High Temperature Short Fatigue Crack Behaviour in a Stainless Steel

Abstract

Continuous low cycle fatigue (LCF) tests with-and without-hold time in push-pull and torsion loading modes and sequential LCF tests in push-pull mode were carried out at 650°C in air on thin tubular specimens of 316 stainless steel; the sequential tests involving pure fatigue (PF) and c r e e p fatigue (CF) loadings. The growth of short fatigue cracks was studied by taking several replicas from the specimen surface which were subsequently observed under a scanning electron microscope. An analysis was done with respect to both crack density and the orientation of microcracks and macrocrack(s) which led to failure.Crack density was higher on the surface of a C F tested specimen than that of a PF tested specimen.Mainly short cracks oriented at 45" to the specimen axis were observed on a torsion fatigue tested specimen surface. For push-pull specimens the microcracks propagated perpendicular to the specimen axis to form macrocracks that propagated in the same direction. On the other hand, for torsion specimens the microcracks which initially propagated at 45" to the specimen axis linked to form macrocracks oriented parallel and perpendicular to the specimen axis. However, the macrocrack responsible for the final fracture was always oriented parallel to the specimen axis. Cumulative damage was dependent on the type of loading (PF or CF) in the first part of sequential tests. In particular microcracks initiated during an initial damage phase observed under sequential LCF tests in PF were found to be healed by oxide formation during the hold times applied in the subsequent C F loading and this produced a total damage summation significantly larger than one. NOMENCLATURE a = half crack length a, = critical half crack length (da/dN) =crack growth rate D,, D, = ratio between the number of applied loading cycles in a sequential test to the number of cycles to failure with that type of loading in a continuous test N = number of cycles Ni =number of cycles necessary to initiate the first microcrack of length 50 pm ytot = total shear strain Ay, = plastic shear strain range At, = plastic axial strain range t , , = total axial strain E~~~~~ = von Mises total equivalent strain x ( N ) =surface density of microcracks

Cyclic creep behaviour and strain classification of a bainite 2.25Cr-1Mo steel at 455oC

“…High temperature equipment is generally subjected to cyclic loadings during operation. [1][2][3][4][5] Hence, the failure mechanism of these kinds of equipment is complicated and often with mixed damages modes of creep, fatigue, as well as their interactions. Under realistic loading conditions, such as nuclear reactor components, stress loading is considered to be more common than strain loading.…”

“…The cumulative deformation for cyclic creep is time-dependent, which is similar to ratcheting. 8,9 According to previous studies, the strain rate can be greatly influenced by a series of test parameters, such as stress amplitude, stress ratio, and duration under hold stress. With the increase of duration under peak stress, a transition in dominant damage type from fatigue to creep can be found, and the fracture mode transformed from brittle to ductile failure correspondingly.…”