PSI- Issue 9

Yu. Matvienko et al. / Procedia Structural Integrity 9 (2018) 16–21 Author name / Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Present innovations in aircraft manufacture include two advanced technologies. The first of them is the cold expansion process that leads to enhance the fatigue life of structures with fastener holes as it shown by Reid (2014). The second technology resides in the creation of integral structures via welding, replacing traditional riveting techniques. This approach, for instance, is described by Schmidt (2005). The application of advanced welding technologies in metallic aircraft structures can potentially reduce the cost and the weight of the final product. An inherent feature of both mentioned technologies is the availability of residual stress fields, which significantly influences the fatigue life of the structure. This influence is positive for cold expanded holes and mostly negative for welded structures. Thus, damage resistance and damage tolerance of structures under residual stress influence are a major issue for the aircraft industry. The comprehensive overview of residual stress influence on life-time of both cold-expanded holes and welded joints is presented by McLung (2007). 2. Cold-expanded holes Experimental investigations are performed for aluminum plates of dimensions 180×30×5 mm 3 , each of which includes centred open hole of nominal diameter 2 0 r = 4.00 mm. Whole set of specimens consists of 8 units. All coupons are manufactured from a single material bar of 2024 aluminum alloy by the same manufacturing process. Annealing treatment of the material is unknown. Absence of residual stresses in all specimens is established by combining the hole drilling method and ESPI measurements of hole diameter increments in principal stress directions. Mechanical properties (elasticity modulus E = 74,000 MPa, yield stress y σ = 330 MPa and Poisson’s ratio μ = 0.33) are determined by tensile tests. A tapered, cylindrical mandrel for the cold-worked process is adopted. The mandrel taper is 1:25 permitting a quite gradual application of loading pressure. Before forcing the hole, the mandrel external surface and internal hole surface are oiled to reduce the friction between the contact surfaces. The forcing load has been applied by using a materials testing machine with a speed equal to 1 mm/min. A split sleeve is not introduced because of a small plane hole diameter. The expansion level is 5% of nominal interference, defined as the ratio of the interference value to the hole radius. Numerical results, obtained by finite element method, evidence that the degree of interference from 4% to 6% leads to the arising circumferential residual stress  res = – (250÷300) MPa at the expanded hole boundary. The exact value of  res depends on the yield limit of used aluminum alloy. It is shown in many works that residual stresses in the inlet surface are always lower than those measured in the outlet surface. This means that fatigue crack always originates from the hole edge corresponding to the inlet face where the lower compression residual stresses arise due to cold expansion. That is why optical interferometric measurements of the local deformation response to small notch length increment are performed in the mandrel entrance (inlet) surface of specimens. To perform measurement procedure, specimens of both types are subjected to uniform uniaxial tension by electro-mechanical testing machine. All experimentally obtained parameters correspond to remote tension  = 80.0 MPa. Drawing of each specimen with loading conditions, co-ordinate system and notation for in-plane displacement components is shown in Figure 1.

Fig. 1. Drawing of the specimen with loading conditions, co-ordinate system and notation involved.