors has not yet been achieved due to the absence of technical expertise and experimental resources. Moreover, even modern machine tools do not provide any guidance to the machine operator regarding the occurrence of chatter during machining. To meet industrial requirements, a computationally fast, easy to use and practical system is developed that detects chatter automatically during milling and thereafter proposes a control strategy to the machine operator. The developed online chatter detection and control system is also validated experimentally with an industrial end-user partner. Apart from the many challenges and the developments discussed above, milling of thin- walled workpieces is also a concern due to changing dynamics during machining. Thus, an enhanced numerical procedure is developed for the selection of chatter free cutting conditions while considering the change in workpiece dynamics along the toolpath using finite element analysis. In order to realize the developed system, MATLAB is used as a programming language. Ge- ometrical modeling and part programming of prismatic parts is done with CATIA. The data acquisition platform for the experimental validation is designed in LABVIEW. Finite element modeling and analysis is implemented with the ANSYS parametric design language (APDL). The developed system is very appealing for industrial application by direct integration with existing CAD/CAM systems and/or modern machine tools. Increase in overall productiv- ity is ensured by optimal selection of cutting conditions and/or toolpath and simultaneous avoidance of repercussions due to their wrong selection.

EPFL2012

Chargement

The present work deals with the large-scale application of the friction welding bonding technology on timber. The investigations that are described in the following were carried out on a prototype of a welding machine that was previously developed for the purpose of this research. By means of this machine timber elements can be welded in a scale that is comparable to structural applications. Detailed experimental experience with the welding of large-sized specimens was not available at the beginning of this work. Preliminary results of initial tests on welded spruce boards indicate a correlation between the size of the sample and the occurrence of larger areas within the interface where no or only an incomplete welding result is attained. Therefore, these scaling effects were investigated in more detail in a first step. The results show that this phenomenon is associated with an internal gas pressure, which is generated at the interface during the evaporation of moisture content and other gaseous by-products. The latter result from the thermal decomposition of wooden cell wall molecules during the friction phase. It can be shown that the scaling effects can be restricted through a reduction of the moisture content or by the arrangement of gas evacuation channels at the interface, which leads to a decrease of the internal gas pressure. Subsequently the potential of the technology with regard to structural applications was estimated and evaluated by means of mechanical studies. First, investigations on the load bearing behaviour of parallel welded connections in form of double lap joints, and in particular the influence of the overlap length on the load capacity, are described. The results show that due to the extremely rigid behaviour of the bond so-called size effects occur, a phenomenon that describes a decrease in nominal strength with increasing size of a load-bearing structure. In addition to the load tests, a numerical strength prediction method is presented, which is based on a probabilistic approach and which allows for a precise prediction of the experimental results and the size effects. Furthermore, it is shown that timber boards with perpendicular fibre orientation can be welded in order to form stiff and resistant connections. Therefore, specimens of different sizes were prepared and tested for their moment transmitting capacity. In this context, a new method was introduced for the determination of a failure criterion of cross-welded joints is presented. Probabilistically determined strength values, which are based on this criterion, show good agreement with the experimental results. The non-linear relationship between the size of the specimens and the nominal strength is confirmed by the results of the numerical calculation. The results from the experiments on cross-wise welded bonds show that the strength of those connections is suitable for an application in timber construction. For this reason small scale prototypes of cross-laminated timber panels were fabricated and examined for their load bearing capacity. For this purpose the most critical load case of in-plane shear is regarded mainly since the load is transmitted only by the moment resisting behaviour of the welded bond. Based on the previously developed probabilistic strength prediction method, the load bearing capacity of the panels can be calculated numerically [...]

EPFL2014

Chargement

Parallel Algorithms for the Solution of Large-Scale Fluid-Structure Interaction Problems in Hemodynamics

Davide Forti

This thesis addresses the development and implementation of efficient and parallel algorithms for the numerical simulation of Fluid-Structure Interaction (FSI) problems in hemodynamics. Indeed, hemodynamic conditions in large arteries are significantly affected by the interaction of the pulsatile blood flow with the arterial wall. The simulation of fluid-structure interaction problems requires the approximation of a coupled system of Partial Differential Equations (PDEs) and the set up of efficient numerical solution strategies. Blood is modeled as an incompressible Newtonian fluid whose dynamics is governed by the Navier-Stokes equations. Different constituive models are used to describe the mechanical response of the arterial wall; specifically, we rely on hyperelastic isotropic and anistotropic material laws. The finite element method is used for the space discretization of both the fluid and structure problems. In particular, for the Navier-Stokes equations we consider a semi-discrete formulation based on the Variational Multiscale (VMS) method. Among a wide range of possible solution strategies for the FSI problem, here we focus on strongly coupled monolithic approaches wherein the nonlinearities are treated in a fully implicit mode. To cope with the high computational complexity of the three dimensional FSI problem, a parallel solution framework is often mandatory. To this end, we develop a new block parallel preconditioner for the coupled linearized FSI system obtained after space and time discretization. The proposed preconditioner, named FaCSI, exploits the factorized form of the FSI Jacobian matrix, the use of static condensation to formally eliminate the interface degrees of freedom of the fluid equations, and the use of a SIMPLE preconditioner for unsteady Navier-Stokes equations. In FSI problems, the different resolution requirements in the fluid and structure physical domains, as well as the presence of complex interface geometries make the use of matching fluid and structure meshes problematic. In such situations, it is much simpler to deal with discretizations that are nonconforming at the interface, provided however that the matching conditions at the interface are properly fulfilled. In this thesis we develop a novel interpolation-based method, named INTERNODES, for numerically solving partial differential equations by Galerkin methods on computational domains that are split into two (or several) subdomains featuring nonconforming interfaces. By this we mean that either a priori independent grids and/or local polynomial degrees are used to discretize each subdomain. INTERNODES can be regarded as an alternative to the mortar element method: it combines the accuracy of the latter with the easiness of implementation in a numerical code. The aforementioned techniques have been applied for the numerical simulation of large-scale fluid-structure interaction problems in the context of biomechanics. The parallel algorithms developed showed scalability up to thousands of cores utilized on high performance computing machines.

EPFL2016

Chargement

Parallel Algorithms for the Solution of Large-Scale Fluid-Structure Interaction Problems in Hemodynamics

Davide Forti

EPFL2016

EPFL2014

EPFL2012