PSI - Issue 8

Matteo Loffredo / Procedia Structural Integrity 8 (2018) 265–275

269

M. Lo ff redo / Structural Integrity Procedia 00 (2017) 000–000

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strains relevant for autofrettage process. Therefore a set of strain controlled uniaxial tests have been performed, all composed by a tensile part, stopped at a certain level of total strain, and by an unloading part where a reversed strain is imposed. The investigated total strain range was ranging from 0 . 75% to 3 . 00% (0 . 75, 1 . 00, 1 . 50, 2 . 00 , 2 . 50, 3 . 00%) as these values are typical of autofrettage process and are the ones above which, according to (Chen (1985)), the reduction of σ (U) y comes to a plateau. Tests were carried out on cylindrical specimens using a servohydraulic standard testing machine with a load cell of 250 kN (Fig.4). The shape of the specimen was chosen to avoid any instabity for the compressive loads involved in the load cycles and to ensure, at the same time, the compatibility of the machine loading capacity with the stress levels required by the test. The measuring tool was composed by an MTS extensometer with a gage length of 10 mm lenght and the acquisition system was an RT2 Trio sistemi. Two loading histories have been set up (Fig.5), both spanning

20

R50

10

22

165

(a) Specimen geometry and dimensions (mm).

(b) Tooling.

Fig. 4. Specimen and testing arrangement.

through all the investigated tensile total strains. Cycles of type 1 superimposed a full loading-unloading cycle and a saw thoot path at unloading. The aim was to check if during unloading the material was still elastic despite of the high hardening modulus. Cycles of type 2 consisted of a set of completely inverted loops with increasing amplitude. The aim was to check the behavior of σ y along the unloading path.

2.3. Experimental results

Fig.6 shows the results related to loading histories of Fig. 5. Results coming from both load cycles revelead a significant presence of the Baschinger e ff ect. It is worth noting that, for large plastic strain, the loss of linearity at unloading starts even when the specimen is still in tension. In addition the reverse yielding level depends on the prior accumulated plastic strain (and, for example, this can be interpreted as a reduction combined with a displacement of the yield locus). Load cycles of type 1 confirmed that, despite the significant hardening modulus the material is still elastic at unloading. Load cycles of type 2 revealed that, after first unloading, σ y does not exhibit further changes and that, in first approximation, the re-loading stress-strain can be considered closed curves. Moreover both load cycles give the hint that all σ − unloading paths are concurrent to a common final level (see also Troiano et al. (2006)).