Entropy production during the fatigue process can serve as a measure of degradation. We postulate that the thermodynamic entropy of metals undergoing repeated cyclic load reaching the point of fracture is a constant, independent of geometry, load and frequency. That is, the necessary and sufficient condition for the final fracture of a metal undergoing fatigue load corresponds to a constant irreversible entropy gain. To examine validity, we present the results of an extensive set of both experimental tests and analytical predictions that involve bending, torsion and tension-compression of aluminium 6061-T6 and stainless steel 304 specimens. The concept of tallying up the entropy generation has application in determining the fatigue life of components undergoing cyclic bending, torsion and tension-compression.

On the thermodynamic entropy of fatigue fracture

Rapid determination of fatigue failure based on temperature evolution: Fully reversed bending load

High-speed, high-resolution infrared thermography, as a noncontact, full-field, and nondestructive technique, was used to study the temperature variations of a cobalt-based ULTIMET alloy subjected to high-cycle fatigue. During each fatigue cycle, the temperature oscillations, which were due to the thermal-elastic-plastic effects, were observed and related to stress-strain analyses. A constitutive model was developed for predicting the thermal and mechanical responses of the ULTIMET alloy subjected to cyclic deformation. The model was constructed in light of internal-state variables, which were developed to characterize the inelastic strain of the material during cyclic loading. The predicted stress-strain and temperature responses were found to be in good agreement with the experimental results. In addition, the change of temperature during fatigue was employed to reveal the accumulation of fatigue damage, and the measured temperature was utilized as an index for fatigue-life prediction.

Characterization of the temperature evolution during high-cycle fatigue of the ULTIMET superalloy : Experiment and theoretical modeling

Fatigue failure of metals undergoing cyclic load is evaluated based on the evolution of surface temperature Aluminum Alloy 6061 and Stainless Steel 304 are selected as testing materials and specimens are subjected to completely reversed torsion load A thermographic technique is used to measure the temperature increase of the specimen due to hysteresis heating during the fatigue testing Experimental results indicate that the initial rate of temperature rise as a function of time can be utilized as an index for prediction of fatigue life An empirical correlation of the form N f = c 1 R θ c 2 with constants c1 and c2 is derived that relates the rate of temperature rise, Rθ, at the beginning of the test to the number of cycles to failure, Nf It is shown that c1 is dependent upon the material properties and stress state whilst c2 is a constant Experimental results are consolidated into a single curve which gives the time to failure as a function of initial slope of temperature rise, thereby enabling fast prediction of fatigue failure

Life prediction of metals undergoing fatigue load based on temperature evolution

The energy dissipated to the surroundings as heat in a unit volume of material per cycle, Q, was recently proposed as fatigue damage index, and it was successfully applied to rationalise fatigue data obtained by carrying out stress-controlled and strain-controlled fatigue tests on AISI 304 L stainless steel plain and hole specimens. In this paper, it is shown that the Q parameter is independent on thermal and mechanical boundary conditions occurring during experiments. After that, additional stress-controlled fatigue tests on plain and notched specimens characterised by smaller notch tip radii than those tested previously have been performed. Present data have been compared with previous ones, and it was found that all available results can be synthesised in terms of the energy parameter Q into a unique scatter band, independently on the testing conditions (stress-controlled or strain-controlled) and on the specimens' geometry (plain or notched). About 100 data were included in the statistical analysis to characterise the energy-based scatter band of the material. Finally, some limitations of applicability of the experimental technique adopted in the present paper are discussed.

A synthesis of the push‐pull fatigue behaviour of plain and notched stainless steel specimens by using the specific heat loss

A method for determining the heat energy of the fatigue process in metals under uniaxial stress: Part 1. Determination of the amount of heat liberated from a fatigue-tested specimen

A method for determining the heat energy of the fatigue process in metals under uniaxial stress: Part 2. Measurement of the temperature of a fatigue specimen by means of thermovision camera-computer system

R.S. Botny

Papers

A method for determining the heat energy of the fatigue process in metals under uniaxial stress: Part 1. Determination of the amount of heat liberated from a fatigue-tested specimen

A method for determining the heat energy of the fatigue process in metals under uniaxial stress: Part 2. Measurement of the temperature of a fatigue specimen by means of thermovision camera-computer system