THE INFLUENCE OF THE K-GRADIENT AND K-MAX(C) LEVEL ON FATIGUE RESPONSE DURING THE K(MAX)(C) THRESHOLD TESTING OF VAN-80 STEEL AND ASTROLOY

Owing to the increased popularity of the K-max(c) threshold test proce dure, work has been undertaken to investigate more closely two importa nt variables of that test: the K gradient and the K-max level. Both Va n 80 steel and Astroloy were tested with various K gradients (-0.08 to -1.2 mm) and under a range of K-max levels (25-55 MPa m(1/2) for the steel and 35-65 MPa m(1/2) for the nickel-based superalloy). Even at 1 5 times the ASTM recommended gradient, there was no influence of testi ng speed on the fatigue response of either material. However, the K-ma x(c) level greatly influenced Astroloy's stage II fatigue response. Th is influence was also reflected in the micromechanisms present on the fracture surface. A similar effect was seen to a lesser degree in Van 80. This effect was clearly linked to the amount of closure present du ring stage II fatigue crack propagation. However, owing to the lack of closure present at high R-ratio levels, there was virtually no influe nce of K-max(c) level on Delta K-Th in either of these materials. (C) 1997 Elsevier Science Ltd.