Issue 30

M. N. James, Frattura ed Integrità Strutturale, 30 (2014) 293-303; DOI: 10.3221/IGF-ESIS.30.36 295 ‘tired’ of carrying the load and became crystalline in nature) versus the correct progressive fissuring mechanism proposed by Rankine [2]. The notion of the material having ‘tired’ of carrying the load led to the term ‘fatigue’ being coined to describe failures under cyclic loading. The Charpy notched bar impact test was subsequently developed as a simple means of assessing the notched toughness of steels . Figure 1 : Fatigue data reported by Wöhler in 1870 [4] from S-N tests on notched and unnotched iron specimens. Whilst the crystallinity theory was completely discredited by 1903 (through microscopic observations of crack initiation and growth via ‘persistent’ slip band cracking made by Humphreys and Ewing [6]) the term ‘fatigue’ has remained in use to the present day to describe the process of crack initiation and/or growth under cyclic loading. Figure 2 : Micrograph of puddled steel circa 1880 showing sharp elongated MnS inclusions. The two phenomena of fatigue and fracture remain of prime concern in ensuring structural integrity and reliability, even though the last 150 years have seen an enormous world-wide effort devoted to understanding, explaining and predicting failure from crack growth and fracture. A strong focus on fracture-safe and fatigue-reliable design is required by the sophistication and complexity of modern engineering components and structures, and their increasingly arduous service requirements. The term ‘fracture-safe and fatigue-reliable design’ was apparently first used by Pellini in guidelines intended for steel structures [7].