Edge chipping resistance of ceramics: Problems of test method

Abstract

An unconventional method for determining the fracture resistance of brittle materials is discussed. This method employs a conical indenter to chip the rectangular edge of the specimen. Particular features of the method are the use of small specimens and the evaluation of the resistance of materials to the nucleation, initiation and propagation of a crack. It is shown that this method is somewhat similar to the Hertzian fracture method and to the way that early man selected stones to make tools and weapons. Measured data of the fracture resistance of ceramics is presented. It is confirmed that if a ceramic material is similar to the model material of linear elastic fracture mechanics (LEFM), then those fracture resistance values are directly proportional to the critical stress intensity factors (baseline). For elastic and inelastic ceramics, R-lines characterizing the fracture resistance to crack growth are plotted. It is shown that proportionality lines (edge chipping resistance versus critical stress intensity factor) may be straight lines for ceramics with similar structure (such as Y-TZP and Mg-PSZ). The effect of rounding of the conical indenter tip (10-800 µm) on chip scar shape is indicated. Other aspects in the fracture behavior of ceramics during edge chipping are also analyzed. The advantages and disadvantages of the method are discussed. Further studies in this mechanico-physical research area are suggested.

Numerical modeling edge chipping tests of ceramics

“…The PATH S trajectory (Fig. 9b and c) consists of two sections: PATH S1 (points 1, 2, 3, 4, 5) representing the primary semicircular crack and PATH S2 (points 5,6,7,8,9,10,11,12) representing the secondary surface crack. Point 5 belongs to both sections and is at the junction between them.…”

“…9b and c) and coordinate Z varying within 0 6 Z 6 2 ⁄ 10 À4 m. The numerical analysis has revealed the following regularity. The partial derivative oU z /oz has a local extremum (a minimum in the range of negative values) at Table 3 Points of the trajectories PATH S1 (1-5) and PATH S2 (5)(6)(7)(8)(9)(10)(11)(12) Fig. 10 shows, as an example, the variation in oU z /oz (e z ) and r z with Z at chosen points (5 and 8) of the oU z /oz trajectory PATHS2 in a silicon-nitride ceramic specimen with d = 125 lm.…”

“…11 shows, as an example, the isolines of the displacement components U z and the trajectory PATH S in the plane XZ superimposed on the top view of a chip in an aluminum oxide ceramic specimen with d = 125 lm. The trajectory consists of two sections: PATH S1 (points 1, 2, 3, 4, 5) representing the primary semicircular crack and PATH S2 (points 5,6,7,8,9,10,11,12) representing the secondary surface crack. This figure also shows the variation in oU z /oz (e z ) and r z with Z at point 5.…”

“…According to the synergistic principles, point 5, which belongs to both the primary semicircular crack (PATH S1) and the secondary surface crack (PATH S2), may be recognized as a bifurcation point at which the system changes from one state to another. Table 4 Points of the trajectories PATH S1 (1-5) and PATH S2 (5)(6)(7)(8)(9)(10)(11) …”

“…It is especially important for study unique expensive materials and for the cases where products are much smaller than standard specimens used in conventional fracture toughness methods based on linear elastic fracture mechanics (LEFM) [5]. A feature of the method under consideration (its version, called the EF method, was substantiated in [2]) is that it allows determining the fracture resistance of brittle materials at all three stages of fracture (crack nucleation, initiation, and extension) simultaneously [6,7]. Unlike this, the LEFM-based test methods involve preparing stress concentrators thus omitting the first stage of fracture, which is only valid for ductile materials.…”