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DNV-OS-C501 Composite Components

Sec.5: Materials - Sandwich Structures

B: Static properties

Sec.5
B. Static properties

Sec.5
B 100   General

Sec.5
B 200   Static properties

Sec.5 B

Table B1 Mechanical static properties for core materials
	Mechanical parameter	Unit	Reference in Appendix D for measurement method
In-plane orthotropic elastic constants
Ext core linear	Tensile modulus of elasticity of core in x-direction in the linear range	[GPa]	B100
Exc core linear	Compressive modulus of elasticity of core in x-direction in the linear range	[GPa]	B200
Eyt core linear	Tensile modulus of elasticity of core transverse to y-direction in the linear range	[GPa]	B100
Eyc core linear	Compressive modulus of elasticity of core transverse to y-direction in the linear range	[GPa]	B200
Gxy core linear	In plane shear modulus of core in the linear range	[GPa]	B300
Ext core non-linear	Tensile modulus of elasticity of core in x-direction in the non-linear range	[GPa]	B100
Exc core non-linear	Compressive modulus of elasticity of core in x-direction in the non-linear range	[GPa]	B200
Eyt core non-linear	Tensile modulus of elasticity of core transverse to y-direction in the non-linear range	[GPa]	B100
Exc core non-linear	Compressive modulus of elasticity of core in y-direction in the non-linear range	[GPa]	B200
Gxy core non-linear	In plane shear modulus of core in the non-linear range	[GPa]	B300
nxy core	Major Poisson's ratio of core	[-]	B100 or B200
nyx core	Minor Poisson's ratio of core	[-]	B100 or B200
In-plane strain (to yield point or to the end of the proportional range)
core linear	Core tensile strain in x-direction		B100
core linear	Core compressive strain in x-direction		B200
core linear	Core tensile strain in y-direction		B100
core linear	Core compressive in y-direction		B200
core linear	Core in-plane shear strain		B400 for balsa B300 for other materials
In-plane strain to failure (all in-plane strain to yield point or to the end of the proportional range, see above)
core non-linear	Core tensile strain in x-direction		B100
core non-linear	Core compressive strain in x-direction		B200
core non-linear	Core tensile strain in y-direction		B100
core non-linear	Core compressive in y-direction		B200
core non-linear	Core in-plane shear		B400 for balsa B300 for other materials
In-plane strength (to yield point or to the end of the proportional range)
core linear	Core tensile stress in the x-direction	[N/mm2] (or MPa)	B100
core linear	Core compressive stress in x-direction	[N/mm2] (or MPa)	B200
core linear	Core tensile stress at failure in the y-direction	[N/mm2] (or MPa)	B100
core linear	Core compressive stress in the y-direction	[N/mm2] (or MPa)	B200
core linear	Core shear stress	[N/mm2] (or MPa)	B400 for balsa B300 for other materials
In-plane strength to failure (all in-plane strength, see above)
core non-linear	Core tensile stress in the x-direction	[N/mm2] (or MPa)	B100
core non-linear	Core compressive in x-direction	[N/mm2] (or MPa)	B200
core non-linear	Core tensile stress in the y-direction.	[N/mm2] (or MPa)	B100
core non-linear	Core compressive in the y-direction.	[N/mm2] (or MPa)	B200
core non-linear	Core shear stress	[N/mm2] (or MPa)	B400 for balsa B300 for other materials
Through thickness elastic constants
Ezt core linear	Core tensile elasticity modulus normal to the core plane in the linear range	[GPa]	B100
Ezc core linear	Core compressive elasticity modulus normal to the core plane in the linear range	[GPa]	B200
Gxz core linear	Core shear modulus normal to the core plane in the linear range	[GPa]	B300
Gyz core linear	Core shear modulus normal to the core plane in the linear range	[GPa]	B300
Ezt core non-linear	Core tensile elasticity modulus normal to the core plane in the non-linear range	[GPa]	B100
Ezc core non-linear	Core compressive elasticity modulus normal to the core plane in the non-linear range	[GPa]	B200
Gxz core non-linear	Core shear modulus normal to the core plane in the non-linear range	[GPa]	B300
Gyz core non-linear	Core shear modulus normal to the core plane in the non-linear range	[GPa]	B300
nxz core	Core Poisson's ratio normal to the core plane	[-]	B100 or B200
nyz core	Core Poisson's ratio normal to the core plane	[-]	B100 or B200
Through thickness strain (to yield point or to the end of the proportional range)
core linear	Core tensile strain normal to the core plane		B100
core linear	Core compression strain at failure normal to the core plane		B200
core linear	Core shear strain at failure normal to the core plane		B400 for balsa B300 for other materials
core linear	Core shear strain normal to the core plane		B400 for balsa B300 for other materials
Through thickness strain to failure
core non-linear	Core tensile strain normal to the core plane		B100
core non-linear	Core compression normal to the core plane		B200
core non-linear	Core shear strain normal to the core plane	[µ-strain] (or %)	B400 for balsa B300 for other materials
core non-linear	Core shear strain normal to the core plane	[µ-strain] (or %)	B400 for balsa B300 for other materials
Through thickness strength (to yield point or to the end of the proportional range)
core linear	Core tensile stress normal to the core plane	[N/mm2] (or MPa)	B100
core linear	Core compressive stress normal to the core plane	[N/mm2] (or MPa)	B200
core linear	Core shear stress normal to the core plane	[N/mm2] (or MPa)	B400 for balsa B300 for other materials
core non-linear	Core shear stress normal to the core plane	[N/mm2] (or MPa)	B400 for balsa B300 for other materials
Through thickness strength to failure
core non-linear	Core tensile stress normal to the core plane	[N/mm2] (or MPa)	B100
core non-linear	Core compressive stress normal to the core plane	[N/mm2] (or MPa)	B200
core non-linear	Core shear stress normal to the core plane	[N/mm2] (or MPa)	B400 for balsa B300 for other materials
core non-linear	Core shear stress normal to the core plane	[N/mm2] (or MPa)	B400 for balsa B300 for other materials
Fracture toughness
GIc core	Mode-I (opening) critical strain energy release rate	[N/m]	B500
GII core	Mode-II (shearing) critical strain energy release rate	[N/m]	B500

