Crippling stress - Plasticity factor

[ArunKumar Bala]

Hi all, How to find the plasticity reduction factor for crippling stress. I have seen such factors only available for Buckling stress in Bruhn C5.7 or C5.8 and the same in Niu. Can anyone suggest a way forward for crippling stress?

And which method is efficient to find the crippling stress of complex shapes in stringers?

 

[rb1957]

I'm surprised you need one, I thought crippling was limited to fcy ? I think Bruhn's cut-offs are somewhat arbitrary, conservative; I note that similar cut-offs are not applied to crippling stresses assembled from the individual flanges, as in McCombs supplement to Bruhn's Analysis of Flight Vehicle Structures.

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

Crippling analysis is covered in:
Nui, Airframe Stress Analysis
Peery, Aircraft Structures
and probably other texts.

Crippling is a strength property, related to compression yielding and failure. The plasticity factor is a reduction of modulus used in buckling calculations.

What is your specific issue re crippling of complex shapes? Just break the cross section into segments.

 

[Stress_Eng]

Allowable buckling stresses include the use of Et. It’s been a while, but if I remember rightly, crippling is based on a section consisting of elements with differing edge support conditions, the individual summation of which gives you an allowable compression stress, at which load redistribution to the stiffer elbow regions of the section occurs. There are numerous explanations on the use of Et in standard buckling equations.

 

[rb1957]

Et is a standard plasticity correction for Euler curve, for columns with a stable cross section.

Crippling is an unstable cross section, so you get a local failure before you have complete buckling of the column.

"Wir hoffen, dass dieses Mal alles gut gehen wird!"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Ng2020]

Crippling is, by definition, an entirely plastic failure mode. Therefore there is no additional plasticity correction factor as you might use when developing buckling critical stresses (which is an elastic mode for thinner elements, and involves plastic deformation for thicker elements)