Identifying Anisotropic Damping Behavior of multi-phases materials


Structural vibrations due for example to the engine functioning or the ground excitation transmitted by the wheels is one of the biggest challenges for engineering applications. In case this kind of behavior is not well mastered, that can cause fatigue failures or unwanted noise. Such product defects would concretely bring an increase of safety risk for passengers, acoustic discomfort as well as an image of bad quality for the OEM in general. . NVH (Noise-Vibration-Harshness) analysis role is to predict accurately the vibrational behavior of the material in terms of stiffness and damping. This involves integrated extensive modeling, simulations, evaluation and optimizing techniques in the design process. Automotive industry is looking continuously for improvement for NVH performances of next generation vehicles through new material development offering better damping capabilities in order to solve all these problems in the better way and with the best compromise among design cycle deployment and costs.

In this sector, Short Fiber Reinforced Plastics (SFRP) are taking place larger and larger due to the well known interest they offer for lightweighting, design flexibility and function’s integration but also to their better damping behavior which helps to get better NVH performances. However, reinforced plastics show also a very complex behavior, with a heterogeneous distribution of local mechanical behavior who influences the structural vibrational modes. This anisotropy in stiffness and damping is frequency dependent and fully driven by the local fiber’s orientations.

Digimat is the perfect simulation tool for Noise and Vibration Simulation allowing to take into account the fiber orientation distribution resulting from the manufacturing process in the design of components and complete vehicle.

Digimat can be used to identify and predict the anisotropic damping behavior of multi-phase materials for automotive applications like the vibrational behavior of a roof front beam. Starting from the interaction of Digimat-MF and Digimat-FE, it is possible to fully identify the material damping performances, evaluating quickly different grades of materials to identify the best candidates for NVH targets and avoiding wasting time in performing costly experimental tests for each of them.

In this application an anisotropic viscoelastic Material has been calibrated in Digimat-MF, and used in Digimat-FE. Digimat-FE generates realistic (stochastic) Representative Volume Elements (RVEs) for a large variety of material microstructures. Based on material input and the microstructure definition, a finite element model is built and run. The results of such FE analysis gives a detailed insight into a typical RVE for a given material grade.

In our case, a virtual DOE through transient dynamic FEA has been performed directly on the RVEs for various frequencies, providing as output several damping vs. frequencies curves for different load direction and fiber orientation tensor. Damping has been measured by the time gap between macro strain and stress answers on the RVEs results. This has allowed to measure the anisotropy level. This material engineering part of the work has allowed to identify the range of damping behavior to be expected from the material depending on the local fiber orientations. This result has been used to feed FEA at the structural engineering level. The lowest and highest frequency dependent damping behaviors has been applied on a roof front beam model in 2 frequency responses FEA in order to obtain the possible range of vibrational response for this given design of the part.

This process allows to characterize numerically and quickly the material’s damping behavior based on Transient Dynamic analysis performed on various RVEs FE models fully created in Digimat-FE. It enriches material knowledge for vibrational performance trough numerical methods that can help material engineers to select materials better and faster for a given NVH target.

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