Role of adhesive interlayer in transverse fracture of brittle layer structures Herzl Chai Department of Solid Mechanics, Materials and Structures Faculty of Engineering, Tel Aviv University, Israel 69978 Brian Lawn Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 2 December 1999; accepted 31 January 2000) 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 prebonding abrasion treatments to ensure uniform flaw states, so as to enable accurate comparisons between crack initiation conditions. The adhesive bonding is strong enough to preclude delamination in our layer system. Of the three transverse crack systems, the subsurface radials generates most easily in systems with large 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. I. INTRODUCTION Brittle layers can be made damage tolerant by joining them together into laminate structures with a weak ad- hesive interphase. 1–4 Practical examples are seashells, natural teeth and dental crowns, car windscreens, and some thermal barrier coatings. The brittle layers afford stiffness, wear resistance and durability; the compliant interlayer provides damage tolerance, by redistributing stresses, and confining fractures within individual brittle layers. The adhesive needs to be weak enough to prevent cracks from penetrating into adjacent layers but strong enough to preclude delamination failures. The damage tolerance properties of such stiff-layer/ soft-foundation structures are especially conspicuous in concentrated loading configurations, e.g., indentation with hard spheres, projectile impacts. In such cases the outer brittle layers are subject to transverse (through- thickness) cracking. Such cracking has been well docu- mented in many ceramic-based layer and coating systems on soft supporting substrates, from examination of sec- tions through indentation sites. 5–17 Two new contact- induced transverse crack systems have been identified in these studies: surface circumferential ring cracks that ini- tiate at the top surface well outside the contact circle (distinct from conventional cone cracks that occur just outside the contact in monolithic materials); and subsur- face median–radial cracks that initiate at the bottom sur- face of the overlayer, i.e., at the interface with the soft support. Normal stresses across the interface plane are compressive below the contact, so delamination cracks J. Mater. Res., Vol. 15, No. 4, Apr 2000 © 2000 Materials Research Society 1017