Issue 33

X. Zhou et alii, Frattura ed Integrità Strutturale, 33 (2015) 209-214; DOI: 10.3221/IGF-ESIS.33.26 210 This paper will focus the attention to the effect of loading history in the threshold regime and especially to the phenomena occurring it the so-called stepwise increasing load amplitude test, which permits the determination of the crack length dependence of the threshold of stress intensity range  K th . The paper is organized in the following way: At first, the stepwise increasing load amplitude test is shortly introduced, then the different applied loading procedures and the obtained results are described, and finally a simple estimation of the extension of the crack where the results are affected by the pre-cracking procedure is presented. T HE STEPWISE INCREASING LOAD AMPLITUDE TEST : he threshold of fatigue crack propagation test is usually measured by a load decreasing technique [3]. In this case a fatigue crack is generated in the Paris regime and then the stress intensity range is slowly reduced till  K th is reached. In order to avoid a history effect a very small reduction rate of  K should be chosen. A complete different technique has been proposed by Pippan [3] and later by Newman [5]. The pre-crack in this case is produced in cyclic compression. Such pre-crack is surely open in the unloaded state. The crack closure during cyclic tension loading has to build-up during crack extension in the following fatigue crack experiment. Figure 1: Illustration of the used stepwise increasing constant load amplitude method. The specimens with existing pre-cracks were loaded with a constant amplitude step-by-step. (a) Schematic of the loading procedure. The amplitude was increased to the next step, when the crack stopped to propagate. The final crack extension Δa and the final stress intensity ΔK are plotted in diagram (c) which characteristics the material resistance against crack propagation (the R-curve). For the last step where stable crack extension occurs, the crack growth rate was measured as a function of ΔK depicted in (b). The load at the start of the stable crack growth corresponds to the long crack threshold value in literature. The loading procedure to determine the threshold  K th and fatigue crack propagation behavior is schematically depicted in Fig. 1. After pre-cracking in compression, the load amplitude at the desired stress ratio, R, is increased in steps until the threshold value of the long crack is reached. If no crack extension takes place, the applied  K is smaller than the effective threshold (because the crack is open, the closure stress intensity at the beginning is in compression, K cl is smaller than -  K effth [6]. If  K is larger than  K effth a crack extension takes place. If during crack extension sufficient crack closure develops, the crack will stop the propagation. When  K is sufficient that the crack closure stress intensity at the threshold T