Reconstruction of fatigue crack growth in AA2024-T3 and AA2198-T8 fastened lap joints David Stanley a , Jonathan Awerbuch a,⇑ , Tein-Min Tan a , Babak Anasori b a Drexel University, Department of Mechanical Engineering and Mechanics, Philadelphia, PA 19104, USA b Drexel University, Department of Material Science and Engineering, Philadelphia, PA 19104, USA article info Article history: Available online 29 July 2015 Keywords: Aluminum/Lithium alloy Fatigue crack initiation site Fatigue crack growth Fatigue striation Fastened lap joint abstract This paper presents a comparative study on the fatigue behavior of a typical third generation Aluminum– Lithium alloy, AA2198-T8, to that of a traditional aerospace aluminum–copper alloy, AA2024-T3 with a focus on developing a methodology for reconstructing the crack growth history. Fatigue testing was conducted using a lap joint configuration designed to induce large amounts of secondary bending for the purpose of accentuating any unique behaviors in these two material systems. Fatigue fracture surface morphology was examined in order to determine the effect of alloying and loading direction relative to plate rolling direction on fatigue crack growth behavior. Crack initiation sites occurred in the vicinity of the fastener hole, at multiple sites along the faying surface, yielding crack tunneling in all specimens, prior to the ductile catastrophic fracture. The fracture surface morphologies in the two alloys were mark- edly different: the AA2024-T3 exhibited substantial meandering fracture surface, with localized fatigue crack progressing along multiple directions and at different rates. The AA2198-T8 alloy exhibited more uniform but shallower fatigue striations, with numerous micro and macro interlaminar cracks. Crack growth rate (da=dN) measurements along the width, thickness and the striation ridge lines indicated that the latter provides the most reliable results in terms of estimating fatigue crack initiation cycle, crack initiation site, and crack growth rate. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction The practical rationale for considering Aluminum–Lithium (Al/Li) alloys was to serve as a potential replacement to traditional aluminum alloys (e.g., 2024, 50xx, and 7075) used in aerospace structures [1–4]. By introducing lithium during the alloying pro- cess, end users obtained substantial weight [2,4–7], corrosion [2,3,8], weldability [5,7–10], and fatigue benefits [2,5,11,12]. Adding 1 weight percent (wt.%) lithium to the alloy yields approx- imately a 3% decrease in density while a 6% increase in Young’s elastic modulus [2,5]. The first and second generation of Al/Li alloys, however, exhibited unwanted material characteristics such as low-transverse fracture toughness, low plane stress fracture toughness, high anisotropy of tensile properties, low ductility, poor corrosion resistance, crack deviation, and microcracking during manufacturing [2,13]. Further developments have produced the third generation of Al/Li alloys, addressing several of these short- comings, by lowering the weight percent of lithium, while intro- ducing additional components (e.g., Zn, Mn, Cu, Mg, Ag, Zr, Sc) during the manufacturing process [14]. Microstructure control during manufacturing is vital to the improvement of the newest generation of Al/Li alloys. The heavy hot rolling to the desired thickness yield highly elongated grains resulting from the inhibition of recrystallization. This inhibition is mainly accomplished by using zirconium (Zr) rich dispersoids, formed on homogenization after casting. The Al/Li alloys are pre- cipitation hardened after a high temperature solution annealing followed by low temperature aging to produce very finely dis- persed precipitates [14,15]. However, the microstructure can also yield unwanted fatigue behavior since grain orientation and grain boundary strength, due to concentration of constituents around the boundary, as shown for example in the second generations, can result in a reduction of fracture toughness [16,17]. Due largely to the effect of microstructure and grain sizes, Al/Li alloys demonstrate unique mechanical behaviors that are rela- tively anisotropic with respect to rolling direction [5,16,18–21]. Elongated grains have been shown to lead to high-angle planar boundaries in the rolling plane [22,23] and thus producing distin- guishable anisotropic effects [5,22]. Generally, as grain size increases, fatigue life decreases; however, this effect has been shown to diminish at higher stresses [19]. Serving as a potential replacement to AA2024-T3 fuselage, the AA2198-T8 alloy should maintain fracture properties and fatigue http://dx.doi.org/10.1016/j.tafmec.2015.06.006 0167-8442/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Theoretical and Applied Fracture Mechanics 82 (2016) 33–50 Contents lists available at ScienceDirect Theoretical and Applied Fracture Mechanics journal homepage: www.elsevier.com/locate/tafmec