Mary L. Hughes

Strengthening of Reinforced Concrete Beams with Externally Bonded Composite Laminates

A particularly challenging problem confronting engineers in the revival of U.S. infrastructure is the rehabilitation of concrete structures. This paper summarizes the results of experimental and analytical studies concerning the flexural strengthening of reinforced concrete beams by the external bonding of high-strength, lightweight fiber reinforced plastic (FRP) plates to the tension face of the beam. Twenty-four large-scale beams were tested experimentally to evaluate the strength enhancement provided by the FRP plates. An inelastic section analysis procedure was developed that accurately predicts the load displacement response of the retrofitted beams. A nonlinear finite element method analysis was also conducted that corroborates the results from the experimental study and inelastic section analysis.

Numerical analysis of high strain rate splitting-tensile tests

Abstract Experimental splitting-tension tests were conducted on 2-in. diameter concrete specimens in a Split Hopkinson Pressure Bar at strain rates of 4.4, 10.6, and 14.7/sec. The specimens were instrumented with electrical resistance strain gages and break circuits to detect crack initiation and growth. Experimental results indicate that there is a shift of crack initiation time relative to the peak stress. Also, experimental strength vs strain rate data reveal that the dynamic tensile strength of concrete is significantly higher than the static tensile strength. A comprehensive numerical analysis was conducted on the splitting-tensile experiments to investigate the effects of varying the uniaxial tensile strength of the concrete on the crack initiation time, stress state, crack growth characteristics, and failure mode in the concrete specimens. The results of the numerical analyses are used to enhance the understanding of concrete tensile strength strain rate sensitivity.

Numerical simulation of high strain rate concrete compression tests

Abstract Dynamic direct compression tests of plain concrete specimens were conducted on a split-Hopkinson pressure bar (SHPB) to investigate the effects of increasing strain rate on the compressive strength of concrete. A comprehensive finite element method study was performed on the SHPB experiments. Both linear and nonlinear analyses were conducted. The results of the numerical analyses disclose the dynamic states of stress in the concrete specimen prior to failure as well as the modes of failure.

High-Speed Penetration of Concrete Using a New Analytical Model of Velocity-Dependent Friction

In this paper, a completely analytical formulation of velocity-dependent friction is used to eliminate the analytical complexity associated with the evaluation of velocity-dependent integrals in the penetration model presented in [5]. The simplification employed is an extension of the method used in [6], in which the velocity regime governing the coefficient of sliding friction was divided into two sectors, producing two distinct velocity-dependent friction coefficients, for low-speed and high-speed friction. In this paper, the velocity regime is divided into a number of intervals, thereby replacing the linear velocity-dependent friction relationship at low speeds with a series of steps; the method is flexible enough also to permit a nonlinear profile for low-speed sliding friction. As a byproduct of the work, a completely analytical estimate for the work done by sliding friction in the problem can be found, permitting the evaluation of projectile mass loss.Copyright

Penetration With High-Speed Friction

Work on friction dates back more than two hundred years. Some of the work applies to dynamic situations in which the influence of speed may be a factor (see Kragelskii [5]). But, most of the work does not apply to the velocity regime observed in high-speed penetration problems. This has become an issue because of the Air Force’s interest in smaller and more versatile air launched weapons. Earlier work focused on conventional friction models. Jones and Rule [2] considered optimal design of the penetrator nose in a high friction environment using Coulomb friction. Later, Jones, et al [3] used constant friction equivalent to target shear strength. While the results of these analyses were largely satisfactory, they did not completely capture the essence of the work done by friction. Recently, the authors [4] proposed a simple velocity-dependent engineering model to account for friction in high-speed penetration events. A critical velocity controlled the initiation of friction and the amount of work done by friction. The results were very satisfactory and correlated very well with experimental observations from range tests. However, there were a number of tedious integrations and numerical evaluation of some difficult integrals. In this paper, several simplifying assumptions are made which replace the numerical integration with closed form solutions. The results, while somewhat different, agree in principle with those given earlier. The same trends are observed and the correlation with independently performed experiments is excellent. Additionally, the zero asymptotic limit is replaced and its effect is considered.Copyright

Nondestructive determination of unknown pile tip elevations using modal analysis

A comprehensive experimental study and three corresponding numerical analyses were conducted to investigate the suitability of a modal analysis approach for identification of unknown pile embedment lengths. A small-scale pile facility containing partially embedded piles of differing lengths, cross section dimensions, and encasement attributes was constructed so that experimental pile response data could be gathered in a controlled laboratory environment. Impact tests were performed at a number of locations on each model pile, and the modal parameters for each were estimated from the resulting frequency response function data. Comparison of modal parameters estimated from model piles with similar cross section dimensions and different buried lengths showed essentially no variation in natural frequency as the buried length increased, in the frequency range that was practical to measure. Modal damping values showed a greater variation with pile embedment depth, but no discernable trends were apparent that wo...

Effect of Compressive Stress on Longitudinal Wave Speed in Cementitious Material

The Air Force has been interested for some time in the development of computer codes that accurately predict the penetrator trajectory created when munitions are fired into concrete and geomaterial targets, as well as the resulting depth of penetration. Recent work has focused on experimental research performed to determine quasistatic, dynamic, unconfined and confined material properties for development of an elastic/viscoplastic constitutive equation. This constitutive equation has shown some promise in predicting stress and strains but lacks a consistent damage parameter to predict damage or fractures exhibited by the target material during experimental impact tests. Current damage level predictors that employ a scalar damage parameter are not sufficient to predict the directional damage or fracture that occurs in simple uniaxial compression tests of concrete and geomaterials. Tensorial or directional damage parameters coupled with constitutive relations are necessary for better understanding and accurate prediction of damage exhibited when munitions impact concrete and geomaterials. The primary objective of the study described herein was to identify, quantify and characterize damage parameters associated with certain constitutive responses of cementitious and geologic materials. To that end, longitudinal wave speed and biaxial strain data were collected simultaneously on a series of grout cubes as they were being loaded to failure in uniaxial compression. The results of these tests, and a comparison to existing related data [1, 2] are presented.Copyright