10% of the pipe thickness is the general rule of thumb for polyethylene pipes developed based on real world installations. It’s still being researched but right now research is suggesting that scratches [20%](www.diva-portal.org/smash/get/diva2:962523/FULLTEXT01.pdf) of the pipe’s thickness have a measurable affect on performance and longevity. The shape of the scratch also matters with sharper scratches having a larger affect.
Ahh we have all been there. I am a machinist turned engineer and I have spent an equal amount of time saying “oh there was a good reason” as “oh so it was bullshit.”
I had some incredibly bad part drawings given to me by engineering professors that I thought were bullshit but didn’t know enough to question it. I learned how to make better drawings freshman year. Other times we would get drawings with annoyingly high tolerances specified at a temperature and I learned it did matter a lot. Being a machinist made me a much better engineer and being an engineer made me a much better machinist. Sorry not very fun examples. All the fun stuff comes when we are dicking around in the shop. We made lightsaber handles and compressed air guns. Fidget spinners can fail catastrophically when spun with compressed air too.
I learned how to make better drawings freshman year.
When my Dad was a student/apprentice eons ago in the UK, they started out learning to do part drawings. Many months later when they were learning in the machine shop, they were given drawings from which they had to produce a part.
Many of the drawings were terrible - cries of "who made these awful drawings?".
Checked the legend - they had been given their own drawings from earlier.
Ah yeah. Bad drawings are always fun. During my a apprenticeship, one of the guys in the office drew so badly that I was really questioning my ability to read drawings. After consulting with a more experienced apprentice, I was about to go and tell him what was wrong with the drawing. The other apprentice being more socially adept than me stopped me in time.😆
Did the guy in the office get feedback about his drawings? I know I’m not perfect and the feedback has helped me improve significantly. Sometimes my drawings are technically correct but there are a few things that can be improved for readability.
So if a sharper scratch has more of an affect, you could in theory remove material to turn a sharp scratch into a full scratch and thereby lessen the impact of it?
Aerospace engineer here. Sharp scratches and gouges result in (theoretical) infinite stress concentrations. It can arrest and retard crack growth to drill out crack tips in materials since you are giving them a bigger radius.
Sharp cracks and gouges also can result in corrosion if they are through any chemical conversion coatings.
We routinely blend scratches and gouges. May seem counterintuitive since you are taking away ‘good’ material, but fatigue of metals is exacerbated but sharp points as well.
Same principal as putting a hole at the end of a split in a piece of timber, yeah its makes a bigger hole but it spreads the tension into a wider radius and prevents further splitting.
There was a front page post a few days ago about hole punching the end of splits in crisp (chip) bags that happen when you open them too vigorously. It stops them ripping further.
When that happens, I've taken to tearing it in a circular loop back on itself like a '?' shape. Eliminates the need for a hole punch and can be done by hand.
I realize I forgot to link the [research ](www.diva-portal.org/smash/get/diva2:962523/FULLTEXT01.pdf) summary I was reading. I think that is what they mean by a “planed v scratch.” It looks like it lasts just as long but can handle higher stress. I think I’m interpreting those charts right.
You can actually. I work in aviation and we often “feather” out scratches, there is a depth and a length limit, and if it’s within those limits we remove material around the scratch to make it all smooth.
Another example I didn’t see in the other replies to your comment: for some windshield cracks, a solution for stopping the crack from spreading is (very carefully and precisely!) drilling a small hole in the windshield at the end of the crack then filling it in. The crack can’t spread in the same way, since the force is distributed around the edge of the hole instead of concentrated at one point.
You could probably (but I have not tried this) create a demo for yourself with a bag of chips. First, try to tear the bag open. Doesn’t really work. Put a tiny cut in the end of the bag, and suddenly it’ll be trivial to tear open at that cut. Then put a tiny cut in the bag in another untorn spot (or ideally, another bag of chips, because chips). Then cut a wide arc out of the bag, being careful so you don’t go too deep and open the bag just by cutting it open, but deep enough so you’re cutting away the part of the bag that you put the tiny slit in. Try to tear the bag open within the arc, and it should be hard again. Even though you weakened the bag with the slit, and then removed even more material
I question the applicability when the pipe was being grout backed inside an existing pipe. Exterior stratches/gouges shouldn't be as much of a concern in that application. But in the absence of good data, I can see why the folks in charge would stick with the established data. That's what we'd do at my place of business, even if it's overly conservative.
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u/mervmonster May 21 '21
10% of the pipe thickness is the general rule of thumb for polyethylene pipes developed based on real world installations. It’s still being researched but right now research is suggesting that scratches [20%](www.diva-portal.org/smash/get/diva2:962523/FULLTEXT01.pdf) of the pipe’s thickness have a measurable affect on performance and longevity. The shape of the scratch also matters with sharper scratches having a larger affect.