No abstract

In the framework of the cooperative Dutch research project BROS II (Continued Research on the Behaviour of Cracks in Thickwalled Steel Constructions), Subprogram 6 [i], a study was made of slow stable crack growth during monotonic loading of precracked steel St.52-3 SENB-and CNT specimens.In order to obtain fracture resistance data (J-curves) from such K specimens, it is necessary that the amount of stable crack extension is known.The most commonly used procedure is the multi-specimen heat-tint method.After termination of the test, the unloaded specimen is heated for several hours at approximately 300 deg C, which causes marking of the cracked area due to oxydation.After breaking, the specimen at liquid nitrogen temperature (-196 deg C), the crack extension can be measured with a traveling microscope, using the nine-point averaging procedure as standardized by the ASTM [2].Through the above procedure, only one crack front is obtained per specimen; and in order to reduce the number of specimens needed to obtain sufficient fracture test toughness data, recorse must be made to single specimen test procedures, e.g., potential drop, unloading compliance, key curve method.When using a single specimen test procedure, the amount of crack extension is determined indirectly, and hence it would be desirable to mark the crack front several times during the loading of the specimen in order to obtain a number of actual crack extensions against which the accuracy of the single specimen test method can be checked.As this cannot be done by fatigue cycling (it can be expected to alter the near tip stress and strain fields as well as sharpen the actual crack tip), recourse can be made to crack front marking by dye penetrants (colourants).The investigation of numerous combinations of dye venetrants led to a combination of three dyes, the specifications of which are summarized in Table i. The dyes are listed in order of subsequent application.These dye penetrants and their solvents were seen to meet the following requirements in order to obtain interpretable "colour fronts": i. Capacity for penetration up to the very tip of the loaded crack.For each of the three penetrants listed in Table i, a separate SENB specimen was loaded up to a certain amount of crack extension. After injection and drying of the dye, the specimen w a s u n l o a d e d and the crack was extended further by fatigue cycling.From the halves of the broken specimens, it was found that for each of the applied dye penetrants, the crack had been entirely marked up to the subsequent fatigued area.2. The dyes should not be soluble in each other's solvent in order to prevent blurring of previous markings by partial redissolvement in a subsequent solvent.The first dye penetrant was sprayed on the fracture surfaces of two specimen halves (originating from earlier fracture mechanics tests, and used here in order to approach actual testing conditions).After measuring the average distance to a Int Journ of Fracture 27 (1985) R94 reference line (using the nine point averaging v...