Inspection of Bonded Composite Structures Through Fairing Materials

TECHNOLOGY AREAS: Materials The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with section 3.5 of the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws. OBJECTIVE: Develop a nondestructive inspection (NDI) solution to detect delaminations and disbonding in complex composite bonded structures through fairing materials (or compounds) applied to the outer mold line (OML). DESCRIPTION: Future aircraft systems will use advanced composite bonded structure concepts that require recurring inspection and maintenance throughout the system lifecycle. After production, much of the underlying support structure is inaccessible; therefore, structural inspections will need to be performed from the outer mold line (OML) using state-of-the-art NDI tools where inspection access is limited to a single side. Many of these structural concepts use bonding processes to assemble graphite epoxy outer mold line (OML) skins to graphite epoxy composite stiffening structures. Skins may range in thickness from 0.100 inch to 0.75 inch thick while bondline thicknesses may range on the order of 0.030 inch to 0.100 inch thick. Of concern are delaminations in the skin and disbonds within the bondline. Methods such as low frequency (less than 1.5 MHz), pulse-echo ultrasonic has demonstrated some success for thicker laminates (greater than 0.150 inch thick) and bare surfaces (i.e. no fairing materials present). Inspection through fairing materials, using conventional ultrasonics, is challenging as the void content prevents efficient ultrasound transmission. To address this challenge, an advanced NDI solution is required to enable inspection of composite skins, adhesive bondlines and the diverse underlying structures by inspecting through these fairing materials. PHASE I: Develop and execute a proof-of-concept and demonstrate the ability to detect 1-inch diameter disbonds between a composite laminate skin and bonded stiffener. In this phase, the stiffener can be simulated using a composite laminate. Two configurations shall be explored in this initial phase: 1) 0.150 inch thick OML laminate adhesively bonded to an 0.150 inch thick inner mold line (IML) laminate. Bondline thicknesses between 0.030 0.050 inch shall be evaluated. Fairing material thicknesses, applied to the OML surface, shall be representative such as between 0.030 inch and 0.150 inch. 2) 0.250 inch thick OML laminate adhesively bonded to an 0.150 inch thick IML laminate. Bondline thicknesses between 0.030 0.050 inch shall be evaluated. Fairing material thicknesses, applied to the OML surface, shall be between 0.030 inch and 0.150 inch. 3) Bondline disbonds may be simulated using flat-bottom holes drilled from the IML surface or other appropriate methods that simulate disbonds at these interfaces. Simulated disbonds shall cover a range of sized from 0.5 to 1.5 inches in diameter, in 0.25 inch increments. PHASE II: Develop, mature and demonstrate a functional prototype capability enabling detection of composite delaminations and bondline disbonds for OML laminate thicknesses (ranging from 0.100 inch to 0.75 inch thick) and possessing bondline thicknesses from 0.030 inch to 0.100 inch. Fairing material shall be representative and shall be applied to the OML surface per specifications and geometries. The capability to reliably detect 0.5 inch diameter disbonds within the OML laminate and/or bondline is the goal and distinguish the disbond depth location. Bondline disbonds may be simulated using flat-bottom holes drilled from the IML surface or other appropriate methods that simulate disbonds at these interfaces. Simulated disbonds shall cover a range of sizes from 0.5 to 1.5 inch diameter in 0.25 inch increments. The capability to scan large areas and generate a 2D map of inspection of results (i.e. C-scan) shall be developed and demonstrated. PHASE III DUAL USE APPLICATIONS: Through interaction with composite aircraft maintenance and engineering users, define and document the system functional requirements and mature the design and capability to address a specific aircraft program office's needs. Design and execute a probability of detection study to validate the detection capability through a range of fairing materials thickness and structural stack-ups/configurations as defined by the customer. Inspection through OML laminate porosity up to 1.5 % shall also be addressed in the final system design and demonstrated in a field level environment. REFERENCES: 1. Advanced Phased-Array Technologies for Ultrasonic Inspection of Complex Composite Parts (ndt.net); 2. Ultrasonic non-destructive evaluation of composites: A review https://www.sciencedirect.com/science/article/abs/pii/S2214785322074296; 3. Nondestructive evaluation of thick-section composites and sandwich structures: A review; 4. Structural Integrity and Durabilty of Advanced Composites, https://www.sciencedirect.com/science/book/9780081001370 KEYWORDS: NDI; Inspection; Bondline Failures; OML; aerodynamic; pulse-echo; composite