Bearing strength failure mode in the bolted joint 1

[kilbchoi]

Hi !

I am looking at the bearing strength of FRP material in the bolted joint. I am wondering what is the main failure mode for the bearing strength. In metal, it is obviously contact compress stress at the contact area.

In my analysis model using NASTRAN, in-plane shear stress is more critical at the point tilted by roughly 30 to 45 degree from the contact center through most of lay-up pattern. Therefore directly applying combined failure criteria such as Hill's criteria, the dorminant mode is in-plane shear strength and the failure point is tiltied as mentioned above.

Looking forward to hearing great comments.

Thanks,

kilbchoi

 

[edrush]

You have made me a very happy man. I am happy because it's not me analyzing this problem.

This sounds like a Phd thesis to me. You can probably find some information in the literature - I'm sure you are not the first person to tackle it.

My suggestion is to break some specimens and then use this as a critera based on fastener force. Personally I don't have much hope that you can get an accurate prediction of failure based on any given failure criteria.

Have you checked Mil-Hbk-17? Maybe it has some recomendations for bearing analysis? You should be able to download it from this site:

http://assist2.daps.dla.mil/quicksearch/

BTW, I don't consider myself an expert, so take this for what you think it's worth.

ed

 

[polymire]

OK
bolt bearing strengths for composites have been sucessfully analyzed and verified by testing, so that there are complete design rules for analyzing bolted joints.

Generally bolted joints are not analyzed in Nastran or other FE modelling. The calculations are "hand calculations" or using a simple PC programe.

There are 6 failure modes that have to be calculated
Tension of the composite
shearout of the composite
bearing of the composite
bolt pull through
bolt bending/failure


Bearing stress =P/dt
P: applied load/bolt
d: bolt diameter
t: composite thickness

Bearing strength =fbr*Z
fbr: bearing strength, this changes with the bolt torque load.
Z: countersunk correction =0.8 (only applies to countersunk)

Shear stress =(P/2ct)*Ks*(ct/ct-A)
P: appled load/bolt
c: distance of center of bolt from edge
t: composite thickness
Ks: Shear stress concentration factor
(ct/ct-A): countersunk correction (only applies for countersunk bolt
A: equivalent area for countersunk bolt

Shear strength = shear strength

Tension stress = (P/((W-D)*t))*Kth*Chs
W: distance between bolt centers or width of coposite
D: bolt hole diameter (major diameter for countersunk)
Kth: tensile concentration factor
Chs: hole size correction factor

Bolt pull through strength generated by testing

As far as I know most of concentration and correction factors are not publicly available and may be subject to security and export restrictions. I do not think MIL-HDBK-17 contains any of properties. Sorry !!

A very good published paper on bolt strengths is...
"Design and Analysis of Bolted and Riveted Joints in Fiberous Composite Structures" L. J. Hart-Smith, international Symposium on Joining and repair of fiber reinforced plastics, 1986

 

[SWComposites]

I agree with the previous responders: attempting to predict bolted joint strength with even moderately complex FE models is an exercise in utter futility. The Tsai-Hill type failure criteria don't work for simple laminates, much less for the highly nonlinear, complex stress states around the fastener hole. I'm currently involved in a series of research projects that has been ongoing for years with the goal of accurately predicting bypass and bearing strengths in carbon fiber composites. We're using extremely large, detailed nonlinear 3D FE models and we still have significant challanges in trying to predict the local failure mechanisms.

Composite laminates loaded in a "bearing" mode can fail by a number of potential modes, depending on the material and layup. There can be a "classical" bearing (crushing) mode where the fastener contacts (the local mode being fiber crushing and matrix cracking). There can be splitting modes at various angles around the hole due to fiber tensile fractures (often preceeded by matrix cracking.

For your situation use simple equations as suggested by 'polymire'. The various papers by Hart-Smith are the best sources for the approximate stress concentration factors around the hole.