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Informal Seminar by Ronald Krueger |
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Date: October 5, 2005
Time: 12:00pm
Location: NIA, Rm 137
Computational Fracture Mechanics for Composites:
State of the Art and Challenges
Ronald Krueger, NIA
Interlaminar fracture mechanics has proven useful for characterizing the onset and growth of delaminations. 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, need to be calculated. In order to predict delamination onset or growth for two-dimensional problems, these calculated G components are compared to interlaminar fracture toughness properties measured over a range from pure mode I loading to pure mode II loading. A quasi static mixed-mode fracture criterion is determined by plotting the interlaminar fracture toughness, Gc, versus the mixed-mode ratio, GII/GT, determined from data generated using pure Mode I DCB (GII/GT=0), pure Mode II 4ENF (GII/GT=1), and mixed-mode MMB tests of varying ratios. A curve fit of these data is performed to determine a mathematical relationship between Gc and GII/GT. Failure is expected when, for a given mixed mode ratio GII/GT, the calculated total energy release rate, GT, exceeds the interlaminar fracture toughness, Gc. Although several specimens have also been suggested for the measurement of the mode III interlaminar fracture toughness property, an interaction criterion incorporating the scissoring shear, however, has not yet been established.
The virtual crack closure technique (VCCT) is widely used for computing energy release rates based on results from continuum (2D) and solid (3D) finite element analyses to provide results on the mode separation required when using the mixed-mode fracture criterion. Although the original publication on VCCT dates back more than a quarter century present techniques typically require geometric non-linear finite element analyses with additional post processing routines that are currently not an integral part in most commercial codes. ABAQUS recently announced the release of a new add-on for ABAQUS 6.5 called VCCT for ABAQUS. The other large commercial finite element codes such as MSC NASTRAN or ANSYS, which are the frequently used in industry do not offer the choice for calculating mixed mode energy release rates using the virtual crack closure technique (VCCT).
Full implementation of Interlaminar Fracture Mechanics (ILFM) in design requires a continuing development effort of codes to calculate energy release rates, advancements in delamination growth criteria under mixed mode conditions, and consideration of three-dimensional geometry and spectrum loading including out-of-plane loads. The objective of this presentation is to demonstrate the state-of-the-art in the areas of delamination characterization, interlaminar fracture mechanics analysis tools and experimental verification of life predictions. The skin/stringer debonding failure was selected as an engineering problem to demonstrate the overall methodology.
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