Composite Bondline Inspection for Structural Integrity

Description:

TECHNOLOGY AREA(S): Air Platform 

OBJECTIVE: Develop a non-destructive bondline inspection technique suitable for assessing structural integrity of high-efficiency composite structures. 

DESCRIPTION: Trends indicate aircraft structural components are increasingly being designed using composite materials. Because continuous-fiber composites allow a material to have great strength in the fiber direction, designers are able to tailor plies to create laminates that have strength in the direction where it is needed. The resulting composite systems, with fiber supported in polymer matrix, have high strength-to-weight efficiency. Current aerospace composite components are often joined using mechanical fasteners, which add weight, increase stress, and essentially damage the component. The drilled fastener holes act as static stress raisers. Structural composites are known to be static notch sensitive due to drilled holes, as opposed to the fatigue notch sensitivity of aluminum. Holes in composite can lead to additional ply build-up to ensure a slow crack growth failure mechanism by greatly surpassing static loads requirements. Minimizing mechanical fasteners in composite structures can reduce weight, manufacturing complexity, and assembly labor. Advancement in composite joining methods is needed. Adhesive bonds are already inherent to composite materials at the laminate level where plies are bonded. Joining composite structure through bonding could minimize or completely replace mechanical fastening methods; however, there currently exists no way to validate the integrity of the bond. To realize aircraft design of primary structure using adhesive bonding, the structural integrity must be ensured throughout the service life. The Army desires an inspection technique capable of detecting any degradation of bondline strength due to combined loading and environmental effects such as temperature and moisture. Previous efforts of Hennige and Cribbs (2008) have explored ultrasonic inspection methods which generate pulse amplitudes that produce strains just below the accepted bond strength. A drawback of this approach is the destructive effect on strength degraded bonds. A truly non-destructive solution is sought which will not degrade the load carrying ability of structure. Possible directions for solutions that can achieve the desired state may include in-situ monitoring methods which have potential for manufacturability, light weight, and reliability. Another avenue for a solution may be a rapid inspection technique to be used with existing maintenance inspections, while remaining cognizant of life-cycle cost associated with the tradeoffs in maintenance and benefits from bondline design. Solutions should be consistent with the Armys desired maintenance free operating period concept and have enough fidelity to ensure bond integrity, and ultimately structural integrity, between inspections. 

PHASE I: Develop an inspection method for bondlines of aerospace composite material systems. This phase should determine limitations of material system, limitations to joint types, limitations for size resolution, limitations to geometric configuration, and precision tolerances of fracture energy for proposed bondline inspection method. At the end of Phase I the Offeror shall perform proof-of-concept testing to show that system can non-destructively inspect a bondline for meeting the minimum threshold for strength. 

PHASE II: Further refine and mature the developed bondline inspection method. This phase should test a variety of materials and joint configurations with good and poor quality bonding to build confidence in the inspection method as a universal solution. Verify detection of any degradation of bondline strength due to environmental effects. Provide analytical and experimental verification that inspection technique has sufficient fidelity (probability of detection and confidence) to ensure structural integrity of bondline between inspection periods. This phase should develop a prototype device as a deliverable. 

PHASE III: Refine the design for commercialization for aerospace applications. A successful Phase II will provide evidence that the technology is promising for both use in field applications and in manufacturing quality assurance. A business case analysis should be conducted. Single or multiple product development will include design of user-interface and software verification and validation. Fully characterize the inspection reliability, including probability of detection and confidence interval. 

REFERENCES: 

1: Hennige, C. W. and Cribbs, R. W., Composite-Bonded Joint Strength Evaluation System, Phase II SBIR Final Report, Project Number 0010195477, 2008. Public: https://www.sbir.gov/sbirsearch/detail/311341

2: Department of Defense Joint Service Specification Guide Aircraft Structures (JSSG-2006)

3: What the Customer Wants. Maintenance-Free and Failure-Free Operating Periods to Improve overall System Availability and Reliability. http://www.dtic.mil/dtic/tr/fulltext/u2/p010429.pdf

4: Kendall, F., Better Buying Power 3.0 http://www.acq.osd.mil/fo/docs/betterBuyingPower3.0(9Apr15).pdf

 

KEYWORDS: Adhesive Bond, Composite Bonding, NDE Bondline, Composite Structures 

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