Specimen size effect on fracture toughness of Pyrowear 53 steel for planetary gears

Abstract A fracture toughness is one of the critical parameters for Pyrowear 53 steel which is widely used in planetary gears of aircraft vehicles. Due to a limited volume of planetary gears, it is impossible to directly assess their plain strain fracture toughness K Ic . Therefore, the main aim of this study was to determine a formula correlating the plain strain fracture toughness K Ic obtained for laboratory compact tension (CT) specimens having a thickness of B = 50 mm with smaller specimens (B = 10 mm) which can be prepared directly from planetary gears. For this reason, different force-based approaches were utilized considering conditional fracture toughness (K Q ), size-insensitive conditional fracture toughness (K Qsi ) and specimen strength ratio (R sc ) values calculated according to ASTM E399 standard. Four different sizes of CT specimens were analyzed, i.e. B = 50, 25, 16 and 10 mm. The K IC value obtained for the B = 50 mm specimen was 101.7 ± 4.5 MPa m 1/2 , while for the smaller specimen sizes, the K Q value gradually decreased from 125.4 ± 10.0 MPa m 1/2 for B = 25 mm to 101.1 ± 3.0 and 86.6 ± 1.3 MPa m 1/2 for B = 16 and 10 mm, respectively. This trend was opposite to the commonly observed relation that the fracture toughness value increases for smaller specimen size with a higher plain stress region. The specimen size effect observed in the current work resulted from the methodology of determining the K Q value based on a force P Q defined at a point of 5% deviation from linearity of load-displacement curve. It has been shown that the fracture toughness of the Pyrowear 53 steel specimens exhibiting a rising R curve was more dependent on a ligament size.