PSI - Issue 13

Mehul Lukhi et al. / Procedia Structural Integrity 13 (2018) 607–612 M. Lukhi et al. / Structural Integrity Procedia 00 (2018) 000–000

610 4

Fig. 5: E ff ect of shape factor on strain-life curve (Lukhi et al., 2018)

Fig. 6: Load sequence e ff ect for block load

loading of two di ff erent strain amplitudes, ∆ 1 and ∆ 2 , are run at a constant strain ratio R = − 1. Under such a block loading sequence, failure occurs according to the Miner-Palmgren rule if

N 2 N f2

N 1 N f1

= 1

(2)

D =

+

is satisfied. Therein, N f1 and N f2 refer to the number of cycles to failure for univariate loading at strain amplitudes ∆ 1 and ∆ 2 , respectively. Furthermore, N 1 and N 2 are the number of cycles which are run at ∆ 1 and ∆ 2 , respectively, in the block loading program. Two loading cases are considered: a case ∆ 1 , 2 = 6% and ∆ 2 , 1 = 5% with a weak di ff erence between both load levels and a case ∆ 1 , 2 = 6% and ∆ 2 , 1 = 3% with a large di ff erence. The load sequence e ff ect for the low-di ff erence case 6%–5%, i. e. the deviation from the Miner-Palmgren line, Eq. (2), is negligible. However, for a larger di ff erence of the load levels ∆ 1 , 2 = 6% and ∆ 2 , 1 = 3%, a pronounced load sequence e ff ect is predicted. It remains open to investigate predictions for multi-axial and non-proportional loading paths. The various grades of NCI are commonly classified according to strength, toughness, metallography of the matrix etc. The e ff ect of material strength on fatigue life can be simulated by varying the ratio σ 0 / E in the present model. Respective values of this ratio were taken from (Meneghetti et al., 2014) for di ff erent grades of NDI, from DI400 till ADI1200. Figure 7 shows that there is hardly any e ff ect of strength on lifetime at high strain amplitude ∆ > 3%. However, at lower strain amplitude, the predicted lifetime increases with increasing material strength. The reason is that the elastic strains are larger for higher strength so that it takes more cycles to accumulate plastic strain and thus to reach a state of cyclic necking. In reality, the associated high local stresses might induce other mechanisms at the microscale. 3.4. E ff ect of strength on fatigue life

3.5. Temperature e ff ects on fatigue life

NCI are used in di ff erent applications like crankshaft in automobiles and wind turbine rotors. In service, these components experience di ff erent temperatures. It is advisable to understand the e ff ect of temperature on the fatigue