PSI - Issue 50

Kirill Guseinov et al. / Procedia Structural Integrity 50 (2023) 105–112 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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To obtain stress-strain curves, the strain components were averaged over a virtual strain gauge. According to the recommendations of Koerber et al. (2010), the deformation averaging area with dimensions of 3 × 3 mm 2 was chosen for cubic specimens. To average the deformations on V- notched specimens, an area with the size of 16×1 mm 2 was used to exclude the strains concentration at the notch root. 3. Results and Discussion 3.1. Failure analysis The failure mechanisms that occurred under various combinations of through-thickness compression and interlaminar shear stresses were identified by visual inspection of the fractured specimens. Interlaminar shear failure was typical for cubic specimens (Figure 3,a) at cut angles α from 1 5° to 45°. However, buckling of cubic specimens with cut angle α = 15° was noted during the tests due to significant compressive loads. A mixed fracture mechanism near the notch flanks was observed for V-notched specimens (Figure 3,b) at loading angle α = 15°. Interlaminar shear failure was observed in the gauge section of the specimen at loading angles α = 20° - 45°.

(a)

(b)

Fig. 3. Failed cubic (a) and V-notched (b) specimens after combined compression-shear test

3.2. Analysis of stress-strain response

Typical stress-strain shear curves under biaxial loading are shown in Figure 4. When testing cubic and V-notched specimens, the obtained shear stress-strain curves differed significantly. This proves the significant influence of the test procedure on the material mechanical response. The reliability of the obtained experimental data under biaxial loading was evaluated by comparison with the experimental results obtained by DeTeresa et al. (2004). It was found out that friction at the cubic specimens ends and small working area affected the initial slope of the curves. These factors affected the yield stress and the deformation diagram more significantly, and therefore, the stress-strain curves (Figure 4,a) even qualitatively differed from the experimental data of DeTeresa et al. (2004). The non-linear monotonically increasing behaviour of the material during biaxial tests on the new fixture was in good agreement with the results of DeTeresa et al. (2004). The solid line on the shear stress-strain curve marks the material behaviour before discontinuity (Figure 4,b). More details about the features of specimen deformation on the new fixture could be found in Guseinov et al. (2022). The apparent interlaminar shear modulus at different loading angles for V-notched specimens was 3.5 GPa, whereas the constant value of 3.8 GPa was obtained on the cubic specimens. Thus, the interlaminar shear modulus was practically independent of the combination of compressive shear loads under proportional loading. However, shear stress-strain curves during tests of cubic specimens no longer reflected the material actual behaviour under biaxial loading.