Core Crush Strength vs Compressive Strength 1

[Miner89r]

Hexcel datasheets for honeycomb core provide allowables for Compressive Strength (both Bare and Stabilized) and also for Crush strength. The crush strength seems to always be lower than the compressive strength. My understanding is that bare compressive strength is for a core specimen tested by itself and stabilized compressive strength is for a core specimen bonded to facesheets, so when analyzing sandwich panels the stabilized compressive strength values are appropriate. What does the Core Crush strength represent and when should crush strength be used instead of stabilized compressive strength?

https://hexcel.com/user_area/content_media/raw/HexWeb_CRIII_DataSheet.pdf

 

[KSshopper]

Honeycomb core can be used to absorb energy by crushing under load. Crush Strength is a property of the core required to analyze an energy absorption problem. See hexcel TSB122 for the analysis method.

Compressive strength is the appropriate property for use in a static strength analysis.

 

[SWComposites]

Crush strength is the stress carried after the point of initial failure (which is more or less cell buckling) as displacement is increased.

 

[WKTaylor]

For what it is worth... From SAE AIR4844C Composites and Metal Bonding Glossary

COMPRESSIVE STRENGTH: The ability of a material to resist a force that tends to crush or buckle. The maximum compressive load sustained by a specimen divided by the original cross-sectional area of the specimen.

CORE CRUSH: A collapse, distortion, or [*permanent*] compression of the core. [* me *]

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Compositepro]

I'm not completely sure, but in my experience, the term core crush refers to a problem that occurs during vacuum-bag/autoclave processing where the core crushes in from the edges, not the faces.

 

[SWComposites]

Cpro, yes, “core crush” can occur during panel cure under pressure. But “crush strength” is different; it is the stress sustained under compression load after max load and initial damage is reached. Crush strength is used for energy absorption analyses.

 

[WKTaylor]

For grins... there are [2] a honeycomb panel variants where crushed core is used in optimum ways.

NOTE1... this applies to aluminum face-sheet, aluminum honeycomb core and is relatively old technology... which are now mostly replace by composites.

When maximum core strength is demanded with relatively light facing sheets... and a relatively thin panel, approaching sheet metal in appearance/utility is required with SOME sheet metal characteristics [may be roll-formed, lightly stretch-formed, punched/drilled, cut/etc]… then a honeycomb sheet is deliberately crushed to ~20% original thickness and is then conventionally bonded with the face sheets. This is called a crushed-core honeycomb panel. Extra thick film adhesive for the face sheets tends to infiltrate and seal a majority of the 'crushed core' providing a very durable/light/thin panel. IF fastened, it is wise to avoid countersinks and install all fasteners with sealant to minimize moisture intrusion and loss of face-sheet bearing/shear/pull-thru strength.

Also... most heavy honeycomb core is typically 'machined' for surface contour matching to 'formed sheet pans/contours' for light weight 'sculpted HC panels'. Well some pretty smart old-guys figured-out that relatively thin/low strength/stiffness HC core [typically ductile 5052 or 5056, NOT 2XXX or not aluminum core]… can be crushed-to-shape/contour in matching dies... which can be matched precisely to the inner surfaces of like-wise-formed-sheet metal pans. Then, hot-bonding with film adhesives in contour controlled tools is no big deal. Some areas of the panel-core will be full depth where others will be crushed almost flat; and panels may/may-not have extra internal doublers along/around edges for added fastener shear-bearing strength and edge-sealing. I think there were also tests of this process where thin sheets of 2024-O[HT-to-W just prior to forming] where co-crushed with the 50xx core [in these same crush-dies?]... and made fairly tightly matched skin and core-parts rapidly and at fairly low-costs [after tooling/processes were stabilized]. I'm a bit unsure of how well this actually worked in high volume production.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]