Fatigue Precracking Time Estimates for Three-Point Bending Specimens
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TECHNICAL ARTICLE—PEER-REVIEWED
Fatigue Precracking Time Estimates for Three-Point Bending Specimens A. Alipour Ghasabi
. A. Motameni . S. Kadioglu
Submitted: 11 June 2019 / Accepted: 13 August 2019 ASM International 2019
Abstract Specimens containing sharp cracks are needed in certain types of mechanical tests, first and foremost for fracture toughness measurement of materials. Their use, however, is not just limited to this type of test. Another category of experiments deals with characterizing nonlinear vibrations of beams containing breathing cracks. To produce cracked beam specimens, fatigue cracks can be grown ahead of sharp notches under controlled loading. ASTM E399 and ASTM E1820 standards provide guidance on such procedures for preparation of fracture toughness test specimens. However, certain issues which might become important in testing of vibrations of cracked beams, such as the time required for specimen preparation is not addressed in these standards. In this article, both low cycle fatigue methods and linear elastic fracture mechanics methods are used to estimate the number of loading cycles required to have a crack of desired length at the notch tip. Calculation results are compared with experimental ones, and the effects of various factors influencing the required number of loading cycles for a certain crack size are discussed. Keywords Crack initiation Crack growth Residual stress Three-point bending
A. Alipour Ghasabi (&) A. Motameni S. Kadioglu Mechanical Engineering Department, Middle East Technical University, 06800 Ankara, Turkey e-mail: [email protected]
Introduction Fatigue, which could be described as failure under cyclic loadings due to initiation and first stable and then unstable propagation of a crack eventually leading to fracture, is a major mechanical failure phenomenon. According to various references [1, 2], fatigue is estimated to cause 50 to 90% of all mechanical failures. It is therefore tried to be avoided at all costs through a range of broad engineering activities such as extensive material testing, applying appropriate design procedures, careful inspection and continuous monitoring of critical members. In contrast to all these efforts to avoid fatigue, a noteworthy exception where a fatigue crack is wanted in a short time is the production of test specimens containing sharp fatigue cracks. Such specimens are needed for example, for fracture toughness tests. In order to produce cracked specimens, fatigue cracks are grown at the tip of sharp notches under controlled loading. This procedure is called fatigue precracking, and ASTM E399 and ASTM E1820 standards provide guidance on such procedures for preparation of various types of fracture toughness test specimens. Another category of experiments, where cracked beam specimens are needed, deals with characterizing nonlinear vibrations of beams containing breathing cracks. Breathing refers to the opening and closing of the crack during vibrations. The reduction and variability (due to opening and closing) of
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