ICF100642OR IMPORTANCE OF CRACK TUNNELING DURING FRACTURE: EXPERIMENTS AND CTOA ANALYSES M. A. James1 and J. C. Newman, Jr.2 1National Research Council Associate and 2Senior Scientist NASA Langley Research Center, Hampton, Virginia, 23681, USA ABSTRACT This paper compares experimental crack-front shapes recorded at various stages of crack extension with area-average crack-extension values during fracture tests conducted on 2024-T351 aluminum alloy plate. Crack-front shapes were determined by fracturing the specimen to a predetermined amount of crack extension, and fatigue cycling the specimen for about 2,000 cycles at a high stress ratio (Pmin/Pmax) to mark the crack-front location. For each shape, the area-average crack length was determined. The evolution of tunneling was used to create a calibration curve that could be used to adjust surface measured crack length values, for a more representative comparison with analyses that use a straight crack-front approximation. The analysis compares much more favorably with the average crack extension than with the surface measured values near maximum load. However, the area average technique tends to over correct crack extension near the crack initiation load. Crack tunneling results show that the area average technique produces more representative crack-length measurements compared to optical based surface measurements. KEYWORDS CTOA, crack extension, unloading compliance, area-average, tunneling, finite-element analysis INTRODUCTION During the 1990’s, as part of a national aging aircraft program, NASA Langley Research Center (LaRC) conducted intensive research on the fracture behavior of thin sheet aluminum alloys [1]. Some of the products of that research were the constant critical crack-tip-opening-angle (CTOA, Yc) fracture criterion, optical/digital imaging means for measuring CTOA, and optical methods for measuring surface crack extension [2]. Two additional means of measuring crack extension, unloading compliance and area average, were not used in favor of the optical methods. This paper presents a comparison of experimental crack-front shapes recorded at various stages of crack extension with area-average crack extension values, and shows how these various measures compare with surface measured values and typical finite-element predictions. BACKGROUND Wells [3] originally proposed the use of the crack-tip-opening displacement (CTOD) or angle (CTOA) during his experimental work. CTOA (Yc) is usually applied as the angle formed by stable tearing material measured at a fixed distance, d, behind the moving crack (typically, d is taken as 1 mm). The fracture methodology developed as part of the NASA program assumes that the critical CTOA is constant and independent of loading and in-plane configuration, as long as the crack length is about 4 times the plate thickness. The criterion has been applied in both two-dimensional (2D) and three-dimensional (3D) finiteelement analysis (FEA). Inherent in 2D FEA is the approximation that the crack front is flat and straight through the thickness of the model. However, neither plane stress nor plane strain accurately capture both
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