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A novel procedure for characterizing fatigue crack growth in CRB specimens via an optical technique


Papers # 2016 Berlin

Cyclic tests with cracked round bar (CRB) specimens have been used for some time to characterize the slow crack growth behavior of polyolefin pipe grade materials. In fact, most recently a standardized test procedure for cyclic CRB testing has been released in ISO 18489. To obtain kinetic data for cyclic crack growth curves, typically three extensometers attached at three angular positions around the specimen circumference (0°, 120° and 240°) are used to measure crack opening displacement (COD) values and to deal with the problem of asymmetric crack growth. From the COD values an average crack length is then calculated using appropriate specimen compliance equations. Utilizing the extensometer technique is, however, limited to a temperature range for which the extensometers are certified. At higher test temperatures and particularly testing materials in harsh liquid environments (e.g., oilfield liquids) an alternative technique for COD measurements at different specimen perimeter positions is needed. Hence, a technique has been developed and implemented in our laboratory in which the entire specimen is rotated in a tensiontorsion electrodynamic fatigue test machine, which allows for crack length measurements at various circumferential positions of the CRB specimen by an optical technique. Applying this technique, a much finer resolution of crack length measurements can be obtained in steps of 20° angular rotation, allowing for a total of 18 crack length measurements over the whole perimeter. The procedure for optical crack length measurement can be used in combination with conventional temperature chambers with a front window over a wide range of temperatures and it can also be used for superimposed tests in environmental liquids, again over a temperature range, as long as the liquids are sufficiently transparent. First results have been generated with polyethylene (PE) and polypropylene (PP) pipe grade materials.

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