Translaminar Fracture Toughness Test Methods and Results from Interlaboratory Tests of Carbon/Epoxy Laminates

Abstract

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.

Measurement of the in situ ply fracture toughness associated with mode I fibre tensile failure in FRP. Part II: Size and lay-up effects

“…Much work has been performed in measuring the fracture toughness of composite laminates using the compact tension (CT) specimen [1][2][3][4], though very little has been done to investigate specimen size or lay-up effects.…”

Fatigue Crack Growth in Two Advanced Titanium Alloys Under Constant and Variable Amplitude Loading at 25 and 175 °c

“…Fatigue crack growth specimens were machined from ≈ 1.5‐mm‐thick sheet material into extended compact tension specimens, EC(T) [15 ], and middle tension, M(T), specimens for constant and variable amplitude loading, respectively. EC(T) specimens were nominally 38 × 142 mm and M(T) specimens were nominally 50 × 180 mm.…”

Composite Energy Storage Flywheel Design for Fatigue Crack Resistance

“…It is an extended compact tension test for Mode I fracture toughness measurement [4][5][6][7]. The geometry of the test specimen and the fiber orientation are shown in Figure 9.…”