Discussion of “Virtual age, is it real?”

The paper by Finkelstein and Cha centres around the attempt to model a system subject to failures after it has experienced a damage and/or failure episode. The paper discusses the validity of the assumptions of virtual age models, and provides helpful examples which elucidate the uses and shortcomings of models using a time transformation to mimic the effect of damage and ageing. We first recast their examples in a uniform notation. We make comments on the nature of empirical vs. physical models, and the advantages and disadvantages of these. We close with a brief note about our own shock/repair model and its relationship to the time transformed failure models discussed by Finkelstein and Cha.