The Effect of High Glass Fiber Content and Reinforcement Combination on Pulse-Echo Ultrasonic Measurement of Composite Ship Structures

Abstract

Ship structures made of glass fiber-reinforced polymer (GFRP) composite laminates are considerably thicker than aircraft and automobile structures and more likely to contain voids. The production characteristics of such composite laminates were investigated in this study by ultrasonic nondestructive evaluation (NDE). The laminate samples were produced from E-glass chopped strand mat (CSM) and woven roving (WR) fabrics with different glass fiber contents of 30–70%. Approximately 300 pulse-echo ultrasonic A-scans were performed on each sample. The laminate samples produced from only CSM tended to contain more voids compared with those produced from a combination of CSM and WR, resulting in the relative density of the former being lower than the design value, particularly for high glass fiber contents of ≥50%. The velocity of the ultrasonic waves through the CSM-only laminates was also lower for higher glass fiber contents, whereas it steadily increased for combined CSM–WR laminates. Burn-off tests of the laminates further revealed that the fabric configuration of the combined CSM–WR laminates was of higher quality, prevented the formation of voids, and improved inter-layer bonding. These findings indicate that combined CSM–WR laminates should be used to achieve more accurate ultrasonic NDE of GFRP composite structures.

Error Analysis of Non-Destructive Ultrasonic Testing of Glass Fiber-Reinforced Polymer Hull Plates

“…For homogeneous materials, including steel, the ultrasonic pulse-echo velocity and impedance for a given material density are available for review [13] and can thus be applied to testing. In a homogeneous material, the ultrasonic propagation speed is constant, and the propagation characteristics are consistent.…”

“…In previous studies conducted by Stone and Clarke [23], Ishii et al [24], and Lin et al [25], voids were confirmed to disrupt the propagation of ultrasonic waves and increase scattering and absorption. In addition, Lee et al [13] observed that a void content of 5% in GFRP hull plates considerably reduced the ultrasonic pulseecho velocity. With an increase in the Gc of plates, there is an increase in the probability of defect formation, i.e., voids [13,26], and these defects lead to the disruption of ultrasonic wave propagation.…”

“…In addition, Lee et al [13] observed that a void content of 5% in GFRP hull plates considerably reduced the ultrasonic pulseecho velocity. With an increase in the Gc of plates, there is an increase in the probability of defect formation, i.e., voids [13,26], and these defects lead to the disruption of ultrasonic wave propagation. Thus, voids can introduce errors into the thickness measurements.…”

“…Thus, voids can introduce errors into the thickness measurements. To improve the accuracy of the NDT results for application to GFRP hull plates, the use of a low-frequency probe in the range of 0.50-2.25 MHz is recommended [13,14,27].…”

“…The use of ultrasonic NDT methods for the examination of GFRP hull structures was investigated in previous studies. In a recent study conducted by Lee et al [13], the influences of the fabrication characteristics of a GFRP hull structure on the ultrasonic NDT results were investigated. Changes in the Gc of the hull plates were confirmed to influence the accuracy of the ultrasonic NDT results.…”