Help me understand the difference between crippling and buckling 1

[sdar79]

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?

 

[rb1957]

"Is this understanding correct?" ... well, sort of, but you're using technical terms without precision. there are two compression tests to determine allowables. one test would be to use a full size structural item, to see when it fails. another is a "short column" test to determine the crippling stress of the section; in this test they make a short piece of the beam and rigidly support the ends so that there is only a short piece of the beam exposed. The difference is crippling refers to short column failure, unlike Euler buckling; buckling usual refers to some compression failure of the whole section, crippling refers to the compression failure a a part of the section ... the standard expression for euler testing/calculation is "stable cross-section" which means no crippling.

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 ?

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

 

[rb1957]

btw, you're only "useless" if you don't ask questions, or if you keep asking the same question. asking questions 'cause something doesn't seem right, or because you don't see the logic, is the opposite of "useless".

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

 

[hokie66]

Not sure if the same terminology is used in aircraft structures as in buildings, but to me, buckling (of several types) occurs in members due to axial compression loading or bending. Crippling is on the other hand due to directly applied loading transverse to the member. So global buckling can occur in axially loaded struts which are relatively long, and local buckling of flanges can occur in bending or compression members. Crippling is generally buckling which occurs at a point load such as a reaction, and is normally due to web slenderness. When a short strut fails, I call that a squash load failure, not crippling.

 

[ESPcomposites]

One thing to consider is the assumption about the edge conditions. The fixed (clamped) edge is the maximum rotational restraint, but that is not usually realized. You may have something closer to a simple support, but likely something in between a simple and clamped support. You can check Bruhn (C6.5) or the appropriate NACA documents. They can give you more accurate K value.

Brian

http://www.espcomposites.com

 

[pattheengineer]

You can buckle without plastic deformation, granted the window is small but still there. Crippling is after the yield strength of the structure has been exceeded and it is permanently buckled.

Generally we are talking about things like Z or J channels as reinforcement for skins.

 

[Andries]

To add to the other posts, crippling is a plastic deformation that occurs when sections with at least one corner (angles, channels, etc.)are compressed. The corners stiffen the column and allow stresses to develop that are high enough to cause a permanent set. As rb1957 said, it is mainly a "short" column phenomenon since short columns are stiffer in compression.

Andries

 

[stressebookllc]

In even more dumber and simpler terms, crippling is local (say a flange) and buckling is more global in nature for the member.

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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Stressing Stresslessly!

 

[stressguyMike]

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.

 

[SWComposites]

Sorry - none of you have it fully correct. Buckling is just elastic instability. Crippling is material failure. Two different things. Crippling (failure) can occur before or after buckling; compact sections will not buckle prior to crippling (really this is just a material compression failure); less compact sections will first buckle, then redistribute load in the section and eventually fail (cripple). In general crippling strength is obtained from test results of various cross section shapes. There is an interaction effect with long column buckling - see J-E column method mentioned above.

 

[LiftDivergence]

Sorry if this is a little long but I think it helps to have a broad view (for me anyway). Also, hopefully I don't put my foot in my mouth as I'm only a couple years into this.

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
//Fight Corrosion!

 

[pattheengineer]

Take a plastic drinking straw and press on it lightly and you can watch it bow slightly, let the pressure off, the straw shouldn't have permanently been damaged if you just put enough pressure on to see a slight bowing. Now if you continue increasing the pressure you put on the straw it will fold and get a permanent crease in it and will never be the same again. The first case is only buckling, the second is crippling caused by buckling. The damage isn't caused by the force directly causing the yield strength to be exceeded but the buckling causing a localized area where the material has yielded.

 

[sdar79]

Thanks everyone for the input. I understand it a bit better now.