Correlations of microstructure with dynamic and quasi-static fracture in a plain carbon steel

Abstract

An investigation was conducted into the effects of temperature, loading rate, and various micro-structural parameters on the initiation of plane strain fracture of a plain carbon AISI 1020 steel. Ferrite and prior austenite grain sizes were chosen as the principal microstructural features to be in-vestigated. The microstructural variations were accomplished by changing the austenitizing tempera-ture and by altering the cooling rate during normalization. Fracture toughness tests were conducted using precracked notched round bars loaded in tension to produce two stress intensity rates,viz.,K ₁ = 1 MPa √m s^-1 andK ₁ = 2 × 10⁶ MPa √m s^-1. In addition, Charpy impact tests along with quasistatic and high rate plasticity tests were conducted. The plasticity tests were done in torsion at shear strain rates of$\overline \gamma = 5.0 \times 10^{ - 4} s^{ - 1 } and \overline \gamma = 1.5 \times 10^{3 } s^{ - 1}$. Testing temperatures covered the range from -150 °C to 150 °C which encompassed fracture initiation modes involving transgranular cleavage to fully ductile fracture. Micromechanical processes involved in void and cleavage micro-crack formation were identified and quantified. For these purposes notched round tensile tests and subsequent metallographic observations along with TEM and SEM observations of the plane strain fracture toughness specimens were performed. The experimental results and quantitative micro-modeling using simple fracture models provide a means of correlating both quasistatic and dynamic fracture toughness with microstructures.