Code of practice for the measurement and analysis of high strain creep-fatigue short crack growth

摘要

There are no procedural standards for the determination of crack growth properties where a reversal of stress is involved, in particular at elevated temperatures. The purpose of this code of practice is to detail the requirements for fatigue and creep-fatigue short crack growth (CFSCG) testing, generally on uniaxial testpieces subject to high strain loading conditions. CFSCG testing may entail the use of a single testpiece to produce data over several strain ranges. Alternatively, data from a number of constant strain range tests may be obtained, as in conventional low cycle fatigue (LCF) endurance, but where a small starter (sharp) defect has been introduced. Procedures for long crack fatigue crack growth and LCF testing are covered by a number of existing codes of practice and standards, and this document does not recommend any alteration to these. CFSCG rates may be determined from testpieces with much larger notch acuities, e.g. uniaxial specimens of the Bridgman type, or reverse-bend specimens with a stress concentrating feature. Thus in addition to recommending best laboratory practice, this document includes sections on engineering requirements and test data analysis. Advice is also given for those circumstances where testpiece material is limited, thus requiring quick methods of data acquisition using block loading techniques. Only isothermal testing at appropriate temperatures under strain-controlled conditions is covered, although the techniques can be extended to non-isothermal (thermo-mechanical) fatigue testing conditions. An appendix covers in more detail: (i) the relation between the shape of short cracks (straight-fronted, thumbnail, etc.) and the specimen area thereby consumed by the fracture surface, and (ii) the relation between this fractional area change and the corresponding change in both the crack-monitoring potential drop signal and drop in peak tension load.