Effect of Test Specimen Size on Graphite Strength

One of the key issues that must be addressed when a sampling and measurement program underwrites the performance of full-size graphite components is the understanding of the influence of material type and specimen size on the measured property. Because of sampling constraints, specimen sizes (geometry and/or volume) may not be compliant with measurement standards and this issue is investigated in the context of graphite strength. Theoretical considerations on stress distribution, together with a review of experimental work based upon Gilsocarbon nuclear graphite IM1-24, are presented for the measurement of strength by three-point bend. A broader historical review of the effects of specimen size on strength, covering studies published in the open literature and hitherto restricted studies published within the U.K. nuclear industry, is also presented, together with some considerations of three-point bend measurement consistency across multiple facilities and some basic trending of three-point bend data. Whereas the findings of this experimental work apply specifically to the grade of graphite selected for investigation, the methods employed in the study provide a model for general application. Furthermore, the review of the wider literature gives some indication of the sensitivity of flexural strength to specimen geometry, volume, and graphite grade.