Lassaad Ben Fekih

Symbolic generation of the kinematics of multibody systems in EasyDyn: From MuPAD to Xcas/Giac

In the EasyDyn multibody open source project, computer algebra has been used from the beginning to generate the expressions of velocities and accelerations of the bodies, by symbolic differentiation of their position. Originally, the MuPAD computer algebra system had been retained because it was freely available for non commercial purposes and showed very good technical features. Unfortunately, MuPAD is nowadays only available through commercial channels and needs to be replaced to keep EasyDyn publicly available. This paper presents why Xcas/Giac is finally selected, among other long-term promising projects like Axiom, Maxima, Sage or Yacas. Among the choice criteria, the accessibility, the portability, the ease of use, the automatic export to C language, and the similarity with the MuPAD language are all considered. The performances of the MuPAD and Xcas/Giac implementations are also compared on some examples.

Robustness Evaluation of Adhesively Bonded Ceramic Quad Flat Chips for Space Applications

Ceramic quad flat packs (CQFP) electronic components have long been used in power devices in the space industry. Soldering CQFPs to a printed circuit board (PCB) is not sufficient and thus the use of adhesives for reinforcement is compulsory. In fact, the risk of failure becomes very high during launch missions due to harsh constant and random accelerations. The paper investigates various distributions of two common thermally conductive and one epoxy adhesives. The intention is to compare the effects of these adhesive bonds on the robustness to vibrations of solder joints. Results are generated from a finite element (FE) model consisting of a CQFP component, an adhesive layer and a PCB. Key outputs such as the maximum deflection of the PCB, the stress in the solder joints and the fatigue lifetime of the assembly are discussed. Recourse to FE simulations allows competitiveness in terms of design cost and delivery time.

Spectral-Based Fatigue Assessment of Ball Grid Arrays under Aerospace Vibratory Environment

Ball grid arrays (BGAs) embedded in aerospace devices should satisfy strict standards in the purpose to ensure their mechanical safety, particularly in fatigue. In fact, critical phases of BGAs service life such as launch lead to high cycle fatigue (HCF) failure due to severe random accelerations.To face this problem, designers are still using experimental qualifications based on deterministic time-domain fatigue methods. This work is motivated principally to study the applicability of the principal spectral fatigue models as cost effective alternative to assess BGA HCF. The study includes an assembly made up of a BGA chip and a support board. Finite element spectrum analysis brings out that the fatigue failure is expected to occur at different interconnect locations like for instance a critical solder joint made of a ductile tin-lead alloy. Among all the studied spectral models, it emerges that the Dirlik’s fatigue prediction is the most relevant in the typical range of the solder fatigue coefficients.

Verification of empirical warp-based design criteria of space electronic boards

Abstract Space electronics are subjected to severe vibration environment. The present paper examines empirical warp-based design rules of electronic boards, i.e., criteria verifying that the ratio of maximum board warp to its length remains below a threshold percentage. An analytical approach assessed that peak stress of the board stems better from its curvature than its warp. The same applies to the adhesive peak stress by investigating a finite element model of an adhesively bonded component. Alternatively, a modified formulation based on board curvature is proposed and a threshold curvature is assessed.

Output-Only Modal Analysis of an Internal Unreachable Module Embedded in a Space Electronic Equipment

An approach is proposed to identify the modal properties of a subsystem made up of an arbitrary chosen inner module of embedded space equipment. An experimental modal analysis was carried out along the equipment transverse direction with references taken onto its outer housing. In parallel, a numerical model using the finite element (FE) method was developed to correlate with the measured results. A static Guyan reduction has led to a set of master degrees of freedom in which the experimental mode shapes were expanded. An updating technique consisting in minimizing the dynamic residual induced by the FE model and the measurements has been investigated. A last verification has consisted in solving the numeric model composed of the new mass and stiffness matrices obtained by means of a minimization of the error in the constitutive equation method.Copyright

Design of Viscous Dynamic Vibration Absorber for a Vertical-Axis Wind Turbine

This paper presents the investigations brought to fruition for the design of a dynamic vibration absorber (DVA) for vertical-axis wind turbine. A first step is devoted to the experimental analysis of the structure, by seeking its modal behaviour in low frequency range. A numerical model of the turbine system consisting of finite elements is developed. Their dynamics and geometrical characteristics are updated, by fitting the first three bending numerical mode shapes with the experimental ones. Finally, a mathematical model of DVA is implemented and the vibration reduction is evaluated with the help of the updated finite element model, considering the modal decomposition of the structure. The results exhibit significant vibration reduction performance evidencing this kind of device. A tuneable anti-vibration device is then designed, with a purpose of simplicity and low-cost production. The possible non-linearity of the DVA is also studied, by comparing behaviours of linear and quadratic selected dampers.Copyright