Crack arrest testing at the micro-scale

Abstract

Crack arrest testing of micro-sized cantilever beams (≈8 × 4 × 6 μm, length, width and height, respectively) was conducted in order to evaluate the suitability of a new method to quantify local crack arrest properties. Chevron notched cantilevers were milled to match the (1 0 0)[0 1¯ 1] crack system in α-iron, where earlier attempts to obtain brittle or rapidly propagating fracture proved difficult. Brittle crack initiation and propagation was achieved by means of the deposition of a layer of SiOX on the surface, acting as a brittle starter. All tests were performed at −75 °C, using an in-house designed cooling system. The cracks arrested after propagation into the iron cantilever. A finite element model was developed to determine the appropriate dimensionless shape factor and provide a rigorous computer analysis of these complexly shaped cantilevers. KQC and KQa, at initiation and arrest respectively, were determined and evaluated. The cantilevers were later displaced further at 40 K to allow evaluation of crack jump lengths and to obtain a more complete analysis of the fracture surfaces. The average fracture toughness was determined to be 3.89 ± 1.00 MPam, and the average arrest toughness to be 2.6 ± 0.86 MPam. The finite element model highlights the effect of small variations in geometry which was larger than anticipated and strongly affects the shape factor, up to a 25% difference in f(a/W). As small variations in geometry are inevitable when milling with FIB, the need for individual models tailored to every cantilever is discussed.