OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.5 Materials – sandwich structures – Page 81
2.4.3 Balsa sandwich structures show a reduction of COV with specimen size. If global properties are needed
the COV of large specimens may be used. If local properties are needed, e.g., for a joint analysis, COV values
of the critical dimensions in the analysis shall be used.
2.5 Shear properties
2.5.1 Shear properties of core materials are difficult to measure. Suitable test methods should be used for the
determination of shear design properties, see Sec.5 [2.2] and App.D. When using data from the literature, it
should be checked that the proper test methods are used.
Guidance note:
Using the block shear tests data to obtain the shear strength of balsa beams and panels will in many relevant cases
overestimates the shear strength by a factor of 2 to 4. Block shear test, such as the one used in ASTM C 273-00 or
ISO 1922, should not be used to obtain design shear strength of balsa cored sandwich beams and panels. The flexural
test method used in ASTM C 393-00 can be used instead, see App.D.
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2.5.2 Ideally the test method shall measure core yield or ultimate strength that is independent of the specimen
geometry and that can be used for all structural geometry. If such a method cannot be found, e.g. for materials
like balsa, corrections may have to be applied to the test results. Typical effects that require corrections are
change in core thickness, change in skin thickness, in-plane size, effects of bending and shear load
superposition.
2.5.3 For polymeric cellular material the effects due to size and bending/shear load superposition on shear
properties are negligible.
2.5.4 For honeycomb materials thickness correction factor should be applied to strengths and moduli, when
using other thickness than those available from test data or from the literature. Mechanical properties are
usually available from manufacturers according to material, density, cell size, and thickness.
2.5.5 For balsa wood material there are two important size effects: core thickness size effect and an in-plane
size effect. Further, the influence of bending and shear load superposition is significant and reduces the shear
strength.
2.5.6 For balsa wood material the value of the ultimate shear strength obtained from test results should only
be used directly for the design of balsa-cored sandwich structures having identical geometrical, physical and
loading characteristics as the test specimens. Otherwise the shear strength should be corrected.
2.5.7 If the core thickness of the component is less than the thickness of the test specimens a core thickness
correction is not necessary. This is a conservative simplification.
2.5.8 The corrected shear strength should be calculated as follows:
For sandwich beams as τcorrected
= τref .
f
tcf
if
b
and for sandwich panels as τ = τ f f f , where,
correcetd
ref .
— τ ref is the mean value of the shear strengths measured from the reference specimen
— f tc is a correction factor for the effect of core thickness
— f i is a correction factor for the in-plane size of the sandwich beam
— f ip is a correction factor for the in-plane size of the sandwich panel
— f b is a correction factor accounting for the effect of bending.
tc
ip
b
2.5.9 The correction factors can be obtained experimentally by testing specimens of at least three different
dimensions. The corrections factors are based on Weibull theory. A method to obtain the correction factors
experimentally is described in McGeorge, D and Hayman B.: “Shear Strength of balsa-cored-sandwich
panels”, in proceedings of the 4th international conference on sandwich construction, Olsson, K-A (Ed), EMAS
Publishing, UK, 1998. The factors are given in Table 5-5.
DET NORSKE VERITAS AS