Previous reports of applications of the AESMART 2000 instrument have concentrated primarily on the detection of crack growth in plate like structures. There are numerous other applications for AE instrumentation such as leak detection, Tribology (friction) studies, and machine tool cutting and grinding applications. These processes create a semi-continuous AE signal and are best studied by the use of RMS or ASL (average signal level) processing of the AE signal.
A new screen shown below can now be utilized in the AESMART 2000 instrument to process the ASL level of the AE signals. The peak amplitude of the ASL signals in the high frequency and low frequency range is detected, then the ratio of the HF/LF is calculated similar to the applications regarding crack growth. The usefulness of these ratios for Tribology and crack growth measurements in the presence of extraneous noise will be illustrated.
The data in the previous screen is the result of placing an SE900-MWB transducer at the center of a 400 X 400mm aluminum plate 5mm thick, then rubbing the plate with different grit size sandpaper. What is plotted is the cumulative average HF/LF ratio as a function of time. The coarse 80 grit gives the lowest ratio, and the 600 grit the highest ratio, with the 240 grit giving a value in between the two. This simple experiment illustrates the possibility of determining surface roughness using the AESMART 2000. It is anticipated that monitoring the ASL ratio of a transducer mounted to a tool post in a metal cutting process would provide the necessary data to measure the efficiency of the cutting process, and determine when a tool should be replaced. Burnish and glide information on hard disk media is another application where the ratio information would give more meaningful data than the simple RMS measurements that are presently being used.
The SMART ASL instrument is equipped with several selectable real time plots. The present configuration utilizes time for the X axis and the following parameters can be plotted as a function of time.
The above parameters can be plotted in real time, then stored in a buffer memory. Data can be transferred to an Excel spreadsheet with a click of the mouse button. The figure below is an example of average ratio data from two grit sizes plotted with the Excel graphics software.
There was a great deal of excitement present in the lab when these experimental results were first observed. Making an observation is one thing, explaining what is really happening is another. It was postulated that since the rubbing of the sandpaper is occurring in a shearing mode, that shear waves generated by this action were responsible for the different results produced by the different grit sizes. We have found in recent experiments (ref 1), that Sv shear waves mode convert into low frequency flexure waves, while Sh shear waves maintain their high frequency characteristics with no mode conversion. An experiment was conducted on a metal bar which was equipped with two shear plates bonded to the end of the bar. One was sensitive to motion perpendicular to the plane of the bar(Sv waves) and the other to particle motion parallel to the bar(Sh waves). 400 grit sandpaper was rubbed on top of the bar and the output of each shear plate in turn was recorded by the SMART ASL. The data was transferred to Excel and the results plotted. The next figure shows these results.
The results show an order of magnitude higher ratio from the Sh shear plate than from the Sv shear plate. This large difference is due to the higher amplitude low frequency signals produced by the Sv shear plate. The dramatic difference observed in the aluminum plate from the SE900-MWB sensor mounted on top of the plate was not observed from these shear plates when different grit sizes were used. The metal bar was much thicker than the aluminum plate, which could be responsible for some of the differences. I believe that the difference in transducer design and placement is the more likely reason for the difference in response.
The primary reason that acoustic emission technology is not being widely applied for the monitoring of bridges, aircraft, offshore platforms and other large structures is the inability of traditional AE methods in recognizing the difference between signals produced by crack growth and those produced by noise. A small degree of success has been achieved in monitoring known cracks in such structures, but global monitoring to detect growing cracks in the presence of high background noise has been unachievable with traditional methods.
Both the AESMART and the AESMART ASL instrument have the capability of detecting and recording crack-like signals (IP sources) in the presence of high amplitude extraneous noise (OOP sources).
An SE9125-M transducer was mounted with petroleum jelly at the center of a 400 X 400mm aluminum plate 5mm thick. A plot of the HF/LF ratio as a function of time was selected for the SMART ASL instrument. 400 size grit sandpaper was rubbed against the top surface of the plate. This resulted in high amplitude signals from both the low frequency and high frequency channels. Simultaneous with the rubbing of the sandpaper, IP pencil lead breaks were made at the edge of the plate to simulate crack propagation. After capture the data was transferred to excel. The graphical results are shown in the next figure.
Note from the above graph that over 1500 events were recorded. Approximately 20 of these were the IP lead breaks made to simulate crack growth and the rest would be considered frictional noise. Note the wide range in the ratios of the IP signals. This is due to the fact that no special attention was paid to the depth on the plate edge that the pencil lead was broken. The large values represent situations where the pencil lead was broken near the center of the plate, and the smaller values where the lead was broken near the top or lower surface of the plate. Some of the very small values are due to reflections of the signal from the opposite edge. As shown by this data, setting the ratio filter at a value of 1 in the AESMART would prevent any of the noise data from the sandpaper rubbing entering the database.
It is difficult to imagine a noise situation on a bridge, aircraft or other structure that would be more severe than the sandpaper test illustrated in the above data. Therefore it should be fairly straight forward to now conduct global surveys of such structures to determine in real time if crack growth is occurring within the sensitivity range of the AE transducer.
Our new URL address for the DECI home page is HYPERLINK http://www.deci.com. Also note that we have a new area code. It has been changed from 714 to 949.
This joint US and Japanese working group meeting will take place August 9-14 1998 on the Big Island Hawaii, at the Royal Waikoloan Hotel. For more information take a look at our home page under conferences.
I will have the AESMART 2000 to show you if you attend the meeting, as well as several new transducer models.