Composite Materials Research Progress

Research Directions in the Fatigue Testing of Polymer Composites 219
Through-transmission ultrasonics basically consists of a transducer for emitting ultrasonic
pulses that is placed at or near one surface and a receiver sensor that is located at the opposite
surface. The technique applies to relatively low frequency sound beams, typically 0.5 MHz to
15 MHz, having a small aperture. The transducer and receiver are coupled to the surfaces or
they are immersed in water together with the composite. The ultrasound waves are attenuated
by defects in the composite and the acoustic attenuation is monitored using the receiver [37].
Figure 12 shows the C-scan of a thermoplastic composite specimen tested in three-point
bending fatigue. The central area is clearly damaged.
Figure 12. C-scan of the central damaged zone in a composite specimen loaded in three-point bending
fatigue.
With classical ultrasonic C-scans, the surface of the object under investigation is scanned
point by point in order to detect and to localise possible defects or possible anomalies. In a Cscan
the transducer is normally kept perpendicular and at a constant distance to the surface of
the object.
A less known but promising technique is the ultrasonic polar scan. With the use of polar
scans we do not aim at the detection and localisation of defects or anomalies, but rather at the
characterisation of the material. Therefore in a polar scan a single representative point of the
object is scanned, under all possible angles θ and ϕ of incidence of the ultrasonic beam, as is
shown in Figure 13. Due to the dimensions of a real ultrasonic beam, a small zone, rather than
a single point of the object is scanned. The distance between transducer and scanned point is
again kept constant, and an acoustic coupling medium, such as water, is used. As is also the
case with classical C-scans, scanning is performed using pulsed signals. Obliquely incident
ultrasonic waves have already been used more or less frequently for purposes of material
characterisation. In each case wave velocities or arrival times of ultrasonic pulses were
measured [38-40]. In a polar scan however, the amplitude of the transmitted beam is
measured. Amplitude measurements are much easier to perform, and can be done with the
most simple ultrasonic apparatus, an advantage for the possible application of the technique in
industrial circumstances.
In the early eighties Van Dreumel and Speijer [41] have shown that ultrasonic polar scans
in principle can visualise in a non-destructive way fibre orientations of the layers in laminates
stacked from unidirectional layers. Unfortunately, after these experiments, polar scans have
been hardly studied or used any more, the reasons for this being mainly the complexity of the
"formation" of a polar scan, and the lack of means at that time for the numerical simulation of
a polar scan.