Blocking effect of twin boundaries on partial dislocation emission from void surfaces

Recent discovery that nanoscale twin boundaries can be introduced in ultrafine-grained metals to improve strength and ductility has renewed interest in the mechanical behavior and deformation mechanisms of these nanostructured materials. By controlling twin boundary spacing, the effect of twin boundaries on void growth is investigated by using atomistic simulation method. The strength is significantly enhanced due to the discontinuous slip system associated with these coherent interfaces. Atomic-scale mechanisms underlying void growth, as well as the interaction between twin boundaries and the void, are revealed in details

Active Shape Control and Phase Coexistence of Dielectric Elastomer Membrane With Patterned Electrodes

Various applications of dielectric elastomers (DEs) have been realized in recent years due to their lightweight, low cost, large actuation and fast response. In this paper, experiments and simulations are performed on the active shape control of DE structures with various two-dimensional patterned electrodes by applying voltage. A DE membrane with a pattern of electrodes is mounted on an air chamber. It is first inflated by air pressure and then further deformed by applying voltage, which actively controls the membrane shape. Under higher voltage, an acrylic membrane with larger actuation can induce shape instability and demonstrate multiphase coexistence behavior. In the framework of electromechanical theory, finite element simulations are carried out and the results are in good agreement with those obtained by experiments

Bonding dissimilar polymer networks in various manufacturing processes

Recently developed devices mimic neuromuscular and neurosensory systems by integrating hydrogels and hydrophobic elastomers. While different methods are developed to bond hydrogels with hydrophobic elastomers, it remains a challenge to coat and print various hydrogels and elastomers of arbitrary shapes, in arbitrary sequences, with strong adhesion. Here we report an approach to meet this challenge. We mix silane coupling agents into the precursors of the networks, and tune the kinetics such that, when the networks form, the coupling agents incorporate into the polymer chains, but do not condensate. After a manufacturing step, the coupling agents condensate, add crosslinks inside the networks, and form bonds between the networks. This approach enables independent bonding and manufacturing. We formulate oxygen-tolerant hydrogel resins for spinning, printing, and coating in the open air. We find that thin elastomer coatings enable hydrogels to sustain high temperatures without boiling

A Conventional Theory of Mechanism-Based Strain Gradient Plasticity

130 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2004.CMSG is also used to study the indentation size effect (ISE) observed in micro and nanoindentation experiments. Different hardness trends with different indenter shapes obtained from CMSG agree very well with the experimental measurement.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

A Conventional Theory of Mechanism-Based Strain Gradient Plasticity

130 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2004.CMSG is also used to study the indentation size effect (ISE) observed in micro and nanoindentation experiments. Different hardness trends with different indenter shapes obtained from CMSG agree very well with the experimental measurement.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

A cohesive zone model for the elevated temperature interfacial debonding and frictional sliding behavior

In this study, a cohesive zone model that accounts for the interfacial decohesion and frictional sliding behavior as well as the effect of the residual stress is formulated and implemented for analyzing fiber push-out problems in metal matrix composites at elevated temperature. The proposed model can explore the coupling between the interfacial debonding and frictional sliding behavior within a simple formulation. It has been demonstrated the model, after proper calibration of the interfacial shear strength by available experimental data in the literature, can successfully account for the temperature dependence of the load–displacement curves in the push-out test. A parametric study to investigate other material parameters on the load–displacement response of the fiber push-out test has also been carried out and the results show that the frictional stress has a significant effect on the peak load and the frictional sliding behavior. The matrix yield strength is also important for the interaction between the matrix and the cohesive interface

Electromechanical Bistable Behavior of a Novel Dielectric Elastomer Actuator

High voltage is required for the existing dielectric elastomer (DE) actuators to convert electrical energy to mechanical energy. However, maintaining high voltage on DE membranes can cause various failures, such as current leakage and electrical breakdown, which limits their practical applications, especially in small-scale devices. To overcome the above drawback of DE actuators, this paper proposes a new actuation method using DE membranes with a properly designed bistable structure. Experiment shows that the actuator only requires a high-voltage pulse to drive the structure forward and backward with electromechanical snap-through instability. The actuator can maintain its stroke when the voltage is removed. An analytical model based on continuum mechanics is developed, showing good agreement with experiment. The study may inspire the design and optimization of DE transducers