66
3
(iii) Both fibres and the matrix deform plastically.
(iv) The fracture of fibres is followed by the fracture of a
composite.
The loss of properties in composites is due to the following factors
in general.
(i) Stress corrosion causing a loss of strength of fibres
(ii) Chemical degradation of the matrix
(iii) Loss of interfacial strength between the fibres and the matrix
(iv) Degradation of the matrix due to overaging, UV effects,
service temperature, and other radiations (such as X-ray)
In general, brittle failure is observed in polymeric composites due
to a very low percentage of elongation at the break, as mentioned in
7 Table 1.6 of 7 Chap. 1. The strengthening effect is primarily
achieved by the fibres in the composite, and it is only achieved when
the ultimate tensile strength of the composite exceeds that of the
matrix alone. In UD composites, strength in the transverse direction is lower than the strength of the matrix due to stress and strain
caused by the fibres in the matrix. The stress and strain concentration causes failure of composites at a much lower strain than the
strain at which an unstrained matrix would fail. However, the modulus in the transverse direction is more than that of the matrix if the
volume fraction of fibres is very high and the fibres are of a high
modulus.
3.1.3 Failure Modes in Polymeric Composites
The failure in composites is a distinct phenomenon. It initiates from
microcrazing of the matrix and leads to surface and core cracks in
the matrix. Furthermore, it causes debonding between the fibre and
the matrix, leading to breaking of the fibres [5, 6]. Finally, delamination of the composite takes place. Composites with low V f (<0.40),
moderate V f (between 0.40 and 0.65), and high V f (>0.65) typically
show a brittle failure with initial cracking leading to debonding and
brittle failure with fibre pull-out, respectively. The possible three
modes of failure under longitudinal compressive load are given here:
(i) Transverse tensile failure
(ii) Fibres microbuckling in extensional mode
(iii) Fibres microbuckling in shear mode
Similarly, the possible three modes of failure under longitudinal
tensile load are as follows:
(i) Tensile forces stretch the matrix more than the fibres, causing
the material to shear at the interface between the matrix and
the fibres.
(ii) Tensile forces near the end of the fibres exceed the tolerances
of the matrix, separating the fibres from the matrix
(iii) Tensile forces can also exceed the tolerances of the fibres
causing the fibres themselves to fracture, leading to material
failure
Chapter 3 · Micromechanics and Macromechanics of Polymeric Composites
3
(iii) Both fibres and the matrix deform plastically.
(iv) The fracture of fibres is followed by the fracture of a
composite.
The loss of properties in composites is due to the following factors
in general.
(i) Stress corrosion causing a loss of strength of fibres
(ii) Chemical degradation of the matrix
(iii) Loss of interfacial strength between the fibres and the matrix
(iv) Degradation of the matrix due to overaging, UV effects,
service temperature, and other radiations (such as X-ray)
In general, brittle failure is observed in polymeric composites due
to a very low percentage of elongation at the break, as mentioned in
7 Table 1.6 of 7 Chap. 1. The strengthening effect is primarily
achieved by the fibres in the composite, and it is only achieved when
the ultimate tensile strength of the composite exceeds that of the
matrix alone. In UD composites, strength in the transverse direction is lower than the strength of the matrix due to stress and strain
caused by the fibres in the matrix. The stress and strain concentration causes failure of composites at a much lower strain than the
strain at which an unstrained matrix would fail. However, the modulus in the transverse direction is more than that of the matrix if the
volume fraction of fibres is very high and the fibres are of a high
modulus.
3.1.3 Failure Modes in Polymeric Composites
The failure in composites is a distinct phenomenon. It initiates from
microcrazing of the matrix and leads to surface and core cracks in
the matrix. Furthermore, it causes debonding between the fibre and
the matrix, leading to breaking of the fibres [5, 6]. Finally, delamination of the composite takes place. Composites with low V f (<0.40),
moderate V f (between 0.40 and 0.65), and high V f (>0.65) typically
show a brittle failure with initial cracking leading to debonding and
brittle failure with fibre pull-out, respectively. The possible three
modes of failure under longitudinal compressive load are given here:
(i) Transverse tensile failure
(ii) Fibres microbuckling in extensional mode
(iii) Fibres microbuckling in shear mode
Similarly, the possible three modes of failure under longitudinal
tensile load are as follows:
(i) Tensile forces stretch the matrix more than the fibres, causing
the material to shear at the interface between the matrix and
the fibres.
(ii) Tensile forces near the end of the fibres exceed the tolerances
of the matrix, separating the fibres from the matrix
(iii) Tensile forces can also exceed the tolerances of the fibres
causing the fibres themselves to fracture, leading to material
failure
Chapter 3 · Micromechanics and Macromechanics of Polymeric Composites