Fatigue behaviour of unitized structures compared to built-up structures

In the last couple of decades, the aerospace industry has been attracted by unitized structures for their potential of economic advantage in terms of low production cost and fuel efficiency from weight reduction due to a reduced number of parts and joints. In recent years, advanced manufacturing technologies, such as additive manufacturing processes and electron beam freeform fabrication, have boosted the feasibility of unitized structure manufacturing. As a result, unitized structures are increasingly becoming common in airframes as they displace conventional ubiquitous built-up structures. One concern with unitized structures is that they may not show the same resistance to fatigue as built-up structures in terms of crack growth rates. For a fatigue crack to cause failure of a built-up structure, it faces the problem of having to jump across the gaps between the different parts of the structure. Those gaps represent natural barriers that slow down crack-growth. However, a unitized structure will likely to have less stress raisers (e.g., fewer holes for fasteners). Hence, experimental as well as numerical analysis approaches are required to determine how a unitized structure compares to an equivalent built-up structure in terms of fatigue crack growth. This paper addresses finite element analysis of modelled unitized structure and equivalent built-up structure under amplitude loading for fatigue crack growth behavior