Accepted Manuscripts

Nitin Garg, Gurudutt Chandrashekar, Farid Alisafaei and Chung-Souk Han
J. Eng. Mater. Technol   doi: 10.1115/1.4043766
Micro-beam bending and nano-indentation experiments illustrate that length scale dependent elastic deformation can be significant in polymers at micron and submicron length scales. Such length scale effects in polymers should also affect the mechanical behavior of reinforced polymer composites, as particle sizes or diameters of fibers are typically in the micron range. Corresponding experiments on particle-reinforced polymer composites have shown increased stiffening with decreasing particle size at the same volume fraction. To examine a possible linkage between the size effects in neat polymers and polymer composites a numerical study is pursued here. Based on a couple stress elasticity theory, a finite element approach for plane strain problems is applied to predict the mechanical behavior of fiber-reinforced epoxy composite materials at micrometer length scale. Numerical results show significant changes in the stress fields and illustrate that with a constant fiber volume fraction the effective elastic modulus increases with decreasing fiber diameter. These results exhibit similar tendencies as in mechanical experiments of particle reinforced polymer composites.
TOPICS: Deformation, Fibers, Polymer composites, Particulate matter, Polymers, Mechanical behavior, Stress, Composite materials, Epoxy resins, Epoxy adhesives, Linkages, Finite element analysis, Elasticity, Elastic moduli, Nanoindentation, Particle size, Plane strain, Microbeams
Bhasker Paliwal, Youssef Hammi, Mei Chandler, Robert D. Moser and Mark F. Horstemeyer
J. Eng. Mater. Technol   doi: 10.1115/1.4043705
A new dynamic strain-rate dependent elasto-viscoplastic damage constitutive model for Ultra-High Performance Concrete (UHPC) is developed by incorporating Duvaut-Lions viscoplasticity generalized to multi-surface plasticity followed by rate-dependent dynamic damage initiation and evolution under multiaxial loading, to our previous elastoplastic-damage model. The predictive capability of the proposed model is compared against experimental results and experimentally observed features from tests on Cor-Tuf concrete, a Reactive Powder Concrete (RPC) and a proprietary UHPC developed by the U.S. Army Corps of Engineers. These experiments were conducted under various compressive loading conditions under low to high confinement and different strain-rates, and model predictions demonstrate excellent agreement with these results.
TOPICS: Concretes, Damage, Plasticity, Engineers, Constitutive equations, Army, Viscoplasticity
Adetokunbo Adedoyin, Koffi Enakoutsa and D.J. Bammann
J. Eng. Mater. Technol   doi: 10.1115/1.4043627
The Evolving Micro-structural Model of Inelasticity (EMMI) previously developed as an improvement over the Bammann-Chiesa-Johnson (BCJ) material model, is well known to describe the macroscopic non-linear behavior of polycrystalline metals subjected to rapid external loads such as those encountered during high rate events possibly near shock regime. The improved model accounts for deformation mechanisms such as thermally activated dislocation motion, generation, annihilation and drag. It also accounts for the effects of material texture, recrystallization and grain growth and void nucleation, growth and coalescence. Material incompatibility, previously disregard in the aforementioned model, manifest themselves as structural misorientation where ductile failure often initiates are currently being considered. To proceed, the representation of material incompatibility is introduced into the EMMI model by incorporating the distribution of the geometrically necessary defects such as dislocations and disclination. To assess the newly proposed formulation, classical elastic solutions of benchmarks problems including far field stress applied to the boundary of body containing a defect, e.g., voids, cracks, and dislocations are used to compute the plastic velocity gradient for various states of the material in terms of assumed values of the internal state variables. The full-field state of the inelastic flow is then computed and the spatial dependence of the dislocations and disclination density is determined. The predicted results shows good agreement with finding of dislocation theory.
TOPICS: Dislocations, Stress, Nucleation (Physics), Shock (Mechanics), Fracture (Materials), Texture (Materials), Dislocation motion, Density, Flow (Dynamics), Deformation, Metals, Drag (Fluid dynamics), Recrystallization, Dislocations (Crystals), Failure
James A. Mills, Hang Xiao and Xi Chen
J. Eng. Mater. Technol   doi: 10.1115/1.4043628
There have been many studies performed with respect to the indentation of thin films affixed to a corresponding substrate base. These studies have primarily focused on determining the mechanical properties of the film. It is the goal of this paper to further understand the role that the film plays and how a potential pre-stressing of this film has on both the film and substrate base. It is equally important to be able to understand the material properties of the substrate since during manufacturing or long term use, the substrate properties may change. In this study, we establish through spherical indentation a framework to characterize the material properties of both the substrate and film as well as a method to determine the prestress of the film. It is proposed that through an initial forward analysis a set of relationships are developed that along with then performing a single spherical indentation test, measuring the indentation force at two prescribed depths, material properties of both the film and substrate can be determined. The problem is further enhanced by also developing the capability of determining any equi-biaxial stress state that may exist in the film. A generalized error sensitivity analysis of this formulation is also performed systematically. This study will enhance the present knowledge of a typical pre-stressed film/substrate system as is commonly used in many of today's engineering and technical applications.
TOPICS: Thin films, Coating processes, Coatings, Manufacturing, Stress, Materials properties, Mechanical properties, Errors, Sensitivity analysis

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