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How Yield Strength and Fracture Toughness Considerations Can Influence Fatigue Design Procedures for Structural Steels

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How Yield Strength and Fracture Toughness Considerations Can Influence Fatigue Design Procedures for Structural Steels How Yield Strength and Fracture Toughness Considerations Can Influence Fatigue Design Procedures for Structural Steels The K,c /cfys ratio provides significant information relevant to fatigue design procedures, and use of NRL Ratio Analysis Diagrams permits determinations based on two relatively simple engineering tests BY T. W. CROOKER AND E. A. LANGE ABSTRACT. The failure of welded fatigue failures in welded structures probable mode of failure, the impor­ structures by fatigue can be viewed are primarily caused by crack propa­ tant factors affecting both fatigue and as a crack growth process operating gation. Numerous laboratory studies fracture, and fatigue design between fixed boundary conditions. The and service failure analyses have re­ procedures are discussed for each lower boundary condition assumes a peatedly shown that fatigue cracks group. rapidly initiated or pre-existing flaw, were initiated very early in the life of and the upper boundary condition the structure or propagated to failure is terminal failure. This paper outlines 1-6 Basic Aspects the modes of failure and methods of from pre-existing flaws. Recognition of this problem has The influence of yield strength and analysis for fatigue design for structural fracture toughness on the fatigue fail­ steels ranging in yield strength from 30 lead to the development of analytical to 300 ksi. It is shown that fatigue crack techniques for fatigue crack propaga­ ure process is largely a manifestation propagation characteristics of steels sel­ tion and the gathering of a sizeable of the role of plasticity in influencing dom vary significantly in response to amount of crack propagation data on the behavior of sharp cracks in struc­ broad changes in yield strength and frac­ a wide variety of structural materials. tural metals. The degree of plasticity ture toughness. However, strength and However, a degree of organization involved in a given situation influences toughness do vary widely among struc­ and perspective has been lacking in both the mechanical behavior of tural steels, and thereby exert profound much of this work. Preoccupation by cracks and the methods of analysis effects on the fatigue failure process. By which can be applied to the problem. applying the principles of the NRL Ratio researchers with crack propagation Analysis Diagram, an analysis is pre­ laws has obscured the fact that crack The degree of plasticity associated propagation is only a portion of the with a crack can be categorized into sented whereby the plane strain fracture 7 toughness to yield strength ratio (Ku/ fatigue failure process. Furthermore, one of three situations. First, there as.) can be employed to determine fa­ it is probably the portion of the fa­ can be gross ductile yielding where the tigue design procedures. The full spec­ tigue failure process least amenable to fully plastic state extends beyond the trum of structural steels is divided into a substantial improvement. vicinity of the crack and dominates three groups on the basis of fracture be­ The failure of welded structures by the behavior of the entire cracked havior: fatigue can be viewed as a crack body. Second, there can be large scale 1. Ductile plastic fracture mode (K,„/ growth process operating between localized yielding where the nominal a,. > 1.5). fixed boundary conditions—Fig. 1. deformations away from the vicinity 2. Elastic-plastic fracture mode (1.5 The lower boundary condition as­ of the crack are elastic. However, a > K,Joys > 0.5). sumes the existence of a rapidly initi­ plastic zone exists at the crack tip 3. Brittle elastic instability fracture ated or pre-existing flaw, and the up­ which, although contained in an elas­ mode C&0/<r»« < 0.5). per boundary condition is terminal tic field, is large in relation to the The probable mode of failure, the im­ failure. The initial conditions primari­ length of the crack and dimensions portant factors affecting both fatigue and ly rely on fabrication procedures, (usually thickness) of the cracked fracture, and fatigue design procedures body. Finally, there can be small scale are discussed for each group. quality control, nondestructive testing, or proof testing. Crack propagation localized yielding where the crack tip Introduction and terminal failure are design and plastic zone is small in relation to the crack size and thickness of the body. It is now generally recognized that material problems, and fatigue design procedures must take into account Figure 2 schematically illustrates both crack propagation and terminal T. W. CROOKER and E. A. LANGE are some of the mechanics of localized with the Strength of Metals Branch, failure. yielding, where the crack tip plastic Metallurgy Division, Naval Research Lab­ This paper provides guidelines for zone is