Determination of double-K criterion for crack propagation in quasi-brittle fracture, Part II: Analytical evaluation and practical measuring methods for three-point bending notched beams

摘要

In this paper, an attempt is made to determine thedouble-K fracture parameters K_(Ic)~(ini) and K_(Ic)~(un) usingthree-point bending notched beams. First, based on theknowledge from extensive investigations which showed that thenonlinearity of P-CMOD curve is mainly associated with crackpropagation, a linear asymptotic superposition assumption isproposed. Then, the critical effective crack length a_c isanalytically evaluated by inserting the secant compliance c_s intothe formula of LEFM. Furthermore, an analytical result offictitious crack with cohesive force in an infinite strip model wasobtained. The double-K fracture parameters K_(Ic)~(ini) andK_(Ic)~(un) as well the critical crack tip opening displacementCTODc were analytically determined. The experimental evidenceshowed that the double-K fracture parameters K_(Ic)~(ini) andK_(Ic)~(un) are size-independent and can be considered as thefracture to determine K_(Ic)~(ini) and K_(Ic)~(un) is quitesimple, without unloading and reloading procedures. So, forperforming this test, a closed-loop testing system is not necessary.