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J. Eng. Mater. Technol. 2019;142(1):011001-011001-12. doi:10.1115/1.4043705.

A new dynamic strain rate-dependent elasto-viscoplastic damage constitutive model for ultrahigh-performance concrete (UHPC) is developed by incorporating Duvaut–Lions viscoplasticity generalized to multisurface 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.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;142(1):011002-011002-7. doi:10.1115/1.4043766.

Microbeam 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.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;142(1):011003-011003-11. 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 prestressing 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. A single spherical indentation test can then be performed, measuring the indentation force at two prescribed depths, and with the relationships developed from the forward analysis, the material properties of both the film and substrate can be determined. The problem is further enhanced by also developing the capability of determining any equibiaxial 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 prestressed film/substrate system as is commonly used in many of today’s engineering and technical applications.

Commentary by Dr. Valentin Fuster

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