Issue 47

M. Peron et alii, Frattura ed Integrità Strutturale, 47 (2019) 425-436; DOI: 10.3221/IGF-ESIS.47.33 428 0.02. The fatigue data, shown in Fig. 2, were fitted by Sobieraj et al. [61] by means of the S-N Basquin relationship [62] with good results being the mean squared error, R 2 , higher than 0.90 for all the notched geometry: d = AN   (1) where Δσ is the stress range, N is the number of cycles to failure, and A and d are constants, listed in Tab. 2. Un-notched Moderate Deep Razor Strain rate (s -1 ) 0.1 0.5 0.1 0.5 0.1 0.5 0.1 0.5 Max axial true stress (MPa) 211 ± 8.2 225 ± 5.4 132 ± 1.1 135 ± 0.4 127 ± 2.3 129 ± 1.4 119 ± 4.9 123 ± 4.3 Table 1 : Tensile properties of notched and un-notched polyetheretherketone (PEEK) specimens under different strain rates. Figure 2 : Experimental S-N curves for the three different notched geometries, i.e. moderate (U-notched with a notch radius of 0.9 mm), deep (U-notched with a notch radius of 0.45 mm) and razor (circumferentially cracked). Modified from Sobieraj et al. [61]. Parameter Moderate (radius 0.9 mm) Deep (radius 0.45 mm) Razor A (MPa) 152 120 131 d -0.043 -0.043 -0.063 R 2 0.95 0.90 0.92 Table 2 : S-N Basquin relationship constants. Data taken from Ref. [61]. A NALYTICAL FRAME WORK SED approach under static loadings he SED criterion states that the failure of a component, subjected to tensile loading, occurs when the total strain energy, W , averaged in a circular control volume of radius R c (surrounding a crack or notch tip) reaches its critical value W c [63]. The critical SED parameters, i.e. the critical radius, R c , and the critical strain energy density, W c , are material dependent”[64], and they can be analytically derived with only few material properties [63]: the ultimate tensile T