Bidimensional numerical study of crack propagation on austempered ductile iron

Resumo

The austempered ductile iron (ADI) has a wide range of applications, due to its high mechanical
strength, fatigue resistance and wear resistance. ADI is composed by an ausferritic matrix with graphite nodules.
The nodule size and amount can be controlled by chemical composition and austenitizing temperature. The
nodules have strength lower than the matrix and can act as stress concentrators and influence on crack
propagation through changing its trajectory that may generate a protective effect. However, according to the
literature, the crack propagation mechanism in ADI is not yet fully understood. In this context, this work
presents a bidimensional numerical model that simulates crack propagation based on Paris law for ADI material
subjected to cyclic load. The model was implemented in Python language and uses the commercial finite element
code ABAQUS. The simulations showed that the presence of a nodule generates a shear load on the crack tip
which is the main parameter responsible for changing the crack direction to the nodule itself. Modifications such
as increasing nodule size and decreasing nodule distance to crack tip intensify this action. In simulations with
two different ADI materials with the same graphite area fraction, increasing the number of nodules causes the
crack to have a shorter lifetime until it intercepts a new nodule. Therefore, this suggests that the protective effect
of nodules in ADI material may be correlated with the number of intercepted nodules.