Design and repeatability analysis of desktop tool for rapid pre-cracking of notched ductile plastic fracture specimens

Abstract

Fracture testing is a useful mechanical testing process to explore the properties and behavior of materials, one that has seen much development and refinement in recent decades. One of the most important steps in preparing samples for testing is the production of a sharp pre-crack to initiate crack propagation in a predictable way. While several methods have been developed for doing this, particularly for metals and brittle plastic materials, a quick and reliable method for more ductile materials is lacking. This paper describes the design and verification of a simple desktop-sized pre-cracking device which safely uses a razor blade and hammer to quickly produce straight and sharp pre-cracks of consistent depth in ductile polymeric material samples. To verify its capability and consistency, a series of tests was performed using both molded and 3-D printed acrylonitrile butadiene styrene (ABS). First, a series of 40 notched 25mm×9.5mm ABS bars was pre-cracked, and the distance under the crack measured on both sides of the bar. Several bars were then broken along the cracks to examine the quality of the pre-crack front. These tests were then repeated 20 times each for two print orientations of fused deposition modeling (FDM) ABS printed at 100% density. All 80 pre-cracks were found to be straight, sharp, and within 1% of the nominal distance under the crack for all samples. The consistency of the pre-cracks throughout the sample cross-section was also observed to be excellent, with all 80 tests showing less than 0.25mm of deviation, even on the highly-anisotropic FDM samples.