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S. Henkel et alii, Frattura ed Integrità Strutturale, 34 (2015) 466-475; DOI: 10.3221/IGF-ESIS.34.52 470 R ESULTS AND DISCUSSION n the biaxial tests without overload, only a small influence of a block-wise or continuously modified loading F X parallel to the crack on the crack growth rate was found. Fig. 4 shows the crack growth data in comparison with the stress intensity parameter  K. For comparison the crack growth values and the scatter band [17] of the uniaxial measurements on the SENB samples are given. For R = 0.1 (Fig. 4a) it was attempted to reduce the load to a  K-value of 8 MPa  m at the beginning of each block. In order to minimize load history effects the further crack growth was carried out under constant force amplitude F Ya at blockwise variation of the static load Fx. In this measurement, the crack growth rate is lower for tensile load in the crack direction X. Probably crack closure effects such as roughness, for example, or plasticity induced crack closure is supported by a T-stress shifted to the tensile region. For the crack growth data at R = 0.5 shown in Fig. 4b), the influence of the variation of T-stress is significantly lower and mostly within the uniaxial scatter band. The gray line in this picture shows data for a continuous ramping in F X while cyclic loading in F Y . a) b) Figure 4 : Crack growth rates and cyclic stress intensity factors  K versus number of cycles under variation of the force parallel to the crack F X for a) R = 0.1 and b) R = 0.5. Fig. 5 shows the average crack length measurement signals of the two crack gages versus cycle number (Fig. 5a) and the corresponding crack growth rates (Fig. 5b) for two consecutive 1.5-fold overloads with approximately the same load at R = 0.5 with and without load in the crack direction F X . The delay before the crack growth rate has returned to the initial level is approximately 103,200 cycles for F X = 40 kN and 77,500 cycles for F X = 0 kN. a) b) Figure 5 : a) Averaged crack length signal of the two gages versus cycle for two 1.5-fold overloads at R = 0.5 with variation of the force parallel to the crack F X , b) crack growth rate versus  K for the two overloads. I