Issue 37

H.A. Richard et alii, Frattura ed Integrità Strutturale, 37 (2016) 80-86; DOI: 10.3221/IGF-ESIS.37.11 84 Figure 4 : Experimental results for PMMA in contrast to the criterion by Richard F ATIGUE C RACK G ROWTH UNDER 2 D - AND 3 D -M IXED -M ODE - LOADING mong the experimental determination of the fracture limit surface presented in Fig. 4 for PMMA the investigation of the threshold value surface of different materials under 2D- and 3D-mixed-mode-conditions is important too, to characterise the crack growth behaviour under combined loading situations. Cyclic comparative stress intensity factor for fatigue crack growth A crack, which is subjected to an arbitrary mixed-mode-loading, is able to propagate under fatigue crack growth conditions, if the local crack front loading combined of mode I, mode II and mode III portions is located in between the threshold value and a critical cyclic stress intensity factor. This condition can be written down by the formula: IC V thI, Δ Δ Δ K K K   (5) Hereby Δ K V is the cyclic comparative stress intensity factor, which can be derived from Eq. 4 using as before α 1 = 1.155 and α 2 = 1.0: 2 III 2 II 2 I I V Δ4 Δ 336 5 Δ 2 1 2 Δ Δ K K , K K K        (6) Experimental determinations of threshold values under combined loading The mixed-mode threshold values were determined using the load rising amplitude test [15, 16, 17]. Before the fatigue test the specimen were pre-cracked under cyclic compression. The advantage of pre-cracking the specimen in cyclic compression are, however, the left residual tensile stresses, which may cause cyclic plastic deformation and crack initiation [15]. The threshold tests were performed at a constant load ratio of R = 0.1 by increasing the load amplitude in A