FTM 181: Should bolts stretch under load

FTM 181: Should bolts stretch under load

This edition of the Fastener Training Minute with Carmen Vertullo was originally published October, 27th, 2022 as “How to develop good methods for tightening stainless steel structural bolting” during episode 181 of Fully Threaded Radio.

Well, hi everybody. This is Carmen Vertullo with the Fastener Training Minute, coming to you from the Fastener Training Institute and the AIM Testing Laboratory in beautiful El Cajon California.

As usual, today’s topic comes from a question from one of our students. Actually this is one of the students who attend our new Fastener Expert mentoring program. I’m going to read you the question, tell you a little bit of background, and then when we return we’ll learn some stuff.

Here’s the question:

I’m seeking a little help with a customer issue installing these cap screws. The part is a 5/8-11 x 2 Grade 8 cap screw, plain. The customer is stating that the cap screws are stretching. The only info I have is that they are torquing the cap screws to 220 to 225 foot pounds, and they sent me some pictures. I have no idea what I’m looking at. I always thought that bolts will naturally stretch when they are torqued down and the only way I know to check would be a proof load test. Is there any other question I should ask before I request samples to test. Please advise.

Well this is an excellent, I guess I would call it a problem, but it’s really not a failure problem, to learn about how bolts work. The customer believes the bolts are stretching, but we all know that bolts naturally stretch when they’re properly installed. It’s not bad for them to stretch, they’re are supposed to stretch. The question is: how much?

When we return we’ll look at the information that we have, and we’ll also make a list of questions to ask so that we can get to the bottom of this problem.

Well, welcome back everybody. This is Carmen Vertullo with the Fastener Training Minute. Today we are talking about an issue that a supplier has with a customer. The customer is complaining that their 5/8-11 x 2 Grade 8 hex cap screws, are stretching. So we don’t know exactly what they mean by stretching. We have to ask that. As you know if you had any of our training classes at the Fastener Training Institute, bolts natural stretch. It’s one of the things they have to do in order to work properly. The question is how much does a stretch. They stretch a little bit and if you remove the load they go back to their normal length. That’s why if you listen to the reading I did from the the email, the customer said that they stretched and the supplier thought they would do a proof load test. A proof load test is a perfect test to know whether or not the fastener can indeed withstand some amount of stretching.

What we do in the proof load test is we apply a load to the fastener, that is, some portion of it’s yield strength, usually about seventy to ninety percent depending on the standard, and then we release that load. We measure the length of the bolt before and after, and make sure that it did not in fact stretch. Now in that range where the fastener stretches but does not stretch it permanently, that’s called the elastic range, and if it goes beyond that that’s called the plastic range. Now it does not go back the way it was. If you look at this on a graphic, it kind of looks like a straight line sort of on slight diagonal going up as the load increases and the stretch increases. At some point it starts to curve over, and goes into what we call the yield or the plastic Zone, and it won’t go back.

This is a property of steel, actually this is a property of all materials from an engineering sense, is called the modulus of elasticity of the material. Elastic modulus for short or the spring constant is another way to say it, because in fact it describes the behavior of materials as if they are Springs. Also sometimes it’s called Young’s modulus. I don’t know who Young was, but I guess Young was the person who figured this out. (Editor’s note: the concept of elasticity was first proposed by German mathematician Leonard Euler in 1727. But it was named Young’s modulus after Robert Young who measured the elastic modulus for different materials in the mid nineteenth century.) Each material has its own characteristic elastic modulus, and for steel that is described as one thousandth of an inch per inch in the grip, for every thirty thousand PSI of load that’s put on the material. Now notice that I said for steel, not Grade 8 steel, not Grade 5, not Grade 2, not A36. All steel is is pretty much the same at this description. And notice I did not say any particular size. It doesn’t matter how large or small the specimen or the bolt is, because it’s based on pounds per square inch, 30,000 pounds per square inch. And when we go about describing this number, it comes out to be a very large number, like it’s about 29 million PSI, 30 million PSI approximately for any kind of normal steel and alloy steel, it about the same.

