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Nondestructive Evaluation of Composite Substrate Below Thick Top Coat Materials



OBJECTIVE: Seeking new and novel nondestructive evaluation technologies to enhance system affordability by characterizing composite substrate structural damage in multi-layered, curved structures through thick specialty top coatings. The geometry of interest include highly curved surfaces, both concave and convex surfaces with one or more thick top coat layers. 

DESCRIPTION: Conventional nondestructive evaluation technologies are limited by the thick top coatings over the structure and therefore required time consuming removal and replace of the coatings to perform the inspection. Additionally, the coatings of interest are highly attenuative to ultrasound by conventional NDE methods. A new and novel technology is required to inspect the composite substrate for damage due to an impact event below thick specialty top coat materials (0.150 – 0.300 inch thick). The new technology must be capable of interrogating the composite substrate in tight and hard to reach locations. The new technology must be easy to use and provide a high degree of confidence in its assessment. The new technology must provide a damage quantification and assessment as well as a “structural integrity map” of the area of interest to aid in any repairs that may be required due to substrate damage found. The area of interest must extend beyond the point of impact to make sure damage to the substrate does not manifest away from that location. Damage to structural composites on military aircraft structures manifests itself by delamination, fiber breakage and matrix degradation. Military aircraft composite structural damage can be produced by a high velocity impact such a bird strike or hail, or a low velocity impact such a tool drop. The impact event may not necessarily produce a visible indication on the top coat surface. The coating removal and re-application process unduly increases inspection/maintenance downtime up to 1000x. Providing a technology solution to provide thru-coating inspection will vastly decrease system maintenance downtime and labor man-hours, and will thus significantly increase system availability and affordability. The Government will have impacted composite specimens with coating applied available for trial experimentation and testing of inspection technology. 

PHASE I: Demonstrate a proof of concept breadboard prototype for identification of delaminations or disbonds of 1.0 inch diameter or greater in composite substrate through thick top coat materials (0.150 – 0.300 inch thick) or demonstrate plan/path forward to achieve the requirements. Develop technology transition plan and business case assessment. Develop plan for technology integration, test and validation with representative structure geometries. 

PHASE II: Demonstrate damage assessment/quantification and structural mapping capabilities using a prototype in a relevant (preferably operational) environment. Demonstrate capability to identify delamination or disbonds of 1.0 inch diameter or greater with coating system intact Update transition plan and business case assessment. Finalize integration plan, test plan, and validation & verification plan to demonstrate and document results on how requirements are met. 

PHASE III: Finalize ruggedized commercial design and obtain certifications (i.e., UL/CE/C1D2) required for use in an operational environment. Complete integration, testing (including linearity and bias test), gauge repeatability and reproducibility study, and validation & verification on an operational asset/environment. Finalize manufacturing/commercialization plan and business case. 


1: "Nondestructive Inspection of Composite Structures: Methods and Practice," David K. HSU, Center for Nondestructive Evaluation, Iowa State University, Ames, Iowa 50011, USA, presented the 17th World Conference on Nondestructive Testing, 25-28 Oct 2008, Shanghai, China.

2: "Metal Defects Sizing and Detection Under Thick Coating Using Microwave NDT," Hong Zhanga, Bin Gaob, Gui Yun Tiana, Wai Lok Wooa, and Libing Baib, NDT & E International, Volume 60, December 2013, Pages 52–61.

3: "Nondestructive Evaluation of Aircraft Composites using Transmissive Terahertz Time Domain Spectroscopy," CD Stoik, MJ Bohn, JL Blackshire, Optics Express, 2008

KEYWORDS: Nondestructive Evaluation/inspection (NDE/I), NDE/I Of Curved Coated Surfaces, NDE/I Of Composite Substrate, NDE/I Of Composite Substrate Structural Integrity Through Coatings 


Charlie Buynak (AFRL/RXCA) 

(937) 255-9807 

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