24-10-2012, 02:28 PM
Residual Strength Characterization of a Curved Integrally-Stiffened Panel
ABSTRACT
Over the years, Finite-element fracture simulation methodology has been very well established at NASA Langley to
predict the residual strength of damaged aircraft structures. This methodology has been experimentally verified at NASA
Langley for structures ranging from laboratory coupons up to full-scale built-up structural components with single and
multiple-site damage cracking. The methodology uses the critical crack-tip-opening-angle (CTOA) fracture criterion to
characterize the fracture behavior of the material. The CTOA fracture criterion assumes that stable crack growth occurs when
the crack-tip angle reaches a constant critical value. The use of the CTOA criterion requires an elastic-plastic, finite-element
analysis. The critical CTOA value is determined by simulating fracture behavior in laboratory specimens, such as a compact
specimen, to obtain the angle that best fits the observed test behavior. The critical CTOA value appears to be independent of
loading, crack length, and in-plane dimensions. However, it is a function of material thickness and local crack-front constraint.
Modeling the local constraint requires either a three-dimensional analysis or a two-dimensional analysis with an
approximation to account for the constraint effects. In recent times as the aircraft industry is leaning towards monolithic
structures with the intension of reducing part count and manufacturing cost, there has been a consistent effort at NASA
Langley to extend critical CTOA based numerical methodology in the analysis of integrally-stiffened panels. In this regard,
a series of fracture tests were conducted on curved aluminum-alloy integrally-stiffened panels. These curved panels were
subjected to uniaxial tension and pressure loading. During the test, applied load-crack extension, out-of-plane displacements
and local deformations around the crack tip region were measured. Compact and middle-crack tension specimens were tested
to determine the critical angle (psi(sub c) using three-dimensional code (ZIP3D) and the plane-strain core height (h(sub c))
using two-dimensional code (STAGS).
ABSTRACT
Over the years, Finite-element fracture simulation methodology has been very well established at NASA Langley to
predict the residual strength of damaged aircraft structures. This methodology has been experimentally verified at NASA
Langley for structures ranging from laboratory coupons up to full-scale built-up structural components with single and
multiple-site damage cracking. The methodology uses the critical crack-tip-opening-angle (CTOA) fracture criterion to
characterize the fracture behavior of the material. The CTOA fracture criterion assumes that stable crack growth occurs when
the crack-tip angle reaches a constant critical value. The use of the CTOA criterion requires an elastic-plastic, finite-element
analysis. The critical CTOA value is determined by simulating fracture behavior in laboratory specimens, such as a compact
specimen, to obtain the angle that best fits the observed test behavior. The critical CTOA value appears to be independent of
loading, crack length, and in-plane dimensions. However, it is a function of material thickness and local crack-front constraint.
Modeling the local constraint requires either a three-dimensional analysis or a two-dimensional analysis with an
approximation to account for the constraint effects. In recent times as the aircraft industry is leaning towards monolithic
structures with the intension of reducing part count and manufacturing cost, there has been a consistent effort at NASA
Langley to extend critical CTOA based numerical methodology in the analysis of integrally-stiffened panels. In this regard,
a series of fracture tests were conducted on curved aluminum-alloy integrally-stiffened panels. These curved panels were
subjected to uniaxial tension and pressure loading. During the test, applied load-crack extension, out-of-plane displacements
and local deformations around the crack tip region were measured. Compact and middle-crack tension specimens were tested
to determine the critical angle (psi(sub c) using three-dimensional code (ZIP3D) and the plane-strain core height (h(sub c))
using two-dimensional code (STAGS).