Fatigue crack growth rate expressed as a function of crack-tip stress-intensity factor range, da/dN versus ΔK, characterizes a material''s resistance to stable crack extension under cyclic loading. Background information on the ration-ale for employing linear elastic fracture mechanics to analyze fatigue crack growth rate data is given in Refs (1) and (2).
In innocuous (inert) environments fatigue crack growth rates are primarily a function of ΔK and force ratio, R, or Kmax and R (Note 1). Temperature and aggressive environments can significantly affect da/dN versus ΔK, and in many cases accentuate R-effects and introduce effects of other loading variables such as cycle frequency and waveform. Attention needs to be given to the proper selection and control of these variables in research studies and in the generation of design data.
Note 18212;ΔK, Kmax, and R are not independent of each other. Specification of any two of these variables is sufficient to define the loading condition. It is customary to specify one of the stress-intensity parameters (ΔK or Kmax) along with the force ratio, R.
Expressing da/dN as a function of ΔK provides results that are independent of planar geometry, thus enabling exchange and comparison of data obtained from a variety of specimen configurations and loading conditions. Moreover, this feature enables da/dN versus ΔK data to be utilized in the design and evaluation of engineering structures. The concept of similitude is assumed, which implies that cracks of differing lengths subjected to the same nominal ΔK will advance by equal increments of crack extension per cycle.
Fatigue crack growth rate data are not always geometry-independent in the strict sense since thickness effects sometimes occur. However, data on the influence of thickness on fatigue crack growth rate are mixed. Fatigue crack growth rates over a wide range of ΔK have been reported to either increase, decrease, or remain unaffected as specimen thickness is increased. Thickness effects can also interact with other variables such as environment and heat treatment. For example, materials may exhibit thickness effects over the terminal range of da/dN versus ΔK, which are associated with either nominal yielding (Note 2) or as Kmax approaches the material fracture toughness. The potential influence of specimen thickness should be considered when generating data for research or design.
Note 28212;This condition should be avoided in tests that conform to the specimen size requirements listed in the appropr......
Copyright ©2024 All Rights Reserved