The paper is the first to highlight what appears to be a unique and unusual failure mechanism that is associated with cracks that nucleated from corrosion pits in aluminium alloy (AA) 7085-T7452 specimens that were tested under a variable amplitude load spectrum. In this study, cracks initially nucleated at corrosion pits and first grew as would be expected, namely at ninety degrees to the surface and perpendicular to the applied load. However, after reaching a depth of approximately 2 mm, these various Mode I cracks transformed into what can be best described as interlayer cracks with their surfaces at an angle of approximately ninety degrees to the initial fatigue crack surface. Analysis of the failures revealed that the maximum value of the stress intensity factor at which this phenomenon occurred, which we have defined as KIL, was substantially less than the fracture toughness for this material. As such, failure was not due to classical Mode I failure, but rather due to K exceeding what we will term KIL. Despite the unusual failures, it was found that, up to the point where this phenomenon occurred, the crack growth versus cycles histories could be reasonably accurately predicted using the small crack growth equation developed by the authors in a prior study on AA 7085-T7452 specimens with a fastener hole.

This paper presents an analytical study of two collinear cracks in a finite thermo-magneto-electroelastic medium. An improved permeable model is proposed and applied alongside the Fourier transform method to reduce the complex boundary-value problem to a set of Fredholm-type integral equations. These equations are further discretized into nonlinear algebraic equations using the Lobatto-Chebyshev integration technique. Explicit solutions for the thermo-magneto-electroelastic fields and the associated intensity factors are derived. The results show that crack size, material properties, and adjustment parameters significantly affect the stress intensity factors, providing a theoretical basis for the fracture analysis of cracked materials under multi-field coupling conditions in practical engineering applications.

Femtosecond laser peening (fs-LP) has attracted interest as a promising surface enhancement technique due to its ability to induce high pressure shock waves without using a confinement layer. Unlike traditional nanosecond laser peening which introduces compressive residual stresses to depths exceeding 1 mm, fs-LP affects primarily the near-surface region. In this study, we investigate the effects of fs-LP on additively manufactured Scalmalloy (R) produced via laser powder bed fusion (L-PBF). Residual stress characterizations using X-ray, neutron diffraction and finite element modelling reveal compressive stresses extending up to 100 mu m beneath the surface. Crosssectional analysis suggests the formation of a 2-3 mu m nanocrystalline surface layer and evidence of surface oxidation. Hardness increased from 2.53 GPa to 2.62 GPa accompanied by a higher amount of dislocation density. Moreover, fatigue testing demonstrated a similar to 1.6 times improvement in fatigue life which is attributed to delayed crack initiation resulting from the combined effects of surface hardening, residual stress and refined grain structure. These findings highlight the unique capabilities of fs-LP process for targeted post-treatment of critical regions in additively manufactured lightweight alloy components, where conventional peening is impractical or overly aggressive.

This paper reviews the fracture mechanics parameters associated with the variability in the crack growth curves associated with forty-two different tests that range from additively manufactured (AM) steels to cold spray additively manufactured (CSAM) 316L steel. As a result of this review, it is found that, to a first approximation, the effects of different building processes and R-ratios on the relationship between ΔK and the crack growth rate (da/dN) can be captured by allowing for changes in the fatigue threshold and the apparent cyclic toughness in the Schwalbe crack driving force (Δκ). Whilst this observation, when taken in conjunction with similar findings for AM Ti-6Al-4V, Inconel 718, Inconel 625, and Boeing Space Intelligence and Weapon Systems (BSI&WS) laser powder bed (LPBF)-built Scalmalloy®, as well as for a range of CSAM pure metals, go a long way in making a point; it is NOT a mathematical proof. It is merely empirical evidence. As a result, this review highlights that for AM and CSAM materials, it is advisable to plot the crack growth rate (da/dN) against both ΔK and Δκ. The observation that, for the AM and CSAM steels examined in this study, the da/dN versus Δκ curves are similar, when coupled with similar observation for a range of other AM materials, supports a prior study that suggested using fracture toughness measurements in conjunction with the flight load spectrum and the operational life requirement to guide the choice of the building process for AM Ti-6Al-4V parts. The observations outlined in this study, when taken together with related findings given in the open literature for AM Ti-6Al-4V, AM Inconel 718, AM Inconel 625, and BSI&WS LPFB-built Scalmalloy®, as well as for a range of CSAM-built pure metals, have implications for the implementation and certification of limited-life AM parts.

