BRIEF REVIEW AND RECONSIDERATION OF FATIGUE-CRACK CLOSURE EFFECT IN MATERIALS

The state-of-the-art of fatigue crack closure effect, involving the me chanisms, measurements, models, evaluation methods, influencing factor s and existing problems is briefly reviewed. Attention is concentrated on a new consideration of closure effect and a re-definition of the e ffective driving force for the propagation of a fatigue crack. A theor etical basis for measuring the closure effect is established. The prop osed closure concept takes into account the role of the tower portion of loading cycle below the opening load. The conventional closure conc ept is proven to be correct only for some extreme cases which are prac tically non-existent for engineering materials. A miniature strain gau ge technique is developed to measure the crack closure. Stress-ratio-c hange tests at threshold level clearly indicate the shortcomings of th e conventional closure concept, and the validity of the newly-proposed closure concept. The application of the proposed concept can give ris e to a new method for the determination of the effective fatigue thres hold of materials. If the fatigue threshold is measured at a sufficien tly negative stress ratio (R<R(cr)) the tensile portion of the cyclic loading just reflects the effective fatigue threshold of the material.