High strength fasteners 1

[izax1]

Does anyone have any experience in using high strength (and then usually low ductility)12.9 quality fasteners for fatigue applications? When I worked in the aerospace industry some years back, 12.9 fasteners were not allowed unless you had very good reasons to use them. Some of my collegaues now say they would not worry about using 12.9 fasteners for fatigue applications. But I don't know. Any one having relevant experience?

 

[MintJulep]

You shouldn't use any fastener in a fatigue application.

Design your joint so that the fastener is not subjected to stress ranges where fatigue is a concern.

 

[israelkk]

MintJulep

Which industry are you working for?

 

[ESPcomposites]

Maybe you should clarify whether it is a tension loaded fastener or shear loaded fastener. Shear loaded fasteners tend to be less of a concern (unless for composites). Fatigue of tension loaded fasteners can be real concern though.

Brian

http://www.espcomposites.com

 

[dinjin]

I think that was the general opinion about supposedly 12.9
fasteners. They have their purpose, but not in high fatique situations.

 

[TVP]

israelkk,

The point that MintJulep is making is that threaded fasteners with sufficient preload can be used in cyclically loaded applications without initiation of cracks and subsequent fatigue crack propagation. Fatigue is a specific phenomenon that is not synonymous with cyclic loading, variable amplitude loading, etc.-- it is a condition that results from excessive stresses (or strains) during cyclic loading, variable amplitude loading, etc.

izax1,

Here are some links to previous Eng-Tips threads and other references on fatigue failure of threaded fasteners:

http://www.eng-tips.com/viewthread.cfm?qid=178349

http://viewmold.com/Products/Techni...ner Theory and Application/Joint-Diagrams.pdf

http://highered.mcgraw-hill.com/sites/dl/free/0073121932/365766/chapter06.pdf

http://www.hexagon.de/rs/review design guideline VDI2230fda.pdf

http://www.attila-alt.de/wp-content/uploads/2010/07/ISSS_Seoul_2009.pdf

 

[ivymike]

TVP, good clarification of Mint's post. When I first read it I also thought "what do you mean?" but after reading your clarification and rereading his post I can agree with the statement... assuming what's meant is along the lines of "you shouldn't design your fasteners such that they are expected to fail and the life 'til failure is something you wish to control."

 

[izax1]

Well, to clearify: My question is not about fatigue life in bolted connections in general, (I have done a lot of that)but specifically the usage of high strength fasteners (in VDI2230 grade 10.9 or 12.9). The 12.9 grade would typically have high strength but very low ductility (low elongation and thus brittle material) which then means very low capacity to redistribute the stresses. Which again, is bad for fatigue. So, I hope I have made this clear: Does anyone have experience with using this type of bolts for fatigue applications?

Thanks for your patience.

 

[MintJulep]

Yes, thank you TVP for perfectly clarifying my point, and for the excellent reference links.

To answer Israelkk's question, I work in an industry where too many things fall off due to poor joint design. Or more accurately, a total lack of any joint design.

 

[drawoh]

izax1,

Most of the fasteners I have seen approved for aircraft are, at best, medium strength. The big issue with aircraft fasteners is that they are inspected and tracked, and you have a reasonable expectation that they will work to specification.

I have ordered grade_12.9 fasteners, and the suppliers have shipped grade_8.8. Will the people in your shop see if anything is wrong?

JHG

 

[CoryPad]

Property class 12.9 screws are used to join rotating driveline components for millions of automobiles every year. These applications are subjected to cyclic loading and can accumulate fatigue damage. Proper joint design, fastener manufacture and joint preloading allow these assemblies to meet durability requirements.

Some additional factors to improve fatigue life for these parts include left hand threads, thread adhesive, and yield point tightening.

Do you have any specific questions?

 

[MintJulep]

How do left-hand threads improve fatigue performance?

 

[TVP]

Brittle fracture is not really a concern with property class 12.9 bolts unless the elongation to fracture is low coupled with high hardness (> 43 HRC). Here are a couple of references with some actual data:

http://www.fatigue.org/Minutes/Fall-2003/Nate-Horn.pdf

http://www.fatigue.org/minutes/Spring-2005/stephens.pdf

http://eb-cat.ds-navi.co.jp/enu/jtekt/tech/ej/img/no166e/166e_03.pdf

 

[CoryPad]

Mint,

Left hand threads can help reduce loosening for rotating parts. Loosening leads to preload loss, which is the large cause of fatigue damage. It is a small effect, but an example of details beyond material tensile strength that can affect lifetime.

 

[izax1]

Thank you very much for valuable input. The links from TVP gave very relvant information.

 

[rob768]

I have read an article in an Allianz book with collected damages saying the fatigue limit of 12.9 grade bolts doesn't vary too much from that of 8.8 grade bolts. The "gain" in higher tensile strength of higher grade bolts is lost due to notch sensitivity and similar aspect. So, you have the risk of designing to higher strength levels, with, effectively, a smaller margin with regards to fatigue.

 

[dinjin]

Do they meet any Aerenautical Standard, Military Standard and have an APL for them?

 

[dinjin]

Cory,
Can you site a study or paper verifying the left hand thread theory? Why aren't all thread left hand if this is true? It is an interesting theory.

 

[CoryPad]

dinjin,

Here are two items on this subject:

Page 10 of:

http://www.ajaxfast.com.au/downloads/VIBRATIONLOOSENINGIEAUS01.pdf

http://www.boltscience.com/pages/failure4.htm

The left hand thread benefit is only valid for rotating parts in the clockwise direction. It doesn't help for counter-clockwise rotation nor for stationary joints.

 

[dinjin]

Cory,
Thanks for the links. Do you think this is some kind of creep phenonomeon caused by which direction the shaft is rotating?
I found the part about jam nuts interesting too. I would like to see the Junkers test on the left hand threads. The Junkers test of the jam nuts was convincing.