Microcantilever investigation of slow crack growth and crack healing in aluminium oxide

Journal article

Slow crack growth (SCG) plays an important role in the fracture of ceramics and glasses. A full understanding of the long term reliability of ceramic components therefore requires knowledge not only of fracture toughness (K_c) but also characteristics of SCG such as the threshold stress intensity factor (K₀) of individual microstructural features. Previous investigations have been limited by the small scale of the microstructure and low crack growth rates involved. Here, the stress intensity factors for crack propagation K_p in sapphire were measured continuously under stable crack growth using chevron-notched microcantilevers, in air and in vacuum. K_p in vacuum was considered close to the fracture toughness K_c, with a value of 2.8 ± 0.1 MPa m^1/2 for the a-plane of sapphire. K_p in air at 54 % relative humidity was reduced substantially by moisture-assisted SCG to around 1.7 MPa m^1/2 and, with crack velocities of ∼20 nm/s, was considered to be close to K₀. Cyclic loading in vacuum enabled quantitative measurements to be made during repeated crack growth and healing for the first time. Cracks healed up along their full length when unloaded but exhibited severe degradation of toughness on reloading. The toughness fell from 2.8 MPa m^1/2 on the a-plane of pristine sapphire to 1.6 MPa m^1/2 after one healing cycle and to successively lower values after further cycles. This technique offers a powerful method of investigating SCG and crack healing at the microstructural scale in different environments, with greater accuracy and on shorter timescales than can be achieved in macroscopic tests.

Authors: Jiang, J; Falco, S; Wang, S; Giuliani, F; Todd, RI

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Elsevier
• Journal: Acta Materialia
• Volume: 273
• Article number: 119914
• Publication date: 2024-04-14
• Acceptance date: 2024-04-10
• DOI: 10.1016/j.actamat.2024.119914
• EISSN: 1873-2453
• ISSN: 1359-6454
• Language: English
• Keywords: environment-assisted cracking; crack healing; fracture toughness; aluminium oxide; slow crack growth; FFR; microcantilevers