Dear researchers----We spent more than two years to prepare a research proposal and it was submitted to a federal funding agency (not NSF). Based on one reviewer’s comments, the program manager rejected our proposal. Our title is “A Multi-Scale Approach of Combining Nano-indentation with Computational Mechanics to Predict Impact Behavior of Structural Composite Materials”. I only list these comments related to nano-indentation. Your frank opinion is really appreciated.
Reviewer said 1. “the nano-indentation method employs low velocity impact methods although the actual impact problems for aircraft are high speed, highly dynamic, and highly non-linear problems.”
2. “It is not clear how the proposed quasi-static method can predict plastic response of a wide range of speed variations.”
3. “The proposal could also be improved by explaining the methodology to capture delamination using nano-indentation techniques on matrices or on fibers as simulation parameters.”
Akanksha Garg, Craig Maloney
(accepted mannuscript in Journal of Mechanics and Physics of Solids)
We perform atomistic simulations to study the mechanism of homogeneous dislocation nucleation in two dimensional (2D) hexagonal crystals during nanoindentation with a circular indenter of radius R. We study both a realistic embedded atom method (EAM) potential for Aluminum in addition to simple pair-wise potentials: Lennard-Jones, Morse, and Hookean springs. The nucleation process is governed by the vanishing of the energy associated with a single energy eigenmode. The critical eigenmode, or dislocation embryo, is found to be localized along a line of atoms with a lateral extent, \xi, at some depth, Y , below the surface. For all interatomic potentials, the scaled critical load, Fc=R, and scaled critical contact length, Cc=R, decrease to R-independent values in the limit of large R. However, despite this \xi & Ydisplay non-trivial scaling with R. We show that although both the interaction potential and the orientation of the lattice affect the prefactors in the scaling relations, all the scaling laws are robust. Furthermore, we show that a stability criterion proposed by vanVliet et. al. based on the minimum eigenvalue, , of the local acoustic tensor predicts the location, orientation, and polarization of the dislocation embryo with a high degree of all crystallographic orientations and interaction potentials, erroneously indicates instability before the true instability occurs.
http://www.sciencedirect.com/science/article/pii/S0022509616302502
http://scitation.aip.org/content/aip/journal/jap/117/11/10.1063/1.491594...
Nanoindentation technique is widely employed in the semiconductor industry to characterize the mechanical properties of thin film materials. Low dielectric constant (low-k) materials, commonly used as interlayer dielectrics of the on-chip interconnects, are structurally fragile and prone to fracture and delamination when subject to concentrated stresses during nanoindentation.Characterization of their mechanical properties by nanoindentation technique is complicated not only by the well-known substrate effect arising from the elastic mismatch between the low-k filmand the substrate but also by the potential material damages. This paper demonstrates the use of a buffer layer structure augmented with a novel analysis procedure to overcome these challenges, allowing us to extend the nanoindentation technique to even thinner films and improve measurement accuracy. The demonstrated approach is not restricted to low-k dielectrics,but is expected to be generically useful for other material systems given proper choice of thebuffer layer.
Note: The manuscript is available free of charge within the first month of publication.
Controllable viscoelastic behavior of vertically aligned carbon nanotube arrays
Kilho Eom, Kihwan Nam, Huihun Jung, Pilhan Kim, Michael S. Strano, Jae-Hee Han, Taeyun Kwon
Abstract
We have characterized the mechanical behavior of aligned carbon nanotube (CNT) arrays that serve as foam-like energy absorbing materials, by using atomic force microscope indentation. It is shown that the mechanical properties (e.g. elastic modulus, adhesion force, and energy dissipation) of aligned CNT arrays are dependent on the length of CNTs as well as chemical environment that surrounds CNT arrays. More remarkably, it is found that CNT array made of CNTs with their length of 10 μm exhibits the excellent damping property (i.e. energy dissipation) higher than that of a conventional composite such as Kevlar. It is also shown that the energy dissipation of CNT arrays during loading–unloading process can be reduced by the solution surrounding CNT array, and that the decrease of energy dissipation for CNT array due to solution depends on the solution type, which mediates the interaction between individual nanotubes. Our study sheds light on the design principles for CNT array-based foam-like materials.
This paper was published at Carbon, and attached in pdf format.
