ME 310: Fatigue Precracking

Hey everyone. Here’s the latest update with everything thus far:

This past month consisted of fatigue precracking of our compact tension (CT) specimens and getting the software of the hydraulic Instron to work. Might not seem so much, but when you have other students wanting to use the Instron…. Well your in line to face delays. But anyways, things are at least much smoother than they were last semester and we finally got the hang of this fatigue precracking.

So what exactly is fatigue precracking? Well it is essentially placing a specimen under a cyclic load for certain amount of time and propagating a small crack at the end of a notch. In our case, we usually fatigue precrack our specimens until the crack grows a certain length and then we place them under the microscope to make sure that the crack on each side of the specimen is of equal length.

So how is this all done?  Well check out this photo:

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The above picture shows one of the CT specimens attached to the 8801 hydraulic Instron. The specimen is connected at the holes by Grade 8 dowel pins that are also attached to the clevises made in house. The ends of each clevis have 1/2-20 holes for the same size bolts, which are then attached to the hydraulic grips. And last but not least: the clip gauge extensometer. This devices measures the crack growth of the specimen and sends it to the da/dN software that calculates the crack growth rate per increasing amount of cycles (a= crack length and N=number of cycles) along with other various parameters. While fatigue precracking a specimen, the following screen is shown:

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This screen displays various parameters and plots that are changing with each ongoing cycle. One of the important things to know when fatiuge precracking is the way you go about doing this procedure. That is, are you letting the Instron run until you click Finish Test or are you setting a limit to the number of cycles or crack length that you want it to stop at? What types of loads will the specimen see during the fatiguing: increasing, decreasing or maybe even constant? Without getting into too much detail, we basically ran our test at a constant delta K, which basically means that our loads were decreasing as the fatigue precracking went on.

And thus so far, we’ve been able to fatigue precrack about 12 specimens. Some of the problems we encountered through the past month is that we initially had no idea how to work the da/dN software when we had finished machining the specimens. I had to contact Instron and other places to get answers to some of the questions we had. We luckily got some help from Professor Vinci in the Material Science department and he helped us a ton on how to work the Instron itself. Another problem we faced was that we broke 3 specimens during the fatigue precracking. We figured out that the software can sometimes go haywire with the some of the calculations it performs and that it can then go at full load and break the specimen as shown in the picture below.

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If you examine closely, the specimen is broken at its centerline where the notch is formed. This isn’t too critical of a problem since we have 36 in total, but I was honestly amazed on how much of a load the Instron can output. We never got to see at what load this specimen broke, but it was definitely higher than 15kN for sure. In order to debug this problem, we just defaulted the coefficients of the calculations then we never had the problem again.

But anyways, that is where we are so far. Next, we will start da/dN testing where we would like to obtain a value for Kmin threshold, which is basically the minimum stress intensity factor needed to begin propagating crack. Stay tuned for more updates!

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