Fatigue crack growth behavior in an aluminum alloy Al–Mg–0.3Sc produced by wire based directed energy deposition process

Abstract

Additive manufacturing (AM) of Al–Mg–Sc alloys has received considerable interest from the aerospace industry owing to their high specific strength and suitability for AM processes. This study has investigated the fatigue crack growth behavior in an Al–Mg–0.3Sc alloy made by wire and arc additive manufacturing. Tests were conducted with two different crack orientations at cyclic load ratios of 0.1 and 0.5. At the lower load ratio, the horizontal crack showed a faster growth rate owing to the smaller grains and coarser second‐phase particles that the crack tip had encountered when it propagated along the material build direction. The anisotropy in crack growth rate was mainly caused by the grain size effect. When the applied stress intensity factor range exceeded the value of 10 MPa m1/2, an isotropic crack growth rate between the two crack orientations was measured. This is due to the microstructural influence being overcome by the governing parameter of fracture mechanics. At the higher load ratio of 0.5, crack growth rate is isotropic, and the threshold stress intensity factor range was much lower than that tested under load ratio 0.1. Finally, the modified Hartman–Schijve equation has been successfully employed to represent the crack growth rates in all three regions.

Durability Analysis of Cold Spray Repairs: Phase I—Effect of Surface Grit Blasting

“…Here, ∆ K = K max – K min , where K max and K min are the maximum and minimum values of the stress intensity factor ( K ) in a cycle, ∆ K thr is the fatigue threshold, n and D are the material constants, and A is the apparent cyclic fracture toughness; see [ 21 ] for more details. The application of this formulation to a wide range of problems can be found in other journal papers [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 ]. The relationship between this formulation and crack-closure-based crack growth equations [ 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 ] is presented in [ 55 ].…”

Thoughts on the Importance of Similitude and Multi-Axial Loads When Assessing the Durability and Damage Tolerance of Adhesively-Bonded Doublers and Repairs

“…It should also be noted that it has long been known [25][26][27] that the durability analysis of operational aircraft requires a linear-elastic fracture-mechanics analysis that uses the small-crack growth curve, as distinct from curves obtained from tests on long cracks. In this context, it has previously been shown [27][28][29][30][31][32][33][34][35][36][37] that the growth of both long and small cracks in a range of both conventionally and additively manufactured materials subjected to both constant and variable loads can often be accurately modeled using the Hartman-Schijve crack growth equation, viz:…”

On the Link between Plastic Wake Induced Crack Closure and the Fatigue Threshold

“…As a result, the DADT assessment of conventionall manufactured, AM and CSAM parts must be based on the appropriate da/dN versus ∆K curve. However, it is now known [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] that the variability in the da/dN versus ∆K curves associated with the growth of long cracks in more than one hundred and twenty independent tests performed on a range of additively manufactured (AM) materialscan often be accounted for by expressing the crack growth rate (da/dN) as a function of ∆κ, where ∆κ is the Schwalbe crack driving force [21], and by allowing for the effect of the manufacturing process on just two parameters, namely the cyclic fatigue threshold and the fracture toughness. Here ∆κ is Schwalbe's crack driving force which is defined as:…”

“…The terms ∆K thr and A in Equation ( 1) are the fatigue threshold and the apparent cyclic fracture toughness respectively. The additively manufactured materials for which this observation has been found to hold include: Ti-6Al-4V [6,7,11,12], Inconel 718 [5], Inconel 625 [5], 316L steel [6,17,20], 304L steel [20], Aermet 100 steel [6], 17-4 Ph steel [15], Scalmalloy [12], an aluminium-scandium-magnesium alloy [4], 18Ni 250 Maraging steel [8,19]. This observation also appears to hold for a range of conventionally manufactured materials [22][23][24][25][26][27][28][29], a range of CSAM materials [20] and for plasma sprayed metals and alloys [30].…”

“…The literature also contains a number of other crack growth equations that are based on the hypothesis that da/dN should be a function of how much ∆K exceeds its threshold value [4,21,[66][67][68][69][70][71][72][73][74][75][76][77][78][79][80]. As such the purpose of the present paper is to attempt to investigate a possible relationship between ∆K eff , as originally defined by Elber [46,47], and the fatigue threshold term ∆K th .…”