PSI - Issue 28

2

Rhys Jones et al. / Procedia Structural Integrity 28 (2020) 364–369 Rhys Jones/ Structural Integrity Procedia 00 (2019) 000–000

365

Nomenclature

total crack (delamination) length, measured from the loading line

a

number of fatigue cycles

N

rate of fatigue crack growth per cycle

da/dN

exponent in the Hartman-Schijve crack-growth equation intercept in the Hartman-Schijve crack-growth equation

n

D A

constant in the Hartman-Schijve equation

FCG

fatigue crack growth stress-intensity factor

K

maximum value of the applied stress-intensity factor in the fatigue cycle minimum value of the applied stress-intensity factor in the fatigue cycle range of the applied stress-intensity factor in the fatigue cycle, as defined below

K max K min

∆ K �� � � ��� � � ��� R

stress ratio (= F min / F max ) coefficient of determination

R 2

The certification requirements associated with such AM replacement parts are required to be consistent with the Joint Services Structural Guidelines JSSG2006 [3] and in the case of the US Air Force (USAF) MIL-STD-1530D [4]. For replacement parts meeting the durability requirements is a vital consideration [5]. The USAF requirements for AM parts are delineated in USAF Structures Bulletin EZ-19-01 [5]. As outlined in [1, 3-6], the airworthiness certification of AM replacement parts requires a durability analysis [4, 5], and the EIDS required for the associated durability analysis is small, i.e. sub mm [3-6]. Indeed, USAF Structures Bulletin EZ-19-01 [5] requires a minimum Equivalent Initial Damage Size (EIDS) of 0.254 mm (0.01 inch). Furthermore, as highlighted in the USAF F-15 program [7], a durability analysis necessitates the use of the associated small crack da/dN versus Δ K curve (A related statement is contained in with such Appendix X3 of the ASTM fatigue test standard E647-13a [8]). In this context, studies such as [1, 6, 9-11] have shown that crack growth in AM parts can often be represented by the Hartman-Schijve crack growth equation, viz: � � � � � ��Δ � � (1) where a is the crack length/depth, N is the number of cycles, D is a material constant, p is another material constant that is often approximately 2, and the crack driving force  κ is as suggested by Schwalbe [12]: Δ � � ��� �� ��� √���� ��� ��� � (2) Here, K is the stress intensity factor, K max and K min are the maximum and minimum values of stress intensity factor seen in the cycle, ∆ K = ( K max - K min ) is the range of the stress intensity factor that is seen in the cycle, ∆ K thr is the “effective fatigue threshold”, and A is the cyclic fracture toughness. As explained in [13], the terms ∆ K thr and A are best interpreted as parameters that are chosen so as to fit the measured da/dN versus ∆ K data. Illiopoulos et al. [9] suggested that, as per conventionally manufactured materials [12, 13], an upper bound on the small crack curve needed for a durability analysis of an AM part that had been stress-relieved could be obtained from