Richard W. Haskins

Tight-binding molecular dynamics study of the role of defects on carbon nanotube moduli and failure

We performed tight-binding molecular dynamics on single-walled carbon nanotubes with and without a variety of defects to study their effect on the nanotube modulus and failure through bond rupture. For a pristine (5,5) nanotube, Youngs modulus was calculated to be approximately 1.1 TPa, and brittle rupture occurred at a strain of 17% under quasistatic loading. The predicted modulus is consistent with values from experimentally derived thermal vibration and pull test measurements. The defects studied consist of moving or removing one or two carbon atoms, and correspond to a 1.4% defect density. The occurrence of a Stone-Wales defect does not significantly affect Youngs modulus, but failure occurs at 15% strain. The occurrence of a pair of separated vacancy defects lowers Youngs modulus by approximately 160 GPa and the critical or rupture strain to 13%. These defects apparently act independently, since one of these defects alone was independently determined to lower Youngs modulus by approximately 90 GPa, also with a critical strain of 13%. When the pair of vacancy defects adjacent, however, Youngs modulus is lowered by only approximately 100 GPa, but with a lower critical strain of 11%. In all cases, there is noticeable strain softening, for instance, leading to an approximately 250 GPa drop in the apparent secant modulus at 10% strain. When a chiral (10,5) nanotube with a vacancy defect was subjected to tensile strain, failure occurred through a continuous spiral-tearing mechanism that maintained a high level of stress (2.5 GPa) even as the nanotube unraveled. Since the statistical likelihood of defects occurring near each other increases with nanotube length, these studies may have important implications for interpreting the experimental distribution of moduli and critical strains.

Million-Atom Count Simulations of the Effects of Carbon Nanotube Length Distributions on Fiber Mechanical Properties

The extraordinary mechanical properties of carbon nanotubes (CNTs) make them prime candidates as a basis for super infrastructure materials. Ab initio, tight binding, and molecular dynamics simulations and recent experiments have shown that CNTs have tensile strengths up to about 15.5 million psi (110 GPa), Young’s modulus of 150 million psi (1 TPa), and density of about 80 lbs/ft3 (1.3 g/cm3). These qualities provide tensile strength-toweight and stiffness-to-weight ratios about 900 times and 30 times, respectively, those of high-strength (100,000- psi) steel. Building macromaterials that maintain these properties is challenging. Molecular defects, voids, foreign inclusions, and, in particular, weak intermolecular bonds have, to date, prevented macromaterials formed from CNTs from having the remarkable strength and stiffness characteristics of CNTs. The van der Waals forces associated with CNTsrepresent a force per unit length between CNTs. Accordingly, one would expect the bond strength between aligned CNTs to increase with overlap length. Real filaments are likely composed of CNTs with some distribution of lengths. To understand the effects that CNT length distributions have on the tensile strength of neat filaments of aligned CNTs, we performed a series of quenched molecular dynamics simulations on high performance computers using Sandia Laboratory’s Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) code. The cross-section of each filament was composed of hexagonal closest-packed (HCP) array CNT strands that formed two HCP rings. The filaments were constructed by placing (5,5) chirality CNTs end to end. While the choice of a single-chirality CNT fiber is currently unrealizable, the use of a singlechirality fiber allowed us to focus only on the effects of CNT lengths on filament response. The lengths of the CNTs were randomly selected to have Gaussian distribution with the average length ranging from 100 to 1,600Å. A series of simulations were performed on filament with lengths ranging from 400 to 6,400Å. For each filament, the strain was increased in small increments and quenched between strain increments. The total tensile force on the filament was recorded and used to determine the uniaxial stress-strain response of the filaments. The results of the simulations quantified the improvements in Young’s modulus, tensile strength, and critical strain as a function of the increase in the average component CNT lengths. These are the first molecular dynamics simulations that the authors are aware of that treat statistical qualities of realistic CNT structures. The simulation results are being used to guide the molecular design of CNT filaments to achieve super (1 million psi) strength. The simulations would be impractical, and perhaps impossible, without massively parallel, highperformance computational platforms and molecular dynamics simulation tools optimized to run on such platforms.

