Methods and apparatuses for fatigue testing of structural components
Methods and apparatuses for multiple degree-of-freedom fatigue testing of a specimen. The apparatus includes a first platform, a second platform, a plurality of actuator assemblies, a load cell, a mounting plate, a specimen support, a direct-strain imaging system, and a local sense and control system. Each actuator assembly includes a servo-control, a position encoder, and a piston that is constructed to move in a linear direction in accordance with the servo-control. Each piston is rotatably connected to the second platform. The load cell is connected to the second platform and constructed to output force measurements in three orthogonal directions and torque measurements about the three orthogonal directions. The mounting plate is constructed to hold a portion a specimen. The specimen support is constructed to hold another portion of the specimen. The direct-strain-imaging system includes a camera that is constructed to record a plurality of images of the specimen during fatigue testing. The local sense and control system constructed to receive: a loading specification, the force measurements and torque measurements from the load cell, and position information from each position encoder, and output control commands to each servo-control of the plurality of actuator assemblies based on the received loading specification. The control commands are updated in time in accordance with the position information from each position encoder and the force measurements and torque measurements from the load cell.
1 . A multiple degree-of-freedom fatigue testing apparatus, comprising:
a first platform;
a second platform;
a plurality of actuator assemblies where each actuator assembly includes a servo-control, a position encoder, and a piston that is constructed to move in a linear direction in accordance with the servo-control, wherein each piston of the plurality of actuator assemblies is rotatably connected to the second platform;
a load cell connected to the second platform and constructed to output force measurements in three orthogonal directions and torque measurements about the three orthogonal directions;
a mounting plate constructed to hold a portion a specimen;
a specimen support constructed to hold another portion of the specimen;
a strain-imaging system comprising a camera constructed to record a plurality of images of the specimen during fatigue testing; and
a local sense and control system constructed to receive: a loading specification, the force measurements and torque measurements from the load cell, and position information from each position encoder, and output control commands to each servo-control of the plurality of actuator assemblies based on the received loading specification, wherein the control commands are updated in time in accordance with the position information from each position encoder and the force measurements and torque measurements from the load cell.
2 . The apparatus of claim 1 , wherein the mounting plate is removably connected to the load cell.
3 . The apparatus of claim 1 , wherein the second platform comprises a resin, a plastic, high-strength steel, carbon fiber, or titanium.
4 . The apparatus of claim 1 , wherein each actuator assembly includes a first joint and a second joint, wherein each actuator assembly is connected to the first platform through the first joint and connected to the second platform through the second joint.
5 . The apparatus of claim 4 , wherein the second joints of the plurality of actuator assemblies are spherical joints, and wherein the second platform includes a plurality of studs each of which includes a ball portion at one end, and wherein each ball portion is connected to a corresponding spherical joint.
6 . The apparatus of claim 1 , wherein the position encoder includes a reading head and a scale, wherein the reading head is connected to a corresponding piston and movable relative to the scale.
7 . The apparatus of claim 1 , wherein the loading specification includes a frame of reference location with respect to a platform frame of reference.
8 . The apparatus of claim 1 , wherein the loading specification also includes either: (i) a waveform evolution for three forces in the three orthogonal directions and three torques about the three orthogonal directions, or (ii) a waveform evolution of six degrees of freedom of a point in space.
9 . The apparatus of claim 1 , wherein the camera of the strain-imaging system is constructed to image a portion of the specimen that is a virtual strain sensor.
10 . The apparatus of claim 9 , wherein the strain-imaging system includes a second camera constructed to image another portion of the specimen that is another virtual strain sensor.