Creep Apparatus
The creep apparatus employs lever action to apply a dead-weight load to a specimen. The lever beam is notched to receive the knife-edge of the fulcrum. The fulcrum is at the top of a column rigidly attached to the base. The specimen is pinned to two "grips" for self-alignment. The lower grip is attached to the base with a screw stud to allow height adjustment during set up. The upper grip pivots on the lever. The distance between the pivot point and the upper grip attachment point is 25.4mm.
Weights are placed on a weight tray suspended from a pin at the opposite end of the lever. The distance from the pivot point to the tray pin is 254mm, thus there is a multiplier of 10 for the load applied to the specimen. A counter weight at the specimen end of the beam is moved to balance the lever system.
A linear variable displacement transformer (LVDT) is located 127mm from the pivot point. The extension of the specimen is one-fifth of the indicated LVDT displacement. The tip of the LVDT core extension rides on the upper surface of the beam. The LVDT is suspended from a rod attached to the base. A signal conditioning unit is connected to the LVDT by a cable. The displacement data from the LVDT are recorded by data acquisition software.
To set up and start a test, the weight tray is removed with the pin remaining in the beam. The specimen is attached to each grip by a pin. The nut on the lower grip is adjusted to make the beam level. The tip of the LVDT probe is aligned on a line marked 127mm from the pivot point. The counter weight is adjusted to balance the beam, then locked by a setscrew. The lower grip nut is adjusted to move the specimen upward until the LVDT display indicates approximately -12V.
The weight tray and first mass are placed on a scale to determine the mass. The tray and mass are attached to the beam and supported so that the specimen is not loaded. The data acquisition system is started and the tray is slowly lowered to apply the load. Subsequent masses are determined and added at periodic intervals. Data are plotted and recorded as displacement vs. time.