The Effect of Rotation on Load-Line Displacement Measurements Using Capacitance Gages in a Compact Tension Specimen

Abstract

Abstract : Non-contacting electrical or optical transducers are attractive for dynamic fracture toughness testing because their frequency response is not limited by the inherent vibration characteristics of the dip gage, which is typically used for quasi-static testing. One such transducer uses the principle of capacitance to measure displacement between the gage and an electrically grounded target. The problem with capacitance transducers is that the reference points used for measurement move as the separating specimen halves rotate under applied load. The purpose of this study was to investigate the effect of specimen rotation on the displacement measured by a capacitive gage, and to determine how close the measured displacement was to the actual load-line displacement. Errors in load-line displacement measurement for contacting (dip gage) and non-contacting capacitive (cap gage) transducers were compared. It was found that both types of transducers show a similar trend of increasing error with increasing displacement, however, the error was always less than the 1% allowable in the ASTM test method for fracture toughness testing. The error in the cap gage measurements was actually less than the clip gage error. Consequently, rotation of the specimen halves does not adversely affect the load-line displacement measurements taken with a capacitance gage.