1. Microscopy

The microscopy is one of the best methods that provide the first hand information on the form of damage. The microscopy can provide the information like:

1. Shape of the fibers,
2. Geometry and uniformity of the fiber spacing,
3. Presence of voids
4. Regions rich or poor in matrix,
5. Fibre alignment.

The microscopy method has limitations like that it can give the information inside the composite. It can give the information on the surface as mentioned above. It cannot give the information like fibre-matrix bond, broken fibres, matrix cracks and delaminations inside the composite.

2. Ultrasonic Inspection

Ultrasonic inspection is a non-destructive method of testing. Using this method one can assess the quality of the composite. The ultrasonic testing method includes the propagation of mechanical waves through the object to be inspected. The mechanical waves propagated are in the range of 100 kHz to 25 MHz. Some of the waves propagated are reflected or transmitted at the other end. The intensity of the waves at the other end is measured by a receiving transducer.

There are two types of waves: Longitudinal and transverse waves. In longitudinal waves direction of oscillation of atoms and the direction of propagation of the wave is along same direction. In the transverse waves the direction of oscillation of atoms is perpendicular to the direction of propagation of wave. The longitudinal waves propagate in all materials whereas the transverse waves propagate only in solid materials. Further, due to Due to the different type of oscillation, transverse waves travel at lower speeds.

When the wave propagating in the material is intercepted by a defect and interfaces (like change from fibre to matrix material and vice a versa or a foreign particle) the energy transmitted through the material also gets reduced due the effect of reflection and attenuation. Thus, one can use both reflection and transmission form of energy for ultrasonic inspection.

The ultrasonic beam requires a transfer medium. In general, water is used as a transfer medium. This is a disadvantage of this method. Further, use of water during the test process can lead to absorption water by composite.

In the recent years the new developments in the ultrasonic testing have made this process very sophisticated and attractive. One can get the complete map or intensity distribution corresponding to the discontinuity in the material. Such a map is called C-scan.

The detailed information on the ultrasonic inspection can be found in ASTM E114-90 for Pulse-Echo method, E214-68(91) for Reflection method, E317-93 for Pulse-Echo and E494-91 for ultrasonic velocities. 

3. X-Ray

X-ray technique is a very useful technique. It uses the electromagnetic waves of extremely of short wavelength. These waves are capable of penetrating solid substances and are affected by discontinuities much as other waves. It should be noted that the polymer composites X-rays. A X-ray opaque penetrant is introduced in the damaged area as a liquid solution or suspension so that it fills the cracks and delaminations and makes them clearly visible on X-ray films as a dark region. Opaque dye penetrant such as tetra-bromo-ethane (TBE) is used in these processes.

It is cautioned that the frequent use of dye penetrant should be avoided. This is because the penetrant actually enhances the crack growth. Thus, under loading the frequent use of penetrant will increase the growth rate. Hence, this technique is treated as effectively destructive.

The regions with lower density such as voids, defects and cracks absorb less radiation. This result in higher intensity of the radiation that reaches a photographic film or plate placed on the far side of the sample. The higher intensity causes the darkening of the film or the plate. Thus, the darker areas of the film indicate the outline of the low density region.

ASTM Standards related with this technique are E 94-93, E 142-92 and E 1316-94.

4. Thermography

This is one of the sophisticated techniques that are used in infrared thermography. The advantage of this technique is that it does not require any interruption for inspection. Thus, it is well suited for fatigue testing.

This technique is based on the principles that the infrared thermography detects the heat generated from a source. In case of damage in composite there are two types of such heat sources. The first one is hysteresis evolving from resin and interface. The second source is heating due to friction between the cracks and delaminated interfaces. Thus, the area which appears hot on the thermographs is the area of damage. Once the area of damage is detected, one can zoom into it and get more details.
ASTM Standard guide for nondestructive testing of polymer matrix composites used in aerospace applications is E2533-09.