Roger H. L. Chen

An ultrasonic method for measuring tensile forces in a seven-wire prestressing strand

Initial tensile forces applied to prestressing strands undergo progressive time-dependent losses. These prestress losses, if unaccounted for, may cause catastrophic failures of prestressed concrete structures. The main objective of this study is to develop a nondestructive evaluation method for measuring prestress forces in the seven-wire prestressing strands which are widely used in various types of prestressed concrete structures. Experimental investigation was conducted by measuring the ultrasonic waves propagating through 1/2 inch-diameter seven-wire strands subjected to different levels of tensile forces. Theoretical analysis was conducted by considering the acoustoelastic effect and the dispersive behavior of a longitudinal transient wave traveling through a long, stressed, circular rod. Results indicate that the velocities of each frequency component of the traveling waves can be related to the tensile force levels in the strand. The stress wave signals were processed using the Wigner-Ville Transform to identify the arrival times of different frequency components. The analytical and experimental results correlate well and high measurement accuracy is observed. The results show a promising progress in the field of nondestructive measurement of tensile forces in the seven-wire steel strands for post-tensioning concrete members.

Nondestructive Detection of Delamination in Thermal Barrier Coatings Using Ultrasonic Technique

Nondestructive testing using an acousto-ultrasonic technique has been utilized to detect the change of material properties and provide early warning of failure of thermal barrier coating (TBC) systems. Testing was performed on Rene N5 and Haynes 230 coupons with an applied NETL-bond coat, as well as on coupons containing both an applied MCrAlY bond coat and 7-YSZ top coat. The coupons were subjected to either cyclic or isothermal testing at 1100°C. Ultrasonic testing was performed before and after thermal testing using piezoelectric sensors with dry contact on the surface of the coatings. Proof-of-concept test results indicated that changes in the properties of the ∼ 40 μ.m bond coat can be detected using the proposed technique. Waveforms generated via Pitch/Catch indicated minor changes within the bond coat applied to Rene N5 substrate after 400∼500 hours of cyclic oxidation at 1100°C. In contrast, marked differences in waveforms and travel time reflected significant crack formation and spallation of the bond coat from the Haynes 230 substrate. Finite element analysis (FEA) simulation of the wave propagation on a simplified TBC system with nonlinear effects was conducted. FEA results clearly show detection of a small embedded void incorporated to simulate delamination. Comparisons between experimental measurements and finite element simulations were used to estimate the material properties of the coatings and the substrate.Copyright

2D Modeling Temperature Development of Mass Concrete Structures at Early Age

In this paper, a 2D finite volume analysis methodology was used to predict temperature development within three different bridge pier caps. MATLAB® was employed to generate a program that solves the governing heat transfer equation where development of thermo-physical concrete properties was defined as a function of degree of hydration. The rate of heat generation was obtained experimentally via adiabatic calorimetry and the activation energy was determined following the ASTM C 1074 procedure to implement equivalent age concept. 2D finite volume analysis results were presented in comparison with the recorded concrete temperatures from the field. Accordingly, temperature time histories at the center and the side surface of the bridge pier caps were predicted reasonably well using the concrete mixture information and the measured concrete hydration properties.

NONDESTRUCTIVE EVALUATION OF CERAMIC CANDLE FILTERS

Nondestructive evaluation (NDE) techniques have been used to reduce the potential mechanical failures and to improve the reliability of a structure. Failure of a structure is usually initiated at some type of flaw in the material. NDE techniques have been developed to determine the presence of flaws larger than an acceptable size and to estimate the remaining stiffness of a damaged structure (Chen, et. al, 1995). Ceramic candle filters have been tested for use in coal-fueled gas turbine systems. They protect gas turbine components from damage due to erosion. A total of one hundred and one candle filters were nondestructively evaluated in this study. Ninety-eight ceramic candle filters and three ceramic composite filters have been nondestructively inspected using dynamic characterization technique. These ceramic filters include twelve unused Coors alumina/mullite, twenty-four unused and fifteen used Schumacher-Dia-Schumalith TF-20, twenty-five unused and nine used Refractron 326, eight unused and three used Refractron 442T, one new Schumacher-T 10-20, and one used Schumacher-Dia-Schumalith F-40. All filters were subjected to a small excitation and the dynamic response was picked up by a piezoelectric accelerometer. The evaluation of experimental results was processed using digital signal analysis technique including various forms of data transformation. The modal parameters for damage assessment for the unexposed (unused) vs. exposed (used) specimen were based on two vibration parameters: natural frequencies and mode shapes. Finite Element models were built for each specimen type to understand its dynamic response. Linear elastic modal analysis was performed using eight nodes, three-dimensional isotropic solid elements. Conclusions based on our study indicate that dynamic characterization is a feasible NDE technique in studying structural properties of ceramic candle filters. It has been shown that the degradation of the filters due to long working hours (or excessive back pulsing conditions and high temperature transient) could be reflected from the shift of vibration frequencies. These shifts are due to changes in structural properties such as stiffness, which are directly related to the Youngs modulus of the candle filters. Further studies are necessary in implementing and verifying the applicability of dynamic NDE characterization methods for actual in-situ conditions, and in establishing a systematic testing procedure for field applications. Also investigations on the filters natural frequency due to the effect of dust cake or due to the change of boundary conditions may provide insight as to how the filter will perform in the field.

Laboratory evaluation of concrete decks and pavements using ground-penetrating radar

This study presents the findings on the use of ground penetrating radar (GPR) to nondestructively assess the condition of bridge decks and pavements with and without asphalt overlay including the thickness effects. Several concrete bridge deck and pavement specimens with varying internal conditions such as with/without reinforcement and with/without delaminations were tested under laboratory conditions. In order to assess the effect of asphalt overlay on the detectability of subsurface delaminations, asphalt layers with thickness ranging from 1 inch to 3 inches were poured on the specimens and tested using GPR. The study also included specimens with debonding at the asphalt/concrete interface. Individual radar waveforms and color intensity plots from these specimens were compared to study the effect of anomalies on the radar waveforms.

Conceptual Design of a Monitoring System for Maglev Guideways

The state-of-the-art in nondestructive testing (NDT) technology has advanced significantly over the past fifteen years and has reached a stage where continuous monitoring of structures can be successfully achieved. The need for the adoption of advanced NDT technology for Maglev is absolutely essential because of the Maglev system safety and performance considerations and high cost of construction. In this paper, several monitoring methods such as Acoustic Emission, Ultrasonics, Ground Penetrating Radar, Infrared Thermography, and Fiber Optic Sensors have been evaluated in terms of their possible use in a Maglev system. Also, monitoring of guideway alignment and detection of debris on the guideway using Lasers and Fiber Optics have been investigated.