Crushing under high strength bolt heads 3

[Philrock]

When using high strength fasteners, crushing under the head can occur, even if you use a steel washer. What's the criterion, in terms of tensile yield strength, for max compressive stress before permanent deformation occurs.

 

[kenvlach]

For structural steels, F_cy ~ F_ty. Exceed F_cy & it yields.

Design Mechanical and Physical Properties of AISI 1025 Carbon Steel.

F_tu, ksi:. . . . . . . . . . . . . 55
F_ty, ksi: . . . . . . . . . . . . 36

F_cy, ksi:. . . . . . . . . . . . . 36

F_su, ksi . . . . . . . . . . . . . 35

E_t, 10³ ksi . . . . . . . 29.0
E_c, 10³ ksi . . . . . . . 29.0
G, 10³ ksi . . . . . . . 11.0

Physical Properties:
?, lb/in.3 . . . . . . . . . . . . . 0.284

From Table 2.2.1.0(b) in Metallic Materials Properties Development and Standardization (MMPDS) Handbook (Jan. 2003).

 

[CoryPad]

Ken's answer really only applies to a compressed tube in which the bolt/nut outer diameter is equal to the tube outer diameter. Most bolted joints have outer diameters larger than the bolt/nut outer diameter.

For bolted joints, the analysis of interest is for the applied surface pressure, which is compared with the allowable surface pressure for the material. While there is a geometry effect, in general, the allowable surface pressure for ductile metals is approximately equal to the Ultimate Tensile Strength. This allowable surface pressure is higher than the compressive yield strength due to the restraining effect of the surrounding material of the clamped part. This is similar to indentation hardness testing. If you have a disc of material that is sufficiently thick and large in diameter, a localized pressure must be approximately 3 times the yield stress to cause indentation. For a bolted joint, you have a similar load case (but with a hole in the object).

There is a good description of allowable surface pressure analysis in VDI 2230 (see faq725-600 for a description of this reference).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tmoose]

If the crushing refers to material deformation under the head, the older Holokrome book listed minimum material hardness to withstand a socket head capscrew's under head pressure directly. The online version does not seem to mention it.

 

[arto]

Try ASTM F-436 Hardened Washers if you're having troubles.

 

[Tmoose]

From Camcar Textron tech manual. Suggested seating torque for SHCS create 75% yield preload and ~ 80,000 psi bearing stress under head. Indentation is not expected if joint material is harder than Rockwell B85.

 

[Philrock]

Thanks for your input guys. I guess I should say a little more about what I'm up to. I'm writing a spreadsheet that will, among other things, tell you how tight you can safely tighten a fastener. Limitations include fastener tensile strength, thread stripping, and crushing under the fastener head. I'm looking for concrete criterea for max allowable compressive stress under the fastener head. Looks like I may have to invest in a copy of VDI 2230 (almost $400 - ouch!!).

 

[CoryPad]

If you are looking for equations and techniques, then you should buy VDI 2230. If you are looking for typical values of allowable surface pressure for various materials, then less expensive journal articles are available (in German language, though):

Ermittlung der Grenzflächenpressung unter einachsialer kontinuierlicher Beanspruchung bei Raumtemperatur, Konstruktion July/August 2002, pages 38-42, available at:

http://www.technikwissen.de/kon/article.php?data

[article_id]=13389

and

Grenzflachenpressung von Eisenwerkstoffen und Leichtmetallen unter kontinuierlicher Beanspruchung, Materialwissenschaft und Werkstofftechnik 2006, 37, No. 10, pages 894-899, available at:

http://www3.interscience.wiley.com/cgi-bin/jissue/113398353

Lastly, I can say that the Bearing Yield Stress values shown in MIL-HDBK-5 are essentially equivalent to the allowable surface pressure values in VDI 2230, and the handbook is free.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.