Just wondering if hardened bolts are more brittle and thus not suitable for certain applications. If so, what are the cases where you would want a grade 5 bolt instead? Is the metal used in fasteners getting better or worse? Chinese steel, unreliable or not to be trusted?

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/6 '67 Barracuda convertible, electronic ignition, 4-OD, street cam, SBP KH discs, 3.55 SG 7.25" 1" t-bars. Bilstein.
340 '67 fastback, Doug Nash 5-speed.
1988 Toyota pickup work truck

I found thisarticle about the history of hardness testing.

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/6 '67 Barracuda convertible, electronic ignition, 4-OD, street cam, SBP KH discs, 3.55 SG 7.25" 1" t-bars. Bilstein.
340 '67 fastback, Doug Nash 5-speed.
1988 Toyota pickup work truck

That answered questions I hadn't even thought of asking. The historical part was interesting, too. I wonder how much testing gets done on construction materials that will be load bearing parts.

Someone who knowingly provides inferior quality parts, such as the example in the referenced article should be held accountable for their actions.

ATB

BC

_________________
'64 Dart GT convertible, 64 Dart 170 2-door post sedan in faded blue and rust (the Az Dart) & a sixty THREE Dart 170 2-door post sedan in faded blue and rust. (future project)
Early Dart Disorder (EDD) is real, and I've got it!

think about bolts like rubber bands. To hold a critical joint tight, the bolt needs to stretch when tightened. The tension from the stretch is what holds the plates of the joint tight and when used, the nut from loosening.

The higher the grade of bolt, the higher the tensile strength is in the steel in the bolt, hence it takes more tightening force to stretch the bolt. Tighten any bolt too much and the steel will go in to permanent elongation (yield- necking) and not have any retention tension.

Basically if you tighten a grade 8 bolt to a grade 5 torque, the proper bolt stretch may not be achieved and the joint may not stay tight.

this article has decent info on bolt torque
http://www.zerofast.com/torque.htm

This is true when looking at clamping force, but bolts also have a shear value that includes their capability to hold or take force at a 90 degree angle to the fastener...so for fasteners that are taking some torque (say for example a built hemi and spool motor mounts, or even suspension bolts), then you have to upgrade the bolts to take that kind of one off "shock" at launch...(you can devalue this if you use multiple fasteners to spread the value around...makes a good arguement for motor plates with 2-3 bolts on each side...).

Food for thought.

-D.Idiot

Slantfin:
[quote]Chinese steel, unreliable or not to be trusted?[/quote]

Ask the airlines… I wouldn’t trust high strength fasteners sourced from China unless fully tested on these shores.

64Ragtop:
[quote] I wonder how much testing gets done on construction materials that will be load bearing parts.[/quote]

Do you mean testing a joint to failure, or testing individual components? Both tests take place as well as computer simulated loadings and cycles to failure. Materials built to spec can be easily assembled to safely perform a task after some mathematical manipulations take place. A structure, joint, or widget is analyzed through several lenses; bending moment, shear, deflection, compression and tensile loading etc. Then a safety factor of “Xâ€￾ is dialed in to insure the item is over designed to be able to handle any unforeseen misuse such as 2000 rpm over red line, too many loads of material warehoused on a bridge while being repaired, wind loads on a structure, over torqueing a threaded fastener, bla, bla, bla… you get my drift. If they build it, someone will abuse it, and hopefully won’t get killed doing it.

Properties such as shear, plasticity, hardness of any material are known values, if made to spec that engineers use to design connections, and structural components of everything be it a building, bridge, car part, or widget. Anything manufactured has been designed and analyzed to death before it comes to market. An engineers’ professional stamp is at stake on everything he designs, and the last thing he wants is a bridge or roof or frame of a car collapsing with his name on it.

