Residual Stress Near Fracture Surface of Alumina Ceramics Measured by X-Ray Method

Abstract

The fracture toughness tests of bluntly notched specimens of sintered alumina were conducted under three point bending. The effect of notch-tip radius on the toughness value was first studied, and then the residual stress beneath the fracture surface was measured by the X-ray method. The depth of the plastic zone left on the fracture surface was determined on the basis of the residual stress distribution. The main results obtained are summarized as follows:
(1) The stress intensity factor K_ρ at the initiation of crack extension was related to the initial notch-tip radius ρ by
K_ρ=K₀(1+ρ/ρ₀)^1/2
where K₀ and ρ₀ are material constants. This was derived through the concept of the fictitious notch-tip radius proposed by Tanaka and Hirose.
(2) The residual stress on the fracture surface measured by X-rays was tension. The tensile residual stress gradually diminished and changed into compression as the depth increased. At a certain depth the residual stress became almost zero.
(3) The size of plastic zone ω_y was determined from the residual stress distribution beneath the fracture surface. The size was related to the fracture toughness K_ρ and the bending stress σ_B by the following equation.
ω_y=0.048(K_ρ/σ_B)²