opening-of-cracks-before-penetrant-testingClosed cracks can be opened through etching. However, this can damage the base materialand make detection more difficult due to background fluorescence.Opening of a crack through a sufficientlyrealistic mechanical loadBurr from grinding orblasting closes the crackGrinding burr closesthe crackTouching crack edgesTouching crack edgesSuitable opening of cracks before or during penetrant testing is of decisive importance to the effectiveness of the process.Crack opens through:- Warping, i.e. "standing-up" of the burr- The burr's adhesion is dissolved- Brittle structural components are broken upCrack opening through plastic deformationdue to compressive stresses during heatingand a pulling-apart of the compressed areaduring coolingExperience has shown that cracks that have been smeared shut can be foundmore easily if a heat-treatment cycle occurs before penetrant testing.Heat treatment causes:- High thermal stress in the burr- A breakdown of residual stresses through creepRotor diskPenetrant is sprayed during spinningEtching results in:- Material removal- Changes in the residual stressesCrack opens through:- Removal of the burr- Dissolving of any adhesion- Deformation of the burr through shifting of the residual stresses Opening of the crack through materialremoval from the crack sides, aided bya capillary effect. Cracks on grain boundaries are opened especially effectivelyif these areas are more sensitive to corrosion