danger-of-surface-flaws-and-volume-flawsThere are less surface flaws to be expected thanvolume flaws. Surface flaws are more dangerous, butvolume flaws are more deceptive."Effective" crack length when the crackcan be detected at thesurface "a2"Volume flaw below the limitof detectibility"a1"Rapid crack growth to thiscrack length "a3"Critical crack length"2ac" with the stressconcentration KIcCrack size thatcan be safelyfound in a non-destructive testTolerable flawsize in the newpartAllowable life span isreachedRelatively slow crackgrowth in this zone untilthe surface is reachedRelatively rapid crackgrowth in this zone afterthe surface is reachedCorresponds to residual life spanto failurePart surfaceDue to the greater stress intensity and accelerating atmospheric influences (corrosion, etc.), the growth rate of surface cracks is considerably higher than that of internal cracks, aside from the fact that they have, by definition,a greater characteristi crack length "a".Crack size "a"for a surfacecrackCrack size "a"for an internalcrack Thickness "d" = const.Part surface "O"Minimum flaw size thatis capable of growth "a"(initial flaw)Part volume "V"Flaw concentration"n F" Volume flaw / surface flawPart thickness "d" [mm]0 4 8 12 16 201086420a=1mma=2mma=4mma=8mmNumber of flaws Z=V x nF = O x d x nFNumber of volume flaws= nF x O (a + d)Number of surface flaws= 2nF x O x aRelative frequency of volume flaws/surface flaws =(d+a)/2aLoad changesCrack length "a"Flaw size "a1"Flaw size "a2"Flaw size "a3"Very rapid crack growth after itbreaks through to the surfaceRapidly accelerated growth ofthe surface crackRelatively slow growth of the internal crackExplanatory diagramfor crack growth in themiddle right diagram