Issue 27

L. Vergani et alii, Frattura ed Integrità Strutturale, 27 (2014) 1-12; DOI: 10.3221/IGF-ESIS.27.01 10 In Fig. 7, focusing the attention on the finite life region, we can notice a larger decrease in the mechanical properties of delaminated specimens, if compared with the undamaged ones. Indeed this kind of specimens clearly shows different mechanical fatigue properties with respect to the other undamaged materials. Instead, as previously mentioned, its high cycle fatigue limit is approximately the same: in [27] it is hypothesized that, at high cycling, active mechanisms of fatigue damage are different than for finite life region. Delamination is active only in this finite stage of S-N curve, while its presence is not influencing mechanical fatigue behaviour at low stress amplitudes. From microscopy magnifications, moreover, it is clear that the Teflon layer is active for the performed tests and cracks start their evolution in the composite from the two edges of this induced damage. The results from the application of thermographic techniques evidenced, for both ΔT/ΔN-σ max and D-mode-σ max plots, a similar value of fatigue limit ( σ D ), for the cases of delaminated fiberglass. This value, moreover, is very similar to the same undamaged material: this supports the thesis that the presence of delamination does not influence the fatigue limit, which can be defined as a characteristic feature of the material. The results of stepwise dynamic tests, carried out on basalt reinforced specimens, lead to a clear bi-linear trend in both the considered plots (i.e. ΔT/ΔN-σ max and D-mode-σ max graphs) based on thermal observations. However, the ΔT/ΔN-σ max method appeared underestimating the fatigue limit, while σ D value from D-mode and energetic technique appeared to be more realistic as fatigue limit of the basalt reinforced composite [19]. C ONCLUDING REMARKS n this review, we presented the application of IR-thermography to different composite materials, in order to relate the thermal response of these materials to their mechanical behaviour, and in particular to the fatigue limit. Glass and basalt fibre reinforced composites were considered and a review of thermographic approaches was discussed. According to the literature, three main thermographic methods are applied to composites to quantify, through this technique, fatigue performances of these materials. One is related to thermographic observations during static tests, while the other two deal with the application of dynamic load, and are based on thermal and energetic considerations. In the present review, applications of this techniques are discussed and compared for the different materials (glass fibre reinforced composites, without and with an induced delamination, and basalt fibre reinforced composites). Thermography is therefore proposed as a valid experimental technique able to quickly estimate mechanical fatigue properties through thermal response of the materials. N OMENCLATURE C p specific heat at constant pressure D damage parameter, defined in Eq. (2) E elastic modulus (stiffness) FRC fibre-reinforced composites IR infrared K 0 thermoelastic constant T 0 average surface temperature of solid TSA thermoelastic stress analysis N number of cycles NCF non-crimp fabrics UD unidirectional ΔT variation in surface temperature with respect to the initial temperature, T 0 λ linear thermal expansion coefficient ρ mass density σ D damage stress, from thermographic observations σ max maximum applied stress during fatigue cycling σ 1 , σ 2 , σ 3 principal stresses σ I , σ II , σ III stress as defined in Fig. 5 I