A recent article published in Sensors proposed an active Lamb wave energy analysis method for detecting interfacial debonding defects in carbon fiber-reinforced polymer (CFRP)-rubber bond structures in rocket motors during service and storage.
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Background
Solid rocket motors (SRMs), extensively used in aerospace and missile systems, employ CFRP composites in motor casings due to their superior mechanical properties. However, the bonding interface between CFRP and viscoelastic materials like rubber is prone to debonding under complex operational and storage conditions, threatening the engine’s structural integrity and reliability.
Current nondestructive testing (NDT) methods like X-ray imaging and infrared thermography have limitations in detecting interfacial defects in multilayered composite materials.
Ultrasonic-guided NDT methods, such as Lamb wave analysis, offer an alternative. Lamb waves can propagate through complex structures, detecting changes in material properties and deep structural defects. Additionally, Lamb wave-based methods offer the advantage of not requiring disassembly or complex equipment, making them suitable for on-field inspections.
In this study, Lamb waves were used to establish a debonding evaluation factor (DEF) to detect CFRP-rubber interface bonding defects in SRMs during both operation and storage.
Methods
Among the symmetric (S) and asymmetric (A) modes of Lamb wave propagation, the A mode was chosen for this analysis due to its greater sensitivity to interfacial debonding damage. To enhance the signal-to-noise ratio, a Hanning window with five cycles was applied to limit the bandwidth of the A mode.
Dispersion curves for both ring-space and flat-plate structures of the CFRP-rubber interface were generated using the open-source GUIGUW software, allowing the extraction of Lamb wave mode velocities at various excitation frequencies in the CFRP-rubber structure.
Lamb wave propagation in CFRP-rubber bonded plates was simulated using the commercial finite element software ABAQUS 2022/Explicit. Both time-domain and frequency-domain analyses were performed to define the DEFs for different debonding scales in the simulation model.
The simulation results were validated through physical experiments by preparing The simulation results were validated by preparing CFRP-rubber sheet specimens matching the simulation model. To induce interfacial debonding damage, polytetrafluoroethylene (PTFE) sheets of varying sizes were placed in the debonding area of these specimens, and PZT sensors were mounted on the bonding elements.
A five-cycle sinusoidal wave modulated by a Hanning window was applied as input to a frequency generator. The peak voltage of this signal was amplified from 5 to 100 V and delivered to the PZT sensor on the bonding member. The signals from the PZT sensors were recorded and analyzed using a digital fluorescence oscilloscope.
Results and Discussion
Lamb wave propagation in the CFRP rubber structure with various sizes of interfacial debonding defects was observed from different angles. Scattering occurred as the incident wave passed through the debonding region, becoming more pronounced as the defect size increased. After traversing the debonding area, the wave continued to propagate in the CFRP sheet without significant energy leakage into the rubber layer.
From the simulation model, the wave packet transmission time was recorded at 672 ms, with a group velocity calculated at 483.33 m/s for an actuator-sensor distance of 325 mm. The maximum error between the simulation results and analytical solutions was 1.1%, confirming the finite element method's accuracy in simulating Lamb wave (A0 mode) propagation in CFRP-rubber bonded plates.
Experimental time-domain signals, processed using continuous wavelet transform (CWT), displayed trends similar to the simulation results concerning the varying size of interfacial debonding defects. Likewise, in the frequency domain, the amplitude of the PZT signal increased with defect size, aligning with simulation findings.
The experimentally calculated DEF values increased with larger interfacial debonding defects, mirroring the simulation results. Some discrepancies were noted, likely due to irregular boundaries in the test specimens and the simplified numerical model's exclusion of electromechanical coupling and bonding layer effects.
Conclusion and Future Prospects
The researchers successfully applied Lamb wave-based analysis to detect interfacial debonding defects in CFRP rubber structures, both numerically and experimentally. They introduced a debonding evaluation factor (DEF) that effectively quantified the extent of debonding damage. The simplicity of the PZT sensor configuration in this method makes it highly promising for integration into current SRM structures, requiring minimal operational complexity.
However, the DEF has limitations, as it does not fully account for variations in material properties like elastic modulus and density or factors such as structural thickness, shape, and boundary conditions. To address these uncertainties, the researchers plan to refine the DEF metric and explore advanced signal processing techniques to enhance the method’s accuracy and robustness for detecting defects in more complex bonded structures.
Journal Reference
Yang, Z., Ren, Y., Shi, Q., Cui, D., Liu, J. (2024). Detection of Debonding Defects in Carbon Fiber-Reinforced Polymer (CFRP)-Rubber Bonded Structures Based on Active Lamb Wave Energy Analysis. Sensors. DOI: 10.3390/s24175567, https://www.mdpi.com/1424-8220/24/17/5567
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