MIL-STD-1530D and the United States Air Force (USAF) Structures Bulletin EZ-SB-19-01 require an ability to predict the growth of naturally occurring three-dimensional cracks with crack depths equal to what they term an equivalent initial damage size (EIDS) of 0.254 mm. This requirement holds for both additively manufactured and conventionally built parts. The authors have previously presented examples of how to perform such predictions for additively manufactured (AM) Ti-6Al-4V; wire arc additively manufactured (WAAM) 18Ni 250 Maraging steel; and Boeing Space, Intelligence and Weapon Systems laser bed powder fusion (LPBF) Scalmalloy®, which is an additively manufactured Aluminium-Scandium-Mg alloy, using the Hartman-Schijve crack growth equation. In these studies, the constants used were as determined from ASTM E647 standard tests on long cracks, and the fatigue threshold term in the Hartman-Schijve equation was set to a small value (namely, 0.1 MPa √m). This paper illustrates how this approach can also be used to predict the growth of naturally occurring three-dimensional cracks in WAAM CP-Ti (commercially pure titanium) specimens built by Solvus Global as well as in WAAM-built Inconel 718. As in the prior studies mentioned above, the constants used in this analysis were taken from prior studies into the growth of long cracks in conventionally manufactured CP-Ti and in AM Inconel 718, and the fatigue threshold term in these analyses was set to 0.1 MPa √m. These studies are complemented via a prediction of the growth of naturally occurring three-dimensional cracks in conventionally built M300 steel.

ABSTRACT This paper presents the results of a preliminary investigation into the corrosion resistance of laser powder bed fusion (LPBF)‐produced Scalmalloy® specimens built by Boeing Space, Intelligence, and Weapons Systems (BSI&WS). The specimens were first exposed to a 5% NaCl salt fog test at 35°C, and a comparison was made with prior tests on the aluminum alloy AA7050‐T7451. The AA7050‐T7451 alloy was chosen since it is widely used in fixed‐wing aircraft, rotary‐wing aircraft, and in space structures. This preliminary study reveals that BSI&WS LPBF‐built Scalmalloy® is significantly more resistant to corrosion pitting than AA7050‐T7451. These prior exposed (Scalmalloy®) specimens were then fatigued tested, and it was shown that exposure for 28 days to a 5% salt spray fog environment at 35°C did not reduce the durability of the specimens. As such, this study, when taken in conjunction with the authors' previous report on the exceptional damage tolerance of Scalmalloy®, reveals that the BIS&WS LPBF Scalmalloy® is particularly attractive for use on a range of both fixed‐ and rotary‐wing military aircraft. It also reinforces the potential for BSI&WS LPBF Scalmalloy® to be used in building parts for attritable aircraft/drones.

This paper is the first to reveal that the conventionally built aluminium alloy (AA) 7085-T7452 has mechanical properties, viz: a yield stress, ultimate strength, and an elongation to failure, that are similar to that of laser powder bed fusion (LPBF) built Scalmalloy®. Following this observation, the growth of cracks that nucleated from corrosion pits in AA7085-T7452 specimens that had been exposed to a 5 wt% NaCl salt fog environment at 35 °C according to ASTM B117-19 standard for fourteen days is then studied. The specimen geometries were chosen to be identical to those associated with a similar study on Boeing Space, Intelligence, and Weapon Systems (BSI&WS) LPBF built Scalmalloy®. This level of prior exposure led to pits in AA7085-T7452 that were approximately 0.5 mm deep with a surface width/diameter of up to approximately 1.5 mm. These pit sizes are broadly consistent with those leading to fatigue crack growth (FCG) in AA 7050-T7451 structural parts on the RAAF F/A-18 Classic Hornet fleet operating in a highly corrosive environment. Fatigue tests on these AA7085-T7452 specimens, under the same spectrum as used in the BSI&WS LPBF Scalmalloy® study, reveals that AA7085-T7452 and Scalmalloy® have similar crack growth histories. This, in turn, leads to the discovery that the growth of naturally occurring three-dimensional (3D) cracks in AA 7085-T7452 could be predicted using the crack growth equation developed for BSI&WS LPBF Scalmalloy®, albeit with allowance made for their different fracture toughness's. These findings suggest that Scalmalloy® may be suitable for printing parts for both current and future attritable aircraft.

