Modelling the Variability and the Anisotropic Behaviour of Crack Growth in SLM Ti-6Al-4V

Abstract

The United States Air Force (USAF) Guidelines for the Durability and Damage Tolerance (DADT) certification of Additive Manufactured (AM) parts states that the most difficult challenge for the certification of an AM part is to establish an accurate prediction of its DADT. How to address this challenge is the focus of the present paper. To this end this paper examines the variability in crack growth in tests on additively manufactured (AM) Ti-6Al-4V specimens built using selective layer melting (SLM). One series of tests analysed involves thirty single edge notch tension specimens with five build orientations and two different post heat treatments. The other test program analysed involved ASTM standard single edge notch specimens with three different build directions. The results of this study highlight the ability of the Hartman–Schijve crack growth equation to capture the variability and the anisotropic behaviour of crack growth in SLM Ti-6Al-4V. It is thus shown that, despite the large variability in crack growth, the intrinsic crack growth equation remains unchanged and that the variability and the anisotropic nature of crack growth in this test program is captured by allowing for changes in both the fatigue threshold and the cyclic fracture toughness.

Computing the Fatigue Life of Cold Spray Repairs to Simulated Corrosion Damage

“…The measured and computed crack growth histories are shown in Figure 8 , Figure 9 and Figure 10 , where we see excellent agreement between the computed and measured crack growth histories. Table 3 reveals that the value of Δ K thr associated with these nine cracks falls within the range 0.0 to 0.3 that is commonly seen for small cracks in conventionally manufactured aluminium alloys [ 1 , 21 , 25 , 26 , 27 , 33 , 34 , 36 , 38 , 39 ]. In contrast, the Δ K thr range seen in [ 37 ] for the growth of long cracks in AA7075-T7351 aluminium alloy specimens tested under a FALSTAFF flight load spectrum was between (approximately) 1.1 and 1.79 MPa √m.…”

“…Here, D and n are material constants; K = K max − K min , where K max and K min are the maximum and minimum values of K in a cycle; K thr is the fatigue threshold; and A is the apparent cyclic fracture toughness. This particular crack growth equation was used as it has been shown [6,21,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] to be able to compute the growth of small and long cracks in a range of both conventionally and additively manufactured materials, and hence is considered to be a valid crack similitude parameter. In this context it should be noted that [23] highlighted that ∆κ is a valid similitude parameter for cracking in structural adhesive.…”

“…As per the requirement, delineated in JSSG206, to follow a "building block approach" the values of D, A, and n in Equation (1) were taken from prior studies [25]; namely, D = 1.86 × 10 −9 , A = 111 MPa √ m, and n = 2. As shown in [6,21,25,26,[39][40][41][42] the variability in the crack growth histories associated with durability analyses can be captured by allowing for variability in the local fatigue threshold (∆K thr ). The initial crack sizes (mm) and values of ∆K thr (MPa √ m) used in the crack growth analyses are given in Table 3, where we see that these sizes, which were taken from fractographic measurements, were significantly smaller than that the mandated minimum EIDS given in [10][11][12].…”

“…Jones et al [39] subsequently reported that the mean value of ∆K thr associated with the twenty cracks studied in [6] was approximately 0.80 MPa √ m, and that the standard deviation (σ) was approximately 0.24 MPa √ m. This gives a mean minus two standard deviations (B basis) value of ∆K thr for cracks that nucleate and grow from surface breaking pits of 0.08 MPa √ m. This value of ∆K thr is similar to the values given in Table 3 for the nine sub-surface interface cracks analysed in the present study.…”

“…It has recently been shown [ 6 ] that for the twenty cracks that nucleated and subsequently grew from the small sub-mm surface pits studied in [ 6 ] that had depths similar to the mandated [ 10 , 11 , 12 ] minimum equivalent initial damage size (EIDS) of 0.254 mm, the fatigue threshold Δ K thr lay in the range 0.35 to 1.2 MPa √m. However, as subsequently discussed in [ 39 ] a group of twenty cracks represents a limited data set, and as such may not necessarily capture the extent of the true variability in Δ K thr . To account for such limited data sets, [ 46 , 47 , 48 , 49 ] suggest the use of a statistical approach whereby the ‘A basis’ and ‘B basis’ properties are determined.…”