Fatigue threshold R-curves predict small crack fatigue behavior of bridging toughened materials

A direct comparison of non-destructive techniques for determining bridging stress distributions

“…Also plotted are bridging stresses measured using fluorescence spectroscopy (solid circles) with error bars indicating 7 1 standard deviation based on the standard deviation of the linear calibration fit. Reprinted from Gallops et al (2011a) with permission from Elsevier. elongated b-phase grains with an average diameter of 0.28 mm with an aspect ratio of $ 7.…”

“…The alumina ceramic is the same as in Fig. 2 and in Gallops et al (2011aGallops et al ( , 2011b. It has nominally equiaxed grains widely ranging in size with a majority having cross-sectional areas of o63 mm 2 (or diameter of o9 mm).…”

“…One recent comparison of independently measured bridging stresses for an Al 2 O 3 ceramic is very encouraging (Gallops et al, 2011a). Indeed, bridging stresses for a fatigue crack were determined by fluorescence spectroscopy and from the R-curve and the results agreed well (Fig.…”

“…For this Si 3 N 4 material the bridging zone is known to be only tens of micrometers in length (Fünfschilling et al, 2009b(Fünfschilling et al, , 2011. Thus, a determination of fatigue thresholds during the bridging zone development and the fatigue threshold R-curve DK TH (Da), as was done in Gallops et al (2011a) for Al 2 O 3 , was impossible. Rather, the steady state fatigue threshold, DK TH , was determined for a fully developed bridging zone.…”

“…2 from Gallops et al (2011a) the laser sampling volumes were determined for both materials. The sampling volumes were calculated based on the absorption coefficients measured using thin samples.…”