PSI - Issue 5

D. Rozumek et al. / Procedia Structural Integrity 5 (2017) 896–903 D. Rozumek et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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a)

b)

Fig. 1. Shape and dimensions of specimen for a) mode I+II, b) mode I+III, dimensions in mm.

2.2. Fatigue tests

Fatigue tests were performed in the low cycle fatigue (LCF) and high cycle fatigue regime (HCF) under the load ratio R = 0 and R = 0.1. The tests for mixed mode I+II were carried out on a MTS 809 servo-hydraulic axial/torsional test machine (Fig. 2a) (Lesiuk et al., 2016). All CTS specimens were pre-cracked under mode I condition, with the same procedure; the initial load was 7.5 kN (R = 0.1) and after 125000 cycles the load has been decreased to the level 5 kN (R = 0.1). After reaching the total number of cycles equal to 200000 the specimens were loaded with the desired mixed-mode loading condition ( θ = 30 and 45 degrees). During the experiments, the constant force amplitude was involved. The maximum value of load was kept on the level 7.5 kN (R = 0.1) with the frequency 5 Hz. The tests for mixed mode I+III were performed on the fatigue test stand MZGS-100 (Fig. 2b) enabling to carry out cyclically variable and static (mean) loading (Rozumek, Marciniak, 2012, Rozumek, B ański , 2012). Bending with torsion were tested for the ratio of torsional to bending moments M T (t) / M B (t) = tan  = 0.577, 1 (  = 30  , 45  ), which corresponded to the nominal amplitude of normal stress  a = 142.4, 174.5 MPa (  max = K t  a ) and the nominal amplitude of shear stress  a = 114.1, 80.7 MPa before the crack initiation. The theoretical stress concentration factor in the specimen under bending K t = 3.76, was estimated with use of the model by Thum et al. (1960). The tests were conducted under controlled force (in the considered case, the amplitude of total moment was controlled) with the loading frequency 29 Hz. The total moment was generated by force on the arm 0.2 m in length. Crack growth was observed on the lateral specimen surface with the optical method. The fatigue crack increments were measured with a micrometer located in the portable microscope with magnification of 25 times and accuracy 0.01 mm. Under bending with torsion, dimension “a” of the crack growth was defined as increments of length and angle of the crack measured on the specimen side surface.