Acoustic Emission Monitoring of In-Service Bridges

An experimental acoustic emission (AE) device, the GARD Acoustic Emission Weld Monitor (AEWM), has been field tested on six bridges during this study. In addition, the device was used to test three other bridges under separate contracts from state highway agencies. The device was evaluated to determine if it could detect fatigue-crack growth on in-service steel bridges. The AEWM employs a proprietary three-step model (filter) to reject noise-related AE activity and detect and locate defects subject to varying stress conditions. The unit uses built-in microprocessors to compare incoming data to the model. If defect-related AE activity is detected, the AEWM will notify the operator and locate the defect in relation to AE sensors placed on the test specimen. The device rejects high background noise rates typical of bridges and detects and locates AE activity from known defects such as cracks and subsurface flaws. The AEWM functioned properly in every field test situation to which it was applied. The device has demonstrated capability to perform AE tests on in-service bridges. It may be used to detect hidden defects or to assist in making repair decisions concerning detected discontinuities. The AEWM and AE testing have the potential for low-cost inspection of critical bridge members

Improved Structural Monitoring with Acoustic Emission Pattern Recognition

A unique acoustic emission monitoring system originally developed for inprocess weld monitoring has been used to monitor fatigue crack growth in a highway bridge during normal traffic loading. The system was able to clearly and reliably detect the presence of fatigue cracks that were adjacent to a row of bolts. The results of the brief experiment show that the signal processing used in this AE system may allow drastic improvements in the ability of acoustic emission to reliably detect propagating bridge flaws under adverse conditions

Deformation mechanisms in compression-loaded, stand-alone plasma-sprayed alumina coatings

Cylindrical, stand-alone tubes of plasma-sprayed alumina were tested in compression in the axial direction at room temperature, using strain gauges to monitor axial and circumferential strains. The primary compression-loading profile used was cyclic loading, with monotonically increased peak stresses. Hysteresis was observed in the stress-strain response on unloading, beginning at a peak stress of 50 MPa. The modulus decreased as the maximum applied stress increased. The stress-strain response was only linear at low stresses; the degree of nonlinearity at high stresses scaled with the stress applied. One-hour dwells at constant stress at room temperature revealed a time-dependent strain response. Using transmission electron microscopy and acoustic emission to investigate deformation mechanisms, the stress-strain response was correlated with crack pop in, growth, and arrest. It is proposed that the numerous defects in plasma-sprayed coatings, including porosity and microcracks, serve as sites for crack nucleation and/or propagation. As these small, nucleated cracks extend under the applied stress, they propagate nearly parallel to the loading direction along interlamellae boundaries. With increasing stress, these cracks ultimately link, resulting in catastrophic failure