The specimen, generally with an hourglass shape to facilitate rupture in the central cross-section, is mounted on two oscillating supports with the constraint of two hinges, Fig. 7.7.
Two equal forces applied to the collars, generate a constant bending moment along the specimen. Taking movement from an electric motor, the system generates a rotating bending, since each fiber is cyclically subjected in variable time to a sinusoidal stress variation, as shown in the same figure. From the point of view of the stress analysis, the only necessary control concerns the measurement of the applied force and the counting of loading cycles until specimen failure.
The symbols have the following meaning, Fig. 7.7:
• ao upper stress
• au lower stress
• R = au /ao stress ratio
|
Fig. 7.7 Scheme of a rotating bending machine and stress variation in the central cross-section of the specimen |
Go + Gu
• om = mean stress
2
Go — Gu
• Ga = 2 amplitude
• Ag = Go — Gu range
• f = frequency
R = — 1 called fully reversed condition, is the first reference test, while R = 0 is another common reference test, called pulsating tension.