The cold spray (CS) process has gained momentum as an additive manufacturing technology, due to its low processing temperatures. Computational modelling can accompany CS experiments to optimise deposition parameters, as well as predict coating properties and their final performance. A commonly used plasticity model is the Johnson–Cook (JC) model; however, its accuracy is limited at the high strain rates typical of cold spray. This study aims to assess the robustness of predictions using a modified JC model, particularly for two material systems of commercially pure titanium (CP-Ti) and Al6061-T6, and feedstock powders of two sizes and three morphologies. CP-Ti powders of spherical and irregular morphologies were sprayed onto CP-Ti substrates using a Titomic TKF1000 cold spray system. The cross-sectional splat profiles and flattening ratios were compared against smoothed particle hydrodynamics (SPH) simulations. The deposition process of particles was simulated using a modified JC model, implemented as an ABAQUS (2020) VUHARD user subroutine programme. The results showed that SPH simulations predicted the depth of impact, the splat profiles and the flattening ratios. Additionally, the simulations indicated that the impacting particle temperature remained below the melting point of CP-Ti throughout the process. Lastly, it was demonstrated that the irregular CP-Ti feedstock showed greater tendency of restitution than spherical feedstock.

This paper presents the results of an extensive investigation into the durability of cold spray repairs to corrosion damage in AA7075-T7351 aluminium alloy specimens where, prior to powder deposition, the surface preparation involved grit blasting. In this context, it is shown that the growth of small naturally occurring cracks in cold spray repairs to simulated corrosion damage can be accurately computed using the Hartman–Schijve crack growth equation in a fashion that is consistent with the requirements delineated in USAF Structures Bulletin EZ-SB-19-01, MIL-STD-1530D, and the US Joint Services Structural Guidelines JSSG2006. The relatively large variation in the da/dN versus ΔK curves associated with low values of da/dN highlights the fact that, before any durability assessment of a cold spray repair to an operational airframe is attempted, it is first necessary to perform a sufficient number of tests so that the worst-case small crack growth curve needed to perform the mandated airworthiness certification analysis can be determined.

Recent studies have proposed a simple formula, which is based on Elber’s original approach to account for R-ratio effects, for determining the crack closure-free ΔKeff versus da/dN curve from the measured R-ratio-dependent ΔK versus da/dN curves. This approach, which is termed “Simple Scaling,” has been shown to collapse the various R-ratio-dependent curves onto a single curve. Indeed, this approach has been verified for a number of tests on metals, polymers, and a medium-entropy alloy. However, it has not yet been used to help assess/determine the closure-free ΔKeff versus da/dN curve. The current paper addresses this shortcoming and illustrates how to use this methodology to assess the ΔKeff versus da/dN curves given in the open literature for tests on a number of steels, aluminum alloys, STOA Ti-6Al-4V, a magnesium alloy, and Rene 95. As such, it would appear to be a useful tool for assessing fatigue crack growth.

The problem of two collinear cracks subject to the quadratic thermo-magneto-electro-elastic loading is addressed in this paper. Taking advantage of the permeable crack models, the Fourier transform and superposition theorem, the exact solutions of physical quantities around two collinear cracks are obtained. It is shown that the effects of the dimensionless quantities between the upper and below crack-face (i.e., h_c ϵ_r and μ_r) on physical quantities of crack-face (i.e., Q_c, B_c , D_c , K_(ϕ^(*+))^Inn, K_(ϕ^(*+))^Out, K_(φ^(*+))^Inn and K_(φ^(*+))^Out)) in numerical results. It should be emphasized that because the theoretical solution obtained in this paper has an explicit form, it is convenient to solve the stress intensity factor. In particular, it is very convenient to solve the influence of various physical quantities on the stress intensity factor of the crack tip. It has more advantages than the numerical method.

This paper mainly studies the fatigue cracks growth of fillet weld specimens in a fashion that is consistent with that used to assess the fatigue performance of complex aerospace structures under operational flight loads. The fatigue test loads were determined using the overall finite element analysis results of the hopper wagon. The actual applied test loads were monitored using strain gauges. The residual stress in the critical region was determined by combining the stress field of the welded specimen obtained by a thermal imager under cyclic loading with the results of the three-dimensional finite element analysis of the specimen. During the fatigue test, a digital camera (with microscope lens) was used in conjunction with infrared measurement technology to obtain the crack growth information. As in prior studies, the three dimensional finite element alternating technique was used to calculate the stress intensity factor in the critical area of the crack in the fillet weld specimen. The Hartman–Schijve crack growth equation was then used, in conjunction with the calculated stress intensity factor solutions, to compute the crack growth history in a fatigue test of a critical welded component in a hopper wagon. The resultant computed crack growth histories are relatively consistent with the test results.

