The invention relates to a method and a device for determining and using a diagnostic tool comprising a temperature
ageing spectrum, the method (P) comprising a step (E1) of receiving at least one
temperature curve relating to a given geographical location, a
processing step (E2) comprising a sequence of consecutive substeps (E2A, E2B, E2C, E2D, E2E), implemented iteratively for different values of a characteristic time associated with a type of material, comprising a substep (E2A) of filtering the
temperature curve using a first-order filter having the characteristic time as the
time constant, a substep (E2B) of applying an
ageing law, a substep (E2C) of performing a cumulative calculation over a given sliding time period, a substep (E2D) of extracting a value corresponding to an
ageing criterion and a substep (E2E) of associating this extracted value with the characteristic time, wherein the pairs of values obtained at the end of the iterations form a temperature ageing spectrum, and a step (E3) of performing, from at least this spectrum, comparisons in order to carry out a diagnosis.