ABSTRACT
In practice, all the variables affecting the aging process are actually not known. The issue of reducing the evolution of all the different degradation factors to a probability law for the component lifetime, which is mathematically exploitable and depending on few parameters, remains open. Most authors consider strictly increasing failure rates, usually in the form of a single-mode, two-parameter Weibull law (see eq.(2) below).This form is actually quite different from the commonly accepted bathtub curve, as it constraints the failure rate to start from a zero value and often leads to estimations of the shape parameters lying between 1 and 2, hence to an exponent lower than 1 in the time-dependent expression of the failure rate.
