Issue34

T. Itoh et alii, Frattura ed Integrità Strutturale, 34 (2015) 487-497; DOI: 10.3221/IGF-ESIS.34.54 491 used push-pull and reversed torsion testing machine. For measurements of axial, torsional and hoop strains, inductive displacement sensor type extensometers are mounted directly onto the specimen. To measure and evaluate axial, torsional and hoop stresses, load cells for axial load and torque and pressure gauge are installed. The maximum loads are  50kN for push-pull and  250N  m for torsion and the maximum inner pressures is 200MPa. By applying these loads, this testing machine can perform the multiaxial fatigue tests under the principal stress ratio range of  1  λ  1. Test control system of this testing machine becomes more complicated than those in common used testing machines since three hydraulic actuators must be controlled simultaneously and independently by employing a test controller program and a system as shown in Fig. 5 (b). In order to raise the precision of control, three types of feedback (FB) loop are applied in this testing machine. First one is from load cell to servo amplifier for axial and torsional control, which operates on load-controlled mode. Second one is from strain gauge to servo amplifier for axial and torsional control, which operates on displacement-controlled mode. The load or the displacement control can be changed by FB selection switch. The last one is from pressure gauge to servo amplifier to control the inner pressure as test control programed. The specimen employed is the hollow cylinder specimens designed for this testing machine of which shapes and dimensions will be shown later. Actuator (torque) Specimen Actuator (axial load) Load cell (axial load & torque) Extensometer (axial & shear displacement) Actuator (inner pressure) Pressure gauge (inner pressure) Analog  to  digital  conversion Servo amplifiers Amplifiers Actuators Servo  valve Axial displacement Shear displacement Internal pressure Sensors Axial load Torque Axial control Torque control Internal pressure PC Digital  to  analog  conversion Feedback (a) (b) Figure 5 : Multiaxial testing machine for push-pull, reversed torsion and inner pressure. (a) Schematic showing of multiaxial testing machine, (b) Over view of control system. T EST MATERIAL AND EXPERIMENTAL PROCEDURE he material tested was Ti-6Al-4V, which was subject to solution treatment at 960°C for 1 hour followed by water cooling, annealing at 705°C for 2 hours and air cooling. Fig. 6 shows the micro structure, which consists of alpha phases (hexagonal close-packed crystal structure) and the dual alpha- and beta-phase mixture (body-centered cubic crystal structure). Figure 6 : Microstructure of Ti-6Al-4V alloy. T alpha+beta alpha