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# Lecture20%20with%20slides - Fracture surface of a fatigue...

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1 1 Lecture 20 Fatigue 1 2 Fracture surface of a fatigue failure 3 Plasticity at crack tip is responsible for cyclic crack growth 4 Fatigue crack striations 5 Data from Paris, as copied from Dowling, “Mechanical behavior of materials” Fatigue crack growth data 6 Paris law for fatigue crack growth 1 2 3

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2 7 Example Center-cracked plate Cast iron: K Ic = 18 MPa m, σ Y =300 MPa da dN = A Δ K I () m A = 4.3x10 -8 m.(MPa m) -4 , m = 4 Initial crack length a o = 0.1 mm. σ min = 0, σ max =50MPa; frequency = 1 Hz Time until failure (in minutes)? 8 Example S g S g K I = Fa / b S g π a Analysis may be a bit awkward unless, F ( a / b ) is essentially constant over range of crack lengths. ..
VK250 Lecture 20 1 F ATIGUE 1 Fatigue failure: accumulation of damage and eventual failure under cyclic loading Old problem in engineering: 1828 mine hoists, mid-1800’s railway axles Very important problem in engineering (a major cause of failure) Must be considered if any possibility of oscillating loads E.g. , Engines Transportation (vibrations in road/rail/flight applications) Marine structures Periodic loading (under-carriage of aircraft, weapons, sports equipment) Two approaches for design against fatigue (i) Propagation of an existing crack (topic of this lecture) (ii) Empirical relationships for total life (including crack initiation) F ATIGUE CRACK GROWTH

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Lecture20%20with%20slides - Fracture surface of a fatigue...

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