Application of fatigue precracking methods and their influence on fracture toughness values

The purpose of this work is to compare three fatigue precracking techniques for SENB specimens containing residual stresses. These are local compression, the use of a high R-ratio in the fatigue cycle and reverse bending. The possible effects of these techniques on the values of fracture toughness are assessed. The main conclusion is that local compression reduces the measured fracture toughness up to 50 % and various materials have different sensitivities to 1 % plastic strain. Local compression produces also greater scatter of values. Reverse bending does not affect the fracture toughness but requires further research and development. A disadvantage caused by the use of a high R-ratio is the essential increase in the total precracking time. Uniform fatigue crack front profiles can be obtained by all three methods.