Dadtruck:
[quote]think about bolts like rubber bands.[/quote]

Yes, but more like Sugar Daddy candy. Remember as a kid getting one, and trying to eat it first licking, than biting & pulling whereas the strain causes the candy to yield almost unperceivable at first, then slowly starts to elongate, and at some point quickly stretches out into a long string where it eventually separates from the chunk still on the stick? That is how steel [url=http://nptel.iitm.ac.in/courses/IIT-MADRAS/Design_Steel_Structures_I/7_beamcolumns/2_concepts_of_limit_state_design.pdf]reacts[/url] to tensile loading beyond its yield point. As long as the tensile load is well within the steel’s ability to rebound to its original length in Dadtruck’s illustration using rubber band the mechanical connection (stuff bolted together) will hold. Go past the yield point, and the connection won’t hold. This is why it is so important to always use a torque wrench when assembling anything that has a torque speck listed.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Actually this is very stringently tested, and this is also why there are very few (I.E. only 3) companies that you can get certified concrete anchors from. Also structural engineers cover their collective rears by requiring certification sheets from the manufacturer and periodic or continuous inspections of anchors and fasteners. I know that if yoiu use Hilti anchors of fastening systems they are currently unbeat as they don't use chinese metal and have a wide range of safety margin built into their numbers...

-D.Idiot

that is a description of yield all the way through, as the candy never has any measureable tension, meaning that when the pulling force is removed, the stretched candy does not return to its original length.
I believe the rubber band example holds, as the stretched rubber band does have measureable tension from beginning to end with the tension or clamping force increasing is the stretch increases. and when the pulling force is removed, it returns to its origina length,,as long as it is not stretched into yield. Take the pull off the candy, it does not return to its original length as it is in continious yield.

Your right candy has no elasticity, but is in a constant plastic state.
I used the candy example because the distance steel will stretch and return is very small, but once it begins to yield elongation is considerable in relation to its rebound. In other words once stretched to plastic state steel won’t fully return to its original dimension unlike rubber. Probably if one were to view steel stressed to failure under an electron microscope it would more resemble taffy pull then a rubber band. I also recall a similar example in the pre computer animation days was used in strength & materials class where we broke a lot of stuff, made a lot of graphs, and cranked out a lot of calculations. It was all great fun.

I guess the point being made is don’t reinvent the wheel using one’s intuition in place of tested science. Follow prescribed methods, and values when using any and particularly high strength fasteners in critical applications.

This is a good topic to promote awareness of proper fastener use, and application.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

I feel much better informed now. Thanks to all who contributed. One question remains in my mind. Is there any harm in using bolts rated higher than spec? Seems I recall a case where using grade 8 bolts instead of grade 5 caused a catastrophe. Something to do with the brittleness of the grade 8 bolts.

ATB

BC

_________________
'64 Dart GT convertible, 64 Dart 170 2-door post sedan in faded blue and rust (the Az Dart) & a sixty THREE Dart 170 2-door post sedan in faded blue and rust. (future project)
Early Dart Disorder (EDD) is real, and I've got it!

Probably not, just keep in mind, higher grade bolts typically require a higher torque value to reach their desired stretch, so don't go by torque values of lesser quality bolts or you could have a problem.....

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There's no such thing as too much cam....only not enough engine!

that is correct and that "very small amount" as you state (measureable with a micrometer when both ends of the bolt are exposed or via ultra sound when only one is exposed) is the increase in bolt length, or stretch that occurs when the correct bolt torque is applied. That rubber band like stretch applies tension (clamp load) to the joint. That clamp load keeps the joint together and also keeps the bolt tight. Bolts that when tightened to specification, and have no elongation and are subject to vibration may need loctite to keep them from backing off.

CNC-Dude is correct, use a grade 8 bolt in place of a grade 5 and use the grade 5 torque setting, and everything else being the same,,the clamp load applied to the joint will be lower

This is all very helpful.
Found more reading material for all you metalheads.


You have to accept their disclaimer to use this torque calculator.

_________________
/6 '67 Barracuda convertible, electronic ignition, 4-OD, street cam, SBP KH discs, 3.55 SG 7.25" 1" t-bars. Bilstein.
340 '67 fastback, Doug Nash 5-speed.
1988 Toyota pickup work truck