Additively manufactured Ti-6Al-4V replacement parts for military aircraft

Requirements and Variability Affecting the Durability of Bonded Joints

“…Applications to bridge and rail steels are given in [50][51][52], to polymers in [53] and to bonded wooden joints in [54]. Equation (1) has also been shown to be able to represent the growth of both long and small cracks in additively-manufactured materials [55][56][57], to laser additive deposition (LAD) and cold spray repairs to metallic structures [58,59]. A feature of this formulation is that the scatter in the crack growth curves can often be accounted for by allowing for the variability in the threshold term ∆K thr [22,42,44,49,56,57].…”

Further Studies into Crack Growth in Additively Manufactured Materials

“…This statement represents a significant development since, until recently, the focus has been on "low hanging fruit," i.e., non-critical parts not affecting the safety of flight. However, as suggested in [2] and is reflected in [1,3] AM offers the potential for the "on-demand" manufacturing of structural parts, albeit with a life that may be less than the original design life but sufficient to ensure continued operational capability until a conventionally manufactured replacement part can be obtained. Structures Bulletin EZ-SB-19-01 [4] subsequently noted that to achieve this goal requires the development of analytical methods that are capable of assessing the damage tolerance of AM parts.…”

Synchrotron radiation microcomputed tomography for assessing internal cracks in cold spray repairs

“…4,6,9 However, as noted in MIL-STD-1530D, 10 it is virtually impossible to specify and analytically model all the types and sizes of initial defects that exist in aircraft structures. 4,6,9 However, as noted in MIL-STD-1530D, 10 it is virtually impossible to specify and analytically model all the types and sizes of initial defects that exist in aircraft structures.…”

“…The paper by Yoshinaka et al, 1 which used synchrotron radiation microcomputed tomography to study the problem of crack growth from internal defects, adds further support to the seminal paper by Schijve, 2 which established that the reduction in the growth of interior cracks was due to the environment, ie, with interior cracks growing in a "vacuum-like" environment (a review of the effect of internal defects in steels and their role in very high cycle fatigue [VHCF] is given by Jeddi and Palin-Luc 3 ). Given that as can be seen in the papers by Yoshinaka et al, 1 Wu et al, 4 and Shiozawa et al, 5 synchrotron radiation microcomputed tomography can be used to study the growth of internal cracks in titanium and aluminium alloys and steels, the paper by Yoshinaka et al 1 has particular relevance to additively manufactured (AM) replacement parts 6 and to cold spray repairs to aircraft structures 7 where interior near-surface defects due to a localized lack of fusion can occur (see Figure 1). In this context, the memo 8 by the Under Secretary, Acquisition and Sustainment stated that, as of March 21, 2019, the DoD will use AM to "enable the transformation of maintenance operations and supply chains, increase logistics resiliency, and improve selfsustainment and readiness for DoD forces."…”

“…The interaction between surface-breaking and nearsurface (subsurface) material discontinuities, ie, porosity due to lack of fusion or keyholing, and the rough surfaces that are associated with both AM replacement parts and cold spray repairs has the potential to complicate the analysis needed for certification. 4,6,9 However, as noted in MIL-STD-1530D, 10 it is virtually impossible to specify and analytically model all the types and sizes of initial defects that exist in aircraft structures. To overcome this difficulty, the concept of a surrogate crack size, also referred to as an equivalent initial defect size (EIDS), that accounts for all of these crack sizes has been introduced both for conventionally manufactured parts 10 and for AM replacement parts.…”