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A. Spagnoli et alii Frattura ed Integrità Strutturale, 25 (2013) 94-101; DOI: 10.3221/IGF-ESIS.25.14 100 experimentally evaluated (the threshold stress intensity th,0 K  for long cracks is equal to 5.21 MPa m and 6.20 MPa m for small-size grain and large-size grain material, respectively). Figure 5a reports, in a normalized form, the above experimental data along with the corresponding theoretical curve of the kinked model. Such a curve (normalized in so that the ratio th th,0 K K   tends to the unity for l d  ) is determined by posing the weighted average effective SIF of Eq. 11 equal to the threshold SIF range for long cracks th,0 K  (the remote SIF ( ) I K   corresponds to the threshold SIF range th K  ), that is :   th ( ) th,0 a K 1 K f , l d ,           (17) It can be seen that the experimentally observed reduction of the threshold stress intensity factor with respect to that of long cracks (reduction occurring as the crack length decreases) is correctly captured by the present model, although to a smaller extent. Further experimental data being analysed concern fatigue crack growth in the Paris regime [23, 24]. Such data are related to fatigue crack propagation in three-point bend specimens made of normal-strength (NS) plain-concrete [23] and high- strength (HS) plain-concrete [24]. For each concrete type, three series of two-dimensional geometrically similar cracked specimens with height h equal to 38, 76, 152mm and to 38, 108, 304mm for NS and HS concrete, respectively, span = 2.5 h , initial crack length l 0 = 0.16 h , thickness = 38 mm. By elaborating the experimental data through a best-fit procedure (see Ref. [13] for details), it is shown that the Paris parameter m is independent of the initial crack length ( m is equal to 10.4 and 8.2 for NS and HS concrete, respectively), while the Paris parameter C turns out to be dependent on such a length. Figure 5b plots the experimental best-fit values of C (for crack growth rate expressed in m / cycle and stress intensity range in MPa m ) as a function of the normalized initial crack length (the characteristic material length is taken to be equal to the maximum aggregate dimension, 12.7 and 9.5mm for NS and HS concrete, respectively), along with the best-fit curves of the present model (see Eq. 16, where the crack-size dependent Paris coefficient is   m ( ) ( ) a a C f , l d , ,                  ). The experimental evidence seems to be well described by the present kinked model in the range of crack sizes being considered. (a) (b) Figure 5 : (a) Fatigue threshold results for mild steel [22]; (b) fatigue crack growth results in the Paris regime for concrete [23, 24]. C ONCLUSIONS n the present paper, irregular morphology of fracture surfaces is described via a two-dimensional model of a periodically-kinked crack, where its kinking is due to a periodic self-balanced microstress field having a length scale, d . On the basis of some geometrical and mechanical arguments, the model allows us to quantify the influence of the deflection degree on the fatigue threshold condition and fatigue crack growth. By correlating the parameter d with a characteristic material length (e.g. average grain size in metals, maximum aggregate dimension in concrete), the possibility of using the present model to describe some experimental findings related to crack size effects in fatigue of materials is explored. Some experimental results related to crack-size effects in fatigue threshold condition for metals as well as I