Fracture toughness of ceramics and semi-brittle alloys using a miniaturized disk-bend test

Abstract

We review herein the measurement of the fracture toughness of brittle and semi-brittle materials via the controlled-flaw method in combination with the miniaturized disk-bend test (MDBT). The specimens utilized are disks 3 mm in diameter and typically range in thickness from 250 to 400 μm. Each specimen is metallographically polished and indented in its center under a known load, F, using a Vickers indenter. The disk is then subjected to biaxial loading, with the indented side in tension, and tested to failure. The fracture stress, of, is calculated from a standard formula if the material is completely brittle, or determined using finite element analysis if the material experiences limited plasticity prior to fracture. Values of the fracture toughness are then obtained by analyzing the data on of vs. F using established equations of fracture mechanics. This includes the empirical relation between fracture resistance, K_R(c), and crack length, c,

where Q is a constant and K_∞ is the crack resistance at “infinite” crack length. It is convincingly shown that this so-called R-curve equation correctly predicts K_∞, which is comparable to the conventionally measured Mode I plain-strain fracture toughness, KIc, of the same material. The fundamental constants in the fracture-mechanics-based equations are discussed, emphasizing the aspects pertinent to the small specimens used in the MDBT. Results are presented on 8 materials: ZnS, glass-ceramic, Si₃N₄, Ti₅Si₃, SiC, Ni₃Ge, NiA1 and Ti-46.5A1-2.1Cr-3.0Nb-0.2W. All are brittle except for the latter two, which undergo slight plastic deformation before fracturing. The resulting values of K_∞ are in excellent agreement with published values derived from conventional measurements, providing considerable confidence in the method.

Key words:

• Small-specimen testing
• Controlled-flaw method
• Fracture toughness
• Brittle materials
• Finite element analysis