Interaction and evolution of short fatigue cracks

A B S T R A C TThe fatigue behaviour of base metal and weld joints of 1Cr-18Ni-9Ti stainless steel has been studied under uniaxial, torsional and 90 • out-of-phase loading. A significant degree of additional hardening is found for both base metal and weld metal under 90 • out-of-phase loading. Both base metal and weld metal have the same cyclic stable stressstrain relationship under torsional cyclic loading and 90 • out-of-phase cyclic loading. Base metal exhibits higher cyclic stress than weld metal under uniaxial loading, and Young's modulus and yield stress of weld metal are smaller than those of base metal. Weld metal exhibited lower fatigue resistance than base metal under uniaxial and torsional loading, but no significant difference was found between the two materials under 90 • out-of-phase loading. A large scatter of fatigue life is observed for weld metal, perhaps because of heterogeneity of the microstructure. The Wang-Brown (WB) damage parameter and the Fatemi-Socie (FS) damage parameter, both based on the shear critical plane approach, were evaluated relative to the fatigue data obtained.Keywords 1Cr-18Ni-9Ti stainless steel; multiaxial fatigue; welded joint. N O M E N C L A T U R Eb, c = fatigue strength exponent and fatigue ductility exponent, respectively b 0 , c 0 = shear fatigue strength exponent and shear fatigue ductility exponent, respectively E, G = elastic modulus and shear modulus, respectively HB = Brinnell hardness k = material constant in the Fatemi-Socie model S = material constant in the Wang-Brown model δ 5 = elongation ε, γ = axial strain and shear strain, respectively ε e , σ e = equivalent strain and equivalent stress, respectively ε * n = normal strain excursion on the critical plane ε f = fatigue ductility coefficient γ f = shear fatigue ductility coefficient ε, γ = axial strain range and shear strain range, respectively γ max = maxmimum shear strain range on the critical plane ν = Poisson's ratio σ , τ = axial stress and shear stress, respectively σ u = ultimate tensile strength σ f = fatigue strength coefficient σ max n = maximum normal stress in a cycle on the critical plane σ y = yield stress τ f = shear fatigue strength coefficient = reduction in areaCorrespondence: X. Chen.

Section: Methodsmentioning

confidence: 86%

Low‐cycle fatigue of 1Cr–18Ni–9Ti stainless steel and related weld metal under axial, torsional and 90° out‐of‐phase loading

Chen

Kim

2004

“…In the description of the crack growth we have adopted the earlier concept 13 of an equivalent crack typical of an applied plastic strain amplitude. A similar concept was adopted later by Zhao et al 23 The length of an equivalent crack is equal to the instantaneous longest crack in the area of observation. The equivalent crack in our steel in the majority of cases was identical to the single real growing crack.…”

Section: Analysis and Discussionmentioning

confidence: 99%

Short crack growth and fatigue life in austenitic‐ferritic duplex stainless steel

Polák

Zezulka

2005

A B S T R A C TThe kinetics of short crack growth has been studied in austenitic-ferritic 2205 duplex stainless steel. Smooth cylindrical specimens and specimens with shallow notch were subjected to constant plastic strain amplitude loading. The crack growth was studied in notched specimens. The notch area has been mechanically and electrolytically polished to facilitate the observation of crack initiation and growth. The initiated cracks were observed in an SEM (scanning electron microscope). The crack growth was studied using long distance QUESTAR optical microscope equipped with high-resolution camera. In constant plastic strain amplitude loading the microcracks were initiated and their growth kinetics has been studied. The characteristic features of the crack growth at different plastic strain amplitudes were recorded. Two approaches to analyse the crack growth rates were adopted. The comparison of the prediction of the fatigue life using the plastic-strain-dependent crack growth rate was compared with Manson-Coffin law and the relation between parameters of this law and parameters of the short crack growth law were established. a = crack length a f = crack length at the end of the fatigue life a i = extrapolated crack length to zero cycle c = fatigue ductility exponent d = exponent of the exponential crack growth law G(n ) = function of the exponent of the cyclic stress-strain curve k g = crack growth coefficient k g0 = parameter of the exponential crack growth law K a = stress intensity amplitude n = the exponent of the cyclic stress-strain curve N = number of cycles N f = number of cycles to fracture N i = number of cycles to crack initiation N g = number of cycles to propagate a crack v 0 = reference crack growth rate ε ap = plastic strain amplitude ε f = fatigue ductility coefficient γ = exponent of the crack growth rate law σ a = stress amplitude J = range of J-integral J 0 = parameter of the crack growth rate law Correspondence: J. Polák.

“…It has been shown that the DESFC growth behaviour is a result of interactive short cracks, and this behaviour should be deemed suitable to describe the collective behaviour of short cracks 13 . Figure 10 shows variations of the DESFC growth rates in five specimens under different loading levels.…”

Section: Microstructural Effectsmentioning

confidence: 99%

“…The present work attempts to reveal the microstructural effects driving short‐crack growth in 1Cr18Ni9Ti stainless weld metal smooth specimens, which were machined from a welded pipe of mixed microstructure during low‐cycle fatigue, therefore it is difficult to distinguish the grains and measure the grain diameter. An experimental study of this material 12 , 13 has been recently undertaken into the mechanism of interaction and evolution of short fatigue cracks. In that study, three new concepts, referred to as effective short fatigue cracks (ESFCs), a dominant effective short fatigue crack (DESFC), and the density of ESFCs, respectively, were introduced to facilitate an understanding of the mechanism of interaction and evolution of short cracks.…”

Section: Introductionmentioning

confidence: 99%

Microstructural effects on the short crack behaviour of a stainless steel weld metal during low‐cycle fatigue

Zhao

Gao

Wang

1999

An experimental study into microstructural effects on short fatigue crack behaviour of 19 stainless steel weld metal smooth specimens during low‐cycle fatigue is performed by a so‐called ‘effective short fatigue crack criterion’. This material has a mixed microstructure in which it is difficult to distinguish the grains and measure the grain diameter. The columnar grain structure is made up of matrix‐rich δ ferrite bands, and the distance between the neighbouring rich δ ferrite bands is an appropriate measurement for characterizing this structure. Particularly, the effective short fatigue cracks (ESFCs) always initiate from the bands of δ ferrite in the matrix in the weakest zone on one of the specimen surface zones which is orientated in accordance with the inner or outer surface of welded pipe from which the specimens were machined. These cracks exhibit characteristics of the microstructural short crack (MSC) and the physically small crack (PSC) stages. The average length of the ESFCs at the transition between MSC and PSC behaviour is ≈40 μm, while the corresponding fatigue life fraction is ≈0.3 at this transition. Different from previous test observations, the growth rate of the dominant effective short fatigue crack in the MSC stage still shows a decrease with fatigue cycling under the present low‐cycle fatigue loading levels. A statistical evolution analysis of the growth rates reveals that the short fatigue crack growth is a damage process that gradually evolves from a non‐ordered (chaotic) to a perfectly independent stochastic process, and then to an ordered (history‐dependent) stochastic state. Correspondingly, the microstructural effects gradually evolve from a weak effect to a strong one in the MSC stage, which maximizes at the transition point. In the PSC stage, the effects gradually evolve from a strong to weak state. This improves our understanding that the short crack behaviour in the PSC stage is mainly related to the loading levels rather than microstructural effects.