ACCURATE CRITICAL STRESS INTENSITY FACTOR GRIFFITH CRACK THEORY MEASUREMENTS BY NUMERICAL TECHNIQUES

摘要

Critical stress intensity factor (K_(Ic)) has been an approximation for fracture toughness using only load-cell measurements. However, artificial man-made cracks several orders of magnitude longer and wider than natural flaws have required a correction factor term (Y) that can be up to about 3 times the recorded experimental value [1-3]. In fact, over 30 years ago a National Academy of Sciences advisory board stated that empiricalK_(Ic) testing was of serious concern and further requested that an accurate bulk fracture toughness method be found [4]. Now that fracture toughness can be calculated accurately by numerical integration from the load/deflection curve as resilience, work of fracture (WOF) and strain energy release (S_(Ic)) [5， 6], K_(Ic) appears to be unnecessary. However, the large body of previous K_(Ic) experimental test results found in the literature offer the opportunity for continued meta analysis with other more practical and accurate fracture toughness results using energy methods and numerical integration. Therefore, K_(Ic) is derived from the classical Griffith Crack Theory [6] to include S_(Ic) as a more accurate term for strain energy release rate (G_(Ic))， along with crack surface energy (γ), crack length (a), modulus (E)， applied stress (σ), Y， crack-tip plastic zone defect region (rp) and yield strength (oys) that can all be determined from load and deflection data. Polymer matrix discontinuous quartz fiber-reinforced composites to accentuate toughness differences were prepared for flexural mechanical testing comprising of 3 mm fibers at different volume percentages from 0-54.0 vol% and at 28.2 vol% with different fiber lengths from 0.0-6.0 mm. Results provided a new correction factor and regression analyses between several numerical integration fracture toughness test methods to support K_(Ic) results. Further, bulk K_(Ic) accurate experimental values are compared with empirical test results found in literature. Also, several fracture toughness mechanisms are discussed especially for fiber-reinforced composites.