PSI - Issue 7

Dalila Dimaggio et al. / Procedia Structural Integrity 7 (2017) 198–205 D. Dimaggio et al. / Structural Integrity Procedia 00 (2017) 000–000

200

3

Fatigue testing was performed on un-notched and notched specimen under strain and stress control with the geometries shown in Figure 1 (notch radius r=1.25mm). Stress concentration factors were calculated by the linear FEA and compared to analytically calculated stress concentration factors using the Neuber [2] and Rainer approaches [3].

Figure 1: Specimen geometries for fatigue tests, K t ≈ 1.0 for strain controlled and K t ≈ 2.0 for load controlled tests (from left to right)

2.3. Testing For the fatigue design approach it was necessary to simulate near-service environmental conditions and near-service load spectra. On this basis, stress controlled tests of notched specimens under bending and axial loading with different strain and stress ratios (R ε and R σ ) under constant amplitude loading at T=120°C were carried out to obtain the mean stress sensitivity of the material and the cyclic material parameters. Moreover, validation tests were realized with a double-stage spectrum to reproduce the near-service load cases. The tests were carried out on two servo-hydraulic test rigs with maximum nominal loads of 25kN (bending tests) and 100kN (all axial tests). In the bending tests the load is applied out of the specimen’s axis to allow a superposition of axial and bending stresses. All test details are summarized in Table 1.

Table 1: Test matrix of the investigated material (CA=constant amplitude loading)

Test Control Type

Test Type

Load Sequence

Specimen Type

Temp. °C

R ε ,R σ

R ε 1 =0 R ε 2 =-1 R ε 3 =0.9

axial axial axial axial axial axial

CA CA CA CA CA CA CA

standard smooth

strain

0 0

120

bending

R σ1 =0.5 R σ2 =0.7

standard notched

stress

R σ,LCF =0 R σ,HCF =0.7

LCF+HCF validation tests

axial

3. Experimental results 3.1. Fatigue testing under constant amplitude loading In Figure 2 S-N lines (Wöhler lines) of the specimens tested in air at T=120 °C under constant amplitude loading are summarized (tests results are normalized to the knee-point of R=0 axial test results).