Other materials, stainless steel, aluminum, titanium, copper alloys will have different elastic moduli, as will plastics and composites and other materials like that. so one one thousandth of an inch per inch in the grip for every 30 thousand PSI of load. So let’s just for the sake of argument pretend that this Grade 8 bolt has three inches in the grip, that is between the head of a bolt and a last thread in the grip there’s three inches and let’s say for the sake of argument because we know that Grade 8 bolts are 150 KSI tensile strength (not yield strength, tensile strength), and we’re going to let’s say we are going to tighten this to just for the sake of argument 50% of its tensile strength. So that means we’re going to tighten it down to 70,000 PSI of load, oh, let’s go a little higher let’s go 90,000 because that will make the math a little easier. So we’re going to put 90,000 PSI of load on this Grade 8 bolt, which is a little bit on the low side, and let’s say that with our three inches of grip, 1/1,000 of an inch multiplied by 3 is 3/1,000. 30,000 into 90,000 goes 3 times, so we’ve got three there, so we technically, or literally I should say, we’re going to stretch this bolt 9/1,000 of an inch. I’ve done this experiment many times, it works perfectly well. As a matter of fact it works so well that that’s how electronic load cells work: they work based on measuring the actual stretch of the material. So we know that this is good science.

Now the question is of course, (and we don’t know the answer because we only have what we were told by the supplier), what does the customer mean by stretching? Obviously they must mean that it’s stretched permanently or how would they know right? there is no way to know unless there’s some permanent damage. so here are some questions we might ask the customer or whoever is doing this work. We know what the torque was by the way, and the torque was about right. I used IFI Fastener Technology Connections to check the proper torque, and for a plain bolt it should be around 225 to 200 foot-pounds, so that is correct. Another question would be: is there a nut and washer involved or is it just going into a tapped hole? What exactly are we holding together? Is the load on this part static or dynamic? That is, once we install it does it hold the load or does the load fluctuate? Does the stretch occur during the tightening of the bolt, or in the use of a bolt after it’s installed? How long was it installed before we saw the problem? This next question is probably the most important one, and it is: was any lubrication used? Because if we lubricated that bolt now the torque that we apply is going to result in a higher tension value which could in fact cause the bolt to stretch.

That’s back to our T = (K D P)/12. Torque equals K (which is a dimensionless coefficient relating to variables such as lubrication and some other stuff) times the diameter of the bolt times the load that is installed. Ask about the torque wrench: what’s the brand the type of torque wrench, the range of the torque wrench, give us a photo of the setting. Most times when I ask this question and I can’t get a quick answer, I know pretty much that this is a torque wrench problem. Are the installers trained on the use of the torque wrench. And why does the customer think these photos are important with shows a photo of the head of the bolt and then some micrographs of the microstructure of the bolt in the threads and some other part, which is completely irrelevant to solving this problem. But it does indicate that the customer has some level of laboratory capability. So something going on there.

And and here’s the next most important thing, and if you take nothing else away from this whole Fastener Training Minute, write this down and remember this. The next question: will a nut go on the bolt thread the whole way after it’s been removed? That is the best indicator of whether or not the bolt has permanently stretched, because if it has stretched, it’s going to stretch in the threads. If it stretches in the threads that is going to change the thread pitch of at least one thread and maybe more, and if that happens even the slightest amount, then the nut will not go on the bolt anymore past that. So that is the key to knowing whether a bolt has permanently stretched. Ask if they did any other testing, a Skidmore test, which is the bolt/tension calibrator, the torque/tension relationship determiner. Did they do a hardness test? How many of these bolts did they install and how many stretched? Did they try another lot and what were the results of that? And then maybe ask what’s the urgency? Is it a safety critical application, and and so on.

So this is a really interesting, I don’t know the answer to this by the way, this thing is in the process so maybe I’ll have the answers for the next Fastener Training Minute.

I suspect that probably they probably used some lubrication ,and as a result the bolts were over-torqued. That is actually malpractice to be rendering a diagnosis without having the information, but that’s the most common cause of this problem.

So now you know all about how bolts stretch why they stretch, and why they’re supposed to stretch, and how to know if they stretch too much.

This has been Carmen Vertullo with your Fastener Training Minute and thanks for listening.

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