Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun
3.5 3.6 Figure 3: Key frames from the video recording of the impact event.
With PP-glass, as with most other types of composite, the energy absorption in impact increases with testing velocity [15]. For a factor of two speed increase this effect would have been small, but it is conceded that the damage might have been slightly less extensive than for an impact at normal speed.
Apart from the damaged however, the remainder of perforation the presence of significant region itself,
largely recovered its original shape. This shape recovery, despite
the hull appears to have damage, is
a
characteristic of composites which differs greatly from the behaviour of metal structures, where significant non- local plastic deformation is often observed. It must be noted that this controlled impact event has significant differences with a real grounding occurrence. However, the resulting overall hull response was reasonably close to what would be expected after a real grounding, especially in relation with the necessary repair size.
While potentially advantageous from the repair viewpoint, the observed shape recovery means that it is not possible to assess the true extent of damage following an impact solely from the appearance of the structure. This underlines the importance of using non-destructive evaluation (NDE) methods to determine the true extent of damage.
Ultrasonic NDE was performed before and after the damage test, as shown in Figures 4 and 5, respectively. An Epoch-4B (Pulse-echo A-scan) with a 500 kHz transducer was calibrated with sample panels of the same characteristics of the RIB hull to allow accurate thickness measurement with the thermoplastic composite. Preliminary thickness measurements were performed and recorded on selected points on the hull (Figure 4) prior to the impact event. These recorded values were compared with the data obtained after the impact to assess the extent of the damage. Further measurements
Figure 4: RIB hull ready for the impact. The marks indicate recorded NDE signal to compare with after- impact values.
Figure 5: Extent of the damage region, as assessed by NDE.
were
performed to finely determine the extent and shape of it. The final contour of the delaminated region is shown in Figure 5.
As it is found with other composites, PP-glass may be affected
by delamination under properties, not certain loading
conditions. This phenomenon comprises layers separation inside the material and significant loss of mechanical continuity and
always followed by obvious visible signs on the outside of the ©2010: The Royal Institution of Naval Architects B-35
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