A critical evaluation of indentation crack lengths in air

Abstract

An extensive overview is presented of Vickers indentation crack lengths in ceramics in air. Measurement of such crack lengths is one of the most common and powerful assessments of the fracture properties of ceramics and the overview provides a critical evaluation of observed behavior as functions of material type and indentation load, and an extensive basis for comparison of results from new materials and analyses.The overview considers single crystals, polycrystals, transforming materials, glasses, and multiphase materials, including cermets, glass-ceramics, and tooth enamel. The coverage extends over structural and electronic ceramics, including oxides, carbides, nitrides, and titanates. The data are presented in a single format for ease of interpretation in terms of idealized indentation fracture and for inter-material comparisons; most data are unique to this work, but the results of selected studies from the published literature are included. The overview considers the precision and accuracy of crack length measurements and demonstrates a simple quantitative evaluation and ranking scheme for ceramic fracture based on load-adjusted crack length and cracking susceptibility. Indentation hardness and cracking threshold are also determined and related to the susceptibility. Material toughness is related to cracking susceptibility by fracture mechanics analyses: typical crack length measurements in air are shown to provide estimates of inert toughness with a relative uncertainty of ±50%. K E Y W O R D Scracks/cracking, fracture, fracture mechanics/toughness, hardness, indentation

Chemomechanical weakening of muscovite quantified with in situ liquid nanoindentation

“…where Dv env = v 0 -v env and DT env = T 0 -T env are environmental corrections that are both positive. Previous data on a range of ceramics via inert [41] and reactive [42] indentation strength measurements suggest that Dv env & 0.2v 0 and DT env & 0.5T 0 , consistent with observations [39] that show c env & 2c 0 [40]. However, the exact details of the c env /c 0 ratio are known to be dependent on test conditions (e.g., indentation load [43] and moisture content [44]).…”

“…In previous work, the dimensionless factor was found to be v 0 = 0.016(H/E) -1/2 ; the initial term in this empirical relationship was determined by averaging data for ''well-behaved'' ceramic materials [39]. For a material in air or liquid, it was later shown that the length of an indentation crack c env in a relaxed residual stress field is given by [40]…”

“…The effects of the environmental conditions on the geometries of subsurface cracks and stress fields can be quantified by combining Eqs. ( 8) and ( 9) to yield [40]…”

Critique of materials‐based models of ductile machining in brittle solids

“…A hemispherical plastic zone of radius b surrounding a contact of surface dimension a creates the field that drives ensuing cracks of size c , depicted in Figure 1 8 . The classical relations between dimensions a and c and indenter load P for materials with well‐developed median/radial cracks have the form The validity of Equations and has been confirmed in a recent review by Cook 9 . A threshold condition for crack suppression may be obtained by setting a c = c c and eliminating load P from these equations:which differs from the BDS formalism in its dependence on the H/E term.…”

Micromechanics of machining and wear in hard and brittle materials

“…Underlying the machining and wear behavior of this class of solids is the competitive interplay between plastic (or quasiplastic) deformation and brittle fracture in concentrated loading. Mechanical machining/wear is essentially a contact problem, more accurately a microcontact or nanocontact problem, fundamentally established in terms of indentation mechanics 1–9 . A principal factor in achieving acceptable surface finish is the scale of individual contacts.…”