Role of Adhesive Interlayer in Transverse Fracture of Brittle Layer Structures

Published

April 1, 2000

Author(s)

H Chai, Brian R. Lawn

Abstract

The role of a soft adhesive interlayer in determining critical conditions for fracture in brittle layer structures from indentation with hard spheres is investigated. A model transparent trilayer system consisting of a glass plate overlayer (thickness range 80 m to 2 mm) joined to a glass plate underlayer (thickness 5.6 mm) by an epoxy adhesive (thickness range 5 m to 8 mm), loaded at its top surface with a hard tungsten carbide sphere (radius 3.96 mm), facilitates in situ observations of the crack initiation and propagation. Whereas in bulk glass fracture occurs by inner Hertzian cone cracking immediately outside the contact circle, the soft adhesive allows the overlayer glass plate to flex, initiating additional transverse fracture modes within the overlayer downward-extending outer ring cracks at the top glass surface well outside the contact, and upward-extending radial cracks at the bottom glass surface (i.e. at the glass/adhesive interface) on median planes containing the contact axis. The top and bottom surfaces of the glass overlayers are given selective pre-bonding abrasion treatments to ensure uniform flaw states, so as to enable accurate comparisons between crack initiation conditions. The adhesive bonding is sufficiently strong as to preclude delamination in our layer system. Of the three transverse crack systems, the subsurface radials generate most easily in systems with larger adhesive thicknesses (and smaller overlayer thicknesses). Semi-empirical relations are specified for the dependence of the critical loads for radial and ring cracking on adhesive as well as overlayer thickness, based on the assumption that crack initiation occurs when the maximum tensile stresses in the flexing glass plate exceed the bulk strength of the (abraded) glass. Coupled with the traditional Auerbach's law for cone crack initiation, these relations afford a basis for the construction of simple design diagrams for brittle layer systems joined by adhesives.

Citation

Journal of Materials Research

Volume

15

Issue

No. 4

Pub Type

Journals

Keywords

adhesives, brittle layers, circumferential cracks, cone cracks, indentation, laminate structures, radial cracks

Citation

Chai, H. and Lawn, B. (2000), Role of Adhesive Interlayer in Transverse Fracture of Brittle Layer Structures, Journal of Materials Research (Accessed April 25, 2024)