Determination of fracture mechanics parameters by measurements of local displacements due to crack length increment (original) (raw)

A B S T R A C T New experimental technique for a determination of the stress intensity factor and the non-singular T-stress is developed, verified and implemented. The approach is based on combining the crack compliance method and optical interferometric measurements of local deformation response on small crack length increment. Initial experimental information has a form of in-plane displacement component values, which are measured by electronic speckle-pattern interferometry at some specific points located near a crack tip. The first four coefficients of Williams' series are derived. A determination of initial experimental data at the nearest vicinity of crack/notch tip is the main feature of the developed approach. In this case, it is not necessary to use complex numerical models, which are connected with geometrical parameters and loading conditions of the object under study in a stage of experimental data interpretation. Moreover, an availability of high-quality interference fringe patterns, which are free from rigid body motions, serves as a reliable indicator of real stress state in the vicinity of the crack tip. Experimental verification of the proposed method is performed for specially designed specimen of double cantilever beam type. Distributions of the stress intensity factor and the T-stress for two cracks in different residual stress fields in the vicinity of friction stir welding joints are presented as an example. a nÀ1 , a n , a n+1 = crack length A n (n = 1, 2, 3, 4) = coefficients of Williams infinite series d ξ = in-plane displacement component in ξ-direction E = Young's modulus K I = stress intensity factor (SIF) in mode I n À 1, n, n + 1 = points of crack tip position N = absolute fringe orders T = T-stress value r, θ = radial and angular distance from the crack tip, respectively U n (r, θ), V n (r, θ) = differences in absolute values of in-plane displacement components for cracks of length a n and a nÀ1 u n , v n = absolute values of in-plane displacement component in x-direction and y-direction, respectively, for a crack of a n length x, y = coordinate axis directed along and normally to the crack line, respectively Δa nÀ1 , Δa n , Δa n+1 = crack length increment ΔN nÀ1 , ΔN nÀ0.5 = relative fringe orders Δ N (v) = total crack opening Δv nÀ1 , Δv nÀ0.5 = crack opening λ = wavelength of laser illumination μ = Poisson's ratio Ψ = sensitivity angle of speckle interferometer