Mapping Strain in Composite Materials using Terahertz Metamaterials

RT&L FOCUS AREA(S): Microelectronics TECHNOLOGY AREA(S): Electronics OBJECTIVE: Design and demonstrate a methodology to detect and map regions of incipient failure in opaque composite materials using terahertz metamaterial laminates whose polarimetric response is sensitive to local strain. DESCRIPTION: Military platforms are increasingly composed of composite materials because they are lighter and less expensive than metals, but their failure is more difficult to predict or detect. It is often the case that regions of high strain go undetected before catastrophic failure, so a means to detect these incipient failures is of critical importance. Recently, a promising solution to this challenge has been proposed [1,2]: metamaterial laminates with a strain-dependent polarimetric response [3,4] may be adhered onto or embedded within a composite, and their polarimetric response may be spectroscopically probed in transmission or reflection geometries. Spatially mapping the polarimetric signature of the metamaterial laminate will reveal the local strain fields within the composite, which will be permanently recorded if the metamaterials break under sufficient stress. Indeed, metamaterial arrays or layers of metamaterial laminates may be designed with different threshold stress responses, so that the amount of strain historically experienced by the composite may be recovered and spatially mapped, thus revealing regions of incipient failure. For other applications which require dynamic monitoring of evolving composite strain fields in real time, self-healing deformable metamaterials are of interest because they can record current levels of strain while reversibly returning to an unstrained signature when the stress is released. Metamaterials operating within the terahertz spectral region (0.1-3 THz) provide a nearly optimal compromise of spatial resolution and material penetration depth in a manner that depends on the unique properties of the composite host. Application of these terahertz metamaterials for non-destructive testing could become a transformative approach to maintenance that will enable longer operating times for systems beyond the current conservative maintenance schedules, increase operational readiness, and at the same time increase safety and confidence. PHASE I: Design and quantitatively assess the synergistic electromagnetic and mechanical performance of a terahertz metamaterial laminates adhered onto or embedded within an opaque composite host, whose polarimetric response is sensitive to the local strain and may be mapped in reflection (preferable) or transmission geometries. Of interest are two types of metamaterials, those with permanently severable break junctions for quantitatively recovering the amount of strain experienced historically, and those that are reversibly deformable for quantitatively recovering the amount of strain currently experienced by the composite. The objective is to demonstrate the feasibility of the concept through a detailed design of both the polarimetric metamaterial laminates and the instrument that will rapidly map the locally-sensed strain fields between 0.1 - 5%. The design must include a quantitative estimate of the achievable spatial resolution (must be less than 5 mm) and strain sensitivity (within 10% of actual strain) for a variety of opaque host materials of practical interest for military platforms. PHASE II: Construct, demonstrate, and deliver the sensor design and exemplar composites containing strain-dependent terahertz metamaterial laminates designed in Phase I. Opaque hosts must be at least 1000 square centimeters in size, and the strain mapping, preferably in a reflection geometry, must be accomplished in less than an hour. The spatial resolution (must be less than 5 mm) and strain sensitivity (within 10% of actual strain) must be demonstrated and quantitatively assessed as a function of at least three opaque host materials in order to ascertain the universality of the technique. Of particular interest is the mapping of strain field extrema, both for historical strain fields using break junction metamaterials and for current strain fields using reversibly deformable metamaterials, spanning the range between 0.1 - 5% for metamaterial laminates adhered onto or embedded within composite material hosts of specific military interest. PHASE III DUAL USE APPLICATIONS: These metamaterials will enable the development of a laminate that can be added to any composite material to provide a fast, low cost, high resolution non-destructive test capability using a compact hand-held sensor. It will find military application via industrial fabrication capability applied to most military vehicle and aircraft programs of record. Adoption by industry will be stimulated by the advantages for commercial vehicles, aircraft, and composite structures. This technology will significantly reduce operating cost, life-cycle costs, and accident costs. REFERENCES: H.O. Everitt et al., Strain Sensing with Metamaterial Composites , Adv. Opt. Mat., DOI:10.1002/adom.201801397 (2019). A.A. Zadeh et al., Enlightening force chains: a review of photoelasticimetry in granular matter , arXiv:1902.11213, (2019). J. Li et al., Mechanically tunable terahertz metamaterials , Appl. Phys. Lett. 102, p. 121101 (2013). Khatib et al. , "Mapping active strain using terahertz metamaterial laminates", APL Photonics 6, 116105 (2021). KEYWORDS: Strain mapping, metamaterial, terahertz, composite material