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Energy must be supplied to accomplish the processes which occur in the plastic zone ahead of the crack during fatigue crack extension. This energy has been measured (1) by using foil strain gages (Ikeda, Izumi, and Fine), (2) from electron-channeling patterns (Davidson and Lankford), (3) by stereoimaging (Davidson and Lankford), and (4) from calorimetric measurement of the heat generated during crack propagation (Gross and Weertman). The experimental results to date are presented and discussed. Both the hysteretic and permanent plastic work have been measured by strain pages. The permanent plastic work and the stored energy from cyclic plastic work are negligible. The work done numerically equals the heat evolved. The crack propagation rate data fit an equation dC/dN = AΔK4/μσ'yU, where A is a universal constant, µ is the shear modulus, σ'y is the cyclic yield stress, and U is the energy per unit area of fatigue crack. The latter is a function of ΔK except when the exponent in the Paris equation is 4.
fatigue crack propagation, energy, hysteretic plastic work, heat evolved
W. P. Murphy Professor of Materials Science and Engineering, Northwestern University, Evanston, Ill.
Staff Scientist, Southwest Research Institute, San Antonio, Tex.