THE KJC TOUGHNESS LIMIT VALUE, KJC (LIMIT), GIVEN BY EQ 12, WILL almost always occur near the point of maximum load in the test record. To avoid specimen failure due to excessive system crack drive at maximum load and beyond, the applied load must decrease equally with the decrease in load bearing capacity of the test specimen. Hence, the test machine must be operated under rigid displacement control. Servo-hydraulic machines can be displacement controlled, using the feedback voltage of a clip gage attached across either the crack mouth or the load-line of a specimen. An alternative is to use the machine actuator voltage feedback for stroke control. The former approach provides infinitely stiff control, and the latter will contain the slightly softer spring-like compliance of the pull rods and clevises. Screw-driven test machines usually have stiff frames and cross-heads, so they perform the same as a servo-hydraulic machine under stroke control. All of the above approaches have been proven to work successfully, provided that the pull rods are of suitable stiffness.