1. Introduction
Glass fiber-reinforced polymer (GFRP) composites have been widely used for decades to build small ships, such as fishing boats, yachts, and patrol boats [
1,
2], owing to their suitable specific strength, corrosion resistance, and excellent workability [
3].
Overall, considerable progress has been achieved in the marine application of tailored materials and optimal structural design technology. While life cycle issues, such as sustainability and environmental protection, have received greater attention in recent times, there is still an attitude of conservatism and hesitation in specifying the application of polymer composites as the solution to several relevant challenges [
1]. With regard to the environment, studies have been conducted to optimize the laminates of composite structures [
2,
4]; however, the design margin remains relatively large because, for example, composite structures, such as ships, are usually built in small shipyards, and the inspection of the production quality does not tend to be sophisticated. Even pleasure and commercial boats that require a higher quality of production are not subjected to quality control and rely only on external surface quality assessment [
5].
Nondestructive evaluation is one of the most commonly used quality evaluation methods and is widely applied to composites [
6,
7]. However, while it is widely employed in the aviation industry for quantitative defect detection and quality characterization of fiber-reinforced polymer (FRP) structures, it is not often applied to GFRPs used to produce ships owing to the much larger thickness of the GFRP structures and the limitations of the measurement environment. This challenge can be addressed by using pulse-echo ultrasonics, which is safe and reliable, enables inspection from only one side of a ship structure, and is convenient for use in a spatially limited environment owing to the portability of the utilized equipment [
8,
9]. Although pulse-echo ultrasonics can also be performed on aircraft for quantitative defect detection and quality characterization, it is more often used for damage repair or maintenance of GFRP ships [
10]. This is because the GFRPs used in ships are generally produced by the hand lay-up method [
11,
12], which tends to unevenly, inadequately, or excessively distribute the polymer resins in the GFRP structure, resulting in more defects in the laminates, such as porosity, voids, and delamination. This causes inconsistency in the quality of the GFRP laminates and affects their performance and the results of ultrasonic nondestructive evaluation (NDE) [
13].
Mouritz [
13] investigated the strength effect of porosity in the GFRP composite structures of ships. His testing of highly porous, thick composites (20–150 mm) revealed that the shear strength decreased with increasing void content. Oh et al. [
14] reported that an increase in the glass fiber content (
Gc) of FRP hull plates caused a decrease in strength at a high impregnation ratio of ≥60% because of the poor quality of the FRP laminate. Through a comparative study of the void content of FRP hull plates, Han et al. [
15] observed a relatively high void volume in high-
Gc laminates and verified the results by burn-off tests.
Additionally, the combination of different reinforcements in GFRPs used for ship structures is not ideal for ultrasonic inspection. E-glass chopped strand mat (CSM) and woven roving (WR) fabrics are the main types of composite fabrics used to produce major ship structures, such as hulls and decks, and both fabrics can be used together. CSM can be exclusively used but is sometimes combined with WR, which has a higher weight per unit area and is interlaced to achieve directional strength characteristics and enhanced impact resistance [
11], which are desirable for large structures. The combination of CSM and WR is also known to improve the interlaminar bonds and reduce porosity [
5]. However, the enhanced strength of CSM–WR laminates can only be achieved through high-quality combination; otherwise, the combination itself may increase the number of voids [
16,
17,
18] and cause inaccurate results of ultrasonic NDE testing.
To improve the structural characteristics of GFRP ships and the accuracy of ultrasonic NDE, it is critical to properly set the receiver’s pulse-echo velocity with respect to the relative density of the test specimen. In other words, it is important to understand the variation of the pulse-echo velocity of the laminates with the design
Gc when producing GFRP ship structures. Generally, the ultrasonic velocity increases with increasing density of a test specimen [
19]. Although the relative density of GFRP structures is lower than that of steel, the irregularity of the crystalline particles of the former is greater, resulting in its greater scattering or absorption of ultrasonic waves [
20]. In addition, the relative density of laminates varies with the
Gc between the reinforcements and polyester, and this affects the number of defects, such as voids, making it more difficult to perform ultrasonic NDE [
21].
