Non-destructive evaluation techniques such as pulse-echo ultrasonics are often used for quality inspection, repair, and maintenance of glass fiber reinforcement plastic (GFRP) composite structures. GFRPs in marine composites have a relatively greater thickness and are mainly produced by the hand lay-up method, rendering it difficult to measure them precisely. For accurate ultrasonic testing (UT) of GFRP specimens such as hull plates, it is important to adequately set the pulse-echo velocity of the receiver. Among the different test conditions, the glass fiber content (Gc) constituting the GFRP laminates is the most important factor. However, as Gc increases, the relative density also increases theoretically; therefore, it is not necessary to increase the pulse-echo velocity. The test conditions of GFRP composites are not only affected by the types of fiberglass, polymer resin, and fabrication method, but ako by the fabrication quality, which can be evaluated by different factors such as the amount of defects.
This study investigated the velocity of pulse-echo as the change in the Gc, which has the greatest impact on the ultrasonic testing of GFRP structures. In this study, the fact that Gc not only affects the mechanical performance of laminates but also the fabrication defects such as voids, has been taken into account. In addition, considering that it is common for two types of fibers to be combined in hull fabrication, prototyping production and experimental observations were conducted. Using e-glass fiber and polyester resin, which are widely used in the construction of composite ships, a prototype comprising approximately 30 - 60wt% Gc was produced with a thickness of 4 - 6 ㎜. And the chopped strand mat (CSM) and the combined laminates of CSM and woven roving (WR) cloth were applied. Gc of all the specimens were accurately measured through burn-off testing in accordance with the ASTM D3171 standard, and the volume fraction of defects such as voids was also measured.
As a result, for the single-material specimens made with CSM only, the pulse-echo velocity increased slightly and then decreased in the normal Gc region (30 - 50wt%);, and it continued to decline even in the high Gc region (50wt% -). For the combined-material specimen made of CSM+WR, the pulse-echo velocity showed an upward tendency in the normal Gc region (30 - 50wt%), and it continued to increase to some extent even in the high Gc region; subsequently, it decreased after Gc exceeded 55wt%. It was confirmed that a decrease in the velocity (a test condition of UT) is related to an increase in the defects such as voids, which increase with a rise in Gc. Furthermore, the combination of fabrics is advantageous for reducing defects since the single-material specimen tends to exhibit an increased number of voids as Gc increases.