PSI - Issue 7

Šulko Miroslav et al. / Procedia Structural Integrity 7 (2017) 262–267

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Šulko Miroslav et Al./ Structural Integrity Procedia 00 (2017) 000–000

material under investigation. The results of micro hardness and surface hardness development (average value from 8 indentations along the circumference of the cross-section) were plotted depending on the degree of fatigue damage calculated according by Palmgren-Miner formula

N n D =

i

(2)

f

where n i represented actual number of loading cycles performed on the specimen on the given loading level of the stress amplitude and N f represented the number of cycles to fatigue fracture for given loading amplitude. During the micro hardness tests was necessary to find the most suitable weight to invoke the force for obtaining indentation. Used weight was in the interval from 100g to 10g see (Fig. 2). The most suitable weight from the point of view of micro structure of the measured materials (size of grains) and the shape of specimens was 20g which was then used for all other measurements.

Fig.2 Development of Vickers micro hardness at the cyclically loaded steel for different used weights

For steel 24CrMoV55 were obtained dependency between development of hardness and the level of fatigue damage for amplitudes of alternating stress cycle of 696MPa (N f = 6200 cycles) and 610MPa (N f = 93000 cycles) - see (Fig. 2, 3)

Fig.3 Development of Vickers micro hardness HV0,02 and Brinell hardness HB1/30/10 depending on the degree of fatigue damage – steel 24CrMoV55