Dan Dumitriu

On the Complexity of the Auxetic Systems

Two major levels of complexity are discussed in a way of understanding the structure and processes that define an auxetic system. The auxeticity and structural complexity is interpreted in the light of Cosserat elasticity which admits degrees of freedom not present in classical elasticity, i.e. the rotation of points in the material, and a couple per unit area or the couple stress. The Young modulus evaluation for a laminated periodic system made up of alternating aluminum and an auxetic material is an example of computing complexity.

On the Damage in Nonlinear Mesoscopic Materials

This paper is devoted to the analysis of the damage in nonlinear mesoscopic materials, which are aggregates of grains which act as rigid vibrating units, while the contacts between them – the bond system – constitute a set of interfaces that control the behaviour of the material. The interfaces are mesoscopic, with a typical size of one micrometer. A constitutive micropolar model for material interfaces is presented. The model is based on damage coupled to plastic or viscoplastic slip, stick and dilatation (separation), and it is able to describe the succesive degradation and failure of an interface.

Direct Dynamics of Gough–Stewart Hexapod Platforms Using the Redundant Parameterisation of Rotations by Full Rotation Matrices

The direct dynamics study of Gough–Stewart hexapod platforms presented in this paper represents a preparatory step towards the direct dynamics study of a double hexapod, i.e., two Gough–Stewart platforms mounted in parallel one above the other. The direct dynamics model used here is based on the redundant parameterisation of rotations by full 3 × 3 rotation matrices. The dynamics of each solid of the hexapod platform comprises 12 scalar differential Eqs. (3 for each translation and 9 for each solid rotation) and 6 algebraic scalar orthogonality equations, plus the algebraic constraints characterizing the joints. For the entire hexapod, the overall differential-algebraic system comprises 156 scalar differential equations and 78 + 72 = 150 scalar algebraic equations. Of course, a number of 150 scalar Lagrange multipliers are introduced in association with fulfilling the algebraic equations. So, our dynamic modelling technique involves an increased number of parameters and equations, but this disadvantage is compensated by the fact that the dynamic equations can be written in a systematic way, being structurally similar for each solid of the hexapod multibody system. From the numerical point of view, the differential-algebraic system is solved by an iterative “shooting method”, using classical adaptive stepsize Runge–Kutta integration. No convergence troubles were encountered so far, when studying the direct dynamics of the Gough–Stewart hexapod platform considered as case study.

A multilayer sonic film

A non-periodic multilayer film was analyzed to show that, despite its non-periodicity, the film exhibits full band-gaps and localized modes at its interfaces, as well as in the sonic composites. The film consists of alternating layers of two different materials that follow a triadic Cantor sequence. The Cantor structure shows extremely low thresholds for subharmonic generation of ultrasonic waves, compared with homogeneous and periodic structures. The coupling between the extended-mode (phonon) and the localized-mode (fracton) vibration regimes explains the generation of full band-gaps, for which there are no propagating Lamb waves. The large enhancement of the nonlinear interaction results from a more favorable frequency and spatial matching of coupled modes. A full band-gap that excludes Love waves is also analyzed.

Simplified 7 DOF Model of Car Vertical Vibrations for Small Pitch and Roll Angles

This paper concerns a simplified 7 DOF model for car vertical vibrations. The classical 7 DOF of the considered 3D vertical model are: the vertical displacement of the gravity center of the car body, the roll and pitch angles of the car body and the four vertical displacements of the wheels centers. Using the x-y-z sequence of rotations parameterization, the Euler’s rotation equations concerning the roll and pitch angle are easily obtained. Small differences are observed between our car body rotation equations concerning the two pitch and roll angles and the same equations provided by Demić et al. [6]. For small pitch and roll angles, no differences were observed with respect with [6]. Also, no differences were observed concerning the other 5 dynamics equations of the 7 DOF model, the ones for the vertical displacements.The simplified 7 DOF car vertical dynamics model, comprising two rotation equations (for pitch and roll angles) and five dynamics equations concerning vertical displacements/accelerations, were integrated/simulated in Matlab. For small pitch and roll angles (a rather flat and smooth straight road profile was considered in our case study), the results obtained using the in-house 7 DOF model Matlab simulator are in very good agreement with the results provided by CARSIM.

