PSI - Issue 42

Lucas Carneiro Araujo et al. / Procedia Structural Integrity 42 (2022) 163–171 Lucas Carneiro Araujo/ Structural Integrity Procedia 00 (2019) 000 – 000 The plane to which the defect’s area is projected is the plane perpendicular to the maximum principal stress direction. In another analysis, Yanase and Endo [8] proposed that the fatigue limit under fully reversed torsion, , could be estimated by a similar type of relation: = 1.21( + 120) (√ ) 1/6 (12) These equations are relatively simple to use, since the value of √ is easily determined for surface defects of known geometry in the specimen. However, to determine the projected area of internal defects, such as non-metallic inclusions or pores, which can exist in large quantities and with different sizes and shapes, it is necessary to conduct a statistical analysis to determine what is the likely largest existing defect in function of the volume of material. This likely biggest defect is denominated, √ [15,21,22]. Defects are separated into surface, subsurface and internal. Considering the worst type of defect, the subsurface one, the adjustment factors 1.43 and 1.20 of Eqs. (11) and (12) are corrected to 1.41 and 1.19 and the calculation of fatigue limits is now obtained by Eqs. (13) and (14), for subsurface defects with estimated value of √ . = 1.41( + 120) (√ ) 1/6 (13) = 1.19( + 120) (√ ) 1/6 (14) 4. Experimental work 4.1. Material and fatigue specimens The experimental campaign was conducted with fatigue specimens made of AISI 4140 steel (DIN 42CrMo4), oil quenched and tempered at 600 °C, with mechanical properties reported in Table 1. The material was removed from crankshafts of thermoelectric generators that failed due to fatigue during operation. These specimens were designed according to ASTM E466-15 with a 10 mm diameter circular section. 4 Two types of specimens were used in this experimental campaign: (i) smooth cylindrical bar specimens, without artificially introduced surface defects and (ii) specimens containing a 550 μ m diameter and depth micro surface hole with a straight bottom.The machining of the micro hole was performed in a CNC XH7132, using a WC micro end mill coated with TiAlN whit 500 µm diameter for lower cutting forces and higher surface quality than mechanical drilling [23]. The cutting parameters were based on the literature [24,25] and improved by trials, resulting in a cutting velocity of 9,4 m/min, feed per tooth of 3,3 µm and a depth setting of the helical course of 25 µm. For higher quality and better surface roughness cutting fluid, Bio100e, were used to ensure that the cut occurred submerged or in a near submerged method to increase the specimens integrity [26,27]. 4.2. Fatigue Tests Fatigue tests were performed under force control in accordance with ASTM E466-15 in an axial (100 kN capacity) and an axial-torsional ( 100 e 1100 ) servo-hydraulic testing machines, producing the following load stories. Table 1. Mechanical properties of AISI 4140 steel. Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) Vickers hardness (kgf/mm 2 ) 710 900 20 320

166