Issue34

S. Henkel et alii, Frattura ed Integrità Strutturale, 34 (2015) 466-475; DOI: 10.3221/IGF-ESIS.34.52 471 More pronounced difference was found at R = 0.1 shown in Fig. 6. The crack length signals and the crack growth rates of five consecutive 1.5-fold overloads are shown. The overloads, in which the sample was loaded with 40 kN tensile force in the X-direction, show a significant increase in the delay effect until the original crack growth rate is achieved again. It rises to about 1.5 ... 1.8 times comparing with the overloads with F X = 0 kN a) b) Figure 6 : a) Averaged crack length signal of the two gages versus number of cycle for fife 1.5-fold overloads at R = 0.1 with variation of the force parallel to the crack F X , b) crack growth rate versus  K for these overloads. For high static preloads at R = 0.8 the corresponding tests are not suitable to detect a clear tendency of the influence of crack parallel stress. Fig. 7 shows the development of the crack gage signal and crack growth rates for one single crack tip for a test with constant load amplitude F Y and eight 1.3-fold overloads with varying F X . The other crack tip kept nearly completely arrested after the first overload, so the crack size became asymmetric. The objective to obtain a significantly larger crack propagation between the overloads as the estimated size of the plastic zone (Dugdale) 0.2 8 Imax pl p K d R           [18] was not fully achieved in this experiment. If the plastic zone of the base load is subtracted it is fulfilled. The results suggest in contrast to the experiments at R = 0.1 a slight reduction in delay with tensile loading in X-direction. That might be caused by a reduction in plastic zone size due to higher in plane constraint. a) b) Figure 7 : a) Crack length signal of the one crack gages versus cycle for 1.3-fold overloads at R = 0.8 with variation of the force parallel to the crack F X , b) crack growth rate versus  K for these overloads.