Procedure for a Drop Tower Testing of Shallow Cracked Single Edge Notched BandSpecimens Jun 89-Feb 90

摘要

Underprediction of the reserve capacity of structures against fracture oftenresult from use of conventional initiation fracture toughness values measured using deeply cracked bend specimens. Conversely, favorable comparisons between prediction and structural fracture behavior are reported when predictions are based on toughness values measured so that the crack tip constraint and loading rate closely match those experienced in service. Thus, considerable attention has been paid to elevated fracture initiation resistance of cracks shallower than the 0.45 to 0.75 a/W used in standardized tests. However, standard procedures for estimating fracture initiation resistance of shallow cracks, particularly at the high loading rates characteristic of certain severe service conditions, have yet to be developed. We developed and validated techniques useful for estimating the load, load line displacement and onset of crack extension during impact tests of single edge notched bend specimens having shallow fatigue cracks (a/W = approx. 0.1). Bending stress distribution across the specimen midway between the support and loading points, determined based on strain measurements and a uniaxial stress-strain relationship, was used to estimate the load imparted to the specimen by the impactor. Load line displacement was estimated using four non-contacting transducers positioned along the underside of the bend specimen while onset of crack extension was inferred from strain readings taken from the elastically loaded region behind the fatigue crack tip. Applicability of these procedures was demonstrated at an impact loading rate of 4.88 m/s.