Effect of Phenolic Matrix Microcracking on the Structural Response of a 3-D Woven Thermal Protection System

The effect of microcracking in the phenolic matrix of a three-dimensional woven thermal protection system (TPS) and the resulting material stiffness reduction was studied via a comparison of finite element analysis results from a linear analysis and an iterative linear analysis. A TPS is necessary to protect space vehicles from the aerodynamic heating of planetary entry. The Heatshield for Extreme Entry Environment Technology (HEEET) project has developed a TPS for use in high heat-flux and pressure missions. The material is a dual-layer continuous dry weave, which is then infiltrated with a low-density phenolic resin matrix to form a rigid ablator. The phenolic resin matrix does not have structural load transfer requirements, and testing has shown that the phenolic resin can fully satisfy thermal requirements when the matrix contains microcracks. Due to high stresses in the through-the-thickness direction of the material, phenolic microcracks may form in the matrix material, which would result in a reduction of stiffness. An exploratory study was conducted to determine if reduction in material stiffness would change the load paths and/or decrease the structural margins. A comparison was performed between a linear finite element analysis that did not take into account phenolic microcracking and an iterative linear finite element analysis that accounted for propagation of phenolic microcracking. Four subcases using varying assumptions were analyzed and the results indicate that the assumed strength at which the phenolic microcracking propagates was the critical parameter for determining the extent of microcracking in the phenolic matrix. Phenolic microcracking does not have an adverse effect on the structural response of the test article and is not a critical failure

"Sticky Hands": learning and generalization for cooperative physical interactions with a humanoid robot

"Sticky Hands" is a physical game for two people involving gentle contact with the hands. The aim is to develop relaxed and elegant motion together, achieve physical sensitivity-improving reactions, and experience an interaction at an intimate yet comfortable level for spiritual development and physical relaxation. We developed a control system for a humanoid robot allowing it to play Sticky Hands with a human partner. We present a real implementation including a physical system, robot control, and a motion learning algorithm based on a generalizable intelligent system capable itself of generalizing observed trajectories' translation, orientation, scale and velocity to new data, operating with scalable speed and storage efficiency bounds, and coping with contact trajectories that evolve over time. Our robot control is capable of physical cooperation in a force domain, using minimal sensor input. We analyze robot-human interaction and relate characteristics of our motion learning algorithm with recorded motion profiles. We discuss our results in the context of realistic motion generation and present a theoretical discussion of stylistic and affective motion generation based on, and motivating cross-disciplinary research in computer graphics, human motion production and motion perception

POLYMER COMPOSITES FOR SENSING AND ACTUATION

This thesis concerns materials for polymer actuators and mechanical sensors. Polymer actuators are a class of artificial muscle with promising actuation performance; however, they are currently limited by the materials used in their fabrication. The metal-foil type mechanical strain gauges are commercially available and well understood; however, typically have gauge factors less than 5.5 [1], cannot be patterned into custom shapes, and only monitor small areas. New materials provide opportunities to improve the performance of both polymer actuators and mechanical sensors. The aim of this research was to develop, characterize, and implement such materials. Specifically, this thesis describes novel composites of exfoliated graphite (EG) blended with elastomeric hosts. The mechanical and electrical properties of these composites were tailored for two specific applications by modifying the EG loading and the elastomer host: compliant electrodes and strain gauges. Compliant electrodes were demonstrated that had ultimate tensile strains greater than 300% and that could withstand more than 106 strain cycles. Composites fabricated with polydimethylsiloxane (PDMS) exhibited conductivities up to 0.2 S/cm, and having tangent moduli less than 1.4 MPa. This modulus is the lowest reported for loaded elastomers above the percolation threshold. Conductivity was increased to more than 12.5 S/cm by fabricating composites with polyisoprene (latex) elastomers, and the tangent moduli remained less than 5 MPa. Actuation strains of polymer actuators were increased 3 fold using the composites as electrodes, compared to using carbon-grease electrodes. This was due to the composites ability to be spincoated with thin insulating layers of PDMS, allowing 30% higher electric fields to be applied. Strain gauges fabricated with these composites exhibited gauge factors (GFs) > 27,000, to the authors knowledge this is the highest GF ever reported. The effects of humidity, temperature and strain were investigated

The role of matrix cracks and fibre/matrix debonding on the stress transfer between fibre and matrix in a single fibre fragmentation test

