bearing strength vs. thickness 1

[Vitalii]

I’m working on shear allowable strength for joints for a few combinations of fasteners and sheet materials. I want to use MMPDS as the main sours of data. Now I take data from MMPDS and draw a diagram. The diagram shows how shear allowable strength depends on a thickness of sheet material. This diagram is typical and can be easily found in lots of books. I put 4 points on the diagram, which are A B C D. Zone between points A and B are bearing critical zone. Zone CD is shear critical. This is clear. And zone between points B and C called transition zone.
Question 1. What kind of failure occurs in this transitional zone?
Question 2. Why the curve changes its inclination so rapidly at point B?

 

[SWComposites]

1. Likely a bending failure of the fastener - failure of the head or threads/collar
2. Because the failure mode changes from the sheet (bearing) to the fastener

 

[aerostress82]

AB & BC: Both are bearing.

CD: Fastener shear failure.

The reason is your material billet thickness.
Different billet thicknesses of the same material have different bearing allowables. And bearing always is linearly proportional to thickness for a constant billet size of any metallic material.

So your material below and above the B point, belong to different billet thicknesses.

Thanks though. Made me think too for a while. Really good question..

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

 

[ESPcomposites]

There is a bit more to it than just the change of bearing strength as a function of thickness due to sheet/plate size.

Per MMPDS, "Bearing design properties are applicable to t/D ratios from 0.25 to 0.50. Bearing design values for conditions of t/D < 0.25 or t/D > 0.50 must be substantiated by tests." What happens is that at certain thicknesses, the through-thickness stress are less uniform and there can be local peaks (due to fastener bending, etc.). So for some thicknesses, there is an interaction between the sheet thickness and the fastener, not accounted for in the provided bearing strength values.

In a related manner (composites are not susceptible to billet size /heat treat effects), consider this comment from CMH-17 (and MIL-HDBK-17). "Bearing strength.....for a specific laminate family, a specific fastener, and equal thickness laminate joining members, the parameter with the greatest influence is D/t." Again, there is an interaction of the fastener size and the bearing strength. Because this is a geometric effect, it is observed for both metals and composites.

Also, the bearing strength as a function of sheet size doesn't change that much for this thickness range (just a few percent usually). If the effect was solely due to this, the change in slope would be pretty subtle and probably imperceptible on the plot.

 

[rb1957]

what is an HL911 ? I couldn't find it in the lisi catalogues. is it CSK ?

another day in paradise, or is paradise one day closer ?

 

[verymadmac]

HL911 is a Reduced head 100 deg CSK 6AL-6V-2S pin

 

[rb1957]

so then it obvious why the allowable falls for thinner sheet ...

another day in paradise, or is paradise one day closer ?

 

[rb1957]

vmm ... where was that ?

another day in paradise, or is paradise one day closer ?

 

[verymadmac]

It was in a spreadsheet buried in the 40 GB of design data I cart about

After a bit of digging it came from a PDF called "HL PIN INDEX" which appears to have come from the Hi-Shear website.

Love the way-back machine

https://web.archive.org/web/20060514051250/http://www.hi-shear.com/brochures/HL Pin Index.pdf

It is also listed in the Voi-Shan Aerospace Fastener Design Manual.

 

[aerostress82]

ESPcomposites
Careful with the D/t. It is mainly related to bending of the fastener. (both for composite and metallic)
Also careful with bearing knockdown factor for blind rivets. Blind rivets and Hi-Loks are also different in bearing behavior. Blind rivets have a knockdown factor to reduce the actual bearing strengths in composites.

I believe what I wrote in my first post above should be the main reason for that graph under "shear loading only" (for Hi-Lok fasteners at least). Please correct me if I'm missing something.

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

 

[SWComposites]

section AB: pure (well, close to) uniform bearing stress thru the material thickness. And basic double shear pin bearing Fbru values can (usually) be used to predict joint strength in this region. Exceptions include blind rivets/fasteners as noted above

section CD: fastener shear failure. fastener material Fsu values can (usually) be used to predict joint strength. Exceptions include blind fasteners and some rivets

section BC: in this region the bearing stress is NOT uniform thru the material thickness. The failure mode(s) is a combination of sheet "bearing" and fastener bending (at the head of collar end). This cannot generally not be predicted with basic material properties; test data is required.

The kinks in the curve are due to changes in failure mode/location.

 

[aerostress82]

I finally checked Al7075 T6 material bearing values for 0.1" and 0.2" (couldn't check it all day because of the busy workday). I remember the bearing strength values of sheets being different, but after your message now went ahead and checked it in detail.

You are right. I remember significant bearing allowable differences between different billet thicknesses. That's what got me.

That linearity at BC region is probably coming from the curve-fitting after the physical test results (if they went ahead and performed more test at tiny increments of thicknesses, they might end up with a curvy region there). So, bending is definitely an effect for isotropic material tests too. Thank you sincerely for your correction on this. Now all is clear.

Spaceship!!
Aerospace Engineer, M.Sc. / Aircraft Stress Engineer

 

[ESPcomposites]

Agree with SW. Fastener bending (associated with D/t) affects the curve in the central region. The fastener bending mode and bearing mode are decoupled for some thicknesses, but coupled for other thickness (due to through-the-thickness stress variation). That is why there is a need to provide valid D/t ranges in MMPDS. These are the D/t ranges where fastener bending is decoupled from the sheet bearing mode.

Calculation: The diameter is not provided in the curve, but assuming Fsu=95 ksi for the fastener, then D is about 0.20 (could be a 3/16 fastener). Per MMDPS, the valid bearing ranges are D/t=0.25 to 0.50. This equates to a thickness of t=0.05 to 0.10. That is exactly where A-B is.

 

[WKTaylor]

Be aware that fastener-shank-to-hole-fit... sloppy clearance or close-tolerance or net-fit or interference fit... will influence to lesser/greater degrees the bearing yield and the fatigue durability of fastener installations.

Regards, Wil Taylor

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