Sec.5 B
207   The static data for adhesive materials are the following:

Sec.5 B

Table B2 Mechanical static properties for adhesive materials
	Mechanical parameter	Unit	Reference in Appendix D for measurement method
In-plane elastic constants
E adhesive linear	Modulus of elasticity of adhesive in the linear range	[N/mm2] (or MPa)	C200 or C300
E adhesive non-linear	Modulus of elasticity of adhesive at the failure point	[N/mm2] (or MPa)	C200 or C300
Gxy adhesive linear	In plane shear modulus of adhesive in the linear range	[N/mm2] (or MPa)	C400
Gxy adhesive non-linear	In plane shear modulus of adhesive at the failure point	[N/mm2] (or MPa)	C400
nxy adhesive	Major Poisson's ratio of adhesive	[-]	C200 or C300
In-plane strain to failure
adhesive	Adhesive tensile strain at failure point		C200
In-plane strength
adhesive	Adhesive flatwise tensile strength	[N/mm2] (or MPa)	C300
adhesive	Adhesive tensile strength	[N/mm2] (or MPa)	C200
adhesive	Adhesive shear strength	[N/mm2] (or MPa)	C400
Fracture toughness
GIc adhesive	Mode-I (opening) critical strain energy release rate	[N/m]	B500 or D200
GIIc adhesive	Mode-II (shearing) critical strain energy release rate	[N/m]	B500 or D200

Sec.5
B 300   Relationship between strength and strain to failure

Sec.5
B 400   Characteristic values

Sec.5
B 500   Shear properties

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 appendix D.

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For sandwich beams as

and for sandwich panels as , where,

—	tref is the mean value of the shear strengths measured from the reference specimen
—	ftc is a correction factor for the effect of core thickness
—	fi is a correction factor for the in-plane size of the sandwich beam
—	fip is a correction factor for the in-plane size of the sandwich panel
—	fb is a correction factor accounting for the effect of bending.

Sec.5 B
509   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 B3.

Sec.5 B

Table B3 Shear strength correction factors
ftc is a correction factor for the effect of core thickness t of a sandwich beam
fi is a correction factor for the in-plane width w of a sandwich beam
fip is a correction factor for the in-plane size b of square panels
fip is a correction factor for the in-plane size ab of rectangular panels
fb is a correction factor accounting for the effect of bending

t_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)

where 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

where C_e and C_g are empirical constants. These empirical constant C_e and C_g 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

Guidance note:
The coefficients are based on Weibull theory. The theory states that





where s_i is the uniform stress at failure acting over a volume V_i.

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_bwt_c .

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t_{ref = 1.52}

and

d = tc + tf

where,

:	the ratio between extensional in-plane strain and shear strain occurring in the core
tc :	the core thickness
lsl :	the shear-loaded length
w :	width
Gc :	the core shear modulus
tf :	the face thickness
Ef :	the face in-plane elastic modulus

In the above equations shear stress values are in MPa and lengths in mm.

Guidance note:
Each correction factor is independent. When no correction is needed, for example when a size effect does not occur because of identical dimensions between reference specimen and structure to design, the corresponding correction factor shall be set to 1 in the above equations.

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Guidance note:
When using data from the literature, it shall be checked that the geometrical, physical and loading characteristics are proper for the structure under consideration.

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Sec.5
B 600   Core skin interface properties