The fracture behaviour (mode I) of concrete beams and in particular, their ductile/brittle transition, was studied within the problem of the interaction between testing machine and specimen.
The tests were carried out by means of a closed loop servocontrolled testing machine. CMOD (crack mouth opening displacement) was chosen as a feedback variable. This is certainly an increasing and monotonous quantity even in the presence of phenomena related to the brittle fracture and, therefore, with a marked instability character. The response of the servocontrol system was examined by submitting to three point-bend tests (T.P.B.T) beams with a different initial compliance obtained by suitably choosing their elastic and dimensional parameters. As the initial compliance decreased, the beams showed a brittle-type fracture with a positive slope softening curve accompanied by a higher elastic energy release than that strictly required for the fracture process, This phenomenon becomes particularly important in the structural analysis where also the local softening phenomena should be taken into due account for a correct project. Finally, the experimental investigation proved that the servocontrol system of the testing machine was able to control the phenomenon instability only if the CMOD rate was imposed at levels compatible with the dynamic response characteristics of the actuator-servovalve group.