Issue 27

L. Vergani et alii, Frattura ed Integrità Strutturale, 27 (2014) 1-12; DOI: 10.3221/IGF-ESIS.27.01 9 since it can offer quick information about fatigue behaviour of the materials, and in particular to their fatigue limit, or to a stress level corresponding to a different damage mechanism. Dynamic tests (stepwise and fatigue) The post-processing of the results obtained from the dynamic stepwise tests confirmed the hypotheses put forth, by observing the results of static tests. In the case of E-glass/epoxy material, [±45°] 10 , the results obtained by stepwise dynamic tests, characterized by shorter and longer steps, were respectively analysed by measuring the D-mode signal and the ΔT/ΔN. In particular, by plotting the results of ΔT/ΔN-σ max and D-mode-σ max we get a bilinear trend, similar to the characteristic trends reported in Fig. 2 and Fig. 3, respectively. The breakup point was found to be 36 MPa. In Tab. 1 the average value of σ D , (34) obtained from static and dynamic thermographic methods, is reported. Instead, a net variation in the thermo mechanical response specimens is detected in delaminated samples tested by stepwise dynamic tests [27]. For these specimens, indeed, fatigue behaviour is deeply influenced by the presence of the localized damage in the thickness. This is clearly visible from experimental fatigue tests (Fig. 7). Fig. 7 shows all the fatigue data available for the considered materials in a normalised S-N curve. In order to have comparable values along vertical axis for all the composites, the ratio between applied stress amplitude and ultimate tensile strength for each material is taken into account. Experimental fatigue data are fitted by means of straight lines, one for each material, to describe the finite life region. Considering the final part of the curve, related to the high cycle fatigue, thus fatigue limit, data of broken and runout specimens of all the considered composites show a flattening trend at approximately 20% of the ultimate tensile strength. This ratio seems a feature for all the composites, able to describe their fatigue strength. Figure 7 : Normalized fatigue curves for all the materials. A linear interpolation is added in the finite life region, and the fatigue limit is plotted in the infite life region. Table 1 : Characteristics and mechanical properties of the tested materials. Damage stress is the average value among those obtained from the different thermographic methods, except for E-glass/epoxy [0°] 10 and [90°] 10 where only the static thermographic method has been applied . Material Layup Fibre content UTS Fatigue strength Damage stress σ D /UTS % E-glass/epoxy [0°] 10 50% wt. 388±4 - 226 58 E-glass/epoxy [90°] 10 50% wt. 59±10 - 33 56 E-glass/epoxy [±45°] 10 50% wt. 142±3 35 34 24 E-glass/epoxy* [±45°] 10 55% wt. 155 ±5 40 37 24 Basalt/epoxy [0°/90°/+45°/-45°] 2s 50% vol. 409±9 - 74 18 *Teflon insert in the mid-section of the material layup to mimic delamination.