IMPLEMENTATION OF INTERLAMINAR FRACTURE MECHANICS IN DESIGN: AN OVERVIEW

Abstract

SUMMARY: The state-of-the-art in the areas of delamination characterization, interlaminar fracturemechanics analysis tools and experimental verification of life predictions is demonstrated usingskin/stringer debonding failure as an engineering problem to describe the overall methodology. KEY TERMS: delamination, fracture toughness, computational fracture mechanics, finite elementanalysis, virtual crack closure technique INTRODUCTION Many composite components in aerospace structures are made of flat or curved panels with co-curedor adhesively-bonded frames and stiffeners. A consistent step-wise approach has been developed over thelast decade which uses experiments to detect the failure mechanism, computational stress analysis todetermine the location of first matrix cracking and computational fracture mechanics to investigate thepotential for delamination growth. Testing of thin skin stiffened panels designed for aircraft fuselageapplications has shown that bond failure at the tip of the frame flange is an important and very likelyfailure mode as shown in Figure 1 [1]. Debonding also occurs when a thin-gage composite fuselage panelis allowed to buckle in service. Comparatively simple specimens consisting of a stringer flange bondedonto a skin have been developed to study skin/stiffener debonding [2, 3] (Figure 2a). The failure thatinitiates at the tip of the flange in these specimens (as shown in Figure 2b) is identical to the failureobserved in the full-scale panels and the frame pull-off specimens [1, 4].The objective of this paper is to demonstrate the state-of-the-art in the areas of delaminationcharacterization, interlaminar fracture mechanics analysis tools and experimental verification of lifepredictions. The advances required in all three areas in order to reach the level of maturity desired forimplementation of this methodology for design and certification of composite components arehighlighted. The skin/stringer debonding failure was selected as an engineering problem to demonstratethe overall methodology.