Fracture Mechanics for Composites State of the Art and Challenges

Interlaminar fracture mechanics has proven useful for characterizing the onset of delaminations in composites and has been used with limited success primarily to investigate onset in fracture toughness specimens and laboratory size coupon type specimens. Future acceptance of the methodology by industry and certification authorities however, requires the successful demonstration of the methodology on the structural level. In this paper, the state-of-the-art in fracture toughness characterization, and interlaminar fracture mechanics analysis tools are described. To demonstrate the application on the structural level, a panel was selected which is reinforced with stringers. Full implementation of interlaminar fracture mechanics in design however remains a challenge and requires a continuing development effort of codes to calculate energy release rates and advancements in delamination onset and growth criteria under mixed mode conditions. 1. BACKGROUND Many composite components in aerospace structures are made of flat or curved panels with co-cured or adhesively bonded frames and stiffeners. Over the last decade a consistent stepwise approach has been developed which uses experiments to detect the failure mechanism, computational stress analysis to determine the location of first matrix cracking and computational fracture mechanics to investigate the potential for delamination growth. Testing of thin skin stiffened panels designed for aircraft fuselage applications has shown that bond failure at the tip of the frame flange is an important and very likely failure mode. Debonding also occurs when a thin-gage composite fuselage panel is allowed to buckle in service. A methodology based on fracture mechanics [1] has proven useful for characterizing the onset and growth of delaminations in composites and has been used with limited success to investigate delamination onset and debonding in simple laboratory coupon type specimens [2, 3]. Future acceptance of a fracture mechanics methodology by industry and certification authorities however, requires the successful demonstration of the methodology on structural level. The objective of this paper is to demonstrate the state-of-the-art in the areas of delamination characterization, interlaminar fracture mechanics analysis tools and demonstrate the application on the structural level for which a panel was selected which is reinforced with stringers. The advances required in all three areas in order to reach the level of maturity desired for implementation of this methodology for design and certification of composite components are highlighted. 1 Presented at the NAFEMS Nordic Seminar: Prediction and Modelling of Failure Using FEA, Copenhagen/Roskilde, Denmark, June 2006. National Institute of Aerospace, 100 Exploration Way, Hampton, VA 23666-6147, USA Email: [email protected] 2. METHODOLOGY 2.1. Interlaminar Fracture Mechanics Interlaminar fracture mechanics has proven useful for characterizing the onset and growth of delaminations [1, 4-6]. When using fracture mechanics, the total strain energy release rate, GT, the mode I component due to interlaminar tension, GI, the mode II component due to interlaminar sliding shear, GII, and the mode III component, GIII, due to interlaminar scissoring shear, as shown in Figure 1, are calculated along the delamination. The calculated GI, GII, and GIII components are then compared to interlaminar fracture toughness values in order to predict delamination onset or growth. Today, the interlaminar fracture toughness values are determined experimentally over a range of mode mixities from pure mode I loading to pure mode II loading [7-10]. A quasi static mixed-mode fracture criterion is determined by plotting the interlaminar fracture toughness, Gc, versus the mixed-mode ratio, GII/GT. The fracture toughness data is generated experimentally using pure Mode I (GII/GT=0) Double Cantilever Beam (DCB), pure Mode II (GII/GT=1) four point End Notched Flexure (4ENF), and Mixed Mode Bending (MMB) tests of varying ratios as shown in Figure 2 for a carbon/epoxy material. A failure criterion – as shown in Figure 2 was suggested by Benzeggah and Kenane [11] using a simple mathematical relationship between Gc and GII/GT ! G c =G Ic + G IIc "G Ic ( ) # G II