Static and dynamic fracture toughness tests at three strain rates were performed on two ASTM A508Cl. 3 steels by using 1T-CT specimens and fatigue pre-cracked instrumented Charpy specimens. The
KJC converted from
JC of the small specimens indicated a wide scatter. When the strain rate increased, the fracture toughness transition curves shifted to a higher temperature region. An increase in strain rate reduced the scatter of
KJC dramatically, especially in the high temperature region, and decreased the lower bound fracture toughness. Fractographic examination of the fractured specimen surfaces indicated that the relationships between
KJC and the stable crack growth, Δ
a0, distance from stable crack front to trigger point,
X, or distance from fatigue crack front to trigger point, Δ
a0+
X, were expressible by a single curve, respectively. The scatter of
KJC was caused by the varience of Δ
a0,
X, and Δ
a0+
X. With increasing strain rate, Δa
0,
X, and Δ
a0+
X decreased significantly, leading to a small scatter of
KJC. The
KJC value at Δ
a0=0,
X=0, and Δ
a0+
X=0 was proposed as the lower bound fracture toughness of a steel and labeled
KJCi. The shape of
KJCi versus temperature curve was controlled by the critical stretch zone width, SZWc. The Weibull slope m of
KJC became larger with increasing strain rate and decreasing temperature. Higher toughness data with larger stable crack extension than 100μm violated the linearity of Weibull plots. In the statistical approach to determine the lower bound fracture toughness in the transition region, much more analytical development is needed. The
KJC value with 3% fracture probability coincided with the
KJCi value in the low temperature region even in a high strain rate.
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