Fatigue Testing Rig

Overview

A fatigue testing rig applies a load to a specimen over and over so the part can be run to failure and its fatigue life recorded. Static strength tells you the load a part survives once. Fatigue tells you how many times it survives a smaller repeated load before a crack starts and grows, which is what governs the life of most rotating and cyclically loaded hardware. The rig pulls and pushes the specimen between two grips while a load cell measures the force and a controller holds the chosen waveform, counting cycles until the specimen breaks or a target count is reached.

The most common form is the servo-hydraulic rig, which uses a hydraulic actuator and a servo valve to apply force across a wide range of amplitude and frequency. Force passes through the Load Frame that reacts it, the Servo-Hydraulic Actuator that applies it, the Load Cell that measures it, and the Specimen Grips and Fixtures that hold the specimen, all driven by the Dynamic Controller and fed by the Hydraulic Power Unit.

Load frame and reaction

The Load Frame gives the actuator something stiff to push against. A heavy Base Platen anchors the lower end, two Strain Column columns rise from it, and an adjustable Adjustable Crosshead spans the top to set the daylight for the specimen length. The columns are sized to stay well within their elastic range at full rated load, because any frame deflection adds to the measured stroke and corrupts a strain-controlled test. Once the crosshead is positioned, a Crosshead Lock clamps it to the columns so it cannot creep under millions of reversals, and Crosshead Lift Cylinder cylinders move it during setup. Four Isolation Mount feet keep the dynamic load from shaking the building floor.

Actuator and servo valve

The Servo-Hydraulic Actuator is a double-acting hydraulic cylinder. Oil pressure on either side of the Piston drives the Piston Rod in tension or compression, and the rod runs in Hydrostatic Bearing bearings that carry it on an oil film with almost no friction and no side play, which keeps the load axial over long runs. A Rod Seal Pack pack contains the oil where the rod leaves the Cylinder Body. The actuator is steered by a two-stage Servo Valve: a Torque Motor deflects the pilot stage from the controller current, the Pilot Spool drives the Main Spool, and the main spool meters oil to each chamber. A Actuator Accumulator Set mounted at the valve supplies the peak flow that fast cycling demands, which the pump cannot deliver on its own.

Hydraulic power unit

The Hydraulic Power Unit supplies the oil. A Variable Displacement Pump driven by the Pump Drive Motor holds system pressure between roughly 210 and 280 bar and varies its flow to match demand. Oil is drawn from the Oil Reservoir, passes through the Supply Manifold to the actuator, and is kept clean by Oil Filter filters, because the close clearances in the servo valve fail quickly on dirty oil. A System Accumulator absorbs pump pulsation and holds reserve volume for the whole rig. The pump and valve put heat into the oil, so an Oil Cooler with a finned Cooler Core and a Cooler Fan pulls that heat back out and holds the oil temperature steady, since oil viscosity drifts with temperature and the valve gain drifts with it.

Measuring load, stroke, and strain

The Load Cell sits in series with the specimen and reads the true applied force. Its Sensing Element is a machined flexure carrying Strain Gauge gauges in a bridge, and a Bridge Amplifier conditions the millivolt output for the controller. The element is made from a high-fatigue-strength steel so the cell itself does not fatigue before the specimens do. A Stroke LVDT inside the actuator reports rod position for stroke control, and for strain-controlled work a clip-on Extensometer measures elongation over a defined gauge length. The three signals, load, stroke, and strain, give the controller its choice of feedback variable and let the test record show the full hysteresis loop of each cycle.

Closed-loop control and cycle counting

The Dynamic Controller runs the test. Its Control Board closes the loop at several kilohertz, comparing the chosen feedback signal against the commanded waveform and correcting the valve every cycle. A Servo Valve Driver turns that command into the exact coil current the torque motor needs, and a Data Acquisition Board samples all channels in step so each cycle can be logged. Tests run in force control for stress-life work or in strain control for low-cycle work, and the controller can switch between them. A Cycle Counter tallies completed cycles and triggers the stop at the target count or when the load drops at specimen fracture, which marks the fatigue life.

Specimen handling and safety

The Specimen Grips and Fixtures hold the specimen on the load line. Hydraulic Wedge Grip wedge grips clamp harder as load rises, so the specimen does not slip through tension and compression reversals, and Grip Jaw inserts are changed to suit the specimen shape. An Alignment Fixture brings the upper and lower load lines coaxial, because any misalignment adds bending that shortens the measured life and scatters the results. Because a specimen releases stored energy when it breaks, the test runs inside a Safety Enclosure with Polycarbonate Window windows that contain fragments, Door Interlock interlocks that cut the actuator when a door opens, and a Emergency Stop that drops pressure at once. Most endurance runs go unattended for days, so the safety chain and the cooling matter as much as the measurement itself.