Issue 30

A. Risitano et alii, Frattura ed Integrità Strutturale, 30 (2014) 201-210; DOI: 10.3221/IGF-ESIS.30.26 202 other researchers [16-28] and now this method is quite normal in use. The analysis of the phenomenon of the heat development has highlighted that the material which is subjected to a dynamic stress higher than its fatigue limit, shows the irreversible phenomena already during the first cycle of application of the fatigue loads. For this reason the authors checked the first loading ramp for the detection of the first micro heat source, index of irreversible deformed states. Then, it seemed almost natural to think about the application of all-out methods able to examine the entire surface of the specimen during the static test, and in particular to rely on infrared thermal sensors for the temperature survey. Since the first analyses of this type which date back to 1987, it has been noted that, using this approach, it was possible to determine the region (stress-strain) in which the material respects the laws of the Thermoelasticity [38, 39]. The Thermal analysis applied to various steels during the static test [29-34] and the check of their fatigue limit using traditional methods, showed that it was possible to define the fatigue limit by determining the point (area) where the slope in the curve changes: the temperature of the hottest point of the surface-applied stress. On the basis of the first results, other researchers [35, 36] tested the procedure on other materials, different from the steels (composite materials).Until now the slope variation point has been determined tracing in the temperature-stress diagram the first constant slope line (perfectly thermoelastic part) and identifying the point where the curve departed from the straight line drawn. This work has a double aim: it wants to confirm the procedure to detect the fatigue limit by adopting different steels and it intends to free the measurement by the feeling and the experience of those who adopt this kind of analysis. It is thought that an analysis of the physical phenomenon could be more appropriate than the analytical study of the function. Therefore, it has been proposed the application of the correlation function to the temperature-time data (solicitation) detected during the static monoaxial traction tests. In order to define a future test protocol, the authors would try to define a process methodology based on a method independent from the performer of the tests. This work describes how using the correlation function for the data collected during a simple static monoaxial traction test, it can be pointed out where the slope changes and, therefore, it can be determined the value of the fatigue limit. It has been verified in fact that the value of the load, which conditioned the loss of the linearity of the temperature-load function, corresponds to the zero of the autocorrelation function whose values have been detected by the sensors used for the test. S URFACE TEMPERATURE DURING A STATIC TRACTION TEST ig. 1 shows the qualitative trend of the result (  ) of a static monoaxial traction test for a steel plate specimen. The same diagram shows the evolution of the surface temperature measured on the specimen during the test. In the diagram, P indicates where the temperature trend is not linear. At this point the stress value is completely different from the corresponding yield limit; generally, the linearity between efforts and deformations ends with the suspension of the applied load and it can be instrumentally detected. This type of instrumental deformations will not be detected if the specimen discharged when the load arrives at point P. The analysis of the surface temperature curve while varying the load (in red in the diagram) is perfectly linear up to point P then the slope changes when it is still far from the classic value of yield. From that point onwards, the slope of the curve varies continuously until the break of the specimen. Analyzing the specimen from a physical point of view, during a static traction test at a constant load speed [N/s], it is possible to characterize the behavior of the first phase as due to the perfectly elastic relation between the change of temperature  T and the stresses (law of Lord Kelvin): ΔT = -K m T σ m (1) where T is the absolute temperature, K m is the thermoelastic constant of the material (for steels 12 3.3 10   [Pa -1 ]) σ m is the average stress applied to the specimen [39]. After this first phase, starting at the point P (second phase) until the break of the specimen, the change of slope is due to the heat developed for the irreversible deformations. Deformations which affect more and more volumes of the sample until its breakage For this area (starting from point P) the following law can be applied [31]: 2 3 0 3 p m a r r r m a r r E t t m K T t t t T K T t                       (2) F