Equivalent Constant Amplitude Concepts Examined Under Fatigue Crack Propagation by Block Loading

Abstract

Abstrad-Fatigue crack growth behaviour under block loading sequences has been studied on the aluminium alloy 2024 T351, using four different equivalent constant amplitude concepts. The root mean squared method gives acceptable results only for relatively long blocks. The equivalent method based on the Pans law (modified to take into account crack closure) gives good predictions for the observed growth rates as does Elbers' method. Finally, a new method based on energy considerations gives excellent results for the studied test conditions. NOMENCLATURE a = crack length BL = block loading da/dN =crack growth rate i = cycle number ECA =equivalent constant amplitude K = stress intensity factor KO = stress intensity factor at crack opening Kmus = maximum stress intensity factor in a block K,, K-= maximum and minimum K levels in a cycle m, rn' = slopes of Pans' law and Elbers' relation respectively N = number of cycles P = load Nq = number of equivalent cycles P , , = maximum load in a block P , , Pd =maximum and minimum loads at cycle i P = crack opening load B = effective mean level Q = energy dissipated per cycle Qi =energy dissipated at cycle i in a block Q,-, = enclosed energy RMS = root mean squared R = load ratio %p = equivalent R ratio R, = load ratio corresponding to cycle i in a block LIE = Elbers' efficiency factor VA = variable amplitude a = specimen compliance AK = stress intensity factor range Qm, = total energy dissipated per block AKem = effective AK 6 =crack mouth opening displacement 998 N. RANGANATHAN AND J. R. DESFORGES

A Dissipated Energy Approach to Fatigue Crack Growth in Ductile Solids and Layered Materials

“…A critical plastic dissipation criterion for fatigue crack extension in ductile solids was first suggested by Rice [4]. Dissipated energy approaches to fatigue crack growth prediction have since been the subject of numerous analytical [5][6][7][8][9][10][11][12][13][14][15] and experimental [16][17][18][19][20][21][22][23][24] investigations. The current approach considers the total plastic dissipation per cycle occurring throughout the reversed plastic zone ahead of the crack, which is a quantity of both theoretical and practical interest.…”

“…Moreover, as opposed to the crack tip stresses and strains, the total plastic dissipation per cycle is a bounded quantity, which allows for straightforward interpretation of numerical results. Finally, numerical results for the total plastic dissipation per cycle can be directly compared to measurements of dissipated energy during fatigue crack growth, which have been reported in the literature by a number of researchers [16][17][18][19][20][21][22][23][24]26].…”

Equivalent constant-amplitude fatigue load method based on the energy equivalence principle

“…As in the RMS method, the number of equivalent cycles is equal to that of cycles in random loading. Distinct from the above methods, an energy-based method was developed by Ranganathan and Desforges (1996) and Ranganathan et al (1992). In the method, the equivalent load amplitude takes the maximum load amplitude during random loading, and the equivalent load ratio is determined by the dissipated energy.…”