Fracture of Alumina with Controlled Pores

Abstract

Fracture from artificial spherical pores, as well as natural defects, in alumina in a grain-size range of 0.8-9.2 µm has been studied experimentally and compared with a fracturemechanics model. Results from fracture-strength measurements have been combined with detailed fractographic analysis to elucidate the ensuing crack instability. Two existing models of possible crack configurations have been extended and contrasted. The semicircular crack as well as the circumferential crack both are described as flaws in the stress-concentrating field of a spherical pore. Surface correction terms afforded by the presence of the pore have been incorporated. A comparative computation shows that fracture occurs more likely from the semicircular crack configuration than the circumferential crack configuration.

Mechanical properties of cBN–Al composite materials

“…The strength appears to decrease with increasing grain size, at least with the batches with 20 and 25 vol.% binder content. This decrease in strength with grain size is a common feature for most ceramics [25] and related to the size of the initial flaw as related to the grain size [26]. With 15 vol.% binder there is no clear trend.…”

Fracture of Ceramics

“…These could be described as sharp notches distributed around the sphere. [63,64] It is obvious that the tip radius of these grooves in front of the pore is much less than 1 lm, giving a very elongated flow.…”

“…For more spherical flaws, which have similar stress concentrations in all directions, the PIA criterion seems to be more appropriate. [63] It should be noted, however, that the influence of the criterion on the equivalent stress is relatively limited: for a material with a Weibull modulus of m=20 and in a biaxial stress state, the FPS equivalent stress is r e,FPS =r I whereas the PIA equivalent stress is r e,PIA =2 1/m r I ≈1.04 r I , i.e. the difference is only four percent.…”

Effects of Pore/Grain-Size Interaction and Porosity on the Fracture of Electroceramics

“…The three-dimensional configuration of defects in ZnO ceramics could be described by a spherical pore with some kind of sharp cracks, such as a circumferential crack, a semicircular crack and a circular crack. [15][16][17][18] The numerical analysis of three-dimensional model is much more complex since we need to consider the special configuration of defects and their arrangement, but it is worthwhile to be investigated. …”

“…Based on fractographic evidence, a model for brittle fracture has to include the interaction of two different defect populations: intrinsic defects on the microstructural level such as grain boundaries are considered as nucleation sites for microcracks, whereas processing defects such as pores or inclusions act as stress concentrators. [15][16][17][18] In a first approximation, a typical flaw in ZnO ceramics is represented by a pore with cusps or sharp grooves, as illustrated in Figure 2(a). Here there are three characteristic length scales: the pore radius R, the grain radius r, and the notch depth c (or the groove angle u).…”