Effect of delamination on the electromagnetic wave absorbing performance of radar absorbing structures
Introduction
Composite materials are widely used in many industrial aspects as well as in military applications due to their high specific modulus and high specific strength. Radar absorbing structures (RAS) are one of the applications that composite materials have shown their effectiveness, especially since the development of carbon nanotubes (CNTs) [1]. CNTs embedded in composite materials are associated with energy dissipation of incident electromagnetic (EM) waves due to its conductive properties. Recently, because of their good electrical properties, CNTs are also utilized in composite materials for EMI shields [2], [3], [4].
RAS works as an electromagnetic wave absorber as well as a load bearing structure [5], [6], [7]. A large number of researches have been conducted on the RAS, and many of them are on different types, shapes, and materials of the RAS. The Salisbury screen requires a thick substrate, and the Dallenbach layer contains additional lossy materials for absorbing electromagnetic (EM) waves at the resonance frequency [8], [9]. A sandwich structured RAS with glass/epoxy and polyurethane has been proposed to improve the mechanical stiffness of RAS [10], and periodic patterns of resistive materials are employed for lightweight RAS [11].
On the other hand, due to the low interlaminar fracture toughness, a major weak point of composite materials is the occurrence of delamination. The delamination may be induced from ‘low velocity impact’ which can cause barely visible impact damage (BVID) during operations and services. Numerous researches have been conducted on BVID and delamination since they are major failure modes in composite materials and hard to be detected with visual inspection [12], [13].
Despite the fact that so many researches have been conducted on radar absorbing structures and composite delamination, no research preceding this study has dealt with how delamination affects the EM wave absorbing performance of RAS.
In this study, the multi-layer Dallenbach radar absorber with glass/epoxy and glass/epoxy-MWCNT was designed to measure the delamination effect in the X-band (8.2–12.4 GHz). For the measurement of the delamination effect, the ‘split specimens’ were designed and fabricated, using the optimized design parameters. The reflection loss measurement test utilizing the ‘split specimen’ method was validated by excellent agreement between the analytical results and the experimental results.
Section snippets
Design of the delamination specimen
The design of RAS was conducted for the understanding of delamination effect on RAS. The Dallenbach absorber used in this study has a characteristic of minimizing the reflection of electromagnetic waves by absorbing the incident waves at the absorption layer and also by offsetting with out-of-phase reflected waves from the interfaces of the absorber, where is the wavelength and is the refractive index. To increase design flexibility and reduce design sensitivity, the multilayer
Delamination effect measurement test
Prior to going further into the delamination effect measurement test, the reflection loss measurement on the selected specimen with no delamination case (no gap between layers) had to be preceded. This process verified that the experimentally measured values matched up with the analysis results, thus the selected specimen and the test method were validated for the analysis of the delamination effect. In terms of structural aspect, this type of test is not a proper approach for testing a
Results of delamination effect measurement tests
The measurement test of RAS was conducted for the understanding of delamination effect on RAS. To evaluate the reflection loss performance of the specimen, measurements were made on 4 different types of delamination locations in thickness-wise. Symbolically the RAS specimen layer stacking sequence is shown as, for example, GFRP/L1//L2,/L3/PEC, with a single slash(/), referring to no delamination between layers and a double slash(//), referring to delamination between layers. The overall results
Conclusions
It was verified that the delamination affected the radar absorbing performance of the RAS. The performance change of the reflection loss was related to the thickness-wise locations and thicknesses of the delamination. The delamination located between GFRP and glass/epoxy-MWCNT had less effect than the delamination located between glass/epoxy-MWCNT and PEC. That was because the additional reflection coefficient term, created by the vacuum, influenced the reflection loss phase shift much less
Acknowledgements
This work was supported by Agency for Defense Development as a part of basic research program under the contract UD130045JD.
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