Browsing by Author "Vassiliou, Michalis F."

Now showing 1 - 3 of 3
  • No Thumbnail Available
    Item
    Development of a pressure-, velocity-, and acceleration-dependent phenomenological friction model using experimental data of sliding tests between 11 polymers and stainless steel
    (2024) Tapia, Nicolas F.; Reyes, Sergio I.; Vassiliou, Michalis F.; Almazan, Jose L.
    This paper experimentally investigates the frictional behavior between stainless steel and 11 polymers. Particularly, the dependence of the friction coefficient on the sliding velocity, pressure, and acceleration is quantified. The novelty of this work lies in quantifying the acceleration-dependent nature of friction, correlating it to the well-documented Stick-Slip effect. The experimental setup consisted of two parallel stiff steel beams, one above the other, with a separation of 95 mm, and steel surfaces welded at the inner sides for sliding the polymers. Cylindrical polymer pads were placed between the stainless-steel surfaces and connected to a dynamic actuator to apply the displacement protocol. The protocol consisted of consecutive nominally constant-velocity ramp cycles covering velocities from 1 mm/s to 300 mm/s (with 20 mm/s increments). An additional vertical force was applied with a hydraulic actuator to reach nominal pressures in the polymers between 5 and 80 MPa. The results showed that the friction coefficient depends on the velocity, pressure, and acceleration of the motion, and a phenomenological model on these three variables is proposed. The velocity dependence can be represented through a logarithmic relationship, while the pressure dependence is through an exponential decay relationship. The acceleration dependence was represented through a linear relationship, which could capture the stick-slip effect. Overall, this work contributes to a better understanding of friction for seismic isolation systems. Since friction is the main source of energy dissipation in such structures, the proposed model will allow a higher accuracy in predicting variables of interest during the dynamic analyses of seismically isolated structures with frictional systems.
  • Thumbnail Image
    Item
    Full-scale shaking table test and numerical modeling of a 3000-liter legged storage tank isolated with a vertical rocking isolation system
    (WILEY, 2022) Reyes, Sergio, I; Almazán Campillay, José Luis; Vassiliou, Michalis F.; Tapia Flores, Nicolás Felipe; Colombo, Jose, I; Llera Martin, Juan Carlos de la
    This paper presents the numerical and experimental evaluation of a vertical-rocking isolation (VRI). This evaluation is done by 1-D shaking table tests performed on a full-scale legged storage tank of 3000-liters capacity and its representation through a simple yet representative rigid lumped-mass model approach. The isolation system setup consisted of four ISO3D-2G devices, each one placed on each leg of the tank, which uses high-damping natural rubber to generate the restoring and dissipative forces. The ISO3D-2G device is vertically flexible and laterally rigid, enabling the isolation mechanism of the rocking motion of the tank. The experiments were carried out using three white noise for the system identification and 17 ground motions inputs for the system validation. The measured variables included the lateral acceleration and displacement of the tank, and the vertical and rotational behavior of the isolation interface. The identification results showed a vertical-rotational coupled fundamental mode that is highly dependent on the amplitude of deformation, with a period varying from 0.5 to more than 1 s, depending on the intensity of the motion. The maximum displacement of the tank at the top remained below 13 cm with total accelerations of nearly 0.3 g, both for motions with Peak Ground Acceleration (PGA) values ranging from 0.3 to 0.8 g. The mean maximum values were predicted with the simplified model with errors of less than 10% and 21% for displacements and accelerations, respectively. Finally, the results show that the behavior of vertical-rocking isolated structures can be predicted by simplified models with reasonable errors and that the development of simple design guidelines and equations for VRI systems is possible.
  • No Thumbnail Available
    Item
    Validation of a three-dimensional finite element constitutive modeling approach for a thermoplastic polyurethane calibrated with uniaxial tests
    (2025) Reyes, Sergio I.; Lopez, Edgar L.; Vassiliou, Michalis F.
    This paper presents the three-dimensional finite element constitutive modeling validation for a Thermoplastic Poly-Urethane (TPU) material. The material parameters are those given in a previous study obtained by calibrating the constitutive model with uniaxial material-level test results (1-D tensile and compression tests). Confined compression tests were performed to estimate the bulk modulus for more accurate material characterization. The parameters were validated by comparing the results of experimental tests on TPU components with those obtained by its equivalent finite element simulations. The TPU component tests comprised cyclic compression tests of (i) a TPU cylinder, (ii) a solid 100 mm diameter TPU ball, and (iii) a 100 mm diameter TPU ball with an 80 mm steel core. In all tests, the constitutive model parameters showed an excellent performance in representing the mechanical behavior of the material observed in the tests, including the nonlinear stiffness and hysteresis. Finally, a briefcase study is presented to illustrate the applicability of the validated constitutive model parameters in modeling a novel TPU damper for structural control before its manufacturing and experimental testing. The validated constitutive modeling approach and available material parameters will allow the performance of reliable and early-stage finite element simulations to prototype the mechanical behavior of TPU components of different shapes and boundary conditions and thus gain relevant insight into the component level response before manufacturing and testing.