STATIC-VIBRATIONAL DESIGN OF A BONNET WITH FRAME TOPOLOGICAL OPTIMIZATION

This paper presents an analytical-experimental methodology in the design and optimisation process of a bonnet for a prototype roadster car named Argento Vivo and built by Pininfarina in co-operation with Honda. A part of this work has been carried out within the European Project HIPOP(High Performance Optimisation). The finite element model has been realised by using the pre-processor MSC/PATRAN with CAD PTC direct interface. At the beginning the bonnet analytical model has been validated by comparing the torsional stiffness calculation by MSC/NASTRAN Solution 101 with the results obtained by Pininfarina Testing Laboratory. Then an analytical modal analysis has been carried out by SOLUTION 103 of MSC/NASTRAN with the frequency and modal shape definition. Then by a pre-test analysis, i.e. by the identification of a reduced analytical model formed by a number of nodes very lower than the complete analytical mode, but able to completely approximate the dynamic behaviour, by using the MAC as control index, a modal experimental analysis has been carried out on the bonnet on the reduced set of points. A considerable importance is given to the methodologies by which the modal parameters are drawn by the frequency response functions (FRF) experimentally obtained by the software LMS Cada-X. At the end a bonnet frame topological optimisation has been carried out by MSC/CONSTRUCT that, with the structure stress behaviour known, acts directly on the material distribution by adding or removing the material in those points on which the stresses reach more or less high values, by creating holes and opening in the area to be optimised, by maintaining the torsional and/or bending stiffness values within the requirements. In this way it is possible to obtain a mass reduction, and therefore cost reduction without jeopardising the static characteristics, or if this occurs, the variations are within defined values. A further model verification for the optimised design has been carried out. This type of applications is perfect for the automotive sector where the structural optimisation, in order to act on the vehicle stiffness with advantages both in terms of stability, safety, comfort and costs is needed almost everyday. The aim is the presentation of a static-vibrational analysis methodology for the bonnet design for a car realised by Pininfarina, named Argento Vivo, built as a prototype and identified as a two-seats roadster car with a complete hidden hard top (Fig.1). The frame is made by aluminium alloy extrusions for a total weight of 87 Kg. The bonnet is made of steel and formed by an outer skin, a bonded lower frame and some reinforcements applied in line with the anchorage points with the frame. The aim of this frame is to stiffen the structure, very flexible, due to its considerable dimensions and its particular shape. The bonnet metal sheet thickness is 0.8 mm. 1. Industrie Pininfarina, Via Lesna 78-80, 10095 Grugliasco (Torino) Italy 2. Industrie Pininfarina, Via Lesna 78-80, 10095 Grugliasco (Torino) Italy 3. University of Karlsruhe – Institute of Machine Design, Kaiserstrasse 12, 76131 Karlsruhe German 4. Industrie Pininfarina, Via Lesna 78-80, 10095 Grugliasco (Torino) Italy Figure 1: Argento Vivo 1. Finite element analytical model and torsional analysis Using the direct interface of CAD model made by ProEngineer with the pre-post processor MSC/PATRAN has carried out the bonnet mesh. The finite element model obtained is characterised by 58369 nodes, 54564 elements CQUAD and 4285 elements CTRIA3. The percentage of 3 nodes triangular elements in the model is about of 5% with respect to the quantity of 4 node elements. Figure 2 shows in particular that points A and B are rigid joints with all the six degrees of freedom blocked, while point D has the translations blocked and the rotations free. This constraint simulates the ball joint used to carry out the experimental test. The force applied to point E has a 100 N value and is downward. The original mathematical model has given a 32.89 N/mm (199.7 Nm/Deg) torsional stiffness value corresponding to a 5.15 mrad torsional angle and a 3.04 mm displacement in the point where experimentally measured. Fig. 3 shows the bonnet finite element model under load with the force application and constraint points viewed from the bottom. Fig. 4 shows the bonnet deformed ”shade” model subjected to torsion, while Fig. 5 shows respectively the local displacement behaviour in z (mm) and the stress behaviour (N/mm2), calculated by using Von Mises cracking assumption. Figure 2: Loads and constraints Constraint Point B