PSI - Issue 42

Martin Matušů et al. / Procedia Structural Integrity 42 (2022) 102 – 109 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The Kohout- Věchet (K&V) formula by Kohout and Věchet (2001) was used for modelling the S-N curve:



N B A C N C  

+  +  

(1)

=   



This approximation well describes the transition into the quasistatic domain and to the fatigue limit (FL) domain; see Fig. 2a. The specimens of the A01 series show a longer lifespan in the area of low-cycle fatigue (LCF), both compared to the short specimen counterpart (A03 series) and compared to the dog bone series (A02 and A04). The S N curves of series A01 and A03 follow the same trend in the region of 100,000 cycles; then the curves of all series A01-04 converge, and only minor differences in lifespan can be observed in the region of high cycle fatigue (HCF). Conventional fatigue limits could be determined for series A01, A02, A03, and A04 as 500MPa, 470MPa, 490MPa and 480MPa, respectively. These fatigue limit values are mainly based on an evaluation that includes run-out specimens as not enough specimens were tested to estimate them properly, especially for the dog-bone specimens. For A01 and A03 the transition to the fatigue limit domain is obvious from the K&V model, but  FL of A02 and A04 are potentially lower since the transition to the FL domain is not supported by more data points.

Fig. 2. a) Experimental S-N curve results and a Kohout- Věchet curve for series A01-A04 with run-out specimens. b) Thermal image from FLIR A315 showing both the loaded and the referential unloaded specimens and the temperature range reached.

For temperature measurements, infrared thermography was used. The FLIR A315 thermal camera was used for research purposes with a NETD (thermal sensitivity) of 50 mK and with a resolution of 320x240 pixels. The surface of the specimen was coated with the high emissivity LabIR black paint coat (HERP-LT-MWIR-BK-11) to increase the precision of the temperature measurement. The specimens were not externally cooled throughout the testing campaign. As an additional precaution, an unloaded reference specimen (see Fig. 2b) was placed in the camera frame to eliminate disturbances and sudden temperature changes in the surroundings of the loaded specimen. 3. Temperature as a fatigue parameter 3.1. Self-heating effect Temperature measurement during cyclic loading documents a self-heating effect. This effect occurs naturally according to the first law of thermodynamics. The work put into the specimen causes a given amount of stress in the active cross section, also inducing internal changes and heat generation. The temperature measured on the surface of a specimen is the sum of internal and heat energy. The internal energy is a measure of the changes in the inner structure of a specimen during cyclic loading.