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So, I have a problem with fatigue cracking originating from hot punched holes. From failure forensic reviews, it seems like surface conditions contributed to some of the failures, especially where there are leftover shear marks and laps from punching operations. I did a quick research but it doesn't seem like there are well established standards regarding the finishes of holes generally in terms of hole surface quality. Am I over thinking and that nobody had the same problem previously? Or are there some standard that I am not already aware of that exists?

One of my thoughts is to establish Ra Rp Rz acceptable ranges that can be applied for the holes. But I have doubts about measurement consistency as those measurements are usually more geared towards flat surfaces. What are your thoughts on this?

Thanks for all your help!!

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  • $\begingroup$ There is lots of research or articles on fatigue analysis especially since some aircraft "fell" out of the sky... So what have you found so far? Also, how is your material loaded? is there extra strengthening to offset that loading? $\endgroup$
    – Solar Mike
    Apr 30, 2019 at 8:42
  • $\begingroup$ The question stated here has nothing to do with load or fatigue directly. We are talking about surface conditions. Also, the items in question are much thicker than aircraft parts if you are curious. Material is generally heat treated steel. Yes, load makes a difference, but it's not what the question is about, and should rightfully not be. $\endgroup$
    – Isa
    Apr 30, 2019 at 13:40
  • $\begingroup$ Fatigue cracking happens on thick or thin materials... But my comment was really about what you have found so far.. and the answer seems to be "nothing"... $\endgroup$
    – Solar Mike
    Apr 30, 2019 at 13:59
  • $\begingroup$ Yes, fatigue happens regardless of thickness, but thicker materials have a smaller chance of hitting the fatigue threshold in its intact condition. Aircraft research do not directly provide more insight in addition to the general principals of fatigue like the SN curve relationships which we already understand. And no, I digress that I didn't research anything. I looked for standards regarding surface finishes of holes, and I stated that I found nothing, which is what this question is about. $\endgroup$
    – Isa
    Apr 30, 2019 at 14:19
  • $\begingroup$ The "Comet" aircraft failures had multiple causes : square window cut-outs in the skin causing stress concentrations , strength and thickness of the aluminum permitted crack growth, poor fracture toughness of the aluminum skin which permitted brittle fracture to start at small ( un-noticed) cracks. $\endgroup$
    – blacksmith37
    Aug 13, 2019 at 0:13

1 Answer 1

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When talking about R values and holes, what you really should be measuring is the S value instead that bases on area. There are generally two techniques you can measure interior surface roughness of holes. One of them is to fill the hole with something and remove it, then measure that piece directly. Alternatively, you can also scan it optically to create a highly accurate 3D impression of the hole and measure Sz Sa Sp that way. R and S values are not equivalents, so care should be taken to review relevant literature on the subject when you are specifying a criteria to meet certain performance requirements.

Good luck!~

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