Continuous servo-controlled alignment of specimens in materials testing

Abstract

A universal testing machine having four degrees of freedom under servo control was successfully used to identify mechanical property degradation in ring and flat coupons exposed to simulated nuclear blast.

Originally conceived as a method of obtaining a uniaxial tension test on a specimen with a curved center line (i.e., cut from a 9-in. ring), the device proved invaluable in the running of tests on standard (i.e., flat ‘dog-bone’) specimens in both tension and compression which were almost devoid of any bending stress.

The ‘crosshead’ of the machine was able to pivot about two axes perpendicular to the primary tension-compression axis. These two axes coincided with the midpoint of the specimen, and rotation about the axes was servo controlled, based on a strain-gage-feedback signal from the specimen. Is addition, it was found necessary to provide the ability to servo control the horizontal movement of the ‘crosshead’ about one of these two axes, based on the shear sensed by a special shear sensor.

Tests on beryllium and fiber-reinforced plastics with up to two percent axial strain showed a peak bending strain of only 30 microstrain, and this only momentarily. The servo systems were also used to good advantage during the ‘closing’ of the structural loop, making possible the testing of very brittle and delicate specimens.