materials Article Computing the Fatigue Life of Cold Spray Repairs to Simulated Corrosion Damage Daren Peng 1 , Caixian Tang 2 , Neil Matthews 2 , Rhys Jones 1,3, * , Sudip Kundu 1,4 , R. K. Singh Raman 1 and Alankar Alankar 4   Citation: Peng, D.; Tang, C.; Matthews, N.; Jones, R.; Kundu, S.; Raman, R.K.S.; Alankar, A. Computing the Fatigue Life of Cold Spray Repairs to Simulated Corrosion Damage. Materials 2021, 14, 4451. https://doi.org/10.3390/ma14164451 Academic Editor: Mohammadreza Daroonparvar Received: 3 July 2021 Accepted: 4 August 2021 Published: 9 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Centre of Expertise in Structural Mechanics, Department of Mechanical and Aeronautical Engineering, Monash University, Wellington Rd, Clayton, VIC 3800, Australia; daren.peng@monash.edu (D.P.); sudip.kundu@monash.edu (S.K.); raman.singh@monash.edu (R.K.S.R.) 2 RUAG Australia, 836 Mountain Highway, Bayswater, VIC 3153, Australia; caixian.tang@ruag.com (C.T.); neil.matthews@ruag.com (N.M.) 3 ARC Industrial Transformation Training Centre on Surface Engineering for Advanced Materials, Faculty of Science, Engineering and Technology, Swinburne University of Technology, John Street, Hawthorn, VIC 3122, Australia 4 Department of Mechanical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, Maharashtra, India; alankar.alankar@iitb.ac.in * Correspondence: rhys.jones@monash.edu Abstract: This paper summarises the findings of an investigation into the durability of cold spray repairs, also known as supersonic particle deposition or SPD repairs, to simulated corrosion damage in AA7075-T7351 aluminium alloy specimens. A feature of this paper is that it is the first to show how to perform the mandatory durability analysis of repaired corroded structures, where the corroded material is first removed by machining and then repaired using cold spray, in a fashion consistent with the requirements delineated in USAF Structures Bulletin EZ-19-01, MIL-STD-1530D, and the US Joint Services Structural Guidelines JSSG2006. Keywords: cold spray repair; skin corrosion; durability; MIL-STD-1530D; EZ-19-01 1. Introduction Cold spray, which is also termed supersonic particle deposition (SPD), is being in- creasingly used to repair military aircraft [1–9]. However, as explained in [1–3,5], to date the majority of applications are limited to non-structural applications. Exceptions to this are the eleven cold spray doublers applied to an F/A-18 full scale fatigue test [1], and the studies into the ability of cold spray to restore the (compressive) load bearing capacity of corroded P3C Orion upper wing skin structures [1,8]. In the case of the doublers applied to the F/A-18 [1] it was found that despite the high stresses, in one case a cold spray doubler was found to experience peak stresses in excess of 250 MPa, and despite the fact that the spectra were measured on an operational combat aircraft, which is a very aggressive spectrum, there was no failure or debonding of the doubler. Indeed, the doublers lasted (intact) for in excess of three design lifetimes, and failure in the fatigue test article occurred well away from any of the cold spray doublers. This coupled with the results associated with the extensive coupon test programs reported in [1,6], whereby cold spray repairs were subjected to fatigue loads representative of those seen by operational aircraft, suggests that cold spray can be used to ensure the structural integrity of load bearing aircraft structures. However, as outlined in USAF Structures Bulletin EZ-19-01 [10], MIL-STD-1530D [11], and the United States Joint Services Structural Guidelines JSSG2006 [12], the airworthiness certification of a cold spray repair to load bearing structure requires a durability life anal- ysis (the term durability is defined in JSSG2006 to be: “Durability is the attribute of an airframe that permits it to resist cracking for a prescribed period of time.”) To this end, the Materials 2021, 14, 4451. https://doi.org/10.3390/ma14164451 https://www.mdpi.com/journal/materials