Help me understand the difference between crippling and buckling
Help me understand the difference between crippling and buckling
(OP)
Let's assume an integrally stiffened I-beam under axial compression. As I understand it, the free-edge flanges will buckle first. As load continues to increase the corners and web will eventually buckle. The crippling stress Fcc of this test sample is basically then a P/A, an average stress of the total effective section that would be higher than the local stress in the flanges but lower than the local stress in the corner and web. Is this understanding correct?
Here's what has me confused. On the aircraft I work with, spar caps are sized to prevent crippling using a method of flanges type technique (McDonnell Douglas method). The crippling stress is independent of the spacing between the integral stiffeners. Typically if I do a buckling check of the free flange (assuming 3 fixed edges and 1 free edge to get the K value) I get a failure stress much higher than the crippling stress for the entire section. This is especially true with short spacings between stiffeners because the buckling stress increases as distance between stiffeners decreases. This doesn't make sense to me though because I thought the free flange was supposed to buckle prior to crippling failure of the total section.
Is the crippling stress used for sizing simply to be conservative, even though it may not be the most accurate? Or am I not understanding something here and crippling failure is something different and unrelated to buckling of the individual flanges?
Here's what has me confused. On the aircraft I work with, spar caps are sized to prevent crippling using a method of flanges type technique (McDonnell Douglas method). The crippling stress is independent of the spacing between the integral stiffeners. Typically if I do a buckling check of the free flange (assuming 3 fixed edges and 1 free edge to get the K value) I get a failure stress much higher than the crippling stress for the entire section. This is especially true with short spacings between stiffeners because the buckling stress increases as distance between stiffeners decreases. This doesn't make sense to me though because I thought the free flange was supposed to buckle prior to crippling failure of the total section.
Is the crippling stress used for sizing simply to be conservative, even though it may not be the most accurate? Or am I not understanding something here and crippling failure is something different and unrelated to buckling of the individual flanges?





RE: Help me understand the difference between crippling and buckling
Research "Johnson-Euler". the practical column allowable curve, plotted stress vs (L/p), starts with the euler curve, but this going to infinite stress for L/p = 0. An engineering answer to this is Johnson-Euler, where Johnson said "the highest compression stress a section can carry is fcc" and then drew a parabola to the euler curve ... the Johnson curve covers the short column behaviour, euler the long column.
clear as mud ?
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RE: Help me understand the difference between crippling and buckling
another day in paradise, or is paradise one day closer ?
RE: Help me understand the difference between crippling and buckling
RE: Help me understand the difference between crippling and buckling
Brian
www.espcomposites.com
RE: Help me understand the difference between crippling and buckling
Generally we are talking about things like Z or J channels as reinforcement for skins.
RE: Help me understand the difference between crippling and buckling
Andries
RE: Help me understand the difference between crippling and buckling
Euler buckling and Johnson both are used for tie rod tube compression analysis, the transitional length ratio, L'/ρ >/< a value, is what decides whether you use a long column or short column analysis. Refer to table C4.1 for example in Bruhn.
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RE: Help me understand the difference between crippling and buckling
Crippling is a method that the analyst employs when he or she has made the determination that a bent-up or formed composite section has post buckling strength (it must be able to take further load at the corners of the section after individual segments have buckled – for a bent up section, the additional load is carried by the corners of the section). By making this assumption, the analyst is, by definition, allowing the segments to buckle and is taking advantage of the post buckling strength of the section. This approach results in the most light weight design. However, it should not be employed for situations for which it was not developed – bending is neither applicable nor necessarily conservative.
The NASA Astronautic Structures Manual affirms this with the following: "When the corners of a thin-walled section in compression are restrained against any lateral movement, the corner material can continue to be loaded even after buckling has occurred in the section. When the stress in the corners exceeds its critical stress, the section loses its ability to support any additional load and fails. The average stress on the section at the failure load is called the crippling stress Fcc." Section C1.3.1
There you have it.
RE: Help me understand the difference between crippling and buckling
RE: Help me understand the difference between crippling and buckling
Here is how I have understood crippling / buckling from the perspective of aircraft structural repair:
As alluded to in the original post, compression failure is dependent on the structural geometry just as much or more than the mechanical strength of the material. Whether the critical mode of failure for a piece of structure is buckling or crippling depends mainly on the stiffness and stability of that structure.
When analyzing an OEM component or a repair section in relation to one another, a value of critical compression strength may be determined. If you have access to the Alteon repair guidelines there is a good summary there, as well as standard references.
Column buckling may be described as a type of failure in long slender members where there is a wave-like deflection and can occur suddenly with little increase in load as it is a stability issue. General characterization is gross deformation with little change to the part cross section. Buckling behavior can be either plastic or elastic.
Crippling failure is analogous to failure of an empty soda can when compressed. There are large changes in the local cross section an lots of plastic deformation. Cross sections which are crippling critical are generally hollow or open.
In the type of work I do, we are generally trying to show that a repair section has at least the same compression strength as the OEM structure. The first step is usually to figure out the regime of compression behavior based on the geometry of the part. This comes down to the slenderness ratio, which is dependent on the radius of gyration and how the part is fixed to the surrounding structure.
The value the critical compression strength Fcr, comes from several places depending on the slenderness ratio. Basically the structure will either be treated as a short, intermediate, or long slender column. Crippling is critical for short columns as this means they are well restrained and the failure will occur locally with changes in cross section, rather than on a gross scale with mainly deflection.
For intermediate columns, the Johnson-Euler equation is used to find Fcr. For long columns, we have the Euler equation or Euler-Engesser equation depending on whether you are looking at elastic or inelastic buckling.
It seems like from your question, you were talking about sizing during initial design. All I can say is that for sizing of a (repair) part, using the crippling strength may not always be conservative. A complete compression analysis will take the geometry into consideration to determine whether what you are looking at can be treated as if buckling is less of a concern than local crippling.
I think comparing the buckling strength of only a free flange to the crippling strength of the entire section as you mentioned is not indicative that the buckling stress is actually that much higher. A buckling check should be made for the whole section depending on the slenderness ratio.
Also, I think crippling is more than just material compression failure. Simple material compression failure in my experience is referred to as "block compression".
Please correct me if needed as my understanding may be improved as well. Thanks.
Keep em' Flying
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RE: Help me understand the difference between crippling and buckling
RE: Help me understand the difference between crippling and buckling