Issue 27

L. Vergani et alii, Frattura ed Integrità Strutturale, 27 (2014) 1-12; DOI: 10.3221/IGF-ESIS.27.01 6 (a) (b) Figure 4 : a) Stepwise loading history. The characteristics of each interval (i.e. maximum applied stress and length of the interval) depend on the tested material. b) Thermal profile in each interval: the temperature characterized by an initial linear increasing region (I), represented with a black continuous line, then followed by a second linear region –represented by a back dashed line- or by a plateau region (region II, represented by a continuous black line) and a final increasing region (region II, represented by a continuous black line). For basalt fibre reinforced composite in epoxy matrix, specimens were tested and subjected to blocks of 10000 cycles per step, and increasing stress amplitude of 10 MPa. Tests were stopped when the specimen failed. As mentioned above, we also performed a fatigue characterization of the studied materials, by carrying out load-controlled dynamic tests according to the standard ASTM D 3479 [28]. For these tests we used an MTS 810 hydraulic machine with a 100 kN load cell, we set a stress ratio equal to 0.1 and a load frequency equal to 10 Hz. We carried out tests at different load levels, to determine the σ max -LogN curve. In our initial studies we chose 5·10 6 cycles as the runout value, chosen as an average value from the literature. In a more recent study we also performed an HCF characterization, by setting the runout value at 10 7 . Then, in view of the obtained results, we made estimations about the materials damage initiation and growth, the materials failure mode, and the life of the materials. C ASE STUDIES n this section, we show a series of case studies. We investigated the damage behaviour on glass fibre- and basalt fibre- reinforced composite materials; for the glass-FRC we also considered the effect of delamination. Here the results are shown on the basis of the type of performed test and analysis. Generally with IR-thermography it is possible to perform both qualitative and quantitative analyses. Indeed, in our study by observing the thermal maps it was possible to locate damage, which appeared as the hottest region. The analysis of the thermal map during an entire test also allowed one to make hypotheses on the failure modes. For instance, during the tests some flashes of lighting were barely visible and oriented like the fibres of the tested materials, hence representing the energy release, due to debonding, at the fibre-matrix interface. We also performed quantitative analyses, by accurately post-processing the thermal data. The temperature can be considered an important energetic parameter, strictly correlated to the damage state of the material. In our data analyses we measured the surface temperature and we averaged the temperature over the scanned area of each specimen, mainly corresponding to the central part of the specimen. Indeed, in our calculations we avoided the upper and lower parts of the specimen surface for the local influence due to the grips, and the external borders for the edge effect. We should stress that the selection of the area, where to average the temperature data, has no influence on the results of the temperature trends. Indeed, we performed some trials, but the trends were globally similar, though upper or lower shifted on the temperature scale. Static tests (continuous and interrupted) and microscopic analyses The results of the static tests were repeatable for all the studied materials. Static tests allowed the determination of the mechanical properties of the materials. The thermal analyses allowed the study of the material behaviour from an energetic viewpoint. In particular, a characteristic thermal trend has been found for all the materials and it is characterized by three regions: I