The paper presents a thermographic method of accelerated determination of theS-Ncurve. In the presented method, theS-Ncurve was developed based on energy-related parameter with the assumption of its dependency on the stress amplitude. The tests made on C45 steel and X5CrNi18-10 steel under reversed bending revealed that theS-Ncurve obtained by accelerated thermographic method fits inside the 95% confidence interval for theS-Ncurve obtained from the full test.

This paper presents an attempt to generalize the description of the course of specimen temperature changes during the tensile test and to connect the value of the thermoelasto-plastic limit stress with the value of a clear (physical) or proof strength (offset yield strength) on the example of tests of the following aluminum alloy sheets used in Poland for airplane structures: 2024-T3 and D16 in three grades: D16ATV, D16CzATV, and D16UTV. A thermographic camera was used for specimen surface temperature measurement during the tensile test. The Portevine–Le Chatelier effect (the so-called PLC effect) was observed for tests of specimens cut from sheet plates, which was strongly reflected in the temperature fluctuations. The course of temperature change during tensile tests was divided into four characteristic stages related to the occurrence of a clear or offset yield strength. It was found that if there is a clear yield strength, the value of the thermoelasto-plastic limit stress was greater than this yield strength. If there was an offset yield strength, the value of the thermoelasto-plastic limit stress was lower than this yield strength. The differences in the aforementioned values of individual yield strengths did not exceed several percent. Thus, it can be concluded that the thermoelasto-plastic limit stress determined on the basis of the course of specimen temperature changes during the tensile test is well correlated with the value of the yield strength of the material.

A new thermographic method that enables simultaneous accelerated determination of the fatigue limit and the S-N curve was presented in this paper. The fatigue limit determination method was based on a constant rate of temperature rise occurring in second phase of the specimen fatigue life. The S-N curve determination method was based on energetic parameter with assumption of its dependency on the stress amplitude. The tests made on X5CrNi18-10 steel under reversed bending revealed that the fatigue limit value obtained from accelerated thermographic tests as compared to the value obtained from full test differs by 5.0 %. The S-N curve obtained by accelerated thermographic method fits inside 95 % confidence interval for the S-N curve obtained from full test.

Selected results of experimental research showing possibilities to combined accelerated methods, i.e. thermographic determination of the fatigue curve and estimating the fatigue limit, were presented in this paper. The tests were performed using the reversed bending fatigue-testing machine with a rotational frequency of 77 Hz. The main item of the test station was the thermographic camera. Test specimens were made of C45 (1.0503) steel drawn bars. The full tests results were presented for the fatigue S-N curve (23 specimens at 10 load levels) and fatigue limit by the Staircase method (25 specimens) based on 10⁷ cycles to failure was determination. The accelerated test for determining the slope of the S-N curve and the fatigue limit were made by a single specimen based on gradually increasing loading. Methods previously presented in the literature are based on the phenomenon of temperature stabilization during most of the fatigue process. The results presented in this work did not show the occurrence of such a phenomenon. For that reason, the new approach to the determination of the slope of S-N curve was proposed. Combined application of both experimental methods, Locati and thermographic, with a new approach to the determination of this slope might allow one to develop automatic equipment for the accelerated determination of the fatigue limit which would contribute to significant reduction of the test costs and time, thus increasing availability of such tests.