A team of researchers from the University of Sheffield’s Department of Mechanical Engineering, including Polymer Centre academic Dr Rachel Tomlinson, have recently developed a method for creating repeatable pre-cracked polymer specimens for fracture testing.
Currently, specimens are prepared manually using a razor blade to create a small pre-crack from a notched sample. These samples are then tested to destruction to obtain fracture properties of the material. The procedure is heavily dependent on the operator and produces pre-cracks of varying length, angle and crack-front shape; all of these factors affect the repeatability and reliability of the subsequent test, as well as posing several safety concerns.
Using a technique first proposed by Tamura et al., Dr Tomlinson’s team developed a methodology to produce consistent pre-cracks that meet the requirements of the various test standards for measuring fracture toughness. The technique involves applying loading in 2 directions; a tensile load to propagate a crack from the notch manufactured during specimen creation, and a compressive load applied across the width of the specimen at the desired distance from the notch to act as a barrier to further crack growth (Figure 1). A variety of compressive loads were investigated to find a suitable load for the test material which prevented crack propagation past the desired length, without causing the crack to then travel parallel to the direction of the compressive load. Once this was established, 40 test samples were prepared using this method and the pre-crack length and shape recorded. The results showed that the developed method was very repeatable, with 35 of the specimens accurate to within 10 microns of the desired crack length. A comparison of the crack fronts produced using the current and proposed methods showed that the developed method also produces a significantly straighter crack front, minimising the need to average the crack length across the width of the specimen and saving additional time.
Original Publication: Nithiananthan Kuppusamy, Rachel A. Tomlinson, Repeatable pre-cracking preparation for fracture testing of polymeric materials, Engineering Fracture Mechanics, Volume 152, February 2016, Pages 81-87, ISSN 0013-7944
Article by Ryan Brown; a PhD Student on the EPSRC Polymers, Soft Matter and Colloids CDT programme. For more information, please contact Dr Joe Gaunt at the Polymer Centre.
 Tamura K, Hasjimoto S. A precrack introducing method in CT-specimens for measuring KIC values of brittle materials. In: 15th International conference of experimental mechanics. ICEM15. Portugal; 2011. p. 7