An Investigation of Corrosion Mitigation Strategies for Aging Post Tensioned Cables

Abstract : Over the past fifty years, the U. S. Army Corps of Engineers has been upgrading its projects by installing high-capacity, post-tensioned foundation anchors. These anchors are typically made with seven-wire strand cables. The purpose of these anchors has been to achieve structural stability for Corps hydraulic concrete structures (e.g., locks, dams, approach walls) and/or to remediate cracked concrete monoliths. Substantial improvements have been made in methods to protect multistrand anchor systems from corrosion since they were first used in Corps projects more than 50 years ago, but the corrosion of older multistrand anchorage systems is still a major concern. Previous technical reports from this ERDC research team have discussed ways to measure and assess corrosion and capacity losses due to corrosion of multistrand cables used for these anchor systems, as well as perform statistical estimates and predictions of the reduced cable capacity. This technical report explores state-of-the art existing corrosion mitigation and repair techniques that are applied in other systems, and turns a critical eye toward how these techniques could be applied for anchors supporting the Corps mass concrete hydraulic structures. Ten techniques were examined and the pros and cons of these methods, with respect to the Corps structure environment, are discussed.

Relating Corroded Seven-Strand, Posttensioned Cable Cross-Sectional Properties to Load Capacity

Multistrand anchors have seen widespread use, providing strength and stability at hydraulic Corps facilities. However, these steel tendons are subject to strength reduction as an effect of corrosion. Methods for evaluating the corroded cable strength do not exist to accurately estimate the time until tendon cables would have to be replaced (at great expense). The following five research tasks are used to address this deficiency: laboratory accelerated corrosion; pull-tests on pristine and laboratory corroded cables; optical scanning; data collection correlated with cross-sectional properties of cables; and development of a method to relate this data to the field. The pull-tests provide measured capacities for seven-strand, posttensioned (PT) cables. An optical scan of the corroded cables provides cross-sectional properties of individual wires within the pulled cables. Trendlines are established for the related peak cable capacities and cross-sectional properties in an effort to determine their correlations. Trendlines for minimum wire area and second-moment short axis diameter are found with low error, making them good predictors of loaded cable capacity. This pull-test dataset has been related back to cable failure in the field, assuming a linear rate of corrosion loss for the cross-sectional properties and required PT capacity.

Corrosion induced loss of capacity of post-tensioned seven wire strand cable used in multistrand anchor systems installed at Corps projects

Abstract : Over the past five decades, the U.S. Army Corps of Engineers has been upgrading its projects by installing high-capacity, post-tensioned foundation anchors, typically with seven-wire strand cables. The purpose of these anchors has been to achieve structural stability for Corps hydraulic concrete structures (e.g., locks, dams, approach walls) and/or to remediate cracked concrete monoliths. Substantial improvements to protect multistrand anchor systems from corrosion have been made in the past five decades, but the corrosion of older multistrand anchorage systems is still a major concern. This report discusses a laboratory-testing program for the estimation of post-tensioning (PT), seven-wire strand cable strength as a function of corroded cross-sectional material loss. Pull tests were performed to gather reduced cable strength measurements. An innovative morphological procedure using digital photography was developed by U.S. Army Engineer Research and Development Center (ERDC) researchers for quantifying the cross-section geometrical properties of cables near their failure locations. The laboratory-testing program also included a successful series pull test to failure on pristine specimens for a control set of data, and the issues encountered are detailed. A statistical assessment of pull-test data to failure of pristine and corroded cables is used to establish a correlation between cross-section properties, corroded and pristine, and the cable strength. An overview of the corrosion process and the variables, ranked by contribution in Corps structures, which determine corrosion rate at each of the multistrand cables, is provided. Further, methods for estimating cable capacity under load were developed using the provided best-fit curves from the laboratory pull tests.