Issue34

E. Marcisz et alii, Frattura ed Integrità Strutturale, 34 (2015) 379-386; DOI: 10.3221/IGF-ESIS.34.42 381 rotation motion of vibrators generate vertical moves of lever which caused loading of specimens. The control and measuring sub-assembly records histories of stresses, strains, energy parameter and hysteresis loops [5]. Figure 3 : Fatigue test stand MZGS-100Ph: 1 – specimen, 2 – clamp, 3 – grip, 4 – bending lever, 5 – rod, 6 – vibrator discs, 7 – springs, 8 – flat springs, 9 – machine base, 10 – engine, 11 – toothed belts. In the closed-loop control system of test stand MZGS-100Ph the amplitude W a of a new energy parameter for determination of fatigue characteristic under cyclic bending was used. The history of normal strain energy density parameter W(t) is defined as [6]             0, 5 pl i i i W t t t (1) where  i pl =  (t i ) for  (t i ) = 0 and i = 1, 2, 3,.... In Eq. (1)  i pl is the plastic strain registered in the moment t i , when the stress  (t i ) is equal to zero, and remains constant to the moment t i+1 when the stress reaches zero again, i.e.  (t i+1 ) = 0. Then the new registered value of plastic strain  i+1 pl replaces the previous one  i pl . This procedure is repeated for each cycle of bending. A sample trajectory of energy parameter is shown in Fig. 4. Fig. 4a presents energy parameter course according to the Smith-Watson-Topper relationship [7], and Fig. 4b the course of modified energy parameter. Fig. 4 allows observing that parameter W aSWT applies to positive values only, while the parameter defined by formula (1) applies both to positive and negative values. Figure 4 : Comparison of energy parameter courses: a) according to Smith-Watson-Topper relationship, b) according to modified energy parameter.