enclosed in an elastic stress oratory, Washington, D. C. the use of thick-section steels in fa­ field. Under these conditions the size Paper based on presentations made at the tigue-loaded structures. Steels ranging of the plastic zone (2r ), i.e., the Conference on Fatigue of Welded Struc­ y tures in Brighton, England, July 6-9, 1970. in yield strength from 30 to 3000 ksi distance it extends directly in front of and the ASME Pressure Vessels and Pip­ are divided into three groups on the the crack tip, is considered to be a ing Division Conference in Denver, Colo., Sept. 16-17, 1970. basis of their fracture behavior. The function of the stress-intensity factor 488-s I OCTOBER 1970 (K), the yield strength of the material increases and the crack tip plastic curve is limited by upper and lower (o-g,s), the stress state at the crack tip zone becomes large in relation to inflection points. The lower inflection (plane stress or plane strain), and the physical dimensions, successive cor­ point is indicative of nonpropagating mode of load application (monotonic rections must be applied and empiri­ fatigue cracks and occurs under ex­ or cyclic) :7 cism creeps into the fracture mechan­ ceedingly low stress intensities, where ics analysis. Nevertheless, the remain­ the crack growth rates are of the 2r = — (X/V )'--plane stress (1) der of this paper is presented using magnitude of the atomic spacing in y ys fracture mechanics terminology be­ the crystal lattice (~ 10~7 to 10-8 cause, despite analytical limitations, it in./cycle). The upper inflection point 2ry = — CKV/o-j^pIane strain (2) provides a common language basis. is caused by the onset of rapid un­ stable crack extension prior to termi­ Fatigue Crack Propagation nal fracture and places a critical limit 2ry = - (A^KJMatigue (3) on the fatigue resistance of structural HIT Crack Propagation Laws and Mechanisms metals. In actuality these equations merely With the recognition that crack The application of fracture provide rough estimates of the scale propagation is more critical than mechanics to fatigue set the trend for of yielding, mainly for the purposes of crack initiation for analyzing fatigue nearly all subsequent fatigue crack calculating an "effective" crack failure in many structures, numerous propagation studies. It is a successful length, based upon the actual crack efforts to develop engineering ap­ approach to crack propagation to the length plus a plastic zone size correc­ proaches to the problem were under­ extent that the stress-intensity factor taken. Much of this work was aimed tion (ry) for fracture mechanics describes the crack tip plasticity. Fa­ analyses.8 at revealing crack propagation laws tigue crack propagation occurs which would remain valid over a through cyclic plastic deformation oc­ Mechanically, large degrees of plas­ sufficiently broad range of experimen­ curring at the crack tip, regardless of ticity tend to inhibit fracture. High tal conditions. In their critical review how far plastic deformations exist be­ fracture toughness is associated with of crack propagation laws, Paris and yond the crack tip region.12'13 The the ability to develop large plastic 9 Erdogan showed that seemingly con­ relationship between gross cyclic plas­ zones or fully plastic yielding before tradictory results could be normalized tic strain and fatigue failure was rec­ fracture. However, fatigue crack prop­ to fit a single power-law curve by ognized a number of years ago and is agation properties do not seem to be relating the fatigue crack growth rate treated in detail by Manson.14 It was as sensitive to such distinctions, which (da/dN) to the stress-intensity factor used as the basis for a hypothesis for will be discussed in a later section. range (AK). The resulting equation fatigue crack propagation by Weiss.15 Analytically, the degree of accuracy was of the power-law form shown in More recently, Lehr and Liu16 have available for treating the mechanics of equation (4): shown that fatigue crack propagation sharp cracks in structural metals de­ m 7 8 da/dN = C(AK) (4) is caused by damage accumulation creases with increasing plasticity. ' due to strain cycling at the crack tip Fatigue and fracture under fully plas­ where C is a material constant and m and that the crack growth rate can be tic loading must be approached large­ is the slope of the power-law curve. 10 u calculated using strain cycling proper­ ly on an empirical basis. Linear-elastic More recent research ' has shown ties. fracture mechanics can be applied that the entire fatigue crack growth Consequently, both the proposed where the plastic zone resides within rate curve for most materials is actu­ mechanisms for fatigue crack propa­ an elastic stress field. However, rigor­ ally of sigmoidal form when plotted gation and the crack propagation laws ous analyses are limited to small scale on log-log coordinates, as shown in which seek to describe the phenom­ yielding. As the degree of yielding Fig. 3. The power-law portion of the enon in engineering terms are based upon cyclic crack tip plasticity.
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