Volume 16, Issue 3 (May 1988)
Strain Measurement Technique for Elevated Temperature Tensile and Creep Testing of Foil-Gage Metals
A technique has been developed for elevated temperature tensile testing of foil-gage metals which employs a modified mechanical extensometer in conjunction with a displacement transducer. The extensometer has been modified by attachment of a counterbalance to reduce the axial load contribution due to the mass of the extensometer to the total maximum test load. In addition, the extensometer grip inserts have been modified to incorporate a conical tip/flat-edge design to minimize induced stresses in the specimen resulting from extensometer attachment.
A room temperature evaluation of the counterbalanced extensometer with modified grip inserts was conducted by comparing the elastic moduli and residuals for titanium alloy (Ti-6Al-4V) specimens determined using bonded, electrical resistance strain gages, to the elastic moduli and residuals determined using the counterbalanced extensometer. Results show the extensometer consistently indicates an elastic modulus within 2.5% of that indicated by the strain gages and an average residual of approximately 2.4 × 10−5 m/m (in./in.).
Demonstration of the counterbalance technique and associated strain measurement system at elevated temperature was accomplished by performing creep tests on 0.102 mm (0.004 in.) thick specimens of Ti-6Al-4V at temperatures of 427°C (800°F) and 538°C (1000°F) and at nominal applied stresses of 310.3 MPa (45 ksi) and 172.4 MPa (25 ksi), respectively. Examination of the resulting creep-curves suggested that the strain measurement system provided actual creep strain values with good accuracy. In addition, the counterbalance technique resulted in a load contribution from the extensometer of less than 0.40% of the applied load levels compared with a 2.21 to 4.48% contribution for specimens tested without a counterbalanced extensometer.