Reconstruction of fatigue crack growth in AA2024-T3 and AA2198-T8 fastened lap joints

• Studied fatigue crack growth rate in fastened lap joints made of two aluminum alloys.
• Examined local fatigue fracture surface morphology and the effect of constituents.
• Characterized fatigue crack growth using striation ridge-lines.
• Measured local fatigue crack growth rate based on fatigue striations.
• Estimated crack initiation cycle using back-calculation algorithm and striation data.

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 markedly 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/dNda/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.