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Plane strain fracture toughness for Mode I loading KIc testing standards impose restrictions on the loads used to prepare the fatigue cracks because experiments have shown that if the loads are too high the resulting toughness measurement might be unconservative. Data displaying the effects of fatigue cracking load level on KQ measurements (“candidate” KIc values) have been reviewed and compared to restrictions imposed by two relevant standards on fatigue cracking load level and on the R ratio (KFmin/KFmax). For the majority of the materials studied these restrictions are sufficiently stringent, but for some materials that display a particular tendency to cyclically soften it may be necessary to increase their severity to the maximum value of stress intensity factor in fatigue KFmax ≯ 0.5 KQ. The basis of the restrictions on R ratio of the two standards is unknown, but relaxation of the limit (≯ 0.1) is difficult to recommend because of lack of relevant data.
Research engineer, The Welding Institute, Research Laboratory, Abington Hall, Abington, Cambridge,
Stock #: JTE11583J