Validation of a Miniature Tensile Strength Measurement System

Abstract

Miniature strength test systems are of use for several reasons. For example, many advanced materials are often in limited supply and the use of small testpieces can accelerate the production of useful data. Also, small sections can be taken from working components and tested to assess material degradation without affecting performance. An important criteria for choice of testpiece size is such that the microstructure within the testpiece is representative of the structure as a whole. However, strain measurement becomes more of a challenge as the testpiece size diminishes. Typically extensometers are used on gauge lengths of 10–25 mm, thus smaller testpieces present some problems in the use of conventional extensometers. Because of these difficulties an alternative method of strain measurement, based on changes in resistance, was used in the current work. A miniature test system, the ETMT (ElectroThermoMechanical Test), developed at the National Physical Laboratory, was used to measure the stress/strain characteristics, particularly at elevated temperatures, of two material types, paniculate reinforced aluminium alloy composites and a Ni-based superalloy, Nimonic 901. The testpiece dimensions were typically 30 × 2 × 1 mm, with a grip separation of 16–20 mm. Elevated temperatures were achieved using a DC heating current. Strain was measured using changes in electrical resistance of a central gauge length of the testpiece, about 2–3 mm in length. Because the strain measurement method was nonstandard the stress/strain data were compared with measurements made from conventional tensile tests using rectangular dogbone testpieces, with a 6 × 3 mm gauge length cross section. The comparison between the ETMT results and those obtained from elevated temperature conventional tests showed that the effects of deformation rate were very important. Thus the use of miniature testpieces can be a cost effective and rapid technique for obtaining material property data, but it is important to report values for all the parameters that can effect the measurement of strength.