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Princeton Junction, NJ, United States

Gonzalez-Nunez M.A.,Mistras Group Inc.
NACE - International Corrosion Conference Series | Year: 2014

The use of acoustic emission (AE) technique for the detection and location of corrosion damages in metallic structures is well known. Systematic tests have been conducted in the laboratory and in the field. The AE method can be used for on-line monitoring to periodically detect corrosion growth during the active corrosion stage or off-line for detection of active corrosion at a specific time. AE signals from the corrosion process on carbon steel are combined high and low frequency signals related to the breakdown and spalling of the oxide film formed during the corrosion process. Although the signals from the corrosion process are low in amplitude compared to other damaging emission mechanisms such as crack growth, they still may be detected depending on the background noise conditions. AE corrosion signals under particular conditions can be of similar amplitudes or much higher than background noise. In the present work different corrosion rates of a carbon steel plate in a 3.5% NaCl solution were studied under controlled conditions. A combination of electrochemical tests such as Potentiostatic and Polarization Resistance experiments were carried out to observe and analyze the change in corrosion rate simultaneously with AE tests. Comparison of corrosion rates and the AE hit rate calculated from both electrochemical and AE tests respectively, was carried out. Characteristic AE signal features were recorded and analyzed at the different corrosion rates using potentiostatic measurements. Preliminary results show some degree of correlation between the different corrosion rates associated with different AE hit rate activity taking place during the corrosion process. © 2014 by NACE International.

Koduru J.P.,Mistras Group Inc. | Koduru J.P.,Pennsylvania State University | Rose J.L.,Pennsylvania State University
Smart Materials and Structures | Year: 2013

Ultrasonic guided wave tomography utilizes an array of permanently mounted transducers to detect and image defects like corrosion, cracks and delamination in structures. It is critical for successful tomography imaging to avoid the influences from external environmental conditions like water loading and changes in temperature. Water loading is particularly challenging as it affects the guided wave propagation in the structure. However, by taking advantage of the physical properties of guided waves it is possible to reduce its effect on the tomography images. Modal points on the dispersion curves can be found that have low out-of-plane displacement in their wave structure and hence no leakage into the liquid on the structure. In this paper, the omnidirectional excitation of desired guided wave modes with annular array transducers is discussed. Guided wave tomography of a steel plate like structure with a corrosion defect is studied under water loading conditions. The influence of water loading is overcome by exciting symmetric guided wave modes (S1) in the structure. Utilizing guided wave mode control it is shown that the defects in the structure can be easily discriminated from any artifacts in the images due to the liquid layer. © 2013 IOP Publishing Ltd.

Graney B.P.,Mistras Group Inc. | Starry K.,IVC Technologies
Materials Evaluation | Year: 2012

All rotating machinery uses bearings to support the load and maintain the clearances between stationary and rotating machinery elements. More than 90% of these machines have rolling element bearings. Unfortunately, rolling element bearings are prone to a myriad of premature failures. A mere 10% of rolling element bearings reach their L10 life, the expected life of 90% of similar bearings under similar operating conditions. Early failures are attributed to lubrication, load and design/application errors, and even pre-existing problems that were not detected during manufacture. A comprehensive condition-based maintenance program incorporating preventative maintenance and predictive maintenance should be in place to detect the onset of wear and deterioration of rolling element bearings. A mature program provides not only indications of wear in the bearings, but also an evaluation of the severity and recommendations for when corrective actions should be taken. The purpose of this paper is to briefly discuss how the high-frequency natural bearing resonance indicator, discrete frequency indicators (acceleration), acceleration timewaveform characteristics, acceleration timewaveform crest factor, and the velocity amplitude of bearing fault frequencies associated with rolling element bearings can be integrated to determine bearing health and the risk of catastrophic failure.

Pollock A.A.,Mistras Group Inc.
Conference Proceedings of the Society for Experimental Mechanics Series | Year: 2011

This paper will review energy aspects of the acoustic emission (AE) phenomenon and its relationship to material properties especially brittleness. The spectral energy density of the AE wave at low frequencies is related to the moment tensor, but this is only a fraction of the total energy converted in the deformation or damage process. The "conversion efficiency" from static elastic energy to dynamic AE energy is governed by the source speed, and this in turn is related to the brittleness of the material. Meanwhile, the spectral bandwidth of the AE near the source is governed by the duration of the source event. The resulting relationships between brittleness and acoustic emissivity will be discussed. Examples will be drawn from metals, fiber reinforced composites and geological materials. A further factor that has a strong influence on a material's damage tolerance is its heterogeneity. This also has a strong influence on its acoustic emissivity, specifically on the amplitude distribution. In a recent development in the practical application of AE to industrial plant monitoring, these factors and others are integrated in a model of the Probability of Detection (POD) for fatigue cracks growing in a mixed mode comprising both ductile and brittle deformation mechanisms. ©2010 Society for Experimental Mechanics Inc.

Hellier C.,Mistras Group Inc.
Welding Journal | Year: 2015

An nondestructive examination (NDE) professional reflects on advancements in the field. The need for quantitative information has proved particularly strong in the defense and nuclear power industries and led to the emergence of quantitative nondestructive evaluation (QNDE) as a new engineering research discipline. Another substantial step forward for the industry was the development of Phased Array Ultrasonic Testing (PAUT), which provides inspectors with quantitative information regarding weld flows. In this, a phased array system utilizes the wave physics principle of phasing, varying the time between a series of outgoing ultrasonic pulses in such a way that the individual wave fronts generated by each element in the array combine with each other to add or cancel energy in predictable ways that effectively steer and shape the sound beam. The programmed pulsing sequence selected by the instrument's operating software then launches a number of individual wave fronts into the test material.

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