# Ultrasonic Attenuation Characteristics of Glass-Fiber-Reinforced Polymer Hull Structure
###### Abstract  
Glass fiber-reinforced polymer (GFRP) ship structures have hull plate thicknesses of 10 mm or more and are fabricated using a higher proportion of resin matrix systems than E-glass fiber reinforcements. Therefore, GFRP is classified as a highly attenuative material, and this characteristic is a major cause of large errors in ultrasonic nondestructive testing for quality inspections. In this study, considering the aforementioned design and fabrication characteristics of GFRP ship structures, hull plate prototypes with various glass fiber weight fractions, glass contents (\(\mathrm{Gc}\)), and laminate thicknesses were fabricated. Then, a pulse-echo ultrasonic test was performed with the fabricated prototypes, and the attenuation characteristics of the GFRP hull plates were investigated by conducting statistical analyses. These results demonstrated that with a variation of 30-50% in the \(\mathrm{Gc}\) used for GFRP structure design, the plate thickness variation had a greater impact than the \(\mathrm{Gc}\) variation on the attenuation characteristics. The increase in \(\mathrm{Gc}\) naturally increased the scattering of ultrasonic waves but did not significantly affect the attenuation coefficient. The effects of the inner voids on the ultrasonic waves were also investigated, and the results confirmed that the laminates in this \(\mathrm{Gc}\) region did not significantly affect attenuation. composite ship; glass fiber-reinforced polymer; ultrasonic; defect addressref=aff1,url=0000-0001-4000-0001-4000-0001-4000-0001-4000-0001-40000-0001-40000-0001-40000-0001-40000-0001-40000-0001-40000-00001-400000-0001-400000-00001-400000-00001-4000001-40000001-4000001-4000001-4000001-4000001 various types of reinforcements, matrices, and fabrics, and the irregularities in the composite material, ultrasonic wave propagation in composite materials is complex. Although the densities of composite structures are lower than that those of steel structures, the irregularity of the crystalline particles in the former is greater, resulting in greater scattering or absorption of ultrasonic waves [5]. Therefore, composite materials are considered highly attenuative materials [5; 6]. Composite structures, such as the glass fiber-reinforced polymers (GFRPs) used in composite ships, have inner defects including voids, pores, foreign objects, and resin-rich regions, depending on environmental factors such as the temperature and humidity as well as the operator's skill level and the techniques employed during fabrication. These inner defects can significantly reduce the flexural and tensile strength [7], compressive strength [8], interlaminar shear strength [9], and fatigue strength [10] of the laminate, thus hindering effective ultrasonic testing. This is because inner defects such as voids interfere with ultrasonic wave propagation and cause the further loss of ultrasonic waves, resulting in increased attenuation characteristics. Therefore, it is also critical to evaluate the relationships between the ultrasonic attenuation characteristics and the inner defect contents. Related research is actively being conducted in the aerospace industry. For instance, [PERSON] et al. [11] and [PERSON] et al. [12] confirmed that voids can increase the attenuation characteristics of carbon fiber-reinforced polymer (CFRP) laminates. CFRP aircraft laminates are typically thin, and their void content is less than 1% when fabricated using the infusion and vacuum-assisted resin transfer molding (VARTM) methods [13]. By contrast, GFRP ship structures have large hull plate thicknesses and require hand lay-up for fabrication, resulting in a high ratio of inner defects such as voids [13; 14]. [PERSON] [13] investigated the effects of artificially increasing the void content by 30% for GFRP composite laminates with thicknesses of 20-150 mm on their void content and ultrasonic attenuation. For increasing the void content, the attenuation characteristics increased accordingly. [PERSON] et al. [1], [PERSON] et al.