Issue 47

M. Peron et alii, Frattura ed Integrità Strutturale, 47 (2019) 425-436; DOI: 10.3221/IGF-ESIS.47.33

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. 

Deep

Razor

Un-notched

Moderate

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) 123 ± 4.3 Table 1 : Tensile properties of notched and un-notched polyetheretherketone (PEEK) specimens under different strain rates. 211 ± 8.2 225 ± 5.4 132 ± 1.1 135 ± 0.4 127 ± 2.3 129 ± 1.4 119 ± 4.9

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].

Moderate (radius 0.9 mm)

Deep (radius 0.45 mm)

Razor

Parameter

131

A (MPa)

152

120

-0.063

d

-0.043

-0.043

0.92

R 2

0.95

0.90

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

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