Abstract
Experimental studies of crack micro-geometry in slow-propagating fractures such as fatigue and stress corrosion are important in the elucidation of crack growth mechanisms. The majority of high resolution fractographic work on metals has been carried out using both scanning and transmission electron microscopy techniques applied to fully-separated fracture surfaces. However, information on little more than the variables of crack surface topography can be obtained in this way. Nielson [1] has used a method of inverse replica preparation for the study by transmission electron microscope of stress corrosion fissuring on the surfaces of stainless steel specimens. This technique is limited by the necessity for a high temperature oxidation treatment and has so far been demonstrated only with small cracks in stainless steels. A method has therefore been developed for the production of accurate inverse replicas of full-size propagating cracks in metals. The scanning electron microscope provides the best means for the examination of these replicas and makes possible the study of the geometries of both crack and crack tip. In practice in the first stage of the replication process, cracks in a stressed specimen are completely filled with a low viscosity thermosetting polymer resin (epoxy biological tissue embedding resins are ideal for this purpose). It is important that liquid contained in stress corrosion cracks be completely removed prior to resin impregnation and a two stage anhydrous methanol wash/vacuum evaporation process is an effective means of attaining this objective. The resin is cured and the specimen then sectioned and polished metallographically before being subjected to a deep chemical or electrolytic etch to remove the metal surrounding the polymer-filled crack. Ideally the etching reagent should dissolve the metal without in any way attacking or disturbing the crack replica. Acidified aqueous ferric chloride solution and bromine in anhydrous methanol have been found to be particularly suitable as etchants for brass and steel specimens respectively. After completion of the etch the specimen is washed, dried with extreme care and vacuum plated for SEM examination. The extreme fragility of most replicas, particularly in the crack tip regions, increases the difficulty of these operations if damage is to be prevented. The disruptive effects of fluid
Published Version
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