The single fibre fragmentation test is commonly used to characterise the fibre/matrix interface. During fragmentation, the stored energy is released resulting in matrix cracking and/or fibre/matrix debonding. Axisymmetric finite element models were formulated to study the impact of matrix cracks and fibre/matrix debonding on the effective stress transfer efficiency (EST) and stress transfer length (STL). At high strains, plastic deformation in the matrix dominated the stress transfer mechanism. The combination of matrix cracking and plasticity reduced the EST and increased STL. For experimental validation, three resins were formulated and the fragmentation of an unsized and uncoupled E-glass fibre examined as a function of matrix properties. Fibre failure was always accompanied by matrix cracking and debonding. With the stiff resin, debonding, transverse matrix cracking and conical crack initiation were observed. With a lower modulus and lower yield strength resin the transverse matrix crack length decreased while that of the conical crack increased. (C) 2011 Elsevier Ltd. All rights reserved

Deformation and Failure of a Multi-Wall Carbon Nanotube Yarn Composite

Forests of multi-walled carbon nanotubes can be twisted and manipulated into continuous fibers or yarns that exhibit many of the characteristics of traditional textiles. Macro-scale analysis and test may provide strength and stiffness predictions for a composite composed of a polymer matrix and low-volume fraction yarns. However, due to the nano-scale of the carbon nanotubes, it is desirable to use atomistic calculations to consider tube-tube interactions and the influence of simulated twist on the effective friction coefficient. This paper reports laboratory test data on the mechanical response of a multi-walled, carbon nanotube yarn/polymer composite from both dynamic and quasi-static tensile tests. Macroscale and nano-scale analysis methods are explored and used to define some of the key structure-property relationships. The measured influence of hot-wet aging on the tensile properties is also reported

An anthropomorphic soft skeleton hand exploiting conditional models for piano playing.

The development of robotic manipulators and hands that show dexterity, adaptability, and subtle behavior comparable to human hands is an unsolved research challenge. In this article, we considered the passive dynamics of mechanically complex systems, such as a skeleton hand, as an approach to improving adaptability, dexterity, and richness of behavioral diversity of such robotic manipulators. With the use of state-of-the-art multimaterial three-dimensional printing technologies, it is possible to design and construct complex passive structures, namely, a complex anthropomorphic skeleton hand that shows anisotropic mechanical stiffness. We introduce a concept, termed the "conditional model," that exploits the anisotropic stiffness of complex soft-rigid hybrid systems. In this approach, the physical configuration, environment conditions, and conditional actuation (applied actuation) resulted in an observable conditional model, allowing joint actuation through passivity-based dynamic interactions. The conditional model approach allowed the physical configuration and actuation to be altered, enabling a single skeleton hand to perform three different phrases of piano music with varying styles and forms and facilitating improved dynamic behaviors and interactions with the piano over those achievable with a rigid end effector

AN INTERPOLATION METHOD FOR DETERMINING THE FREQUENCIES OF PARAMETERIZED LARGE-SCALE STRUCTURES

Parametric Model Order Reduction (pMOR) is an emerging category of models developed with the aim of describing reduced first and second-order dynamical systems. The use of a pROM turns out useful in a variety of applications spanning from the analysis of Micro-Electro-Mechanical Systems (MEMS) to the optimization of complex mechanical systems because they allow predicting the dynamical behavior at any values of the quantities of interest within the design space, e.g. material properties, geometric features or loading conditions. The process underlying the construction of a pROM using an SVD-based method [18] accounts for three basic phases: a) construction of several local ROMs (Reduced Order Models); b) projection of the state-space vector onto a common subspace spanned by several transformation matrices derived in the first step; c) use of an interpolation method capable of capturing for one or more parameters the values of the quantity of interest. One of the major difficulties encountered in this process has been identified at the level of the interpolation method and can be encapsulated in the following contradiction: if the number of detailed finite element analyses is high then an interpolation method can better describe the system for a given choice of a parameter but the time of computation is higher. In this paper is proposed a method for removing the above contradiction by introducing a new interpolation method (RSDM). This method allows to restore and make available to the interpolation tool certain natural components belonging to the matrices of the full FE model that are related on one side, to the process of reduction and on the other side, to the characteristics of a solid in the FE theory. This approach shows higher accuracy than methods used for the assessment of the system’s eigenbehavior. To confirm the usefulness of the RSDM a Hexapod will be analyzed