On Creep Fatigue Interaction of Components at Elevated Temperature

Abstract

The accurate assessment of creep–fatigue interaction is an important issue for industrial components operating with large cyclic thermal and mechanical loads. An extensive review of different aspects of creep fatigue interaction is proposed in this paper. The introduction of a high temperature creep dwell within the loading cycle has relevant impact on the structural behavior. Different mechanisms can occur, including the cyclically enhanced creep, the creep enhanced plasticity and creep ratchetting due to the creep fatigue interaction. A series of crucial parameters for crack initiation assessment can be identified, such as the start of dwell stress, the creep strain, and the total strain range. A comparison between the ASME NH and R5 is proposed, and the principal differences in calculating the aforementioned parameters are outlined. The linear matching method (LMM) framework is also presented and reviewed, as a direct method capable of calculating these parameters and assessing also the steady state cycle response due to creep and cyclic plasticity interaction. Two numerical examples are presented, the first one is a cruciform weldment subjected to cyclic bending moment and uniform high temperature with different dwell times. The second numerical example considers creep fatigue response on a long fiber reinforced metal matrix composite (MMC), which is subjected to a cycling uniform thermal field and a constant transverse mechanical load. All the results demonstrate that the LMM is capable of providing accurate solutions, and also relaxing the conservatisms of the design codes. Furthermore, as a direct method, it is more efficient than standard inelastic incremental finite element analysis.

Creep-fatigue and cyclically enhanced creep mechanisms in aluminium based metal matrix composites

“…Instead reverse plasticity behaviour is expected for the square cross-section. By introducing a creep dwell within the cyclic thermal loading at the tensile peak, different scenarios arise as reported in (Barbera et al, 2016b) for metallic structures at elevated temperature. Here a new mechanism is seen when the off-axis constant mechanical load in Figure 4-b, is applied.…”

“…A better understanding of the micro material scale is necessary to ensure that certain types of failure mechanism do not arise, such as low cycle fatigue (LCF) crack initiation, ratchetting, cyclically enhanced creep or creep ratchetting. This involves the determination of the shakedown limit, ratchet limit, plastic strain range for LCF assessment, and creep cyclic plasticity interaction (Barbera et al, 2016b;Giugliano et al, 2017;Giugliano and Chen, 2016). The schematic representation of the stress-strain material response due to cyclic loading with creep dwell at the tensile peak, as reported in (Barbera et al, 2016b), clarifies the importance of the aforementioned design limits.…”

“…This involves the determination of the shakedown limit, ratchet limit, plastic strain range for LCF assessment, and creep cyclic plasticity interaction (Barbera et al, 2016b;Giugliano et al, 2017;Giugliano and Chen, 2016). The schematic representation of the stress-strain material response due to cyclic loading with creep dwell at the tensile peak, as reported in (Barbera et al, 2016b), clarifies the importance of the aforementioned design limits. Indeed when the load level is below the elastic limit, no plastic strain occurs at the first cycle and the subsequent creep stress relaxation does not cause any plasticity during the following unloading and loading phases.…”

“…Four types of damage's interactions can occur i.e. pure fatigue, transgranular competing, mixed interaction and pure creep (Barbera et al, 2016b), depending on strain range and dwell time (Hales, 1980;Plumbridge, 1987). A better understanding of the micro material scale is necessary to ensure that certain types of failure mechanism do not arise, such as low cycle fatigue (LCF) crack initiation, ratchetting, cyclically enhanced creep or creep ratchetting.…”

Recent Developments of the Linear Matching Method Framework for Structural Integrity Assessment

“…A series of parameters is required to fully assess the creep-fatigue interaction, which leads to the crack initiation at the most critical location. These parameters are discussed by [14], and are the total strain range, the stress at the start of the dwell, the creep stress drop, the associated equivalent creep strain increment and the elastic follow up factor. Contrary to the R5 procedure, which bases on the calculation of many of the aforementioned parameters on a series of approximate quantities obtained by the elastic solution, the LMM fully considers the creep-fatigue interaction during the cycle and accurately calculates the equivalent elastic, plastic and creep strains and associated stresses by adopting robust solid mechanics concepts.…”