An Energy-Based Axial Isothermal- Mechanical Fatigue Lifing Procedure

Abstract

An energy-based fatigue lifing procedure for the determination of full-life and critical-life of in-service structures subjected to axial isothermal-mechanical fatigue (IMF) has been developed. The foundation of this procedure is the energy-based axial room-temperature fatigue model, which states: the total strain energy density accumulated during both a monotonic fracture event and a fatigue process is the same material property. The energy-based axial IMF lifing framework is composed of the following entities: (1) the development of an axial IMF testing capability; (2) the creation of a testing procedure capable of assessing the strain energy accrued during both a monotonic fracture process and a fatigue process at various elevated temperatures; and (3), the incorporation of the effect of temperature into the axial fatigue lifing model. Both an axial IMF capability and a detailed testing procedure were created. The axial IMF capability was employed in conjunction with the monotonic fracture curve testing procedure to produce fifteen fracture curves at four operating temperatures. The strain energy densities for these fracture curves were compared, leading to the assumption of constant monotonic fracture energy at operating temperatures below the creep activation temperature.

A Modified Closed Form Energy Based Framework for Fatigue Life Assessment for Aluminum 6061-T6—Damaging Energy Approach

“…The framework can be easily extended to the cases considering the effects of mean stress [6], multiaxial stresses [17], torsional stress [8], and temperature [9] and in finite element mod eling for LCF life prediction [18,19]. From the obtained average plastic strain range and the average plastic energy, the cyclic parameters of the Ramberg-Osgood equation were computed.…”

“…The fatigue test is performed at the ideal frequency for optimum energy calcu lation to accurately capture the value of the fatigue toughness [10], There is another modification of the previous energy based model; the hysteresis stress-strain loop of a cycle is expressed by the Ramberg-Osgood constitutive relationship [12] rather than by the hysterically used hyperbolic function [1][2][3][4][5][6][7][8][9][10], The benefit of using the Ramberg-Osgood constitutive equation is that the stress range can be related to the fatigue life by a power law. The main modification of the previous framework is to take in to account the average dam aging energy instead of the damaging energy of any random cycle at particular stress range.…”

“…In order to measure the area, the image of the true stress-strain curve was processed by matlab image processing tools. The usage of the image processing tool enables to calculate the area of a hysteresis loop directly without incorporating the curve-fit method where the two parameters of the curve-fit equation need to be determined from the experimen tal data by a complicated optimization method [5][6][7][8][9][10]. 9, the proc essed image of the true stress-strain plot is shown where the gray region represents the area under the true stress-strain curve, and the black region indicates the rest of the image area of the plot.…”

“…The corresponding plastic energy history of aluminum 6061-T6 is shown in Fig. (8) and (9) was computed by dividing the area under the curve in Fig. The data in Figs.…”

“…12, the S-N curve obtained by the previous energy based framework[5][6][7][8][9][10] is also plotted and a little deviation is found to exist between the experimental a ) P la s tic s tr a in r a n g e h is to r y a nd (b) p la s t ic e n e r g y h is t o r y o f a lu m in u m 6 0 6 1 -T 6 d e v e lo p e d f r o m a f a t ig u e t e s t o f s tr e s s a m p lit u d e o f 8 3 % o f y ie ld s t r e n g t h . 8 T ru e s t r e s s -s t r a in c u r v e o f a lu m in u m 6 0 6 1 -T 6 F ig .…”