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High temperature and total strain controlled low cycle fatigue tests for smooth and notched specimen were carried out for the nickel base superalloy Rene80. Two different material batches were tested, where smooth specimen with a preferential grain orientation distribution showed distinct higher lifetimes compared to smooth specimen made of material with random grain orientation. However, for notched specimen no lifetime improvement could be determined for both batches.
Polycrystalline finite element simulation were conducted to understand the influence of different grain orientation distribution on the mechanical behaviour of the specimen in order to explain the observed lifetime behaviour. For smooth specimen the texture of the grains lead to lower Young’s moduli in loading direction and therefore to lower stresses during total strain testing, when compared to a random grain orientation distribution. Lower stresses in combination with also observed lower Schmid factors of the grains, delays the crack initiation and extends therefore the lifetime for smooth specimen with textured grain orientation.
For the notched specimen the simulations reveal an inverse behaviour. The texture of the grains leads to a slightly higher stiffness when compared to random grain orientation distribution. Whereas the bulk of the notched specimen, where a uniaxial stress state can be assumed, shows low stiffness (similar to the smooth specimen) the interaction of varying principal stress along the notch with constant grain texture leads to areas at the notch where the stiffness is significantly increased. It follows that high stresses occur within the notch, which are similar to the stresses in a specimen of the random orientation distribution. Also no differences in the Schmid factor for both orientations distributions could be found. Due to minor differences in mechanical behaviour the lifetimes of textured and random orientated are comparable under same test conditions.
| Speaker Country | United Kingdom |
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