Techniques for Dynamic Fracture Toughness Measurements,

摘要

The dynamic stress intensity factor K sub D is generally understood to depend upon crack velocity and is an important material property in the study of dynamic failure. This report addresses the problem of experimentally determining K sub D as a function of crack velocity for brittle opaque materials in the range from K sub a to K sub b. A number of techniques were investigated for measuring the dynamic stress intensity factor. The first used a birefringent coating bonded to the material,dynamic reflective photoelasticity,and a high speed framing camera. The resulting isochromatic fringes (contours of constant principal stress differences) were analyzed to obtain K sub D. The second technique employed dynamic holographic interferometry from which isopachic fringes (contours of constant principal stress sums) were generated to determine K sub D values. A third technique referred to as the static equivalent method was used for estimating the dynamic stress intensity factor based on static measurements and correction factors for crack velocity effects and dynamic conditions. All were employed in the testing of a birefringent polyester material, Homalite 100,and the results were compared with those obtained using conventional dynamic photoelasticity. All three techniques are directly applicable to testing non-birefringent opaque materials,but only the static equivalent method was found to be useful for testing of rock. This method was applied to pink Westerly granite and an estimate of crack velocity vs. K sub D was obtained.