Viscoelastic Finite Line Contact of Thin Bonded Layered Solids of Low Elastic Modulus

A new finite element-based contact mechanics analysis of layered viscoelastic solids of low elastic modulus is presented. The methodology is based on the Maxwell viscoelastic element, with stress relaxation taken into account by the Prony series’ representation of the bulk and shear material moduli. Simultaneous solutions for deviatoric and volumetric stresses were obtained under instantaneous elastic and subsequent viscoelastic relaxation, at multiples of the relaxation time of a Highly Filled Carbon Polymer (HFCP) layer. The results of the analysis were validated by a constructed, multi-layered sandwich sensor comprising the HFCP sensing elements covered by a protective silicone rubber surface of very low elastic modulus. The combined numerical-experimental approach, and the validated viscoelastic layered contact mechanics represent the original contribution of this paper, not hitherto reported in literature.

Stress Relaxation Measurement of Agar Using a Polymer-Based Microfluidic Device

In light of the significance of the viscoelastic property of agar to cell-based tissue engineering, this paper presents the stress relaxation measurement of agar using a polymer-based microfluidic device. Comprised of a single polymer rectangular microstructure and a set of electrolyte-enabled distributed transducers, this device is capable of detecting continuous distributed static and dynamic loads. In the measurement, an agar specimen is placed on the device and a rigid probe is utilized to press the specimen against the device with a step displacement input. Consequently, the stress relaxation behavior of the specimen translates to time-dependent continuous distributed loads acting on the device and is further registered as discrete resistance changes by the device. Two agar specimens of 1% and 3% in concentration, respectively, are measured using this device; and the data analysis is conducted on the measured results to extract Young’s relaxation modulus, which is further expressed by a Prony-series representation of the Maxwell model with two exponential terms. The results demonstrate the feasibility of using this device to measure the stress relaxation behavior of soft materials.

On the Dynamics of Gas Lubricated Triboelements

Gas lubricated triboelements are ubiquitous in many applications, from gas turbines seals, high speed dental drills, cryogenic refrigerators, oil-free bearings for turbo chargers, to read-write self acting heads in magnetic recording. In all these application the film must be maintained sufficiently precise especially in the presence of disturbances. Hence, it is insufficient to analyze such triboelements quasi statically, where a complete dynamic analysis of the system is needed. Numerous techniques have been developed over the past five decades, some are purely numerical, and others are semi-analytical. Complete analytical methods are practically non-existent because of the non-linear nature of the Reynolds equation. This work discusses a few of the common techniques, explores their intricate, and highlights limitations and flaws in implementation. Particularly: (1) in the so called “orbit codes” the equations of motion and the Reynolds equation must be solved simultaneously (and not “side-by-side”), (2) a common representation of linearized stiffness and damping is shown to be produce unrealistic results even in the simplest application, and (3) for the first time a mathematical justification is given for the use of Prony series representation of the hereditary stiffness in the so called “step-jump” method.