elaboration of a research paper on analysis of composite sandwiches 1

[trish129]

In my pursuit to obtain the properties for input to FEA of flexural behaviour of carbon fiber prepreg sandwich panel(aluminum core),I came across a research paper titled:

Fabrication, testing and analysis of composite sandwich beams by Isaac M. Daniel *, Jandro L. Abot

""The sandwich beam face sheets were unidirectional eight-ply carbon/epoxy laminates (AS4/3501-6). The composite sheets were fabricated by autoclave molding.Uniaxial tensile and compressive tests were conducted primarily in the longitudinal direction in order to obtain
the relevant constitutive behavior of the facing material.The longitudinal tensile and compressive stress-strain behavior for the AS4-3501-6 carbon/epoxy is shown in Fig. 1. It can be seen that the material exhibits a char-acteristic sti€ening nonlinearity in tension and softening nonlinearity in compression. The basic properties of the unidirectional material are tabulated in Table 1."""

---> 1.what tests are to be conducted on the cured pre-preg sheets and as per what standards?
2.are all the mechanical properties listed in table 1 obtained from a single test?
3.are all the mechanical properties listed in table 1 used in analytical calculations and FEA?


""The core material used in this study was aluminum honeycomb PAMG 8.1-3/16-001-P-5052 (Plascore Co.,5052 aluminum alloy, 130 kg/m3
, 8.1 lb/ft3 density, 4.76 mm, 3/16 in cell size, 0.025 mm, 0.001 in foil gauge).Specimens of 2.5 cm (1 in) thickness were obtained. The
material is highly anisotropic. The in-plane stiffnesses E1 and E2 and the out-of-plane stiffness E3 (along the cell axis) were obtained by means of fexural and pure compression tests. The out-of-plane shear modulus was obtained by means of a rail-shear test. Honeycomb core properties are tabulated in Table 2."""

-----> 1.how would the values of E1,E2 and E3 be obtained from flexural and compression tests when it is known that ASTM D7250 provides only 3 properties ,G,U and D....?what tests were performed to obtain these?

2.what is the rail shear test and why is it performed?

3.isnt this a deplication of effort? performing flexural test for obtaining the prinipal elastic moduli and performed again?...


the paper is attached for reference...

 

[RPstress]

Attached is a list of basic tests done for a new material combination. Each test usually takes about 10 specimens. The 'constitutive properties' testing in the paper seem to have just been the basic lamina tests (the first six rows of the attached and RT/AR only), although they don't list an ILSS. Test specs may be ISO, ASTM, NASA or in-house (such as Airbus AITM or Boeing BSS). Each row of the attached needs a different test spec.

The attached list is fairly comprehensive. The only test not really needed for stressing a structure is the kbai (which is to evaluate how much stronger laminates are in bending than direct stress; in a pinch you can just use the lesser of tension and compression as the allowable bending surface stress, though you will add a little bit of weight needlessly), and of course the fastener tests assume the use of fasteners. All of the properties they list are useful.

Usually rail shear (e.g. ASTM C 273) is used for core shear modulus and strength but a beam shear test can be used (it's a bit cheaper, less accurate and is trickier to carry out well).

[tt] Rail shear (sorry about the dots, needed to stop multiple spaces being collapsed into one):
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[/tt]
I have no idea how they measured the in-plane moduli of the core. It's not a property that any core supplier reports as far as I'm aware. It's occasionally used by people doing fairly exotic nonlinear analysis interested in details of how energy is absorbed. Their values of 8 and 1 MPa are slightly lower than I'd have thought (but not much). Apart from this paper I have only one other source which documents the in-plane properties. Personally I have only used it when looking at the properties of a filled honeycomb and I've used the geometry to calculate modulus assuming the corners where foil changes in direction are not free to rotate. At 8 pcf their core is quite heavy but not outrageously so (wall thickness should be 3 thou/.08 mm with 3/16" cells).