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FATIGUE TESTS
Introduction to Fatigue of Structures and Materials
Fatigue failures in metallic structures are a well-known technical problem. Already in the 19th century several serious fatigue failures were reported and the first laboratory investigations were carried out. Noteworthy research on fatigue was done by August Wöhler. He recognized that a single load application, far below thestatic strength of a structure, did not do any damage to the structure. But if the same load was repeated many times it could induce a complete failure. In the 19th century fatigue was thought to be a mysterious phenomenon in the material because fatigue damage could not be seen. Failure apparently occurred without any previous warning. In the 20th century, we have learned that repeated loadapplications can start a fatigue mechanism in the material leading to nucleation of a small crack, followed by crack growth, and ultimately to complete failure. The history of engineering structures until now has been marked by numerous fatigue failures of machinery, moving vehicles, welded structures, aircraft, etc. From time to time such failures have caused catastrophic accidents, such as an explosionof a pressure vessel, a collapse of a bridge, or another complete failure of a large structure. Many fatigue problems did not reach the headlines of the news papers but the economic impact of non-catastrophic fatigue failures has been tremendous. Fatigue of structures is now generally recognized as a significant problem. The history of fatigue covering a time span from 1837 to 1994 was reviewed inan extensive paper by Walter Schütz. Historical milestone papers were collected by Hanewinkel and Zenner and Sanfor. John Mann compiled 21075 literature sources on fatigue problems covering the period from 1838 to 1969 in four books. Since that time the number of publications on fatigue has still considerably increased and it may be estimated to be around 100,000 in the year 2000. Fortunately,consulting the literature on specific topics can now be done with computerized literature retrieval systems. As a result of extensive research and practical experience, much knowledge has been gained about fatigue of structures and the fatigue mechanism in the material. Qualitatively our understanding of fatigue problems is fairly well developed in the 20th century as discussed in a survey paper bythe author (this paper is copied on the CD attached to this book). Much has been learned from laboratory research. However, accident investigations have also highly contributed to the present state of the art. Fatigue failures in service can be most instructive and provide convincing evidence that fatigue may be a serious problem. The analysis of failures often reveals various weaknessescontributing to an insufficient fatigue resistance of a structure. This will be illustrated here by a case history. The front wheel of a heavy motorcycle completely collapsed. Ten spokes of the light alloy casting were broken. Examination of the failure surfaces indicated that fatigue cracks occurred in all spokes. Why was the fatigue life of this wheel insufficient? A first question of a failure analysismust be: Was the failure a symptomatic failure or was it an incidental case? If it is a symptomatic failure, all motorcycles of the same type are in danger and immediate action is required. However, the failure may be an incidental case for some special reason applicable to that single motorcycle only: for instance, unusual and severe damage of the material surface. In the case of this motorcycle,the same failure had occurred in several wheels in different countries, although predominantly in motorcycles of the police. The wheel shown in Figure collapsed when a policeman suddenly had to use the brakes to stop before a railway crossing.

He survived after some heavy shocks. The two most common questions usually put forward after a fatigue failure is: (i) was the fatigue resistance of...