Chapter 11 Westland Lynx Rotor Hub (1998): Progression Marking LEFM Analysis 11.1 Introduction In November 1998 a Westland Lynx helicopter, operated by the Royal Netherlands Navy (RNLN), lost a rotor blade and then the rotor head just before take-off from the RNLN base at Den Helder, the Netherlands. Figure 11.1a shows the helicopter at an earlier date with an indication of the failure location. The failure was caused by fatigue and fracture of the so-called yellow arm of the rotor hub. Figure 11.1b shows the outboard part of the fracture, which was across an elliptical plane section. The rotor hub, with GKN Westland (GKNW) designation M323, failed after 3591.9 service hours, well below the overall design safe retirement life of 5000 h minimum and 8600 h at the failure location [144]. The yellow arm was one of four arms of the rotor hub, which was a monolithic Ti-6Al-4V forging (α+β) processed and heat-treated to obtain a microstructure con- sisting of primary α and transformed β [119, 145]. An extensive investigation found no evidence for failure due to poor material processing (microstructural anomalies) and properties. Also, the hub final machining, surface finishing (shot peening) and dimensions were within the specifications [119, 145], and no service damage was found. Attention then concentrated on fractography of the failure location in the yellow arm, followed by similar fractography of fatigue fracture surfaces from (i) a full-scale, 4-stress-level variable amplitude (VA) test on another arm, with hub designation M6; and (ii) representatively shot-peened specimens made from the M326 yellow arm and tested under fully reversed stressing (R −1) by GKNW. Finally, the fractographic and microstructural results and test stress levels were used to estimate the fatigue stress level(s) responsible for the service failure. © The Author(s), under exclusive licence to Springer Nature B.V., part of Springer Nature 2019 R. Wanhill et al., Fatigue Crack Growth Failure and Lifing Analyses for Metallic Aircraft Structures and Components, SpringerBriefs in Applied Sciences and Technology, https://doi.org/10.1007/978-94-024-1675-6_11 83