Orientation Effects on the Measurement and Analysis of Critical CTOA in an Aluminum Alloy Sheet

Abstract

Fracture tests were conducted on 76.2mm wide, 2.3mm thick middle crack tension (M(T)) specimens machined from 2024-T3 aluminum sheet. The specimens were tested in the T-L orientation and comparisons were made to similar tests conducted in the L-T orientation. Measurement of critical crack tip opening angle (CTOA), applied stress, and crack front shape were made as a function of crack extension. A two-dimensional, elastic-plastic finite element analysis was used to simulate the fracture behavior for both orientations. The results indicate that the T-L orientation had a 10% lower stress at fracture than similar tests conducted in the L-T orientation. Correspondingly, the critical CTOA in the T-L tests reached a constant value of 4.7° after 2–3mm of crack extension and the L-T tests reached a value of 6°. The fracture surface of the T-L specimens were observed to remain flat, while those of the L-T specimens transitioned to a 45° slant fracture after about 2–3mm of crack extension. The tunneling behavior of the two orientations also differed; the T-L specimens reached a deeply tunneled stabilized crack front shape, while the L-T specimens were observed to have only a small amount of tunneling once the crack began to grow on the 45° slant. The two-dimensional, elastic-plastic finite element analysis was able to simulate the fracture behavior for both the T-L and L-T orientations.

Microtopography for ductile fracture process characterization Part 2: application for CTOA analysis

“…Investigations by Lloyd and Piascik [3] and Sutton et al [2] clearly show that CTOA can have strong dependence on through-thickness position, even in thin sheet materials. Their work also show substantial transient variation in CTOA immediately following crack initiation until the crack has advanced -at the specimen surface -the order of one sheet thickness in 7075 aluminum alloy.…”

“…Around 1989, about the same time of investigation into CTOA as a crack growth parameter b by Sutton et al [2] and Guan et al [5] (others had explored CTOA before this time), INEEL developed the microtopography system of measurement and analysis. Microtopography is a unique method for assessing the deformations associated with the ductile fracture process.…”

“…Investigations over the past ten years by different researchers, e.g. Dawicke et al [1], Sutton et al [2], and Lloyd and Piascik [3] show that the CTOA is a credible parameter that adequately characterizes the ductile fracture material response following crack initiation.…”

Recent Advances in Durability and Damage Tolerance Methodology at NASA Langley Research Center

“…[19][20][21][22] The CTOA criterion assumes that a crack will stably tear when the angle made by the crack faces reaches a critical value some distance behind the crack tip, d, as determined by coupon tests (e.g., compact tension, center notch tension). The relationship between CTOA and crack tip opening displacement (CTOD) is also shown in Figure 5.…”

“…During ASIP, crack tip plasticity, three-dimensional constraints around the crack tip, crack tunneling, slant crack growth, and crack growth through sections of varying thickness were all considered. [19][20][21][22] Recent research directions include modeling the crack branching process followed as a lead crack enters an integrally-stiffened region, crack growth through sections of reinforced material, and implications of cyclic crack tip opening displacement (ΔCTOD) in fatigue life estimation. The ACT program was undertaken in three phases.…”