PSI - Issue 66

Grzegorz Glodek et al. / Procedia Structural Integrity 66 (2024) 331–336 Author name / Structural Integrity Procedia 00 (2025) 000 – 000

332 2

1. Introduction Fretting is a tribological phenomenon which occurs when two bodies in contact under pressure are subjected to relative oscillatory tangential motion with small displacement amplitude. It is characterized by highly localized stress concentrations at the contact edges which leads to surface damage and formation of micro cracks. When combined with cyclic bulk stresses, fretting fatigue occurs, where failure of structural components is induced by crack nucleation under fretting conditions [1]. One of the most safety-critical applications at risk due to fretting fatigue failure are turbine blades, where this damage occurs at the contact interface between the dovetail roots of the blades and the central rotating disc. Ruiz et. al. [2] were the first two investigate this issue, using a two-actuator fatigue set-up to accurately replicate the complex loading conditions occurring in real-life assembly. Their findings revealed that cracks always imitated at the edge o contact at constant angle, and that the depth of fretting damage was directly correlated with a reduction in fatigue lifetime. Subsequent studies on fretting fatigue in dovetail joints were conducted by Golden et.al. [3] and Conner [4], who introduced a modified test set-up, where a special fixture is used to hold replaceable fretting pads, representing the turbine central disc. In their approach, a specially designed fixture was used to hold replaceable fretting pads, simulating the turbine’s c entral disc, in contact with a sample representing the blade. Their results indicated that the dovetail angle had minimal impact on fretting fatigue life, but the application of coatings significantly improved the fretting fatigue performance. In recent years, there has been growing interest in producing turbine blades using additive manufacturing (AM), as it offers numerous advantages, including reduced material waste and greater design flexibility. Siemens has demonstrated the feasibility of this approach by manufacturing gas turbine blades from a nickel superalloy and successfully testing them under operational conditions [5]. However, before AM materials can be widely implemented in such critical applications, their safety must be rigorously validated. While significant research has focused on the fatigue properties of AM materials, there is limited information regarding their fretting fatigue performance. Lavella and Botto [6] investigated the fretting fatigue response of AM intermetallic Ti-48Al-2Cr-2Nb alloy at elevated temperatures, finding that the material's lifetime was highly sensitive to variations in the applied load. Unusually, cracks propagated in the gross slip regime, likely due to the material’s high crack propagation rate. Talemi [7] performed a series of finite element (FE) simulations to better understand the influence of subsurface porosities, introduced during the AM process, on fretting fatigue response. His results show that while the defects do not affect the contact stresses significantly, majority of the cracks initiated at the porosities rather than at the contact edges. This study contributes to a broader project investigating the fretting fatigue behavior of Ti-6Al-4V alloy produced using AM technique known as laser powder bed fusion (LPBF), focusing on both numerical and experimental analysis in a dovetail joint configuration test set-up. 2. Methodology 2.1. Experimental set-up Additively manufactured Ti-6Al-4V (AM-Ti64) dovetail geometry samples were produced using 3D systems ProX 320 using a layer height of 60 µm and linear energy density equal to 0.2 J/mm. The parts subsequently underwent heat treatment in Argon atmosphere at 850°C for 2 hours, followed by furnace cooling. The final dimensions were achieved through wire EDM cutting with a wire diameter of 0.15, resulting in an average surface roughness (Ra) of 0.9 µm. Tensile tests determined that the stiffness of the material is equal to 108.57 GPa, the yield stress is equal to 951 MPa and the strain at fracture lies in the range between 0.11 and 0.15 These properties conform to the ASTM E8 standard for Grade 5 Ti64 alloy [8]. Fretting pads are manufactured from wrought Ti64 using EDM and feature a cylindrical contact edge creating an incomplete contact interface. The assembled test setup is presented in Figure 1, where the stainless steel fixture, clamped at the top of the fatigue bench, holds the fretting pads. These pads are secured with bolts to prevent unwanted movement during testing. Fatigue tests were conducted on Zwick Roell HA100 servo hydraulic fatigue bench at load ratio R = 0.1 and frequency of 25 Hz. Application of the fatigue loading to the specimen generates cyclic, relative loads at the contact interface between the specimen and the pads, resulting in fretting fatigue conditions.