determination of honeycomb sandwich Directional properties from test results

[trish129]

Hello,

I am performing a bending test as per ASTM D7250 of a honeycomb sandwich panel comprising the following :

Face Skin
Material: carbon fiber epoxyPREPREG
Uni directional
1 skin consists of

3 plies of 0 degree

3 plies of 45 degree

3 plies of -45 degree

3 plies of 90

i.e ,12 plies layed up in 1 skin.both skins of nearly same thicknes and same material


Quasi -isotropic configuration


Aluminium Core
thicknes: 1 inch

The tests would provide me the following properties:
Core Shear Modulus,G
Flexural Stiffness,D
Transverse Shear rigidity,U

panel orientation in test: flat-wise
panel length along the core-ribbon direction



1.what information do these three properties convey about a particular sandwich PREPARED by given fabrication process(oven,autoclaceve)?

2.how can these properties be incorporated in the finite element analysis of this particular configuration? that is,how can these properties be RESOLVED into the directional properties for inputs in the FEA program?

thank you,​

 

[adfergusson]

I'm not a big fan of this and other flexural test methods (ASTM or otherwise); they're pretty much only good for quality control or as in intermediate step for validating an FEA modelling approach before going to larger scale tests. I've performed this test and other similar tests as a student, and continue to come across students using tests like these on composite sandwich panels who want to know why they don't see the results/behaviour they expected. As an example of one of the many issues with such tests that I reckon will be of particular note (based on what you've said about your panel):

1. Assuming a standard prepreg ply thickness of 0.125mm; you've got a pretty thin skin on a comparatively thick core. Consequently, the rollers on your test rig will readily indent into you panels; this is particularly problematic for four point bending as at a fairly low load, the indentation of the rollers will be sufficient for them to constrain the deformation of your panel. At this point you will see your panel become a lot stiffer.

To check this for yourself; put a strain gauge on the top and bottom surface between the two central rollers. Initially, they will both record increasing strain with load/displacement. Once the the rollers start to indent into the panel you will see the reading in the strain gauge on the compressive surface drop towards zero while the tensile gauge continues to record increasing strain. When the rollers start to interact with the panel in this way, the compressive skin in the panel essentially becomes the neutral axis of bending; hence the apparent (and artificial) increase in panel stiffness.

Back to your specific points

1.what information do these three properties convey about a particular sandwich PREPARED by given fabrication process(oven,autoclaceve)?

^Not much.

1.what information do these three properties convey about a particular sandwich PREPARED by given fabrication process(oven,autoclaeve)?

Once again; not much.

I appreciate that my above two comments are not particularly helpful, so:

If you're looking to compare the performance of panel made in an autoclave vs out-of-'clave then I'd suggest that starting with a sandwich panel test is not the way to go. Assuming you're using the same prepreg processed with, and without, an autoclave then there are a number of differences to consider, including:

1) Volume fraction (Vf); this will be higher in the autoclaved panel. This will increase the moduli and strength of the laminate, but the laminate will be a bit thinner. Potential for reduction in fatigue life and or strain to transverse tensile failure in resin.

2) Increased porosity in out-of-clave samples; probably won't make much difference in one-off tests of your laminate but can be very significant when it comes to fatigue/damage.

3) Fibres in your laminate will bend/curve slightly inwards between cells in your honeycomb. This will knock down laminate performance with the result that laminates produced on a caul plate without a honeycomb core will outperform the laminates that make up the skin of your sandwich panel. In order to get an idea of the effect of this you could start by curing a laminate (not necessarily the one you've described) by itself and also cure the same laminate on the honeycomb core but with a layer of peel ply between the skin and core so that you can remove the skin from the core for testing; then do some of the standard battery of test methods for composite laminates to get an idea of the knockdown in performance the laminate will exhibit. Compressive failure of laminates with misalignment is a non-trivial matter and there are almost as many opinions on this as there are academic papers on the subject....caveat emptor.

Anyway, ^a few points to consider.... composite materials make things much more complicated on the experimental side as well as the numerical side.

 

[rb1957]

"2.how can these properties be incorporated in the finite element analysis of this particular configuration? that is,how can these properties be RESOLVED into the directional properties for inputs in the FEA program?" ... you can model the beam with plate elements with laminate property. laminate properties allow you to build up the element stiffness matrix according to your lay-up. different FE codes do this differently, read your user manual.


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

 

[stressebookllc]

Try this link:

http://www.stressebook.com/sandwich-panel-long-beam-and-short-beam-tests/

These tests have stood the time and the specifications are widely used in the aerospace industry.

@adfergusson: I believe the what you are talking about is more pertinent to short beam tests, here the dominant strain is out of plane. Core shear, and this is how the failure will happen.

For flexure tests, the coupon lengths have evolved into the current recommendations to ensure the top skin in most cases fails in compression and flexure. Then there are also the statistical data reductions out of the various failure mode variations.

http://www.stressebook.com

Stressing Stresslessly!

 

[trish129]

thank you for your replies fellows...

Given the fact that i am new to the world of composites and learning the ropes of the trade,especially its mechanics, i would like to know more...

1)curing pre-preg plies(12) in autoclave,making them into 2 skins and bonding them to the upper and lower surface of the honeycomb VIA an adhesive called cold bonding or co-bonding?

2)i fail to understand the FINE difference between core shear modulus and transverse shear rigidity for a given sandwich panel..what is the physical significance of these? do these hold for the skins or core or the whole fabricated panel itself?

3)in bending test,the applied load is out-of plane i.e transverse to the sandwich beam longitudinal axis(L core ribbon direction in my case).i intend to orient the panel FLATWISE.does it run counter to ASTM D7250?

4)ASTM D7250 suggests two methods(1) when the facing modulus is known (2) when the facing modulus is NOT known...i intend to go with the second one,though i can test the CURED face skins YET i turn a blind eye to it and want to go on with the second one so that i test the panel independent of the tests used to determine facing properties.and for this ,the standard D7250 suggests performing a short span test(c393) and a long span test(d7249).for the long span the d7249 recommends a specimen size of 24 inch length and 3 in width and a 22 in support span.and for the short beam test it recommends 8 inch specimn length and 6 inch support span ..

i am going by this.just wanted to know IF,by mistake,i happen to cut a panel of length 540 mm for the long beam test.how would it affect my test results?

5)if it is known that the major loading on the structure is going to be that of bending loads,what would these values(D,U,G),flexural stiffness G in particular,tell for panels processed by different methods(out of autoclave,autoclave and cold bonding)? what would the comparison of these values suggest about these processed?

6)i go back to my main question..after getting the values for D,U,G from the bending test D7250...how do i get the values for input in FEA program? because the properties i get would be REPRESENTATIVE of the whole sandwich panel is there any way to resolve them into the directional properties for the FEA?


7)adfergusson:why wouldnt the adopted approach(bending tests of panels fabricated by different methods to determine which one is good in strength and stiffness) do? especially given the circumstances that this approach has went into the pipeline..

8)rb1957: can you please elaborate a bit more on your reply in the light of the foregoing..

9)stressbookllc: amazing resource...loads of thanks..