Fracture tests were performed with carbon/polymer laminates and analyzed for the purpose of developing translaminar fracture toughness test and analysis procedures. Notched specimens were tested of two types of symmetrical layups — quasi-isotropic [0/45/90] and [0/90]; two carbon fiber/epoxy materials — a relatively brittle T300 fiber/976 epoxy and a tougher AS4 fiber/977-2 epoxy; two laminate thicknesses — 2 mm and 4 mm; and three specimen configurations — the standard three-point bend and compact configurations, and an extended compact specimen with arm-height to specimen-width ratio of 1.9. Stress and displacement expressions were obtained for the extended compact specimen, including those for stress intensity factor, K, and crack mouth opening displacement, V, in terms of relative notch length, a/W, and for a/W in terms of V. Relationships for the bending stresses that control self-similar and off-axis cracking for the extended compact specimen were derived.
Damage was characterized in the tests, including that associated with arm breakage in the standard compact specimen and load-point damage in the bend specimen. Two types of notch-tip damage were characterized using radiography, that which extends perpendicular to the notch in predominantly 0‡ fiber layups, and that which occurs ahead of the notch in quasi-isotropic and 90‡ fiber layups.
The applied K at maximum load, Kmax, determined in a way that took account of the effective crack growth up to the maximum load point, was used as a measure of fracture toughness. For deviations from the linear P-V plot corresponding to Δa/W ≤ 0.04, Kmax gave consistent measurements of fracture toughness. This criterion also excluded tests with damage of the type that violates the basic concept of fracture toughness measurement. Plots of Kmax vs δa/W showed increasing resistance to crack growth for quasi-isotropic layups and constant resistance to crack growth for predominantly 90‡ fiber layups.