PSI - Issue 5

G. Lesiuk et al. / Procedia Structural Integrity 5 (2017) 904–911 Lesiuk et al./ Structural Integrity Procedia 00 (2017) 000 – 000

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where Y 1 is correction coefficients,  n is the nominal stress range, a 0 is notch length.

3. Experimental results

In presented approach, the authors focused only on the Paris regime. Therefore the dissipated energy in hysteresis loop area is considered as a main crack driving force. The determination of the energy dissipated in the unit volume of material is faced with many difficulties, particularly in the case of a high cycle fatigue. The hysteresis loop area is then very small and the algorithms should be of a great accuracy. This work has been realized with the help of a software package using the programming language Hewlett – Packard Agilent HP VEE version 5.0. The numerical algorithm is described in papers of Szata (2002) and developed by Lesiuk et al. (2017). For presentation the efficiency of the proposed model, the 41Cr4 steel (0.38%C, 0.69%Mn, 0.28%Si, 0.98%Cr, 0.029%P, 0.008%S) and 42CrMo4 (0.39%C, 0.64%Mn, 0.26%Si, 0.18%Mo, 0.92%Cr, 0.027%P, 0.004%S) steel have been tested. The 355J2W (<0.13%C, <0.4%Si, 0.2%-0.6%Mn, 0.4%Cr-0.8%Cr, <0.035%P, <0.03%S) steel has been also tested for  J energy representation of KFFD. The 42CrMo4 steel was in normalized state and 41Cr4 steel was heat treated (Q+T700°C). The chosen (for comparison) materials have similar static properties and the similarity of FCGR is also expected. In the Table 1 contains some monotonic quasi-static tension properties of the tested steel. For experiments the CT specimens were performed in accordance with the ASTM E647. During tests following signals were registered: force, displacements, crack opening displacement (COD). Amid applying of monotonically arising loading, the crack length size was determined by compliance procedures. The function of plane stress elastic compliance for CT specimens is described by formula (ASTM E647) = 0 + 1 + 2 2 + 3 3 + 4 4 + 5 5 , (18) where: − crack length; − specimen width (for tested CT specimen it is 50 mm). Coefficients C 0 , C 1 , C 2 , C 3 , C 4 , C 5 are fully described by ASTM E647 depending on measurement localization of COD. The u x quantity is defined as: = √ 1 + 1 (19) where: − specimen thickness [mm]; − elastic modulus [MPa]; − COD [mm]; − force [N]; ⁄ − displacement versus force curve slope measured during the test [mm/N]. The CT specimens (initial a/W =0.5, thickness t =10mm for 42CrMo4 steel and 18 mm for 41Cr4 steel) were cut from steel plate in rolling directions. The thru-straight notch was prepared using electro-discharging machine (EDM). Before the main investigation the fatigue pre-crack was made preserving all condition of loading described in ASTM E647. In case of constant amplitude test, the stress ratio R =0.1 and R =0.5 was by sinusoidal loading with frequency f =12.5 Hz ( F max =4.5 kN for thickness t =10mm) and ( F max =10kN for thickness t =18mm). The crack length size was monitored using elastic compliance (18) variations. Periodically, the crack length was controlled and adjusted using digitalized stereoscopic microscope with digital camera coupled with the tensile machine MTS 810 (Fig. 4a). During the experiments the dissipated energy was calculated in-situ in HPVEE environment. The computer screen during tests is shown in Fig. 4b. Apart of this, the LQSM (linear quadratic spline method) was used for calculation of the crack closure level for specimen ( R =0.1, 42Cr Mo4 steel). The details of this method and algorithm is described in the authors’ paper Lesiuk et al. (2017). The specimens subjected to bending had an external unilateral sharp notch 5 mm in depth, with the rounding radius ρ = 0.5 mm. The specimen dimensions were: length L =120 mm, width W =20 mm and thickness t =4 mm. The tests were performed on the fatigue test stand MZGS-100 with the loading frequency 29 Hz (Rozumek, Macha, 2009). Table 1. Monotonic quasi-static tension properties of the materials. Materials  y (MPa)  u (MPa) E (GPa) A 5 (%) HRC 42CrMo4 690 660 418 981 970 566 212 210 215 12 11 31 28 25 30 41Cr4 S355J2W