Strain-Controlled Fatigue

Strain-controlled fatigue testing, also known as low cycle fatigue (LCF), per ASTM E606 was conducted in a closed-loop, servo-controlled, electro-hydraulic testing machine. A process control computer controlled fully reversed (R=-1) constant strain or stress amplitude in a sinusoidal or triangular waveform. Each test started in strain-control mode with a frequency of 0.05-3.0 Hz. After the stress-strain loops exhibited constant stress peaks, the control mode was changed to load control at the corresponding stress amplitude and the frequency was increased to 5-50 Hz. During strain control, specimen failure was defined as a 50% drop in the tensile peak load from the peak tensile load observed in stable stress-strain loops. During the load control, failure was defined as complete separation of the specimen.

Various test materials were manufactured by industrial sources representing different materials and processing conditions with densities of 6.8 g/cm3 to full density.

The LCF test specimens were machined from bars into smooth cylindrical axial fatigue specimens. Before testing, the gauge sections of all the fatigue specimens were mechanically polished in the loading direction using emery paper. After polishing, a thin band of M-coat D acrylic coating was applied along the central gauge section. The purpose of the M-coat application was to prevent scratching of the smooth specimen surface by the knife edges of the strain extensometer and thus reduce the incidence of knife-edge failures.