Development of the Effective SIF Range Concept for Predicting the Fatigue Crack Growth Rate at Different Stress Ratios and Maximum SIF Values. Part 1. Fitting Unified Elber-Type Diagrams and Detecting Anomalies in Some Materials (Marci Effect)
摘要
The development of Elber’s effective stress intensity factor (SIF) concept for predicting fatigue crack growth (FCG) rates under various stress ratios R and maximum SIF values over the last 40 years is discussed in detail. Using ten state-of-the-art Al, Ti, and Ni superalloys as examples, this paper analyzes the relevance and feasibility of further using Elber’s concept by combining FCG curves with different SIF values Kmax and positive R values into a unified ΔKeff–da/dN diagram via empirical Elber-type spline functions. For negative R values, it is proposed to collapse all FCG curves into a single one along the ordinate (FCG rate) axis using a new “vertical intercept-shift method” (VISM), which allows integrating the contributions of crack closure and other factors with high correlation (r2 ≈ 0.94–0.96) for most experimental FCG curves of Al2024-T3. Examples of anomalous FCG behavior at high Kmax values (Marci effect) for different materials are provided, yielding the problem formulation for the alternative substantiation of this effect via modified short-crack ΔKeff–da/dN diagram for structural alloys envisaged in Part 2 of this article.