Properties of PH 13-8 Mo Steel for Fatigue Application in Helicopters

Abstract

While many fixed-wing aircraft have adopted damage-tolerant design in recent years, helicopter design is still based predominantly on a safe life approach, in which relatively simple Stress Life (S-N) data underpins the tools used for life prediction. Due to their unique loading, helicopter structures experience a high number of loading cycles as compared to fixed-wing aircraft, and this presents a more challenging fatigue life management problem. To minimise the fatigue damage, the helicopter community tends to design components such that most of the loading experienced falls below the fatigue limit of the selected material. These materials are usually of high strength and have good fatigue properties, although the large number of cycles experienced by some components raises the possibility of fatigue in the “gigacycle” regime where the fatigue limit drops to a new, lower level. This paper discusses the suitability of a high-quality PH 13-8 Mo steel for critical helicopter usage, using a simulated application in Australian service to evaluate its fatigue performance particularly at high R ratio and other properties such as density, corrosion properties and cost in terms of the operational environment experienced in Australian helicopter operations.