The effect of lateral crack growth on the strength of contact flaws in brittle materials

Abstract

The effect of lateral cracks on strength controlling contact flaws in brittle materials is examined. Inert strength studies using controlled indentation flaws on a range of ceramic, glass, and single crystal materials reveal significant increases in strength at large contact loads, above the predicted load dependence extrapolated from strength measurements at low indentation loads. The increases are explained by the growth of lateral cracks decohesing the plastic deformation zone associated with the contact from the elastically restraining matrix, thereby reducing the residual stress field driving the strength controlling radial cracks. A strength formulation is developed from indentation fracture mechanics which permits inert strengths to be described over the full range of contact loads. The formulation takes account of the decreased constraint of the plastic deformation zone by lateral crack growth as well as post-contact nonequilibrium growth of the radial cracks. Simple extensions permit the strengths of specimens controlled by impact flaws to be described, as well as those failing under nonequilibrium (fatigue) conditions. The implications for materials evaluation using indentation techniques are discussed and the dangers of unqualified use of strength measurements at large indentation loads pointed out. The work reinforces the conclusion that a full understanding of the residual stress field at dominant contact flaws is necessary to describe the strength of brittle materials.

Effect of Flaw State on the Strength of Brittle Coatings on Soft Substrates

“…(In these fits, there does appear to be some systematic deviation from classical Q Ϫ1/3 dependence at higher Q, likely due to stress relaxation from formation of lateral cracks at high loads. 26 ) We obtain ϭ 0.65 and ϭ 0.046 for glass. These values may be compared with previous determinations of the geometry-dependent coefficient ϭ 0.86 (from Lawn and Marshall 27 ) and 0.77 (from Braun et al 28 ) and material-dependent coefficient ϭ 0.032 (from Lawn and Marshall 27 ) and 0.049 (from Lawn et al 19 ) for extending Vickers flaws in monolithic glass flexure specimens.…”

Effects of ion implantation on the mechanical behavior of GaN films

“…Based on the minimum loads to induce radial cracking and surface flaking, the relative component strength and resistance to erosion and wear of the samples under study can be deduced [38]. Namely, GaN possesses the highest strength and resistance to wear.…”

Final Report to the Office of Naval Research on Precision Engineering

“…No decrease in X with increasing indentation load could be found. Evidently, the decrease in X due to relaxation effects associated with lateral 3 crack growth [24] is not significant for the range of indentation loads 5 N 5 P < 80 N. It must be recognized that the value of X obtained here is for post-indentation crack growth.The operative value of X may be different for the formation of virgin indentation cracks. Lawn et.…”