State Variable Methods of Assessment, Prognosis, and Control of Composite and Bonded Structures

Abstract

It is generally recognized that the determination of material state is the baseline foundation for the assessment and prognosis of current and future performance of structural materials in which distributed damage leads to progressive property degradation. Many classes of composite materials, both naturally occurring and engineering designs, are governed by such behavior. However, finding appropriate measureable physical variables to make such assessments, especially for asmanufactured and subsequent real-time assessment and prognosis conditions is challenging. Measurement and analysis of multiple single defect initiation, interaction, and collective effects is the most rigorous current methodology, and is the foundation for certification and quality assessment in the aerospace industry. But it can be difficult (or impossible) to find all of the defects in as-manufactured composites or (especially) to assess their growth and interaction during real-time service, and to conduct the proper analysis of their collective effect on strength and life as a function of real-time load and environmental history of a given structure. Global state variables are well suited to this challenge, but finding suitable measurables and methods is challenging. The current paper will examine several aspects of this problem for structural composite materials, bonded joints, and additive manufactured composite elements with complex shapes. Recent experience and results of development efforts in our group to exploit electrical and dielectric methods will be described. Applications to materials aging, degradation during highly nonlinear deformation, bonded joints, and "materials in the loop" structural control concepts and examples will be presented [1].