Study of the effect of contact force model on the dynamic response of mechanical systems with dry clearance joints : computational and experimental approaches

The main objective of this work is to present a computational and experimental study on the contact forces developed in revolute clearance joints. For this purpose, a well-known slider-crank mechanism with a revolute clearance joint between the connecting rod and slider is utilized. The intra-joint contact forces that generated at this clearance joints are computed by considered several different elastic and dissipative approaches, namely those based on the Hertz contact theory and the ESDU tribology-based for cylindrical contacts, along with a hysteresis-type dissipative damping. The normal contact force is augmented with the dry Coulomb’s friction force. In addition, an experimental apparatus is use to obtained some experimental data in order to verify and validate the computational models. From the outcomes reported in this paper, it is concluded that the selection of the appropriate contact force model with proper dissipative damping plays a significant role in the dynamic response of mechanical systems involving contact events at low or moderate impact velocities.Fundação para a Ciência e a Tecnologia (FCT

Functional Development of Engineered Skeletal Muscle from Adult and Neonatal Rats

A myooid is a three-dimensional skeletal muscle construct cultured from mammalian myoblasts and fibroblasts. The purpose was to compare over several weeks in culture the morphology, excitability, and contractility of myooids developed from neonatal and adult rat cells. The hypotheses tested were as follows: (1) baseline forces of myooids correlate with the cross-sectional area (CSA) of the myooids composed of fibroblasts, and (2) peak isometric tetanic forces normalized by total CSA (specific Po) of neonatal and adult rat myooids are not different. Electrical field stimulation was used to measure the excitability and peak tetanic forces. The proportion of the CSA composed of fibroblasts was greater for neonatal (40%) than adult (17%) myooids. For all myooids the baseline passive force normalized by fibroblast CSA (mean = 5.5 kPa) correlated with the fibroblast CSA (r2 = 0.74). A two-element cylindrical model was analyzed to determine the contributions of fibroblasts and myotubes to the baseline force. At each measurement period, the specific Po of the adult myooids was greater than that of the neonatal myooids. The specific Po of the adult myooids was ~1% of the control value for adult muscles and did not change with time in culture, while that of neonatal myooids increased.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/63318/1/107632701753213192.pd