TECHNOLOGY AREAS: Materials/Processes

ACQUISITION PROGRAM: Joint Strike Fighter

OBJECTIVE: Develop a methodology for the real time monitoring of full field strain measurement.

DESCRIPTION: The development and certification of composite aircraft structure requires a building block approach that requires many thousands of coupons to validate the performance of the composite. The development of analytic tools to predict complex behavior � especially that near stress concentrations � is costly and cumbersome. Furthermore, in order to be effective, these tools must be calibrated with extensive data, and are limited in their use to specific applications. Since the predominant limitation of current methods of composite test is the ability to measure strain response in regions of concentration, the use of full-field strain measurement techniques could reduce costly experimental programs through better understanding of material behavior. Coupling full-field strain measurements with stress models will result in accurate stress-strain relations and material failure criteria. Nonlinear matrix-dominated constitutive properties could be directly measured, and not inferred from model behavior. This could enable the implementation of smaller test matrices and simpler specimen designs compared to traditional methods. By lowering the cost to validate the use of composite materials, the barriers to implementation for new materials, analysis methods, and structural concepts would all be reduced.

Proposed solutions must measure full-field strain in real time and not require significant post-test processing (as is the case today). In addition, proposed methods must demonstrate the ability to be correlated with linear and non-linear analysis tools using a variety of stress concentration types. Real time measurement capability will allow test engineers to change data acquisition rates during test and capture critical information, without current limitations of computational speed or storage space. Monitoring strain values at critical regions during test will allow for increasing frame rates prior to failure onset. Furthermore, the use of real-time data capture and analysis will allow for test control based on concentrated stress fields. This capability does not exist presently, and could provide an excellent means of dramatically reducing the need for extensive testing of stress concentrations.

PHASE I: Develop a methodology and demonstrate feasibility of the proposed approach for a full field strain measurement system.

PHASE II: Based on Phase I results, develop a methodology for assessment of stress-strain relations and failure criteria for composites through the development of software for strain control based on real-time strain measurements.

PHASE III: Complete the development of the control software as a stand-alone system for use with the full-field strain measurement system.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The developed technology will reduce the cost of correlating analysis tools with experimental validation date. This will reduce the cost of transitioning new composite technology � especially structural composite materials with limited application. This will provide transition opportunities for this technology to both commercial as well as military aircraft and other composite material applications.

REFERENCES:
1. J. H. Gosse and S. Christensen. "Strain Invariant Failure Criteria for Polymers in Composite Materials". AIAA-2001-1184.

2. S. Padmanabhan and A.V. Kumar, "Inverse Problem for Estimation of Loads and Support Constraints from Structural Response Data". AIAA-2007-1199.

3. Dulieu-Barton, J.M. (2008) Full-field experimental stress/strain analysis of sandwich structures. In, Torres Marques, A. (ed.) Advanced School on Sandwich Structures. Porto, Portugal, University of Porto, 1-25.

KEYWORDS: Composite; Material Design; Strain Measurement; Stress Concentrations; Full-Field Strain; Experimental Stress Analysis; Experimental Mechanics; Composite Material Testing