Abstract
Understanding and characterizing crack growth is central to meeting the damage tolerance and durability requirements delineated in USAF Structures Bulletin EZ-SB-19-01 for the utilization of additive manufacturing (AM) in the sustainment of aging aircraft. In this context, the present paper discusses the effect of different AM processes, different build directions, and the variability in the crack growth rates related to AM Ti-6Al-4V, AM Inconel 625, and AM 17-4 PH stainless steel. This study reveals that crack growth in these three AM materials can be captured using the Hartman–Schijve crack growth equation and that the variability in the various da/dN versus ΔK curves can be modeled by allowing the terms ΔKthr and A to vary. It is also shown that for the AM Ti-6AL-4V processes considered, the variability in the cyclic fracture toughness appears to be greatest for specimens manufactured using selective layer melting (SLM).
Highlights
The recent memo by the Under Secretary, Acquisition and Sustainment [1] enunciated that as ofMarch 21, 2019, additive manufacturing (AM) is used to “enable the transformation of maintenance operations and supply chains, increase logistics resiliency, and improve self-sustainment and readiness”.This memo further stated that: “AM parts or AM repair processes can be used in both critical and non-critical applications
The subsequent USAF Structures Bulletin EZ-SB-19-01 [4], which established the requirements for the Durability and Damage Tolerance (DADT) certification of aircraft structural metallic parts fabricated from an additive manufacturing (AM) process, stated that the Equivalent Initial Damage Sizes (EIDS) for durability crack growth analysis of durability critical and fatigue critical parts shall be based on a probability of exceeding the EIDS of 0.001, but not less than 0.01 inches (0.254 mm)
There are references from the US Federal Aviation Authority (FAA) and the U.S Navy’s data repositories. Keywords used in these searches were Additive Manufacturing, AM, durability, damage tolerance, Hartman–Schijve, small cracks, crack growth in operational aircraft, full-scale fatigue tests, and aircraft certification
Summary
The recent memo by the Under Secretary, Acquisition and Sustainment [1] enunciated that as of. The subsequent USAF Structures Bulletin EZ-SB-19-01 [4], which established the requirements for the Durability and Damage Tolerance (DADT) certification of aircraft structural metallic parts fabricated from an additive manufacturing (AM) process, stated that the EIDS for durability crack growth analysis of durability critical and fatigue critical parts shall be based on a probability of exceeding the EIDS of 0.001, but not less than 0.01 inches (0.254 mm). Understanding the variability in crack growth, including the effect of different build directions, the interaction between the surface roughness and manufacturing defects on or in proximity to the surface, and the ability to determine an upper bound on the crack growth curve are essential steps in the certification of additively manufactured replacement parts. It is shown that the crack growth in these particular additively manufactured materials can be modeled using the Hartman–Schijve crack growth equation and that the variability of the relevant curves can be captured by allowing the terms ∆Kthr and A to vary
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