Monotonic Tension, Strain Controlled Fatigue and Fracture Toughness Properties of a Ductile Iron 2003-01-0832

The objective of this work is to test and develop monotonic tensile properties and strain controlled fatigue properties of a cast ductile iron. The test data and the related material constants will be used in conjunction with vehicle loading data to perform finite element stress-strain analysis and fatigue life prediction analysis to aid in the design of automotive components made from ductile iron. Currently, such material property data does not exist in the literature for this particular grade of ductile iron.

Monotonic tension and fully reversed strain controlled fatigue tests were conducted by following ASTM E-8, ASTM E-606, and SAE J-1099 on samples machined from the cast ductile iron. Monotonic tensile properties were obtained, including Young's modulus, yield strength, ultimate tensile strength, elongation, reduction in area, strength coefficient K, and strain hardening exponent n. Fully reversed strain controlled fatigue properties were also obtained, including cyclic strength coefficient K′, cyclic strain hardening exponent n′, Young's Modulus as a function of fatigue cycles, mean stress as a function of cycles, fatigue strength coefficient, fatigue strength exponent, fatigue ductility coefficient, and fatigue ductility exponent. Four repetitive tests were conducted at each strain level. The ductile iron showed cyclic hardening behavior. Fatigue life scatter was about a factor of two for the five strain amplitude levels tested.

Chemistry, microstructure, hardness, nodularity and nodule count was characterized for the ductile iron. Finally fractography analysis was done using a scanning electron microscope which revealed the fatigue region and transgranular cleavage fracture region on the fracture surface of tested samples. Fracture toughness of the ductile iron was estimated using the stress intensity factor equation of a surface crack in a solid round bar under tension and based on the fatigue crack depth at fast fracture and the maximum stress applied.

In conclusion, monotonic tension and strain controlled fatigue tests were successfully performed. The fatigue performance of the material was characterized, and the material's fatigue properties will be used in durability design and fatigue life prediction.