4.1 The determination of the superplastic properties of a metallic sheet material is important for the observation, development and comparison of superplastic materials. It is also necessary to predict the correct forming parameters during an SPF process. SPF tensile testing has peculiar characteristics compared to conventional mechanical testing, which distort the true values of stress, strain, strain hardening, and strain rate at the very large elongations encountered in an SPF pull test, consequently conventional mechanical test methods cannot be used. This test method addresses those characteristics by optimizing the shape of the test coupon and specifying a new test procedure.

4.2 The evaluation of a superplastic material can be divided into two parts. Firstly, the basic superplastic-forming (SPF) properties of the material are measured using the four parameters of stress, temperature, strain, and strain rate. These are obtained using conversions from the raw data of a tensile test. Secondly, derived properties useful to define an SPF material are obtained from the basic properties using specific equations.

4.3 The test coupon undergoes an essentially uniform and constant neck along its length, and S and e are assumed in this standard to be valid. However at the junction to the clamp sections of the test coupon the cross section reduces from the original value to the final value, over a length of approximately 4 % at each end. Also, there are local small instabilities of cross section over the gauge length. These contribute to an error in the calculated values of ε and σ. In the absence of currently available extensometers that could operate in the high temperature environment of an SPF test, ε and σ are to be inferred from crosshead extension and force.

4.4 The derived term m is widely used to describe the SPF properties of a material. It should be used with caution, as it is dependent on strain, strain rate and temperature. Many references in the literature do not identify the strain condition at which the readings were taken, or allow multiple strains to be used in the determination of m.

4.5 Many superplastic alloys exhibit strain hardening. However, the conventional strain hardening exponent n as defined in Test Method E646 is not valid for superplastic materials as strain hardening in the latter is usually a coefficient of strain, rather than an exponent. The mechanism of strain hardening in superplastic flow is essentially due to grain growth, and although the stress/strain relationship is often linear, it is not universal for all superplastic materials. Consequently, there is no simple definition of a strain hardening coefficient and this standard does not define one. Consideration of strain hardening in superplastic deformation is discussed in Ghosh and Hamilton's, "Influences of Material Parameters and Microstructure on Superplastic Forming."3

4.6 It is assumed no local necking takes place and the cross section of the test coupon is constant over the entire gauge length. For some materials, cavitation inside the material increases the volume of the gauge section as the test progresses, and the true cross-sectional area has to be compensated for any strain. For other materials, the coupon can develop a ribbed or other local texture, and in this case, the minimum cross section has to be measured. During the test there is an increasingly non uniform cross section at each end of the test coupon where the gauge section transitions to the original width at the clamp section. This effect is small and can usually be ignored.