OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.5 Materials – sandwich structures – Page 82
Table 5-5 Shear strength correction factors
f tc is a correction factor for the effect of core thickness t of a sandwich beam
f i is a correction factor for the in-plane width w of a sandwich beam
f ip is a correction factor for the in-plane size b of square panels
f ip is a correction factor for the in-plane size ab of rectangular panels
f b is a correction factor accounting for the effect of bending
τ ref is the mean value of the shear strengths measured from the reference sandwich specimen.
f b can be derived as follows:
The ratio between shear strain and bending strain for a beam subject to four point bending is given by the
following formula (derived from sandwich beam theory)
ε l
bG
ct
c
=
γ t E d
where
ε
γ
is the ratio between extensional in-plane strain and shear strain occurring in the core.
A simple failure criterion in terms of shear strain and in-plane normal strain can be chosen
ε γ
+ = 1
C C
where C ε and C γ are empirical constants. These empirical constant C ε and C γ are determined by fitting the
previous equation to measured data.
Solving the equations simultaneously for and multiplying by G c , one obtains the shear stress as a function
of the ε ration where a 1 and a 2 are constants.
γ
τ G
c
f
b
= =
τ ε
ref
a1
+ a
2
γ
Guidance note:
The coefficients are based on Weibull theory. The theory states that
where σ i is the uniform stress at failure acting over a volume V i .
ε
γ
σ
σ
1
2
ε
The equation describes the dependence of the failure stress on the loaded volume, and was originally developed for
the failure of brittle materials such as ceramics. In a balsa-cored sandwich beam, one can expect the core failure of a
shear-loaded beam to be controlled by randomly distributed defects within the loaded volume. For a 4-point bending
specimen, the shear-loaded volume is V=2L b wt c .
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
corrected
2.5.10 Other methods to correct the shear strength may be used if they are backed by experimental evidence.
f
2.5.11 For specifically predicting the shear strength at failure of balsa-cored sandwich beams or panels made
out of end-grain balsa type of density 150 kg/m 3 , and provided that the ratio between extensional in-plane strain
ε
and shear strain occurring in the core, , remains between 0.37 and 1.1 , the following correction factors may
γ
be used:
γ
f
⎛ V2
⎞
=
⎜
V
⎟
⎝ 1 ⎠
1
m
f
f
f
f
f
tc
i
τ
=
τ
τ
=
τ
ip,square
ip,rec tan gular
b
=
ref
corrected
τ
τ
ref
τ
=
τ
ref
⎛ t
=
⎜
⎝ t
⎛ l
= ⎜
⎝
corrected
ref
τ
=
τ
=
a
corrected
1
ref
ref
c
slref
l
sl
G
c
ε
+ a
γ
⎞
⎟
⎠
w
w
1
m tc
ref
⎛ b
= ⎜
⎝ b
ref
⎛ 2b
= ⎜
⎝ ab
2
⎞
⎟
⎠
⎟
⎠
⎞
2
ref
1
mi
2
m ip
⎞
⎟
⎠
1
m ip
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