Topological and Kinematic Analysis of a 6-DOF Driving Simulator

Driving simulators represent a novel way to reproduce the movements of a vehicle with the purpose of testing different cars, to prevent dangerous situations, entertainment and also to train the professional drivers. More, they are used to render the effects of the interactions between the car and the road, as a consequence of the constrains imposed by the road: different folds, cavities. This work presents the topological synthesis, the kinematic analysis and the virtual modeling of driving simulator. The mechanical structure of the simulator is modeled in the Matlab/Simulink environment, generally used the advantages of numerical solutions.

Aspects concerning development of reconfigurable driving simulators

This paper define and analyze the new concept of reconfigurable simulator avatar. A simulator avatar is a fully integrated computer model of a driving simulator. The simulator is a useful tool for driving simulators development because it allows implementation and validation of different working hypotheses and layouts for driving simulators topologies. As a case study three avatars for three different configuration of ATV vehicle were developed.

Influence of Real Car Longitudinal Acceleration Regime on the Vertical Vibrations of a Dynamic Car Simulator

For dynamic car simulators, it is obvious that the longitudinal motion cannot be fully reproduced, the possibility to simulate longitudinal motions being generally reduced to less than 1m. In our case, a four post shaker is intended to be built as dynamic car simulator. Each post shaker, one under each car wheel, is aimed to generate mainly vertical vibrations and lateral motion. Since the displacements/motion in the longitudinal direction can only be partially reproduced, our problem was how to compensate the impossibility of fully reproducing the longitudinal motion by at least taking into account the influence of the longitudinal motion on the vertical vibrations.In this paper a simplified 4 DOF “bicycle” model is used for vertical dynamics, instead of a 7 DOF full car vertical dynamics model, which will be considered in a further study.On the other hand, the vertical vibrations are different for the same car riding on the same road, but for different acceleration regimes: 1) null acceleration (e.g., 60km/h constant speed); 2) uniform acceleration from 10 to 110 km/h (during 8 seconds), followed by uniform deceleration from 110 to 10 km/h (during another 8 sec), then uniform acceleration from 10 to 60 km/h (during the last 4 sec of the simulation).In order to reproduce in the dynamic car simulator the vertical vibrations of the above-mentioned longitudinal motion regimes (involving displacements of tens of meters), the following steps are proposed: a) direct dynamics CARSIM computer simulations of the car motion and its interaction with the road; b) inverse dynamics of car vertical model (4 degrees of freedom), using as input the following parameters computed in step a): the vertical displacements and velocities of the sprung mass and of the front and rear wheel centers as well as the pitch angle of the sprung mass and its rate. These inverse dynamics computer simulations are performed using an in-house Matlab software programming only the 4-DOF vertical car/road interaction (2D “bicycle” model in the pitch plane, no roll motion considered), without considering any longitudinal motion.The output of these inverse dynamics computer simulations, using the in-house Matlab software, is the “modified/distorted road profile”. Thus, the modifications brought to the real road profile are aimed to compensate the lack of the longitudinal degree of freedom in the dynamic car simulator, in order to reproduce the vertical vibrations of the above-mentioned longitudinal motion regimes (so that to encounter the same car vertical displacements and accelerations for the in-house Matlab simulation, as for the CARSIM simulation).Results are presented in order to show how the real road profile is modified/distorted in order to cope with this impossibility to simulate the road profile in dynamic car simulators, without huge costs.Copyright

On the nonlocal simulation of nanoindentation problems

The nanoscopic scale can be considered as a lower bound to the mesoscopic scale, for which the nonlocal elasticity and the lattice dynamics yield to the same motion equations and the total potential energy expressions. Nonlocal elasticity takes better into account the long-range effects of all points of the body on one point of the body. This paper presents a nonlocal approach of the elastic nanoindentation problem. Starting to the local representation, the nonlocal solution for contact pressures is developed, obtaining finite values in all points near the contact boundary, which accords better with the experimental results.