TAILIEUCHUNG - Mechanical Properties of Engineered Materials 2008 Part 15

Tham khảo tài liệu 'mechanical properties of engineered materials 2008 part 15', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 14 Fatigue of Materials INTRODUCTION Fatigue is the response of a material to cyclic loading by the initiation and propagation of cracks. Fatigue has been estimated to account for up to 8090 of mechanical failures in engineering structures and components Illston et al. 1979 . It is therefore not surprising that a considerable amount of research has been carried out to investigate the initiation and propagation of cracks by fatigue. A summary of prior work on fatigue can be found in a comprehensive text by Suresh 1999 . This chapter will therefore present only a general overview of the subject. The earliest work on fatigue was carried out in the middle of the 19th century following the advent of the industrial revolution. Albert 1838 conducted a series of tests on mining cables which were observed to fail after being subjected to loads that were below the design loads. However Wohler 1858-1871 was the first to carry out systematic investigations of fatigue. He showed that fatigue life was not determined by the maximum load but by the load range. Wohler proposed the use of S N curves of stress amplitude Sa Fig. or stress range AS Fig. versus the number of cycles to failure Nf for design against fatigue. Such data are still obtained from machines of the type shown in Fig. . He also identified a Copyright 2003 Marcel Dekker Inc. Time -------- - Í Ư Time----- - íờ Figure Basic definitions of stress parameters that are used in the characterization of fatigue cycles. From Callister 2000 reprinted with permission of John Wiley Sons. fatigue limit below which smooth specimens appeared to have an infinite fatigue life. Rankine 1843 of mechanical engineering fame the Rankine cycle noted the characteristic brittle appearance of material broken under repeated loading and suggested that this type of failure was due to recrys- Copyright 2003 Marcel Dekker Inc. Figure Schematic of rotating bending test machine. From Keyser 1973 reprinted with .

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