## 4 Experimental Data and Discussion  
Data from [43; 44] using [PERSON]'s definition of \(K_{\alpha\!q}\) (Equation (4)) indicate that, for the test conditions and structural steels considered, the experimentally measured \(da\)/\(dN\) vs. \(\Delta K_{l}\) and \(da\)/\(dN\) vs. \(\Delta K_{\alpha\!q}\) were approximately similar, a trend also found by [PERSON] using [PERSON]'s \(K_{\alpha\!q}\) definition and testing biaxially loaded plates [45]. Using CTS specimens and [PERSON]'s \(K_{\alpha\!q}\) definition of Equations (11) and (12), [46] notes that \(C\) and \(m\) in Equation (2) depend on the angle of load direction and initial pre-crack, i.e., on the initial values of \(K_{l\!l}\)/\(K_{l\!l}\). Such a dependence is also shown in [33]; that is a weak dependence, which is not surprising given that in CTS specimens, after the initial shift in the crack path, most of the FCG takes place in near-mode-I conditions. Similar conclusions were obtained by [PERSON] et al. [47]. As shown in Section 2, substantial differences between \(K_{\alpha\!q}\) definitions are found. What are the consequences of that diversity? The mixed-mode fatigue crack propagation tests may be performed using a travelling microscope measuring horizontal and vertical coordinates. Preferably, to monitor both surfaces of the specimen, two travelling microscopes may be used. The measurement of the crack path's \(x\), \(y\) coordinates in a mixed-mode fatigue crack propagation test is a tedious and time-consuming task. It involves the identification of the number of cycles needed for the crack tip to move from location \(x_{1}\), \(y_{1}\) to \(x_{2}\), \(y_{2}\). As a simplification, the distance between points 1 and 2 is assumed to be a straight line and therefore:  
\[\Delta a\approx\frac{\sqrt{\left(x_{2}-x_{1}\right)^{2}+\left(y_{2}-y_{1} \right)^{2}}}{\Delta N} \tag{23}\]  
A higher accuracy is obtained if more points are used to define the path, implying more frequent measurements of coordinates associated with lower values of \(\Delta a\), which can be achieved with automatic systems and high-resolution digital cameras. The crack growth rate \(da\)/\(dN\) is then defined as:  
\[\frac{da}{dN}\approx\frac{\Delta a}{\Delta N} \tag{24}\]  
In tests reported in [5; 6], Al alloy AA6082 T6 CT specimens modified with holes, along with cracked 4-point bend specimens subjected to asymmetrical loading promoting mixed-mode situations, were subjected to FCG tests where the crack path and crack growth rate were measured. As in [48; 49; 50], for the CT specimens with holes, \(K_{\alpha\!q}\) values were not far from \(K_{l}\), and \(da\)/\(dN\) vs. \(\Delta K_{\alpha\!q}\) data were close to \(da\)/\(dN\) vs. \(\Delta K_{l}\) data. Figure 4 shows the crack path in a CT specimen modified with a hole [51]. Figure 4: CT specimen modified with a hole, showing the crack path. FCG takes place under the predominant mode I situation, [PERSON] et al. [51]. Research showing a good predictability of FCG was also presented by [PERSON] in [52; 53], where, again, the values of KII were typically smaller or much smaller than the values of \(K_{l}\). However, for the 4-point bend specimens, a great diversity of mixity values was achieved in [5], and substantial differences between mode-I and mixed-mode FCG could be identified. The difference between _da/dN_ vs. \(\Delta K_{eq}\) and _da/dN_ vs. \(\Delta K_{l}\) data increases with the mixity value. According to a suitable choice of loading points, 4-point bend tests may produce pure mode II situations. As an example, Figure 5 shows a 4-point bend specimen, tested under initial pure mode II. Of interest here is the initial direction of the crack growth, starting from the precrack--in this test, the crack growth takes place at an angle of approx. \(90^{\circ}\). Details of the experiments may be found in [5]. The present note highlights the need for data clarifying what is the best definition for \(K_{eq}\).