Detectability of Slow Crack Growth in Bridge Seels by Acoustic Emission
Marvin A. Hamstad, Joseph D. McColskey
The detectabillity of slow crack growth of cracks in bridge steels has been studied by use of acoustic emission testing technology. Fatigue crack-growth rates of nominally 1x10-4 mm/cycle (4x10-6 in./cycle) and 1x10-3 mm/cycle (4x10-5 in/cycle) were monitored with an eight channel, 12-bit waveform-based acoustic emission measurement system. Eight acoustic emission sensors were arranged on plates nominally 25 mm (1 in.) thick (A588, A572, A36,A7,A514 steels and 2024 T351 aluminum alloy). Four of the sensors were typical resonant commercial acoustic emission sensors, and four sensors were wideband high sensitivity sensors developed at NIST. The resonant sensors were bandpassed from 100-300 kHz and the wideband sensors from 50 kHz to 1.5 MHz. Each recorded acoustic emission event produced eight simultaneous waveforms. A representative series of event waveforms from each test was analyzed. Crack events were sorted from extraneous acoustic emission events visually, using the relative arrival sequence of sets of whole waveforms. The extraneous events normally originated in the specimen grip region. Typically at the lower crack-growth rate several thousand cycles were required in order to obtain one valid crack acoustic emission signal. The results from A514 steel and 2024 aluminum were exceptions. The A514 steel had 20 crack acoustic emission events over eight consecutive cycles and none over the other 12 461 cycles examined. The 2024 aluminum had a crack event approximately every 12 cycles. At the higher crack growth rate, approximately one or two cycles were required for a valid event from A514 steel and 2024 aluminum. The other steels required 18 to 130 cycles for a valid event. The behavior of crack generated acoustic emission will be discussed in terms of rise times of acoustic emission sources as related to material microstructure. In addition, indications of classes of crack events as observed with the wideband sensors will be discussed. Supporting experiments and results from plate specimens with large lateral dimensions (533 x 457mm [21 x 18 in.])are presented. These studies on steel 6.4 mm (0.25 in.) thick demonstrate the significance of attenuation of the signals from a sensor located at the crack tip to a sensor approximately 102 mm (4 in.) away. These supporting experiments also demonstrate the changes in signal amplitudes due to acoustic emission source radiation pattern effects as well as signal reflections.