One common concern with the manual testing of chevron notched specimens to determine fracture toughness is that it does not provide for precise control over the loading rate. This study examines the effect of loading rate on the measured fracture toughness of 20 identical hardened AISI 4140 steel samples. Eight samples were tested in a servohydraulic testing machine using a four-point 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 similar to a torque wrench, that measures the maximum moment required to fracture a chevron notched four-point bend sample. Samples were tested in this device using fast, natural and slow loading rates corresponding to testing times of 0.7, 3.5 and 23 set, respectively.
Both sets of tests reveal that loading rate has no effect on the measured fracture toughness.
Moreover, it is shown that the loading rate introduced by the Toughness Tool is easily controlled to within k 15%. Therefore, the loading rate variability introduced by the new device is much smaller than that provided for in ASTM E1304, the standard for chevron notch fracture toughness testing, in which the loading rate range is permitted to vary over a factor of four.