240
9
In carbon epoxy composites, carbon fibre content is
calculated using a chemical decomposition method. In this
method, a carbon fibre composite is placed in concentrated
nitric acid, and the mixture is boiled. The matrix gets dissolved
in concentrated nitric acid, and carbon is decanted as a residue,
washed, and dried. The dried carbon fibre is weighed to
calculate V f . In the case of carbon fibre- reinforced polyimide
composites, hydrazine hydrate is used in place of concentrated
nitric acid. In the case of thermoplastic composites, solution
methods are used. In PP matrix composites, the PP is dissolved
in xylene.
There is also a nondestructive testing (NDT) method for
evaluating the reinforcement content in the composite. This
technique uses selective radiation where samples can be tested
for glass content in 2 minutes with an accuracy of ±1%. The
selective radiation can separate out glass fibre from all other
fillers, additives, and pigments used in composites.
9.3.2 Mechanical Testing
Mechanical testing (tensile test, flexural test, and impact test, etc.)
of composites refers to the determination of material properties
through tests conducted on suitably designed specimens. The
understanding of material response over an entire range of loads is
necessary for designing the product and efficiently utilizing the
material. The macromechanical properties of a composite may be
computed through a micromechanical analysis, and the design of a
composite material may be started with knowledge of the constituent material properties. However, the validity of a micromechanical
analysis is checked through experiments. Other purposes of
mechanical characterization are to check the adequacy of the fabrication procedure to be adopted and to assure uniformity of the
material.
1. Hardness—The hardness testing of plastics is most often
measured by the Rockwell hardness test. It measures the
resistance of the plastic toward indentation, thereby providing
an empirical value of hardness. These hardness values do not
necessarily correlate to other properties or fundamental
characteristics. Rockwell hardness is generally chosen for
“harder” plastics and is carried out as per ASTM D785 for
plastics and composites. The samples are prepared in the form
of small discs of rectangular geometry with a dimension of
50 × 30 × 3 mm 3 .
2. Tensile Strength and Modulus—The failure of composites is a
cumulative effect of a series of microscopic failures that include
the failure of the matrix, the reinforcement, and interphases. In
composites, fibres share the major load. Composites having
uniaxially oriented fibres fail when the fibres suffer from
fracture. In cases where the matrix cracks before the fibres fail,
the ultimate strength of the composite is controlled by the fibre
Chapter 9 · Characterization and Testing of Polymeric Composites
9
In carbon epoxy composites, carbon fibre content is
calculated using a chemical decomposition method. In this
method, a carbon fibre composite is placed in concentrated
nitric acid, and the mixture is boiled. The matrix gets dissolved
in concentrated nitric acid, and carbon is decanted as a residue,
washed, and dried. The dried carbon fibre is weighed to
calculate V f . In the case of carbon fibre- reinforced polyimide
composites, hydrazine hydrate is used in place of concentrated
nitric acid. In the case of thermoplastic composites, solution
methods are used. In PP matrix composites, the PP is dissolved
in xylene.
There is also a nondestructive testing (NDT) method for
evaluating the reinforcement content in the composite. This
technique uses selective radiation where samples can be tested
for glass content in 2 minutes with an accuracy of ±1%. The
selective radiation can separate out glass fibre from all other
fillers, additives, and pigments used in composites.
9.3.2 Mechanical Testing
Mechanical testing (tensile test, flexural test, and impact test, etc.)
of composites refers to the determination of material properties
through tests conducted on suitably designed specimens. The
understanding of material response over an entire range of loads is
necessary for designing the product and efficiently utilizing the
material. The macromechanical properties of a composite may be
computed through a micromechanical analysis, and the design of a
composite material may be started with knowledge of the constituent material properties. However, the validity of a micromechanical
analysis is checked through experiments. Other purposes of
mechanical characterization are to check the adequacy of the fabrication procedure to be adopted and to assure uniformity of the
material.
1. Hardness—The hardness testing of plastics is most often
measured by the Rockwell hardness test. It measures the
resistance of the plastic toward indentation, thereby providing
an empirical value of hardness. These hardness values do not
necessarily correlate to other properties or fundamental
characteristics. Rockwell hardness is generally chosen for
“harder” plastics and is carried out as per ASTM D785 for
plastics and composites. The samples are prepared in the form
of small discs of rectangular geometry with a dimension of
50 × 30 × 3 mm 3 .
2. Tensile Strength and Modulus—The failure of composites is a
cumulative effect of a series of microscopic failures that include
the failure of the matrix, the reinforcement, and interphases. In
composites, fibres share the major load. Composites having
uniaxially oriented fibres fail when the fibres suffer from
fracture. In cases where the matrix cracks before the fibres fail,
the ultimate strength of the composite is controlled by the fibre
Chapter 9 · Characterization and Testing of Polymeric Composites