Influence of porosity on ultrasonic wave velocity, attenuation and interlaminar interface echoes in composite laminates: Finite element simulations and measurements

Abstract

The influence of porosity on the ultrasonic wave propagation in unidirectional carbon-fiber-reinforced composite laminates is investigated based on the two-dimensional finite element analysis and measurements. Random distributions of pores with different contents and size are considered in the analysis, together with the effects of viscoelastic plies and interlaminar resin-rich regions. The transient reflection waveforms are calculated from the frequency-domain finite-element solutions by the inverse Fourier transform. As the measures for porosity characterization, the ultrasonic wave velocity, attenuation coefficient, and interlaminar interface echo characteristics are examined for 24-ply unidirectional composite laminates. As a result, the wave velocity decreases with the porosity content in a manner insensitive to the pore size. On the other hand, the attenuation coefficient increases both with the porosity content and with the pore size. The time-frequency analysis of the reflection waveforms shows that the temporal decay rate of interlaminar interface echoes at the stop-band frequency is a good indicator of the porosity content. The measured porosity-content dependence of the wave velocity is better reproduced by the numerical simulations when the interlayer interfacial stiffnesses are adjusted according to the porosity content, indicating that not only the porosity features but also the interlaminar interfacial properties vary with curing conditions.

Ultrasonic bandgaps in composite laminates: Experimental observation and Floquet-Bloch wave analysis

“…In the temporal reflection waveforms, this can be observed as persistent echoes from the interlaminar interfaces. 10 In assessing the structural integrity of composite laminate structures, the quality of interlaminar interfaces is of equal or higher importance compared to the mechanical properties of plies themselves. In recent investigations, a method has been proposed [11][12][13][14][15] to evaluate the equivalent stiffnesses of interlaminar interfaces of composite laminates based on the reflection or transmission spectra of ultrasonic waves in the frequency range where the wavelength is comparable to the double ply thickness.…”

“…Furthermore, it has been shown that the porosity content in the laminate can be evaluated from the characteristics of interlaminar interface echoes in the frequency range of bandgap. 10 These studies highlight the importance of gaining fundamental understanding of the ultrasonic bandgap features of composite laminates in order to utilize them for nondestructive materials characterization. In the above studies, [10][11][12][13][14][15] however, conventional carbon fiber-reinforced composite laminates were used for experimental observations, whose interlaminar resin regions were so thin (only a few microns) that the interlaminar scattering was very weak.…”

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

“…Furthermore, internal defects such as voids can be introduced due to the fabrication process of the composite material. 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.…”

Defect Characteristics and Online Detection Techniques During Manufacturing of FRPs Using Automated Fiber Placement: A Review

“…For thermoplastics, the voids generated during the laying process are the final defects due to the in-situ curing technique. For thermosetting plastics, the defects before curing can significantly affect the formation of voids after curing [ 93 , 94 , 95 , 96 , 97 ]. Their formation mechanisms are entirely different from the positional defects.…”