Robert M. Ebeling

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.

Sand-to-Concrete Interface Response to Complex Load Paths in a Large Displacement Shear Box

The large displacement shear box (LDSB) allows testing of interfaces as large as 711 by 406 mm (28 by 16 in.) with maximum interface displacements of 305 mm (12 in.). This device has been used to investigate the response of a variety of interfaces, including clay-geomembrane interfaces for which large displacements are important. The most recent application of the LDSB was to study the response of several sand-to-concrete interfaces under complex loading paths. In this application, the relevant characteristics of the LDSB are its geometry, which reduces the significance of end effects, its ability to apply monotonic and cyclic loading, and its ability to apply simultaneous changes in shear and normal stresses so that complex loading paths can be followed. This paper describes the main features of the LDSB, as well as the testing procedures and results of the sand-to-concrete interface tests that were performed. A procedure for normalizing the interface shear test data is also presented. This procedure facilitates comparative evaluations of interface response to different types of loading. The test results formed the basis for development of an extended hyperbolic model for interfaces that has been implemented in finite element analyses of soil-structure interaction problems.

The role of non-linear deformation analyses in the design of a reinforced soil berm at red river U-frame lock No.1

This paper describes a design application of non-linear deformation analysis to a complex soil-structure-foundation interaction problem through use of a finite element analysis. The problem consists of a proposed renovation to an existing soil-founded U-frame lock structure consisting of construction of a densely reinforced soil berm adjacent to an existing lock wall. Major questions facing the designer involve reduction of the earth pressure on the lock wall, layout of the reinforcing in the soil berm, and collateral effects of berm construction on the U-frame lock structure. A non-linear deformation analysis played a central role in addressing all of these questions. Berm construction and four operational load cases were used to understand the performance of the reinforced berm and to discern interactions among the lock, the backfill, the foundation strata of the U-frame lock, the reinforced berm, and the foundation strata of the reinforced berm. Insight gained from the soil-structure-foundation interaction analyses led to an alteration to the proposed reinforcement layout to enhance the performance of the reinforced soil berm.

Corroded Anchor Structure Stability/Reliability (CAS_Stab-R) software for hydraulic structures

This report describes software that provides a probabilistic estimate of time-to-failure for a corroding anchor strand system. These anchor systems are installed to preserve and extend the service life of U.S. Army Corps of Engineers hydraulic structures. Corrosion reduces the cross-section area of steel cables until the cable capacity is less than the tension force applied when the anchor cable was initially installed. When enough material is lost from the cable anchor that the cable capacity is less than the lock-off load of the anchor, the anchor will fail and no longer provide stability to the structure. A series of unique pull-test experiments conducted by Ebeling et al. (2016) at the U.S. Army Engineer Research and Development Center provided the required statistical relationships of reduced seven-strand cable capacity to (1) corroded cross-section area and to (2) corroded cross-section minimum short axis diameter for failed cable strands. The software product Corroded Anchorage Structural Stability and Reliability (CAS_Stab-R) produces probabilistic Remaining Anchor Life time estimates for anchor cables based upon the direct corrosion rate for the installation. CAS_Stab-R can also perform a probabilistic analysis to determine the Probability of Unsatisfactory Performance for a structural model cross section founded on rock. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. DESTROY THIS REPORT WHEN NO LONGER NEEDED. DO NOT RETURN IT TO THE ORIGINATOR. ERDC/ITL TR-17-3 iii

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.

Design of Very High-Strength Aligned and Interconnected Carbon Nanotube Fibers Based on Molecular Dynamics Simulations

The principal objective of this work is to implement a new material development paradigm using atomistic simulations to guide the molecular design of materials. Traditional empirical macroscopic material development studies omit the fundamental insight needed to understand material behavior at the atomic and molecular levels where material response begins. The new paradigm relies heavily on a tight integration between simulation and experimental efforts to design and process new materials with nanometer-scale precision. Exploiting nanotechnology requires atomic-molecular-level material design and the ability to process these materials with atomic-molecular-level precision. Processing materials with nanoscale precision poses formidable theoretical, computational, and experimental challenges to developing advanced materials. High performance computers and advanced physics-based simulations can complement experimental efforts to design, test, synthesize, and analyze novel materials and innovative structural designs. This method can be applied to a wide range of material designs. As a proof of concept, we began our work on the design of novel carbon nanotube-based materials. The mechanical properties of carbon nanotubes such as low-density, high-stiffness, and exceptional strength make them ideal candidates for reinforcement material in a wide range of high performance composites. Molecular dynamics simulations are used to predict the tensile response of fibers composed of aligned carbon nanotubes with intermolecular bonds of interstitial carbon atoms. The effects of bond density and carbon nanotube length distribution on fiber strength and stiffness are investigated. Results indicate that including cross link atoms between the carbon nanotubes in the strands significantly increases the load transfer between the carbon nanotubes and prevents them from slipping. This increases the elastic modulus and yield strength of the fibers by an order-of-magnitude. Carbon nanotube-based materials appear poised to affect civil and military engineering significantly over the next two decades by providing materials with an order-of- magnitude improvement in strength-to-weight and stiffness-to-weight ratios over existing materials.

Stress-Strain Behavior of and Hyperbolic Parameters for Structured/Cemented Silts

Isotropically consolidated-drained triaxial compression tests were performed on undisturbed structured/cemented silt to gain a better understanding of the shear behavior of and hyperbolic stress-strain parameters for this material. The results show that the hyperbolic stress-strain parameters for the structured/cemented silt are anisotropic and differ significantly from the hyperbolic parameters developed for silt reconstituted at the field total unit weight and water. As the confining pressure in a triaxial compression test approaches the preconsolidation pressure, the effects of the structure/cementation are reduced due to bond breakage, and the structured/cemented silt exhibits a stress-strain behavior similar to that of reconstituted silt.