Illustrated Case Studies in the Maintenance Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
|One way to tell a fatigued bolt versus an overtorqued bolt.
The manner in which a load is applied to a stud bolt will leave different patterns on the fracture surface, such as in the following two cases.
The figure to the left shows a stud bolt that had been in service for well over four years with a potential of nearly 20 million cycles. This stud bolt was replaced with the one shown below. It was in service for only seven days with a potential of nearly 91 thousand cycles. There is a dramatic difference between the appearance of the two studs.
There is a definite science behind the appearance. When cracks finally do start, they will start as a small void and fan out. As the crack stops between load cycles it leaves a scar, or "beachmark." Kind of like the line of debris which is left at the sea shore when the tide goes out, hence, its name. If you were to put the stud bolt to the left under an electron microscope you might just be able to count the (possibly) 20 million beach marks which are not discernible to the naked eye. The crack started at the very top, the magnitude was very very low. It progressed straight down and failed at the bottom. It is obvious that as the crack progressed downward, the shape of the crack front (i.e. beachmark) changed. It is roughly straight across near the middle of the bolt. This is because the relative area of the bolt still intact and able to carry the load is now smaller than before and therefore the stress is much greater. The texture also changes near the final failure when there is very little area of the bolt still intact, and the ratio of the stress in the area still intact is extraordinarily large.
In the photo to the right, the fracture surface is completely different in appearance. The crack started at the top and progressed downwards. Final failure was at the bottom. Notice how the signs of any beachmarks like those in the picture above are missing. There is a definite discolored region going across the top of the stud bolt (10:00 & 2:00 positions). This region would indicate that the crack initiation loading was considerably higher than the loading in the example above (i.e. straight across instead of progressively larger ring shapes). The final fracture area in the lower third region (8:00 & 4:00) is considerable in size, and very rough in appearance. The roughness of the fracture surface has to do with the same stress ratio as indicated in the previous example. However in this case, for the same amount of area, there is too much stress. Final rupture involved this entire region and was brittle in nature.
When this stud bolt was tightened an impact wrench was used instead of a torque wrench. It had been tightened until it "rang out."
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