In this study, the effect of the production characteristics of GFRP laminates used for ship construction on the NDE of ships was investigated. For this purpose, laminate specimens similar to the hull plates of GFRP ship structures were produced with normal (30–50%) and high (50–70%) values using the hand lay-up method for producing CSM and WR fabrics. The same design was used to produce combined CSM–WR specimens. The laminates were 4–6 mm thick and were examined by pulse-echo ultrasonic A-scanning, followed by vernier caliper measurements and accurate
Gc measurements through burn-off tests [
15]. The A-scanning results were analyzed to understand the effects of the
Gc variation and combined reinforcements on the pulse-echo velocity and the effects of internal defects in the laminates, such as voids, on the ultrasonic waves.
4. Discussion
The change in the pulse-echo velocity of the single-material specimens with increasing glass fiber content (
Gc) was negligible, with the velocity decreasing as
Gc further increased above 38.22%. Continuous increase in the velocity was observed for the combined-material specimens even for
Gc values above 50%. While the relative density and ultrasonic velocity for laminates generally tend to increase with increasing E-glass fiber content [
13,
19], the tendency was different for the single-material specimens. However, we observed increased data spread in the high-
Gc region for both types of specimens. It is believed that the density of the laminates did not actually increase normally with increasing void volume after a certain point in
Gc, and that the internal voids also impeded the ultrasonic wave.
Figure 9 shows the void volume contents of the two types of laminate specimens determined by burn-off tests and the variation of the actual relative density calculated from the void volume. The single-material and combined-material specimens exhibited increases in their void volume with
Gc exceeding approximately 37.74% and 38.18%, respectively (
Figure 9a). However, the increase in the single-material specimens was larger than that in the combined-material specimens. The difference was more evident beyond
Gc values of 45–50%, with the void volume reaching as high as 5.07% (
Figure 9a). This increase in void volume with increasing
Gc caused a reduction in the increase in the actual relative density (
Figure 9b), resulting in defects, such as voids, inside the laminates. The defects impeded the transmission of ultrasonic waves and decreased the ultrasonic measurement accuracy, with significant effects on the laminate density and pulse-echo velocity.
To better understand the cause of the clear difference between the void developments of the two types of specimens with
Gc, the burn-off test results were divided into a normal
Gc area of 30–50% and a high-
Gc area of >50% and plotted as violin charts, as shown in
Figure 10.
In the region of
Gc < 50%, both types of specimens exhibit void volume developments of 1–2%, with that of the single-material specimens being slightly larger. This is believed to be the cause of the lower pulse-echo velocity of the single-material specimens for a given
Gc (
Figure 7). In the high-
Gc region (>50%), the single-material and combined-material specimens exhibit different void volume development trends. The variation of the void volume of the former is widely distributed over 2–5%, with the median value being 3.39%. The variation range for the combined-material specimens is narrower, with a median value of 2.54%. The combination of CSM and WR might have improved the bonding between the laminate layers for a given
Gc [
32], resulting in a higher-quality material. This bonding effect is more evident for high-
Gc values.
5. Conclusions
The effect of production quality of GFRP laminates on their transmission of ultrasonic waves was investigated. In addition to the glass fiber contents commonly applied to GFRP laminates used to produce ship structures, such as hull plates and decks, CSM and WR fabrics were also combined to achieve Gc contents of 30–70%, such as those used for stiffeners. Ultrasonic NDE was conducted on the laminate specimens.
Overall, the single-material specimens tended to have a lower pulse-echo velocity for a given Gc value. The actual relative densities of the CSM-only specimens calculated from the void volumes determined by burn-off tests were found to be lower than the theoretical design values. This was more evident for Gc values of ≥50% owing to the increased voids in the laminates, resulting in a decreased ultrasonic velocity. The combined CSM–WR laminates also contained more voids in the higher-Gc region, although the void development was less than that of the CSM-only laminates. The present results indicate that, for a given glass fiber content, the configuration of the fiber reinforcement can be used to prevent the development of voids, thereby improving the quality of the laminates. This is specifically the case for high-Gc values compared with the values of 30–50% commonly used to produce GFRP ship structures. Hence, to achieve more accurate NDE of GFRP structures, it is important to consider not only the relative density but also the glass fiber content and nature of the GFRP fabric.