CTOA of a stable crack in a thin aluminum fracture specimen

Abstract

A crack tip opening angle (CTOA) resistance curve was generated from the moiré interferometry data of thin single edge notched (SEN) and central notched (CN), 2024-T3 aluminum fracture specimens. This CTOA resistance curve, which has a steady state value of 6°, was then used to propagate the cracks in elastic–plastic finite element models of the CN specimen and a CN specimen with a simulated multiple site damage. The CTOA of curved crack growth in a biaxial fracture specimen scattered between 4° and 8° but the resultant crack tip opening displacement, which is the vector sum of the mode-I and the mode-II crack tip sliding displacement, remained a constant 0.18 mm. The CTOA of a rapidly propagating crack in 1.6 mm thick, 7075-T6 SEN specimens increased from 4.5° at a low-crack velocity to a constant 7° at the terminal crack velocity.

Introduction

A ductile fracture criterion, which relates to the plastic strain field in the crack tip plastic zone, is the crack tip bluntness and was quantified by the crack opening displacement (COD) criterion advanced by Wells in 1963 [1]. This COD criterion, which was initially related to Irwin’s plastic zone estimate [2], r_y, and later to the Dugdale plastic zone [3], was promoted mainly by The Welding Institute in the 60s and 70s [4] for fracture assessment of thick-walled pressure vessels. Since the COD along a surface crack front in a thick plate cannot be readily measured and must be computed, its acceptance was hindered by the lack of easily accessible three-dimensional (3D) elastic–plastic computer codes with the sensitivity to accurately map the crack tip bluntness. The crack tip opening angle (CTOA), which uniquely characterizes the crack tip strain field [5], is a local field parameter which in theory can be readily measured on the surface and computed in the interior of a 3D crack. Shih et al. [5] as well as Kaninnen et al. [6] in the 70s concluded that the CTOA was a computationally attractive operational parameter and an alternative to the J-integral criterion which subsequently dominated the ductile fracture research of the 80s and 90s.

The CTOA criterion was used in the 80s to analyze axial crack propagation in a subscale gas transmission line [7]. The rupturing profile of a propagating crack in a pressurized 2-in. diameter, schedule 10, carbon steel pipe was recorded with a high-speed framing camera and was processed through a special software [8] which yielded the CTOA and flap motion. The CTOA criterion was also used in a ring model of the rupturing pipe and a thermal hydraulic depressurization code to simulate the large-scale burst tests of 48-in. diameter×0.720-in. thick, X70 line pipes [9]. The excellent agreement between the measured and the computed CTOA versus crack velocity relations demonstrated that the CTOA was an effective static and dynamic fracture parameter for pipe rupture [10].

A renewed research thrust in the CTOA criterion was undertaken in the early 90s in response to the NASA Aircraft Integrity Program. The thin aluminum specimens considered were ideally suited for the CTOA criterion. Newman et al. showed that the CTOA criterion will predict the residual strength in laboratory specimens [11], [12] as well as in the FAA/NASA wide panels with multiple site damages (MSDs) [13]. In the following, the authors’ findings on the utility of CTOA in similar aluminum fracture specimens are presented.