Classes of imperfect repair models based on reduction of failure intensity or virtual age

This article proposes two new classes of imperfect repair models. The (conditional) failure intensity before the first repair is a continuous function of time. The repair effect is characterized by the change induced on the failure intensity before and after failure. In the first class of models, repair effect is expressed by a reduction of failure intensity. In the second class, repair effect is expressed by a reduction of the system virtual age. In each case, several particular cases are studied, which take into account the possibility of a markovian memory property. For almost every model studied, there exists a minimal wear intensity, i.e. a maximal lower bound for failure intensity. The classification presented here involves existing models and allows the proposition of new ones. The models are compared in terms of wear-out speed. Finally, a numerical statistical study on the quality of the model parameters estimators is presented. (C) 2003 Elsevier Ltd. All rights reserved.