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Wednesday, January 4, 2012

Ultrasonic Inspection for Fibre Reinforced Plastic & Composite

The most common method of NDT for fibre reinforced plastic and composite materials is ultrasonic inspection. The measurement of ultrasonic parameters can provide a wealth of information on the quality of composites.

Ultrasonics can generally detect delaminations, inclusions, matrix macro-cracks, and voids in composites structures. The ultrasonic method itself uses longitudinal, shear, Lamb, Rayleigh, or guided waves for various measurements on composite materials. Wave parameters, including acoustic attenuation and speed, can be used to determine materials properties and characteristics, such as void fraction, stiffness, and, if the density is known, moduli. The reader is referred to other texts and articles to obtain detailed information on the fundamental aspects of ultrasonics as well as advanced methodologies for measurements using ultrasound.

Table 2 provides some typical acoustic characteristics of materials, and Table 3 gives the relative wavelength as a function of the frequency. For example, at the interface between water and a graphite/epoxy interface, the transmission and reflection of sound energy will be 152% and 52%, respectively.

The sound energy in the fibre reinforced plastic and composite material will be greater than in the water. At an interface between graphite/epoxy and air, the transmission is 0.017%, and the reflection is 100%. It is this change in the acoustic impedance that allows ultrasound to be used to detect defects in materials. Delaminations and cracks represent air interfaces that transmit very little sound and provide a large reflection. Inclusions must have a sufficient difference in acoustic impedance from the composite material in order to be detected.

In solid homogeneous material, both longitudinal (compression) and transverse (shear) waves can be created and monitored independently. In composite materials, however, the separation of longitudinal and transverse waves is much more difficult. Composites are anisotropic media. The plane waves moving through the anisotropic media are often only quasi-longitudinal or quasi-transverse (containing both longitudinal and shear characteristics), because of the wave interactions at the many matrix and the reinforcing material interfaces. In fiber-reinforced composites, the ultrasonic waves can travel along fibers, and therefore, the technique can be highly sensitive to the fiber orientation in the structure. This feature can be used as a means of characterization of the structure.  However, for the general inspection of fiber-reinforced composites, the most common inspection is with ultrasonic waves perpendicular to the fiber tape or fabric planes. As the ultrasonic beam passes through a material, it will be attenuated due to scattering, absorption, and beam spreading.

The anisotropic properties of composites can make them strong scatters of ultrasound, depending on the wavelength. As shown in Table 3, for ultrasonic frequencies between 1 and 10 MHz, the wavelength will be between 0.3 and 0.03 mm (0.01 and 0.001 in.). Features in the composite larger than one-tenth the wavelength will contribute to scatter. Features smaller than this will not be detectable. Absorption occurs due to the conversion of the sound energy into heat. Beam spreading is due to the geometric principles of the ultrasonic beam size, frequency, angle, and distance

The most common inspections of composite structures are through-transmission (TT) ultrasonics and pulse echo (PE) ultrasonics. The TT method uses two transducers, one on each side of the part, to measure the acoustic attenuation through the structure. One transducer is the transmitter and is electronically pulsed to produce an ultrasonic signal, and the other transducer is the receiver, typically aligned opposite to the pulser. The coupling media of the ultrasound to the composite is usually water. This may be performed by immersing the sample in a water tank or by using water-squirter systems.

For TT examinations, the pulser often employs a tone burst (several cycles of the waveform), so that significant ultrasonic power is available. Through transmission is the most common ultrasonic examination method for fiber-reinforced composites. Trough transmission is relatively simple to implement, much more forgiving than automated PE on alignment of the transducers to the part under test, typically has a wide dynamic range (>120 dB on some systems), and easily detects problems in multilayered structures. By comparing TT signal loss in adjacent good areas, porosity, unbonds, wrinkles, delaminations, and most inclusions can be detected. In the case of porosity, the signal loss can be correlated to standards. For example, porosity in the 1 to 2% range corresponds to a change of 4 to 8 dB in signal level for approximately 6 mm (0.25 in.) of graphite/epoxy material at 2.25 MHz.

Through transmission methods cannot determine the depth or layer of detected defects. This must be accomplished using pulse-echo ultrasonic methods. However, data from TT methods are normally presented in the C-scan mode, which presents an image of the part with gray scale or color values relating to the attenuation experienced. This is perhaps the easiest ultrasonic presentation to interpret. Pulse-echo ultrasonic methods typically use a single transducer as both the pulser and receiver of the ultrasound signal. The ultrasound is commonly coupled into the part using a couplant, such as water, glycerin, and other materials. Typical noncontact systems used to couple PE transducers to the parts are hand-held devices,immersion tanks, water-bubbler systems, or water-squirter systems.

 Automated ultrasonic scanning systems take many forms and sizes. They range from small tabletop immersion tanks to large overhead gantry systems.
Automated ultrasonic two-channel flat panel through-transmission system

Overhead gantry automated ultrasonic scanning system