Planar‐biaxial low‐cycle fatigue behavior of the nickel‐base alloy Inconel 718 at elevated temperatures under selected loading conditions

Abstract

The isothermal planar‐biaxial fatigue behavior was studied for two different disk batches of nickel‐base superalloy Inconel 718 using cruciform specimens at 400°C and 630°C under equi‐biaxial and shear loading. Additionally, non‐proportional tests were performed. The planar‐biaxial test results were compared with uniaxial reference tests using the von Mises equivalent strain hypothesis, a shear strain parameter of the critical plane, and a modified crack‐opening‐displacement strain range approach. Additionally, the crack initiation mechanism was analyzed. Using a modified crack‐opening‐displacement strain range approach, the low‐cycle fatigue lifetimes of the proportional planar‐biaxial tests (i.e., lifetimes up to 40,000 cycles) were described within a scatter band of two. Thus, it was better than using the equivalent strain of von Mises or a shear strain parameter. The fatigue crack initiation took place at oxidized primary carbides at the surface. The crack paths were presented.

Planar-Biaxial Thermo-mechanical Fatigue Behavior of Nickel-Base Superalloy Inconel 718 Under Proportional Loading

“…The same trend of the mean stress development was also presented in investigations by Deng et al, [12] Sun and Yuan, [31] and Schlesinger et al [32] 2. Planar-biaxial fatigue tests The normalized force amplitudes of the planar-biaxial TMF tests are presented in Figure 7.…”

“…The normalized mean stresses vs. the number of cycles are presented in Figure 6. In contrast to LCF tests, [12] the material developed significant mean stresses over the number of cycles. For the mechanical strain-controlled tests, the mean stresses were negative for IP and positive for OP loads.…”

“…Quasi-static, uniaxial LCF, and planar-biaxial LCF material properties of the batches at the minimum and maximum temperatures of the TMF temperature cycle of 400 °C and 630 °C were presented by the authors in a recent study. [12] The results of warm-tensile tests showed that the quasi-static strength of material B was slightly higher compared to material A. The average grain size of the batches with 8 lm, or 11 according to ASTM E112, was comparable for both batches.…”

“…The lifetimes of the uniaxial TMF tests are presented in Figure 9(a) together with data of isothermal LCF tests at 400 °C and 630 °C on the same material, presented in another investigation by the authors. [12] At high mechanical strain amplitudes, the IP tests had lower lifetimes than the respective OP tests. Here, the lifetimes of the OP tests were around the lifetime of the 630 °C LCF tests.…”

“…Such an influence of multiaxiality was also reported for planar-biaxial LCF tests at 400 °C and 630 °C performed by the authors. [12] This deformation behavior can be explained by common equivalent stress hypotheses, like that of von Mises. Here, under shear loading, the stresses are lower since no hydrostatic stresses exist.…”