244
9
Universal Testing Machine (UTM), as shown in . Fig. 9.8. The
test method is similar to the flexural test method, except the
thickness of the specimen is larger. The ratio of span/thickness
of the test specimen is >16:1.
Impact Strength—Composites have good dampening
properties. Long fibre-reinforced composites have good impact
strength. The impact energy in composites is absorbed by the
initiation of a number of cracks in the resin matrix on impact.
Since the impact energy dissipates fast, the impact damages are
localized.
Basically, impact strength is a measure of fracture toughness. High-impact materials are resistant to fracture. In short
fibre composites, the impact resistance is improved if short
fibres are directionally oriented and are set perpendicular to
the impact force.
In randomly oriented fibre-reinforced composites, a good
bond between the fibres and the resin matrix reduces the
impact strength. This is contrary to long fibre-reinforced
composites where a good bond between the fibre and the
matrix imposes the impact strength. The poor bonding in
randomly oriented short fibre-reinforced composites creates
sites for absorbing shock rather transmitting it. This causes the
absorption and diversifaction of impact energy to be fast,
improving the impact strength of short fibre-reinforced
composites. Notched impact strength is tested as per ASTM
D-256 on a pendulum-type impact testing machine. In
notched impact test, a notch is created at the centre of one
longitudinal edge of the sample. The hammer is impacted just
at notch during the test.
The stress-intensity factor (K) is used to determine the
fracture toughness of most of the materials. There are three
modes of fracture as illustrated in . Fig. 9.10. The three modes
of fracture are depicted by Roman numeral subscripts (I, II,
and III) and are described here.
5 Mode I: The crack plane is normal to the direction of a
tensile force. It is the most common mode. It is denoted as
K I .
5 Mode II: Shear force is applied on the crack plane due to a
sliding motion. It is denoted as K II .
5 Mode III: Shear force is experienced due to a twisting
motion at crack plane. It is denoted as K III .
Fracture toughness is determined using ASTM E 399. The
stress intensity factor is a function of loading, crack size, and
structural geometry.
6. Fatigue Strength—The fatigue properties of a material represent
its response to cyclic loading, which is a common occurrence
in many practical applications. It is well recognized that the
strength of a material is significantly reduced under cyclic
loads. In the case of cyclic loading, the orientation of the fibres,
selection of the matrix, type of loading (such as tension–tension, tension–compression, and compression–compression),
Chapter 9 · Characterization and Testing of Polymeric Composites
9
Universal Testing Machine (UTM), as shown in . Fig. 9.8. The
test method is similar to the flexural test method, except the
thickness of the specimen is larger. The ratio of span/thickness
of the test specimen is >16:1.
Impact Strength—Composites have good dampening
properties. Long fibre-reinforced composites have good impact
strength. The impact energy in composites is absorbed by the
initiation of a number of cracks in the resin matrix on impact.
Since the impact energy dissipates fast, the impact damages are
localized.
Basically, impact strength is a measure of fracture toughness. High-impact materials are resistant to fracture. In short
fibre composites, the impact resistance is improved if short
fibres are directionally oriented and are set perpendicular to
the impact force.
In randomly oriented fibre-reinforced composites, a good
bond between the fibres and the resin matrix reduces the
impact strength. This is contrary to long fibre-reinforced
composites where a good bond between the fibre and the
matrix imposes the impact strength. The poor bonding in
randomly oriented short fibre-reinforced composites creates
sites for absorbing shock rather transmitting it. This causes the
absorption and diversifaction of impact energy to be fast,
improving the impact strength of short fibre-reinforced
composites. Notched impact strength is tested as per ASTM
D-256 on a pendulum-type impact testing machine. In
notched impact test, a notch is created at the centre of one
longitudinal edge of the sample. The hammer is impacted just
at notch during the test.
The stress-intensity factor (K) is used to determine the
fracture toughness of most of the materials. There are three
modes of fracture as illustrated in . Fig. 9.10. The three modes
of fracture are depicted by Roman numeral subscripts (I, II,
and III) and are described here.
5 Mode I: The crack plane is normal to the direction of a
tensile force. It is the most common mode. It is denoted as
K I .
5 Mode II: Shear force is applied on the crack plane due to a
sliding motion. It is denoted as K II .
5 Mode III: Shear force is experienced due to a twisting
motion at crack plane. It is denoted as K III .
Fracture toughness is determined using ASTM E 399. The
stress intensity factor is a function of loading, crack size, and
structural geometry.
6. Fatigue Strength—The fatigue properties of a material represent
its response to cyclic loading, which is a common occurrence
in many practical applications. It is well recognized that the
strength of a material is significantly reduced under cyclic
loads. In the case of cyclic loading, the orientation of the fibres,
selection of the matrix, type of loading (such as tension–tension, tension–compression, and compression–compression),
Chapter 9 · Characterization and Testing of Polymeric Composites