Influence Of The Irreversibility Of Interface Constitutive Relations On The Propagation Of Fatigue Cracks

Abstract

Prediction of life in fatigue is still based on purely phenomenological or empiric
relations, in most cases in Engineering practice. The Paris’ law is one of the few consolidated
tools to calculate propagation velocity of fatigue cracks based on Fracture Mechanics,
although being also a phenomenological relation.
The cohesive interface method has been used intensively lately as a tool to simulate
cracking process in metals with great success. More recently, few attempts to use this method
to predict life in fatigue have been done. Such works follow the general idea that during the
loading-unloading, cohesive law should present some hysteresis, function of parameters that
measure damage during cycling process.
In this work, the cohesive surface method will be used as an attempt to model the rupture
in fatigue. However, dissipation during cyclic loading in the present work will not introduce
new damage parameters, being a residual opening after unloading the only source of
irreversibility. Such hypothesis is based on the fact that oxidation films develop after opening.
The unloading path may have also an important effect on monotonic crack propagation,
since local unloading, near the crack tip, are expected.
To simulate propagation, the cohesive surface was implemented in a Element code.
Preliminary results for a 7075-T6 aluminum show good data fitting, indicating that the
hypothesis is feasible. In the examples analyzed, only Mode I of propagation was considered.
Plastic strains on the crack tip were small, indicating the validity of the Linear Elastic Fracture
Mechanics. Dynamic and monotonic crack propagation are also considered here, showing that
the local unloading near crack tip may have an important effect on the kinematics of the
propagation.