A new method of determining the stress intensity factor K from isochromatic fringe loops

Abstract

This paper describes the four parameter method of analysis for determining the stress intensity factor K. Dynamic photoelastic isochromatic fringe patterns associated with cracks propagating in centerpin loaded, eccentric-pin-loaded and crack-line-loaded SEN specimens of Homalite 100 were recorded. Data was obtained for tests over a range of crack velocities from arrest to the terminal velocity of 14,900 in. sec (378 m sec). Six measurements describing the size and shape of the experimental isochromatic loops were used to determine the stress intensity factor K by employing a comparison function to match analytical and experimental results. The analytical isochromatic loops were generated with a Westergaard stress function of the form Z(z) = K √2πz {1 + β( z a}) and a superimposed normal stress σ Ox = αK|√2 πz which acts parallel to the direction of crack extension. Results were obtained by the computer program (FRACTURE) for different values of the four parameters to give 8925 analytical fringe loops. Another computer program (SEARCH) was used to find a small group of solution which given very low values of the comparison function f c . The final solution which contains the value of K was obtained from the small group by selecting the most consistent solution. The results obtained for Homalite 100 show that K min and K k are nearly the same and that ȧ increases abruptly from 0 to about 10,000 in./sec (254 msec) for modest increases in K above 400 psi √in. (4.4 × 10 5 Nm 3 2 ). Further increases in crack velocity require significant increases in K until terminal velocity is