Multiaxial loading experiments

Further work will be focusing on lifetime prediction methods for gas turbine materials (superalloys) and experiments on fracture mechanics in superalloys as well as finite element calculations of multiaxial loaded tubes and validation of multiaxial tests (tension, torsion, internal pressure). 

Under the action of multiaxial loads, crack precursors wifi develop in a manner that reflects their particular loading experiences. It may be assumed that a crack precursor is present at every point, and that the crack precursor may occur in any orientation. By computing the rate at which each possible precursor will develop, it can be determined (Mars, 2002) which particular precursor(s) will develop the fastest. The orientation of this precursor identifies the critical plane on which cracks will develop, which is needed in order to accurately estimate fatigue life. 

The fact that the energy release rate of a small crack surrounded by homogenously strained material scales linearly with the size of the crack, for all multiaxial loading states, has been established from experience (Gough and Muhr, 2005), and can be established mathematically by considering the balance of configurational stresses (Ait-Bachir et al., 2012). 

In a recent study, Saintier et al. ° investigated the multiaxial effects on fatigue crack nucleation and growth in natural mbber. They found that the same mechanisms of decohesion and cavitation of inclusions that cause crack nucleation and crack growth in uniaxial experiments were responsible for the crack behavior in multiaxial experiments. They studied crack orientations for nonproportional multiaxial fatigue loadings and found them to be related to the direction of the maximum first principal stress of a cycle when material plane rotations are taken into account. This method accounts for material rotations in the analysis due to the displacement of planes associated with large strain conditions. 

By definition, ESC is influenced by the level of the applied multiaxial stress, ft is expected that below an assigned value of stress in a specific medium ESC will not occur. Fatigue crack growth experiments at various constant levels of A K can be applied to study the influence of the crack-tip loading on the ESC behavior in a systematic way. As an example, cast CT specimens of PMMA (Mn = 4.6 x 105) were tested in air and IPA at different levels of AK (0.6, 0.7, 0.8, 0.9 MPa -v/m). The tests were performed as described earlier, with the application of the medium after a certain time of cyclic loading in air. From the plots of the crack length ratio vs. number of cycles, the following observations can be made (Fig. 23) ... 

While lab tests often employ simple uniaxial loads, rubber components in service commonly experience states that combine loads from multiple directions. Such loads are said to be multiaxial. Fracture and fatigue processes occurring under such loads are governed by the same general principles as for uniaxial loads, but additional consideration must be made to account for the way in which local effects on crack precursors depend on orientation. 

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