Hyperelastic Material Parameter Determination and Numerical Study of TPU and PDMS Dampers

Dampers provide safety by controlling unwanted motion that is caused due to the conversion of mechanical work into another form of energy (e.g., heat). State-of-the-art materials are elastomers and include thermoplastic elastomers. For the polymer-appropriate replacement of multi-component shock absorbers comprising mounts, rods, hydraulic fluids, pneumatic devices, or electro-magnetic devices, among others, in-depth insights into the mechanical characteristics of damper materials are required. The ultimate objective is to reduce complexity by utilizing inherent material damping rather than structural (multi-component) damping properties. The objective of this work was to compare the damping behavior of different elastomeric materials including thermoplastic poly(urethane) (TPU) and silicone rubber blends (mixtures of different poly(dimethylsiloxane) (PDMS)). Therefore, the materials were hyper- and viscoelastic characterized, a finite element calculation of a ball drop test was performed, and for validation, the rebound resilience was measured experimentally. The results revealed that the material parameter determination methodology is reliable, and the data that were applied for simulation led to realistic predictions. Interestingly, the rebound resilience of the mixture of soft and hard PDMS (50:50) wt% was the highest, and the lowest values were measured for TPU.

Improvement of the mechanical and damping behavior of nylon by integration of nanoclay platelets

Nylon is used as a gear material thanks to its beneficial characteristics, such as self-lubrication, noiseless and fail-safe operation. Poor resistance to heat, dimensional stability, shock and impact loads are major drawbacks of nylon when used in engineering applications. The addition of a nanofiller to a nylon matrix can enhance its mechanical and vibrational properties. Montmorillonite nanoclay (Cloisite 15 A, Cloisite 20 A and Cloisite 30B) modified with ammonium salt was incorporated into the Nylon 6 matrix by solution mixing and melt mixing. Nanoclay with 1, 2 and 3 wt.-% were added to the nylon matrix and the resulting mechanical and free vibration characteristics were determined. The experimental results of the mechanical and free vibration behavior were compared with the ANSYS results. Tensile strength, modulus of elasticity, specific strength, specific stiffness, natural frequency and damping factor were found to increase as the weight percentage of the nanoclay in the nylon matrix increased. Cloisite 30B nanocomposite shows better mechanical and free vibration characteristics when compared with pure Nylon 6, Cloisite 15 A and Cloisite 20 A nanocomposites. The Cloisite 30B nano-composite was prepared with 2 wt.-% shows maximum mechanical and free vibration performance.

Nonlinear stability analysis and numerical continuation of bifurcations of a rotor supported by floating ring bearings

Floating ring bearings have been widely used, over the last decades, in rotors of automotive turbochargers because of their improved damping behavior and their good emergency-operating capabilities. They also offer a cost-effective design and have good assembly properties. Nevertheless, rotors with floating ring bearings show vibration effects of nonlinear nature induced by self-excited oscillations originating from the bearing oil films (oil whirl/whip phenomena) and may exhibit various nonlinear vibration effects which may cause damage to the rotor. In order to investigate these dynamic phenomena, this paper has developed a nonlinear model of a perfectly balanced rigid rotor supported by two identical floating ring bearings with consideration of their vibration behavior mainly governed by fluid dynamics. The dimensionless hydrodynamic forces of floating ring bearings have been derived based on the short bearing theory and the half Sommerfeld solution. Using the numerical continuation approach, different bifurcations are detected when a control parameter, the journal speed, is varied. Depending on the system’s physical parameters, the rotor can show stable or unstable limit cycles which themselves may collapse beyond a certain rotor speed to exhibit a fold bifurcation. Bifurcation analysis is performed to investigate the occurring instabilities and nonlinear phenomena. Such results explain the instabilities characteristics of the floating ring bearing in high-speed applications. It has also been found that the selection of the bearing modulus plays an important role in the characterization of the rotor stability threshold speed and bifurcation sequences. An understanding of the system’s nonlinear behavior serves as the basis for new and rational criteria for the design and the safe operation of rotating machines.

Impact of convection on the damping of an oscillating droplet during viscosity measurement using the ISS-EML facility

Oscillating droplet experiments are conducted using the Electromagnetic Levitation (EML) facility under microgravity conditions. The droplet of molten metal is internally stirred concurrently with the pulse excitation initiating shape oscillations, allowing viscosity measurement of the liquid melts based on the damping rate of the oscillating droplet. We experimentally investigate the impact of convection on the droplet’s damping behavior. The effective viscosity arises and increases as the internal convective flow becomes transitional or turbulent, up to 2–8 times higher than the intrinsic molecular viscosity. The enhanced effective viscosity decays when the stirring has stopped, and an overshoot decay pattern is identified at higher Reynolds numbers, which presents a faster decay rate as the constraint of flow domain size becomes influential. By discriminating the impact of convection on the viscosity results, the intrinsic viscosity can be evaluated with improved measurement accuracy.