Torsion Fatigue Test Rig
Investigators: John Bomidi, Nick Weinzapfel, Sina Moghaddam
Figure 1: Torsion fatigue test rig
The torsion fatigue test rig (TFTR) was constructed to experimentally characterize the fatigue behavior of materials that fail due to oscillating shear stresses. It consists of a base, hydraulic rotary actuator, and torque cell. Given the horizontal configuration of the rig, light weight mechanical adapters were designed to interface with the torsion bar specimens to minimize the interference of bending moments on the tests results. The adapters are essentially custom designed collet fixtures with base flanges for mounting, and they use off-the-shelf collets and clamping nuts to retain the specimen and transmit torque. Tests are generally carried out in torque controlled mode and the prescribed torque levels/amplitudes are controlled through commercial software.
Figure 2: CAD model of custom mechanical adapters
First the ultimate strength of the material in shear is determined through quasi-static ramp loading. Then tests are performed at several fully reversed torque amplitudes below the ultimate strength in order to define the stress-vs-life torsion fatigue curve. Tests are repeated at each stress level to characterize the scatter in the data. The image below illustrates preliminary results obtained with the TFTR.
Figure 3: Preliminary stress-life data from torsion fatigue experiments
Observations made of the failed specimens below indicate that as the number of fatigue cycles to failure increases, the final fracture surface becomes increasingly smooth and well-defined. This is due to the fact that the strain energy applied to the specimens in static strength testing is large enough to simultaneously activate failure on multiple planes of weakness within the material. In high cycle fatigue, however, the applied energy is significantly lower and initiates failure only on planes which are critically oriented or contain a material defect. Over the course of hundreds of thousands or millions of stress cycles, one of these sites will become predominant and direct the path to final failure. Most of the life of a high cycle torsion fatigue specimen is spent in the process of initiating a mesoscale crack on the exterior surface near the minimum diameter cross-section where the stresses are the highest. Each of the failed high cycle fatigue specimens contains a characteristic short crack that is oriented along a direction of maximum shear stress (either parallel or perpendicular to the specimen axis direction). Following the crack initiation event, mode I or opening mode crack growth takes over at significantly faster rate. This causes the crack to traverse a helical course around the perimeter and propagate through the core, leading immediately to the final failure.
Figure 4: Observed fracture surfaces from static strength and fatigue tests
Figure 5: Detailed images of a high cycle torsion fatigue specimen