ABSTRACT
Use of the diagram is most simply illustrated if all factors of stress, crack size and toughness are known so that an Sr-Kr state point can be plotted.
If K is estimated as in LEFM (with no plastic zone correction) and KIc is known, then the ordinate Kr = Kj KIe is known. If the 'local' limit load Fe is estimated, then the abscissa Sr= FIFe is known (or MIMe or PIPe for moment and pressure). The resulting point is then clearly seen to be within the diagram (i.e. safe) or outside it (i.e. unsafe). Obviously J can be directly estimated from eqn. (5.12) and compared to J Ie or J R if so desired, but the object of the diagram is to ensure that J < Jeri! and F < Fc whilst estimating only Kby LEFM and Fe by limit analysis. Since crack size affects both axes it is either necessary to iterate or better to explore the effect of increasing
one of the variables, e.g. crack length at known stress levels. A locus is then plotted within the diagram that shows the sensitivity of failure to changes of the variable. Such a line is shown later in Fig. 5.18, for one particular case. If the crack length is fixed and the stress level is increased (proportionally for all loading terms) then the margin of safety or stress level is represented by OF/OA (Fig. 5.18), and one of the main virtues of this form of diagram is the insight it gives to the separate effects of the several variables.
