Continuous damage mechanics — A tool to describe phenomena before crack initiation

Fatigue crack propagation in engineering materials has been the subject of considerable research, and extensive review articles have appeared over the past several years. Most of these investigations focused on the behaviour of ‘long’ fatigue cracks, even though the characteristics associated with the extension of small cracks in metals and alloys remain relatively unexplored, despite their unquestionable importance from an engineering standpoint. In this review, the mechanics and micromechanisms of the subcritical growth of short fatigue cracks are examined, and aspects of their propagation behaviour are contrasted with those of long cracks in terms of fracture mechanics, microstructure, and environment. Cracks are defined as being short (i) when their length is small compared to relevant microstructural dimensions (a continuum mechanics limitation), (ii) when their length is small compared to the scale of local plasticity (a linear elastic fracture mechanics limitation), or (iii) when they are s...

Propagation of short fatigue cracks

Over the past 15 years important advances have been made in the experimental study of the microstructural changes occurring during the non-linear steady-state creep of single phase crystalline matter at elevated temperatures. Curiously, although the results of these painstaking studies have gone a long way toward elucidating the mechanism of this phenomenon, they have been largely ignored in favour of some simple dislocation mechanisms that are not only inconsistent with these observations, but are also unable to describe correctly the known phenomenology. This review concentrates primarily on the recent experiments on microstructural alterations occurring during creep; however, it also surveys the many mechanistic models that attempt to describe this phenomenon, and finds them all deficient.

Steady-state creep of single-phase crystalline matter at high temperature

A crystallographic mechanism for fatigue crack propagation through grain boundaries

The effects of grain size and crack length on the threshold condition of fatigue growth of small cracks are analysed both theoretically and experimentally. The theoretical model is based on the assumption that the threshold condition is determined by whether the crack-tip slip band blocked by the grain boundary propagates into an adjacent grain or not. By comparing the model analysis with the experiments of mild steel, it was found that the threshold stress estimated from the fatigue limit of the smooth specimen gave the effective components of the applied stress. When the threshold stress normalized by the fatigue limit of the smooth specimen was correlated to the crack length normalized by a certain crack length, the theoretical relation was found to agree well with the experimental data. From a further analysis of the published data of various materials, the theoretical normalized relation was found to give a lower bound of the threshold values of the stress and the stress intensity factor. A deviation from the theoretical relation seen in the cases of hard metals was explained through an extension of the model by regarding that hard metals contained initial, inherent flaws even in the smooth specimen.

Fatigue growth threshold of small cracks

For designing structures which are to be subjected to variable loads and temperatures, it is desirable to know the creep properties of materials under such conditions. Formulas are derived to predict creep in the transient and steady stages and creep rupture time under variable loads and temperatures from data obtained in simple creep or creep rupture tests. The concepts of equivalent stress and equivalent temperature are introduced and the validity of the formulas is discussed on the basis of the results of experiments performed with heat resistant alloys.

Lifetime of Structures Subjected to Varying Load and Temperature

Application of J-Integral to High-Temperature Crack Propagation: Part I—Creep Crack Propagation

A model of crack-tip slip band blocked by grain boundary

Biaxial tension-compression fatigue tests were conducted with cruciform shaped specimens in a closed-loop servo hydraulic testing machine. The effects of static and cyclic non-singular stresses acting parallel to the crack plane on the crack growth rate are discussed based on the experimental observations of crack opening behaviour and fractography. Those non-singular stresses did affect the growth rate significantly under certain conditions. The range of crack-tip opening displacement was found to be a better parameter in correlating the growth rate than the stress intensity range or its effective range. The rate tended to increase with increasing non-singular stress which is correlated to the opening displacement range. This tendency was explained by the shift of fracture mechanisms to a more brittle type due to a higher elevation of hydrostatic stress near the crack tip for the case of a larger non-singular stress term.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1460-2695.1979.tb01354.x

Fatigue crack propagation in biaxial stress fields

A collaborative study was conducted by the Subcommittee of the Japanese Society of Materials Science Committee on High Temperature Strength, in order to examine thermal fatigue properties of four Japanese high temperature alloys, and to determine short tensile, creep and isothermal fatigue properties.



Thermal fatigue lives of SUS 304 and Hastelloy X were found to be shorter than isothermal fatigue lives at the maximum temperature of the thermal cycle. Intergranular cracks were observed more frequently in in-phase thermal fatigue than in isothermal fatigue at the maximum temperature. The lower bounds of out-of-phase thermal fatigue life and in-phase thermal fatigue life were nearly equal to Manson's pc-type fatigue life and his cp-type fatigue life, respectively. An empirical formula previously reported by the authors was found to be useful as an approximate life prediction of thermal fatigue based on isothermal fatigue lives.

Shuji Taira

Papers

Lifetime of Structures Subjected to Varying Load and Temperature

Application of J-Integral to High-Temperature Crack Propagation: Part I—Creep Crack Propagation

A model of crack-tip slip band blocked by grain boundary

Fatigue crack propagation in biaxial stress fields

Collaborative study on thermal fatigue properties of high temperature alloys in japan