A. Pratellesi

On the effect of testing uncertainties in the homologation tests of motorcycle helmets according to ECE 22.05

Industrial products designed for a specific task need to prove their capability to fulfil their design goal through experimental homologation procedures. Homologation standards define the necessary steps for these experimental tests in a very detailed manner but still are not entirely capable of accounting for any imprecisions in the definition of the testing procedure and variability in its carrying out. This paper addresses the issue of testing uncertainties that are related to the homologation standard ECE 22.05 for motorcycle helmets. Finite Element simulations are performed in the framework of an uncertainty analysis based on fuzzy-valued testing parameters, which are an adequate and very practicable method to represent the homologation uncertainties. Through the uncertainty analysis, large uncertainties are found for the relevant homologation quantities. They are entirely in agreement with the homologation standards. Moreover, the most important testing parameters are identified and recommendations for improving the homologation standards are formulated.

Stochastic BEM for the Vibroacoustic Analysis of Three-Dimensional Structures

Nowadays, extending the NVH prediction reliability to the whole frequency range is an attractive goal of vibroacoustics. Deterministic methodologies are well established for the low-frequency range, but, decreasing the wavelength, energy-based methods are necessary. In such a range, a crucial role is played by small perturbations which highly influence the response sensitivity. Moreover, taking into account these variations allows to make the product design more robust and even quicker. Introducing geometrical uncertainties within the classic BEM formulation allows to obtain the so-called stochastic BEM. As a result, the solution shows deterministic behaviour at low frequencies; decreasing the wavelength, the effect of the uncertainties smooths the response. Consequently, it is possible to obtain an averaged trend over the whole frequency range which asymptotically tends to the deterministic one. In this paper, we deal with three-dimensional acoustic SBEM. First, the formulation and its basic assumptions are presented. Secondly, they are applied to academic cases to show its potentialities in predicting vibroacoustic behaviour over a wide frequency range.

Optimization of the Global Static and Dynamic Performance of a Vehicle Body by Means of Response Surface Models

Concept FE models of the vehicle structure are often used to optimize it in terms of static and dynamic stiffness, as they are parametric and computationally inexpensive. On the other hand they introduce modeling errors with respect to their detailed FE equivalents due to the simplifications made. Even worse, the link between the concept and the detailed FE model can be sometimes lost after optimization.The aim of this paper is to present and validate an alternative optimization approach that uses the detailed FE model of the vehicle body-in-white instead of its concept representation. Structural modifications of this model were applied in two different ways — by local joint modifications and by using mesh morphing techniques. The first choice was motivated by the strong influence of the structural joints on the global vehicle performance. For this type of modification the plate thicknesses of the most influent car body joints were changed. In the second case the overall car dimensions were modified.The drawback of using detailed FE models of the vehicle body is that they can be times bigger than their concept counterparts and can thus require considerably more time for structural analysis. To make the approach proposed in this work a feasible alternative for optimization in the concept phase response surface models were introduced. With them the global static and dynamic performance of the body-in-white was represented by means of approximating polynomials. Optimization on such mathematical models is fast, so the choice of the optimization algorithm is not limited only among local-search strategies.In the current study Genetic Algorithm was used to increase the chances for finding better design alternatives. Two different optimization problems were defined and solved. Their final solutions were presented and compared in terms of structural modifications and resulting responses. The approach in this paper can be successfully used in the concept phase as it is fast and reliable and at the same time it avoids the problems typical for concept models.Copyright

A stochastic BEM formulation for vibro-acoustic analysis of structures in the mid-to-high frequency range

Predicting the acoustic behaviour of a complex structure in the mid-frequency range is a challenging task due to the complexity of the structure and the involved physical phenomena. Since, in the high- and even mid-frequency range, the variability of the product may have a significant effect on its NVH behaviour, it is important to include this sensitiveness in numerical models. In other respects a detailed description of the structures is necessary for structure borne contributions. The approach proposed in this paper provides a robust method for the vibroacoustic analysis of 3D acoustic domains coupled with structural components in the mid-frequency range. The method is based on a probabilistic approach which introduces uncertainties in the Boundary Element formulation: random geometrical parameters are introduced into the integral representations in order to model the increasing sensitivity of the harmonic vibrational responses to any parameter perturbation when the frequency increases. The stochastic formulation is solved in terms of the expectations of the square boundary kinematic unknowns and is valid in the entire frequency domain. The effectiveness of the formulation is demonstrated in this paper with a numericalapplication to a 3D model of an acoustic cavity coupled with a vibrating structure and with an internal source.