Correlation Between Fatigue Crack Propagation and Low Cycle Fatigue Properties

Abstract

Apprrvad for public rcltoMg DUtrihution Unlimitod »// tmtmtaumlmiiim ■ >L^w*mpiipipi»i|B^ppppfB»!»Tiwpppppw^w(«iii^i ii wpw» wwpuuww" "i .1 miwrnfmrnim mi" "". > mi^lPBnmnwaiiinjiiiii^iii ■> m^i^mm^mm FOREWORD Recent requirements for increased strength and service life of machines and structures have been m>t by the use of higher strength materials and new fabrication and joining methods. Simultaneously, failures due to fracture have increased raiative to those resulting from excessive deformation. Frequently service conditions are such that low temperature brittle fracture, fatigue fracture, and high temperature creep rupture must be considered in a single system. National concern with increased safety, reliability, and cost has focused attention upon these problems. Methods are now available to predict both fatigue crack initiation life and crack propagation life. Paradoxically the materials properties required for long fatigue crark initiation life are incompatible with the requirements of high fracture toughness. Thus, the conflicting design approaches and requirements placed on the material are confusing and often impossible to satisfy. Numerous publications dealing with a variety of fracture problems have led to many new and useful developments. However, the synthesis of the concepts into methods for design, testing and inspection has lagged. This program of study is intended to contribute to the integration, correlation, and organizaticn of mechanics and materials concepts and research information into a form that will permit enlightened decisions to be made regarding fracture control. Reports are in preparation in three categories: 1. Research reports designed to explore, study and integrate isolated and/or conflicting concepts and methods dealing with life prediction, 2. Reports to introduce and summarize the state-of-the-art concepts and methods in particular areas, and 3. Example problems and solutions intended to illustrate the use of these concepts in decision making.

A tentative explanation for two parameters, C and m, in Paris equation of fatigue crack growth

“…The scatter of the data from the correlation line in Fig. The mechanical properties may affect the fatigue crack growth rate through the variation in the crack opening behavior or through the exponent, m. In this study, the basic log(daMN) vs. log(AK,fr) curves are 566 K. Tanaka, S. Matsuoka estimated for four steels according to the cumulative damage criterion [4,3,25] by using the low cycle fatigue data. It follows from this that the fatigue crack growth is relatively insensitive to yield strength and microstructure [2,3].…”

“…The mechanism is described essentially in the same way as proposed by Lehr and Liu [4] and developed by Majumdar and Morrow [25]. Figure 3 shows a schematic representation of the stress and strain history experienced by an element ahead of a propagating crack in a plate subjected to repeated tension so that AK is constant.…”

“…Majudmar and Morrow [25] assume that there is a critical width of element ahead of the crack tip to which macroscopic failure criteria may not be applied, being determined by the microstructural units responsible for strengthening in the metal. In reality, this may not be true and the crack closure may be accompanied by a change in the shape of the cyclic plastic zone.…”

“…Majudmar and Morrow [25] assume that there is a critical width of element ahead of the crack tip to which macroscopic failure criteria may not be applied, being determined by the microstructural units responsible for strengthening in the metal. Majumdar and Morrow [25] assume implicitly that U is equal to unity, independently of AK, and that the coefficient, C,n, in (3) becomes a function of AK. The latter assumption actually implies that the maximum strain amplitude suffered in the cyclic plastic zone should be dependent on AK.…”

“…The most realistic mechanism for fatigue crack growth is that predicting a AK 2 dependence of crack growth rate, which resulted either from a cumulative damage criterion [4,3,25] or from a crack opening displacement (COD) criterion [5]. This is explicitly or implicitly based on two assumptions; small scale yielding and the preservation of similarity in strain distribution in the plastically deformed region at the advancing crack tips.…”