Date of Award

2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Sundaresan, Mannur Dr.

Abstract

Over 80% of components in a system that undergo fatigue cycling fail due to cracking. Acoustic emission (AE) has become a major resource in non-destructive testing (NDT) for the detection, location, and quantification of cracking events. Normally, AE an event is referred to as crack growth, fretting, or friction. As an AE signal progress through material, the waveform converts into several modes; additionally, attenuation and dispersion may also change the waveform. Furthermore, the noise may intrude into the signal in such a way that by time the signal from the source arrives at the sensor the waveform becomes very complex. Due to the complexity of the signal, the source of the acoustic emission event is very difficult to determine. In the following research, the aim is to develop relationships between signal content and high-speed digital imaging to assist in determining key components of the AE source. Thick aluminum bar samples are experimentally subject to cyclic loading utilizing three point bending setup to observe the crack growth. The crack growth thus generated is monitored simultaneously with the help of a high-speed camera system to record the expansion of the crack tip, and by piezoelectric (PZT) sensors bonded over the specimen surface to collect the AE signals. Analysis is provided to differentiate between the AE signals occurring due to crack growth and fretting occurring during the closing of the crack. Furthermore, a 3D model was studied to differentiate between surface and interior crack growth signals. The fracture results show clear indication of the interior crack growth propagating at a different rate to the crack growth along the free surface of the specimen. This phenomenon is attributed to the plane stress and plane strain conditions in the specimen. This important result comes from the video recordings showing the future of the crack path as it propagates over the surface.

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