Is it true that the higher the number of plies is, the safer is a brittle laminate?

It is well known that, for-plastic materials, the more numerous are the elements in which the structure is divided, the higher is the improvement in structural reliability. This principle is generally assumed even for brittle materials, for which plastic adaptations are not possible. Here, the cumulative functions of plate strength for brittle laminated plates, composed by a variable number of plies under in-plane or out-of-plane bending, are derived. We consider materials characterized by a volume size effect and an area size effect, as well as materials that do not exhibit any size effect. For the same maximum stress in the sound state, lamination is beneficial under any circumstances for plates made of a material with volume size effect. The same is true for materials with area size effect only when the laminate is in-plane loaded; otherwise, lamination leads to an increasing of the surface area under tensile stress, while stress redistribution can counterbalance this negative effect only for the extreme lower quantiles of the strength distribution. When no size effect is present, the gain from lamination is only appreciable on the left-hand-side tail of the cumulative function. These findings do not confirm that increasing the number of plies leads to a safer design under any circumstances, in contrast to the commonly accepted principle for brittle glass in architectural applications, which is characterized by an area size effect. One should consider the structural reliability as a whole and compare, on a statistical basis, what is the real gain obtainable through lamination in terms of probability of survival under the applied design load, in order to define the optimal number of plies for any specific structural application.