The behavior of a 7.6 mm thick 2000 series aluminum plate alloy was investigated. Fracture tests were conducted on 304.8 mm and 101.6 mm wide M(T) specimens and 152.4 mm and 101.6 C(T) specimens (the 101.6 mm wide C(T) specimen had 10% side grooves). Two-dimensional and three-dimensional, elastic-plastic finite element simulations used the critical CTOA criterion to simulate the fracture behavior. A plane strain core was used in the two-dimensional analyses to approximate the three-dimensional constraint. The results from this study indicate: (A) The three-dimensional finite element analyses required a critical CTOA of 5.75° to simulate the fracture behavior of the 101.6 mm and 304.8 mm wide M(T) specimens and the 152.4 C(T) specimen. This angle was about the upper limit of the surface CTOA measurements. (B) The three-dimensional finite element analyses required a critical CTOA of 3.6° to simulate the fracture behavior of the 101.6 mm C(T) specimen with side grooves. This angle was about the upper limit of the microtopography through-thickness CTOA measurements. (C) A plane strain core height of PSC = 4 mm was required for the two-dimensional analyses to match the fracture behavior obtained from the three-dimensional analyses. This height agreed with the distance that a three-dimensional analysis indicated was the start of plane strain like behavior. (D) For large M(T) specimens (W > 1000 mm) the two-dimensional plane strain core analysis predicted a failure stress between the plane stress and plane strain conditions and provided a good approximation of the three- dimensional analyses. (E) The experimental measurements and analytical results show good agreement when the specimens sizes meet the uncracked ligament to thickness ratio (b/B > 4) determined by Newman et. al [14]. This indicates that there is a minimum size “laboratory specimen” that can be used to determine the material behavior needed to predict fracture in large specimens and structures.