Crack‐Interface Grain Bridging as a Fracture Resistance I, Mechanism in Ceramics: I, Experimental Study on Alumina

Abstract

Direct microscopic evidence is presented in support of an explanation of R-curve behavior in monophase ceramics by grain-localized bridging across the newly formed crack interface. In situ observations are made of crack growth in tapered cantilever beam and indented flexure specimens of a coarsegrained alumina. The fractures are observed to be highly stable, typical of a material with a strongly increasing resistance characteristic, but are discontinuous at the microstructural level. Associated with this discontinuity is the appearance of overlapping segments in the surface fracture trace around bridging grains; the mean spacing of such "activity sites" along the trace is about 2 to 5 grain diameters. These segments link up with the primary crack beneath the specimen surface, and continue to evolve toward rupture of the bridge as fracture proceeds. The bridges remain active at large distances, of order 100 grain diameters or more, behind the crack tip. Scanning electron microscopy of some of the bridging sites demonstrates that secondary (interface-adjacent) microfracture and frictional tractions are important elements in the bridge separation process. Evidence is sought, but none found, for some of the more popular alternative models of toughening, notably frontal-zone microcracking and cracktiphnternal-stress interaction. It is suggested that the crackinterface bridging mechanism may be a general phenomenon in nontransforming ceramics.

Fracture and Fatigue Behavior at Ambient and Elevated Temperatures of Alumina Bonded with Copper/Niobium/Copper Interlayers

“…When intergranular fracture is predominant in bulk alumina, frictional tractions that develop along the fractured grain boundaries in the crack wake are known to sustain load during monotonic loading, thereby shielding the crack tip from a portion of the applied driving force and resulting in rising toughness with crack extension. [49][50][51] The result of such behavior is illustrated in Fig. 8 in the form of a rising R-curve for this Coors AD995 alumina.…”

Comportamiento mecánico de materiales cerámicos estructurales

“…A particular of this mechanism is based on the interlocking produced between the surfaces of the crack due to frictional sliding between grains on the intergranular fracture surfaces. In this case, rupture of the grains which produce this interlocking for the maximum opening of the crack does not occur; this has been described as particularly effective in alumina materials with large grain size (>40µm, 5,[46][47]. Toughening by crack-bridging is conditioned by the state of residual tensions of the particle and the matrix, since this determines whether the crack surrounds or draws nearer to the toughening ligament.…”

Fractal Fracture Mechanics Applied to Materials Engineering

“…It corresponds to the classical equation (70) corrected for a rugged surface with Hurst's exponent H. Experimental results [1,2] show that J0 and the crack resistance 0 R rise non-linearly and it is well known that this rising of the J-R curve is correlated to the ruggedness of the cracked surface [3,4]. .…”

“…For its correction it will be considered that the elastic energy released rate G can be expressed as a function of 0 G according to equation (4 …”

“…As an example of this limitation, there is the problem of stable crack growth, characterized by the J-R curve [1,2]. The rising of this curve has been analyzed by qualitative arguments [1,2,3,4] but no definite explanation in the realm of EPFM has been provided.…”