PURPOSE: To accurately measure the fatigue crack propagation lower limit stress expansion coefficient range of a metal material within a short time.
CONSTITUTION: In an experiment repeatedly applying load to a metal material to measure a fatigue crack length and calculating the relation between a stress expansion coefficient range and a crack propagation speed. At first, measurement is performed in an area II where the relation between both of them is obedient to Paris equation and the regression equation of both of them is calculated. Next, the load is gradually lowered and a stress expansion coefficient range wherein at least one of three crack propagation speeds measured in an area I becomes 1/3 the crack propagation speed calculated according to the regression equation and the crack propagation speed to said range are set to measured values. A fatigue crack propagation lower limit stress expansion coefficient range is determined by a nonlinear optimization method using the measured values of the areas I, II.
SETO KOJI
OMIYA SHINICHI
