Bond Shear Allowable vs. Shear Stress Limits 2

[kth42]

Hello All,

In Nastran what is the difference between the Bond Shear Allowable in the property card vs the Shear Limit Stress in the material card? I did a quick search of the forum, but didn't find anything.

Thanks in advance for your help.
Kris

 

[blakmax]

Hi Kris

There are two answers to your question.

The short answer is that the Shear Limit Stress is the maximum average stress which the adhesive can sustain without failure, and the Bond Shear Allowable is the value of average shear stress below which you are advised that you should keep the average stress below to avoid bond failure. The allowable is derived by applying knock-down factors to the limit stress to take account for different adherend thicknesses, different adherend elastic moduli, different adherend coefficients of thermal expansion, different overlap lengths and different service operating temperatures. The allowable is usually generated by an extensive test program and is mainly based on lap-shear strength tests such as ASTM D1102. Use of the allowable stress method must be backed up by an extensive test program to demonstrate structural integrity for all joint configurations under all operating conditions (especially temperature).

The long answer is that the "allowable" stress is a meaningless parameter because it is based on an average shear stress design methodology. It has been known since 1936 (Volkersen) that the shear stress distribution in a bonded joint is not uniform, with shear stress peaks at each end of the joint and a shear stress trough in the middle of the joint which (if the overlap length is adequate) will decay to zero. The average stress approach assumes that the adhesive is uniformly stressed along the bond length which is false.

To demonstrate the inadequacy of the allowable stress method, consider this. I undertake a design of a joint and I find that the stress exceeds the "allowable" stress, so I double the overlap length to reduce the calculated average stress to the required value. In practice, provided the actual shear stress has decayed to zero in the middle of my joint, then I have added bond overlap to the area of the joint where the shear stress is zero. I do not change the actual shear stress in the joint at all.

Hence it is possible to create an unconservative design. The only reason the average shear stress approach does not result in bond failures is that the knock down factors are so severe that failure is prevented by the conservatism of the value and the extent of testing, not the level of rigour of the design.

There actually is a far more rigorous design methodology based on the load capacity of the adhesive where the design methodology (not the adhesive allowables) takes into account all of the factors which require knock down factors in the average shear stress method. This approach is based on strain energy.

I suggest that you read DOT/FAA/AR – TN06/57, May 2007, which is available through the FAA Tech Center, or at

http://www.tc.faa.gov/its/worldpac/techrpt/artn0657.pdf

Regards

Max Davis

 

[SWComposites]

1) RTFM

2) what Max says is all correct and good info

3) specific to your NASTRAN question, assuming you are refering to PCOMP and MAT8 cards:
-the SB value on the PCOMP card is the allowable shear stress of the bonding material (allowable interlaminar shear stress)
- the SB value on the MAT8 card is the allowable shear stress of the composite laminate bonding material (allowable interlaminar shear stress)
- the value on the MAT8 card over rides the value on the PCOMP card if it is not blank. Both are an interlaminar shear stress value.

4) now, what value to use for SB, and whether the margin calculated using the SB value is correct, are different and much more complicated questions, and can only be answered after specific details of your material, structure and analysis are provided (but in general it is my opinion that interlaminar stress margins are almost worthless, as interlaminar failure is a fracture process).

SW

 

[kth42]

Thank you both for your responses.

"RTFM" - that hurts, but it was deserved.

Thanks again.