Crack Paths 2012

Outer layer

Diffusion zone

Substrate

Figure 1. Microstructure of a coated specimen in the section perpendicular to the

specimen surface (SEM)

Biaxial fatigue experiments were carried out on cylindrical specimens (the gauge

length 16.4 m mand the diameter 8.5 m m )by means of a resonance testing machine

MZGS-200 operating in the load-control regime. Three loading regimes were

employed: (i) symmetric (R = -1) bending, (ii) symmetric torsion and (iii) their

a § τa.

synchronous in-phase combination with equal bending and torsion amplitudes

Both loading components had a sinusoidal shape of the loading cycle and were applied

at room temperature at frequency f ≈ 30 Hz up to a final rupture. Cylindrical button-end

specimens (the gauge length of 15 m mand the diameter of 6 m m ) were used in low

cycle fatigue tests at 800 °C in air [6]. The specimens were fatigued in a computer

controlled electro-hydraulic testing system at total strain rate of 2x10-3 s-1 with a fully

reversed total strain cycle (Rε = -1). Heating was provided by a three-zone resistance

furnace and monitored by three thermocouples attached to both specimen ends and to

the upper part of the gauge section.

R E S U L T S

Biaxial fatigue tests

The results of fatigue experiments are plotted in Fig. 2 in terms of stress amplitude vs.

number of cycles to failure Nf. The combined bending-torsion equivalent stress, ekv,

was calculated as

ekv = (a2 + 3τa2)0.5. In consistence with the results of high

temperature push-pull experiments (see below), the presence of D A Cleads to a

decrease of the bending fatigue strength in the low-cycle fatigue (LCF) region

(Nf = 103 - 105 cycles). On the other hand, the presence of the coating seems to slightly

improve the LCFresistance in the case of torsional loading. The results for combined

bending-torsion loading seem to follow those of the bending experiments: the D A C

somewhat reduces the LCF life. On the other hand, the high-cycle fatigue (HCF)

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