THE EFFECT OF LOADING RATE ON CHEVRON-NOTCH FRACTURE-TOUGHNESS MEASUREMENTS

One common concern with the manual testing of chevron notched specimen s to determine fracture toughness is that it does not provide for prec ise control over the loading rate. This study examines the effect of l oading rate on the measured fracture toughness of 20 identical hardene d AISI 4140 steel samples. Eight samples were tested in a servohydraul ic testing machine using a four-paint bend fixture and four different ram speeds to produce total testing times of 1.1, 4.4, 16.7 and 69 sec . In addition 12 samples were tested in the Toughness Tool, a device s imilar to a torque wrench, that measures the maximum moment required t o fracture a chevron notched four-point bend sample. Samples were test ed in this device using fast, natural and slow loading rates correspon ding to testing times of 0.7, 3.5 and 23 sec, respectively. Both sets of tests reveal that loading rate has no effect on the measured fractu re toughness. Moreover, it is shown that the loading rate introduced b y the Toughness Tool is easily controlled to within +/- 15%. Therefore , the loading rate variability introduced by the new device is much sm aller than that provided for in ASTM E1304, the standard for chevron n otch fracture toughness testing, in which the loading rate range is pe rmitted to vary over a factor of four.