241
9
alone. Therefore, the tensile strength of the composite is found
as per Eq. 9.3.
s s
c
f f
= V
(9.3)
where:
5 σ c = Ultimate tensile strength of composite
5 σ f = Tensile strength of fibre
5 V f = Volume fraction of fibre
If the matrix does not fail before the fibres fail, the load is shared
by the matrix until the fibres fail, and the ultimate tensile strength
of composite is given by Eq. 9.4.
s s
c
f f
m
f
f
=
+ -
(
)
é
ë
ê
ù
û
ú
V
E
E
V
1
(9.4)
where:
5 σ c = Ultimate tensile strength of composite
5 σ f = Tensile strength of fibre
5 V f = Volume fraction of fibre
5 E m = Tensile modulus of matrix
5 E f = Tensile modulus of fibre
In many applications such as pressure vessels and storage tanks,
the utility of the composite is controlled by the matrix cracking and
not by the strength of the reinforcement. The stress at which the
matrix cracks is given by Eq. 9.5.
s s
c
m f
m
f
f
=
+ -
(
)
é
ë
ê
ù
û
ú
V
E
E
V
1
(9.5)
where:
5 σ c = Ultimate tensile strength of composite
5 σ m = Tensile strength of matrix
5 v f = Volume fraction of fibre
5 E m = Tensile modulus of matrix
5 E f = Tensile modulus of fibre
In actual cases, the matrix fails much before the failure of the fibres
mainly because of the stress concentration, fibre debonding, and
strain magnification at microlevels. The tensile test is carried out as
per ASTM D3039 on a Universal Testing Machine (UTM), as shown
in . Fig. 9.8.
This test provides the ultimate tensile strength, ultimate tensile
strain, modulus of elasticity, and elongation at a break in the test
direction.
3. Flexural Strength and Modulus—The flexural strength of a
composite is dependent on the outer layers of the reinforcement. A well-cured monolithic composite will have higher
flexural strength compared to its tensile strength. Flexural
strength and modulus are tested as per ASTM 749 D. This
3-point bend test is shown in . Fig. 9.9
Point to Ponder…
In a 3-point bend test, the upper
portion of the sample is under
compression, the bottom portion
is under tension, and at the centre
of the sample there is no load.
9.3 · Testing
9
alone. Therefore, the tensile strength of the composite is found
as per Eq. 9.3.
s s
c
f f
= V
(9.3)
where:
5 σ c = Ultimate tensile strength of composite
5 σ f = Tensile strength of fibre
5 V f = Volume fraction of fibre
If the matrix does not fail before the fibres fail, the load is shared
by the matrix until the fibres fail, and the ultimate tensile strength
of composite is given by Eq. 9.4.
s s
c
f f
m
f
f
=
+ -
(
)
é
ë
ê
ù
û
ú
V
E
E
V
1
(9.4)
where:
5 σ c = Ultimate tensile strength of composite
5 σ f = Tensile strength of fibre
5 V f = Volume fraction of fibre
5 E m = Tensile modulus of matrix
5 E f = Tensile modulus of fibre
In many applications such as pressure vessels and storage tanks,
the utility of the composite is controlled by the matrix cracking and
not by the strength of the reinforcement. The stress at which the
matrix cracks is given by Eq. 9.5.
s s
c
m f
m
f
f
=
+ -
(
)
é
ë
ê
ù
û
ú
V
E
E
V
1
(9.5)
where:
5 σ c = Ultimate tensile strength of composite
5 σ m = Tensile strength of matrix
5 v f = Volume fraction of fibre
5 E m = Tensile modulus of matrix
5 E f = Tensile modulus of fibre
In actual cases, the matrix fails much before the failure of the fibres
mainly because of the stress concentration, fibre debonding, and
strain magnification at microlevels. The tensile test is carried out as
per ASTM D3039 on a Universal Testing Machine (UTM), as shown
in . Fig. 9.8.
This test provides the ultimate tensile strength, ultimate tensile
strain, modulus of elasticity, and elongation at a break in the test
direction.
3. Flexural Strength and Modulus—The flexural strength of a
composite is dependent on the outer layers of the reinforcement. A well-cured monolithic composite will have higher
flexural strength compared to its tensile strength. Flexural
strength and modulus are tested as per ASTM 749 D. This
3-point bend test is shown in . Fig. 9.9
Point to Ponder…
In a 3-point bend test, the upper
portion of the sample is under
compression, the bottom portion
is under tension, and at the centre
of the sample there is no load.
9.3 · Testing