The durability assessment of additively manufactured parts needs to account for both surface-breaking material discontinuities and surface-breaking porosity and how these material discontinuities interact with parts that have been left in the as-built state. Furthermore, to be consistent with the airworthiness standards associated with the certification of metallic parts on military aircraft the durability analysis must be able to predict crack growth, as distinct from using a crack growth analysis in which parameters are adjusted so as to match measured data. To partially address this, the authors recently showed how the durability of wire arc additively manufactured (WAAM) 18Ni-250 maraging steel specimens, where failure was due to the interaction of small surface-breaking cracks with surface roughness, could be predicted using the Hartman–Schijve variant of the NASGRO crack growth equation. This paper illustrates how the same equation, with the same material parameters, can be used to predict the durability of a specimen where failure is due to surface-breaking porosity.

This paper first presents the results of an experimental study into the damage tolerance of AA7075-T6, which is widely used in both fixed- and rotary-wing aircraft, space structures, and laser bed powder fusion (LBPF) Scalmalloy specimens built by Boeing Space, Intelligence, and Weapons Systems. To this end, four single edge notch AA7075-T6 specimens and four identical single edge notch LBPF Scalmalloy specimens were tested. The resultant crack growth curves reveal that Boeing Space, Intelligence, and Weapons Systems AM-built Scalmalloy is more damage tolerant than conventionally built AA7075-T6. This finding leads to the observation that the da/dN versus ΔK curves associated with Scalmalloy and conventionally manufactured AA2024-T3 are similar. These findings highlight the potential for Boeing Space, Intelligence, and Weapons Systems AM-built Scalmalloy to be used to extend the operational lives of military aircraft by the on-demand printing of limited-life Scalmalloy replacement parts.

This paper first examines crack growth in a range of tests on additively manufactured (AM) and conventionally manufactured Inconel 718. It is shown that whereas when the crack growth rate (da/dN) is plotted as a function of the range of the stress intensity factor (ΔK), the crack growth curves exhibit considerable scatter/variability, when da/dN is expressed in terms of the Schwalbe crack driving force (Δκ), then each of the 33 different curves essentially collapse onto a single curve. This relationship appears to hold over approximately six orders of magnitude in da/dN. The same phenomenon also appears to hold for 20 room temperature tests on both conventionally and additively manufactured Inconel 625. Given that the 53 studies examined in this paper were taken from a wide cross section of research studies it would appear that the variability in the da/dN and ΔK curves can (to a first approximation) be accounted for by allowing for the variability in the fatigue threshold and the cyclic fracture toughness terms in the Schwalbe crack driving force. As such, the materials science community is challenged to address the fundamental science underpinning this observation.

USAF MIL-STD 1530D states that the design and certification approval of military aircraft requires analytical tools that are capable of modeling crack growth, and that the role of testing is to validate or correct the durability and damage tolerance analyses. As such analysis and experimental testing are intimately interconnected. As a result this article presents a methodology for performing the necessary experimental testing and analyses/assessments in a mutually consistent fashion, and in a fashion that is consistent with the mandated building block approach delineated in the US Joint Services Structural Guidelines JSSG2006, USAF MIL-STD-1530D, and NASA Handbook NASA-HDBK-5010.

Motivated by the need for an efficient fatigue crack growth prediction infrastructure for both legacy and novel materials, we have initiated the development of an automated computational framework capable of determining crack growth model parameters for an equationally defined model. As a first step in addressing this need, the present paper focuses on the Hartman-Schijve crack growth variant of the NASGRO equation by exploring and comparing various global optimization methods for parameter determination and evaluating their performance by using both synthetic and actual data. It also introduces the concept of the total least-squares minimization criterion within the context of crack growth modeling. The development of an open-source software library and an application implementing the approach are also described and are available for distribution to the technical community.

