Abstract: In this work, more specimens were tested at equal conditions (static compression test) and, during the tests, the released heat for irreversible phenomena was monitored by means of the analysis of the temperature surface of the specimen. In this way, it was possible to estimate the average value of the macroscopic stress (“critical stress”) for which local micro cracks begin. The compression static tests (load - machine time) and the related thermal analysis (temperature-machine time) of spots located on the specimen face of the specimens, highlighted the possibility to estimate the value of the compressive load to which there was loss of linearity in the temperature - machine time diagram (Δt-t). This effect is due to internal heat generated for irreversible phenomena (internal micro fractures). The results show that the “critical stress” has values practically coincident for the points (spots) located in different zones (center or corners of the specimens' surface).