K. J. Miller

Fatigue cracks at notches

Abstract The failure of a component or specimen due to a fatigue crack growing from a notch is considered. Previous methods of analysis involving stress and strain concentration factors are shown to be inadequate. By defining equivalent cracks in notched and un-notched situations as cracks with equal growth rates, the concept of notch contribution to crack length is introduced. Theoretical notch contributions are obtained for a variety of central and edge elliptical notches via stress intensity factor solutions. These results when extended to a very wide range of general notch shapes can be reduced to a useful and simple design rule e = 7·69l √( D ϱ ) where e is the contribution to a crack of length l growing from a notch of depth D and root radius ϱ. This rule combines the size and shape effects long known to affect fatigue behaviour and defines the extent of the notch field as 0·13√(Dϱ) . The fatigue crack propagation lives of a wide variety of notches were estimated by this rule and comparisons with experimental values revealed very small errors normally well within the scatter of fatigue lives. The design rule is extended to enable the conventional stress intensity factor method to be employed. A fatigue concentration factor is proposed which takes into account the presence of a fatigue crack which all previous methods have ignored.

Short Fatigue Cracks

For more than a century, metal fatigue research has been associated with producing S-N endurance curves from experiments on plain and notched specimens. Recently a detailed interpretation of S-N curves generated from smooth specimen tests has become possible because of our increased understanding of the behaviour of very small cracks ie cracks growing to about 500 pm in depth, which can involve a high proportion of the fatigue lifetime at high endurances. This recent knowledge has been due to the development of elastic-plastic fracture mechanics of crack growth, new techniques to monitor the growth of small surface cracks, and not least, from commercial pressures to use advanced materials.

A critical comparison of proposed parameters for high-strain fatigue crack growth

A number of elastic-plastic fracture mechanics parameters have been suggested for the description of fatigue crack growth, of which three are considered in this paper: the J-integral, crack-tip opening displacement, and plastic zone size. A critical comparison of the effectiveness of each parameter may be devised from a study of Mode I fatigue crack growth rates under various states of biaxial stress. Data for one heat of AISI 316 austenitic stainless steel at 20°C and 550°C showed that, if crack closure is taken into account, the cyclic plastic zone size is the most suitable correlating parameter for stress levels up to the yield value. A second heat of 316 stainless steel showed a preference for crack opening displacement control of crack growth. The analysis is based on the Dugdale model for crack-tip plasticity.

Electrochemistry of deformed smooth surfaces and short corrosion fatigue crack growth behaviour

AbstractThe polarisation characteristics for a deformed smooth surface of a 0·2% carbon steel in an artificial sea water have been determined under static and cyclic loading conditions. The influence of strain level and loading frequency on anodic and cathodic Tafel constants, corrosion current density, and corrosion potential is described. Based on these data, the conditions for corrosion fatigue testing which corresponded to a maximum synergism between surface deformation and anodic dissolution were determined. Corrosion fatigue tests, which were conducted under both constant potential and constant current density conditions have shown that a process of metal dissolution plays a determining role in the short corrosion fatigue crack growth behaviour. An experimentally based criterion is proposed involving the development of a short corrosion fatigue crack, of characteristic size, which is associated with the spacing between the major microstructural barriers. This criterion is a function of both shear st...

Fatigue-Life Predictions Including the Effects of Hold Time and Multiaxial Loads on Crack-Coalescence Behavior

Two aspects of crack-coalescence behavior are reported. The first concerns a regime frequently referred to in the literature as creep-fatigue interactions but which in this paper is essentially a time-dependent, fatigue-failure process. The second relates to crack coalescence under a wide range of different multiaxial stress-strain states. In the framework of the first approach, a fatigue-crack growth model is derived based on experimental observations during high-temperature, high-strain, reversed-bend, hold-time tests on AISI 316 stainless steel. Essential features of these tests are the compressive and the tensile 60-min hold periods on different surfaces, which induce, respectively, transgranular-short and intergranular-long cracks. The latter, more damaging cracks involve the coalescence of numerous short cracks to form a dominant Stage II crack that leads to failure. Then, in the framework of the second approach, the crack-coalescence model is advanced to predict the fatigue lifetimes for multiaxial, variable amplitude, proportional loading of a medium carbon steel commonly used to manufacture engineering components. It is shown that under high strain fatigue conditions the models used for the calculations of lifetime must necessarily involve crack-coalescence behavior if unsafe lifetime predictions are to be avoided.