Ratcheting behaviour and mean stress considerations in uniaxial low‐cycle fatigue of Inconel 718 at 649 °C

Abstract

A B S T R A C TThe ratcheting behaviour of Inconel 718 was investigated at 649 • C under uniaxial cyclic loading. Stress-control tests have been conducted at various combinations of stress amplitude and mean stress. The ratcheting strain at failure increases with increasing mean stress for a given stress amplitude and with decreasing stress amplitude for a given mean stress. Fatigue lives were correlated using three mean stress models: the Goodman equation, the Smith-Watson-Topper (SWT) parameter and the Walker parameter. It has been shown that the Goodman equation and the SWT parameter do not correlate life data, while the Walker parameter yields acceptable correlation. The SWT parameter was modified to incorporate the ratcheting effect. The new parameter is found to yield correlation similar to that of the Walker parameter.Keywords Goodman equation; mean stress effect; ratcheting strain; SWT parameter; uniaxial fatigue; Walker parameter. N O M E N C L A T U R Eσ a = stress amplitude σ m = mean stress σ eq a = equivalent stress amplitude σ max = maximum stress R = stress ratio (R = σ min /σ max ) ε r = ratcheting strain N = number of cycles N f = number of cycles to failure σ f = fatigue strength coefficient b = fatigue strength exponent γ = Walker exponent I N T R O D U C T I O NMany engineering components are subjected to cyclic loading in which the fatigue process takes place under stress-control conditions. Fatigue cycles may not be fully reversed. In these circumstances, the mean stress effect has to be taken into account in fatigue life prediction. A phenomenon that occurs in the presence of mean stress in the low-cycle fatigue regime is ratcheting, which results from the accumulation of plastic strain. Ratcheting causes fatigue damage of the material and shortens the life of engineering components significantly, unless a shakedown state is reached in the early stage of life. Naturally, this important phenomenon has received the attention of many researchers, and numerous studies have been carried out on the deformation aspect of the phenomenon. The Armstrong-Frederick nonlinear kinematic hardening rule 1 is widely used in ratcheting analysis. The decomposed model by Chaboche 2 and the multilinear model by Ohno and Wang 3 belong to this category. These models have been successful for predicting uniaxial ratcheting, but they tend to overpredict ratcheting under multiaxial loading. Despite many modified models suggested 4,5 to resolve the shortcomings, the prediction of ratcheting strain under multiaxial loading still remains a challenging problem. Not only that, there are many other variables influencing ratcheting behaviour which need to be further investigated. For all these and other reasons, the 1076

Ratcheting and low cycle fatigue behavior of SA333 steel and their life prediction

“…This dimensional alteration of the specimen helps to increase the maximum true stress to such a high value that instability and necking is often observed. Similar trends of ratcheting strain accumulation have been reported for many materials; for example Inconel 718 at 649°C [30]. Fig.…”

“…Therefore the fatigue life is not appreciably affected by the introduction of a mean strain as compared with the symmetric strain cycling test [40,41]. Whereas, stress controlled cycling with mean stress causes accumulation of permanent strain (ratcheting strain) which impose an additional damage, resulting shorter in fatigue life [11,12,30,39]. So, ratcheting effects should be taken into account during prediction of fatigue life in case of asymmetric stress cycling.…”

Fatigue life estimation in presence of ratcheting phenomenon for AISI 304LN stainless steel tested under uniaxial cyclic loading

“…(2) A new stress-based fatigue life estimation equation is proposed in this investigation. The proposed equation satisfactorily predicts the ratcheting life in the range of 10 2 -10 5 cycles for the investigated material as well as for few other materials like a copper alloy: Elbrodor-NIB (Lim et al, 2009) and Inconel 718 (Park et al, 2007). (Walker, 1970) 1058.431 À0.0876 Inconel 718 (Smith et al, 1970) 972.849 À0.05…”

“…Determination of Basquin's coefficient for three different materials: (a) 304LN stainless steel; (b) Elbrodur-NIB(Lim et al, 2009); (c) Inconel 718(Park et al, 2007).Goodman and the SWT parameter-based equations show better fatigue life prediction as compared to the Goodman equation; but none of these life predicting equations are able to predict fatigue life completely satisfactorily. This demands the re-tuning of power law coefficient and exponent for predicting fatigue life signifying importance of cyclic plasticity.…”

A constitutive model for uniaxial/multiaxial ratcheting behavior of a duplex stainless steel

“…Parallel to the ratcheting experiments [1][2][3][4][5][6][7][8][9][10][11][12][13] which have provided valuable information for uniaxial/multiaxial ratcheting characteristics of a variety of materials, various constitutive models have been developed to simulate the cyclic plastic response of materials as accurate as possible. The accuracy in simulating ratcheting behavior is mainly related with the modeling of anisotropic strain hardening.…”