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.

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Timber frameworks are one of the most important and widespread types of structures. Their configurations and joints are usually complex and require a high level of craftsmanship to assemble. In the field of restoration, a good understanding of the structural behaviour is necessary and is often based on assessment techniques dedicated to wood characterisation. This paper presents the use of experimental modal analysis for finite element updating. To do this, several timber beams in a free supported condition were analysed in order to extract their bending natural characteristics (frequency, damping and mode shapes). Corresponding ABAQUS finite element models were derived which included the effects of local defects (holes, cracks and wood nodes), moisture and structural decay. To achieve the modal updating, additional simulations were performed in order to study the sensitivity of the mechanical parameters. With the intent to estimate their mechanical properties, a procedure of modal updating was carried out in MatLab with a Python script. This was created to extract the modal information from the ABAQUS modal analysis results to be compared with the experimental results. The updating was based on a minimum of unconstrained multivariable function using a derivative-free method. The objective function was selected from the conventional comparison tools (absolute or relative frequency difference, and/or modal assurance criterion). This testing technique was used to determine the dynamic mechanical properties of timber beams, such as the anisotropic Young's Moduli and damping ratio. To verify the modulus, a series of static 4-point bending tests and STS04 classifications were conducted. The results also revealed that local defects have a negligible influence on natural frequencies. The results demonstrate that this assessment tool offers an effective method to obtain the mechanical properties of timber elements, especially when on-site and non-destructive techniques are needed, for example when retrofitting an existing structure.

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.

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.