Fatigue & Fracture of Engineering Materials & StructuresVolume 46, Issue 4 p. 1638-1640 LETTER TO THE EDITOR Further thoughts on EIDS and the durability analysis of WAAM 18Ni 250 steel with rough surfaces Daren Peng, Daren Peng orcid.org/0000-0003-4586-7268 ARC Training Centre on Surface Engineering for Advanced Materials (SEAM), School of Engineering, Swinburne University of Technology, Hawthorn, Victoria, Australia Centre of Expertise for Structural Mechanics, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria, AustraliaSearch for more papers by this authorRhys Jones, Corresponding Author Rhys Jones [email protected] orcid.org/0000-0003-3197-2796 ARC Training Centre on Surface Engineering for Advanced Materials (SEAM), School of Engineering, Swinburne University of Technology, Hawthorn, Victoria, Australia Centre of Expertise for Structural Mechanics, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria, Australia Correspondence Jones Rhys, ARC Training Centre on Surface Engineering for Advanced Materials (SEAM), School of Engineering, Swinburne University of Technology, John Street, Hawthorn, VIC, 3122, Australia. Email: [email protected]Search for more papers by this authorAndrew S. M. Ang, Andrew S. M. Ang ARC Training Centre on Surface Engineering for Advanced Materials (SEAM), School of Engineering, Swinburne University of Technology, Hawthorn, Victoria, AustraliaSearch for more papers by this authorAlex Michelson, Alex Michelson Solvus Global, Worcester, Massachusetts, USASearch for more papers by this authorVictor Champagne, Victor Champagne US Army Research Laboratory, U.S. Army Combat Capabilities Development Command Weapons and Materials Research Directorate, Aberdeen, Maryland, USASearch for more papers by this authorAaron Birt, Aaron Birt Solvus Global, Worcester, Massachusetts, USASearch for more papers by this author Daren Peng, Daren Peng orcid.org/0000-0003-4586-7268 ARC Training Centre on Surface Engineering for Advanced Materials (SEAM), School of Engineering, Swinburne University of Technology, Hawthorn, Victoria, Australia Centre of Expertise for Structural Mechanics, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria, AustraliaSearch for more papers by this authorRhys Jones, Corresponding Author Rhys Jones [email protected] orcid.org/0000-0003-3197-2796 ARC Training Centre on Surface Engineering for Advanced Materials (SEAM), School of Engineering, Swinburne University of Technology, Hawthorn, Victoria, Australia Centre of Expertise for Structural Mechanics, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria, Australia Correspondence Jones Rhys, ARC Training Centre on Surface Engineering for Advanced Materials (SEAM), School of Engineering, Swinburne University of Technology, John Street, Hawthorn, VIC, 3122, Australia. Email: [email protected]Search for more papers by this authorAndrew S. M. Ang, Andrew S. M. Ang ARC Training Centre on Surface Engineering for Advanced Materials (SEAM), School of Engineering, Swinburne University of Technology, Hawthorn, Victoria, AustraliaSearch for more papers by this authorAlex Michelson, Alex Michelson Solvus Global, Worcester, Massachusetts, USASearch for more papers by this authorVictor Champagne, Victor Champagne US Army Research Laboratory, U.S. Army Combat Capabilities Development Command Weapons and Materials Research Directorate, Aberdeen, Maryland, USASearch for more papers by this authorAaron Birt, Aaron Birt Solvus Global, Worcester, Massachusetts, USASearch for more papers by this author First published: 01 January 2023 https://doi.org/10.1111/ffe.13938Citations: 1 Funding information: Rhys Jones, Andrew Ang, and Daren Peng would like to acknowledge funding provided by the US Army International Technology Center, Indo-Pacific (ITC-IPAC), Tokyo, contract no. FA520921P0164. The findings and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the ITC-IPAC. Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. REFERENCES 1Peng D, Jones R, Ang ASM, et al. Computing the durability of WAAM 18Ni 250 Maraging steel specimens. Fatigue Fract Engineering Mater Struct. 2022; 45(12): 1-11. 2 Structures bulletin EZ-SB-19-01. Durability and damage tolerance certification for additive manufacturing of aircraft structural metallic parts, Wright Patterson Air Force Base, OH, USA, 10 June 2019. Available online: https://daytonaero.com/usaf-structures-bulletins-library/ 3 MIL-STD-1530D, Department of defense standard practice: Aircraft structural integrity program (ASIP). Available online: http://everyspec.com/MIL-STD/MIL-STD%85/download.php?spec%3DMIL-STD-1530D, (accessed on 17th September 2022). 4 Department of defense joint service specification guide. Aircraft Structures, JSSG-2006, October 1998. Available online: http://everyspec.com/USAF/USAF-General/JSSG-2006_10206/ 5 NASA-HDBK-5010, fracture control handbook for payloads, experiments, and similar hardware, May 2005, revalidated 2012. Available online at https://standards.nasa.gov/standard/nasa/nasa-hdbk-5010 Citing Literature Volume46, Issue4April 2023Pages 1638-1640 ReferencesRelatedInformation

This paper discussed the fracture analysis of two collinear cracks under a thermo-magnetic-electric-elastic field. Taking advantage of the permeable crack models and the singular integral equation, the analytical solutions of important parameters around two collinear cracks are obtained. An example is used to demonstrate the method provided in this paper. The effects of the dimensionless quantities between the upper and lower crack surfaces, such as heat flux on the crack surface, electric displacement, magnetic induction per unit thickness, and the corresponding intensity factors near the inner and outer crack tips, are shown in the case analysis. It should be emphasized that because the theoretical solution obtained in this paper has an explicit form, it is convenient to solve the stress intensity factor. In particular, it is very convenient to solve the influence of various physical quantities on the stress intensity factor of the crack tip.