A cylindrical-punch indentation technique has been developed as a means of measuring the nonlinear elastic responses of materials - more specifically, for measuring the moduli of elasticity of materials in cases in which these moduli vary with applied loads. This technique offers no advantage for characterizing materials that exhibit purely linear elastic responses (constant moduli of elasticity, independent of applied loads). However, the technique offers a significant advantage for characterizing such important materials as plasma-sprayed thermal-barrier coatings, which, in cyclic loading, exhibit nonlinear elasticity with hysteresis related to compaction and sliding within their microstructures.
A specimen to be tested by the cylindrical-punch indentation technique is prepared by standard metallographic procedures. The specimen is mounted on a load-versus-displacement-measuring apparatus, which could be any of a variety of indentation-type hardness testers or other conventional mechanical testing instruments. In the indentation test, the flat end of a round cylindrical punch is pushed into the polished, flat surface of the specimen. To minimize impression creep (a time-dependent plastic deformation that could contribute a large error to the modulus data), the specimen is preconditioned by pre-indenting it at a load greater than the load to be applied during the subsequent test. Thereafter, the applied load is varied according to the specification for the test and the punch displacement is measured as a function of the applied load. The modulus of elasticity (for example, see figure) and, if desired, other aspects of the elastic response of the specimen material are computed from the displacement-versus-load data with corrections, if necessary, for the elastic response of the punch and the rest of the testing apparatus.
The flat-bottom cylindrical punch used in this technique offers important advantages over the pointed indenters used in traditional hardness testing: A pointed indenter is well suited to measuring hardness but is ill suited to measuring the modulus of elasticity of a specimen because the contact area is unknown and varies during the test, so that there is no simple relationship between applied load and applied stress. In addition, a pointed indenter causes significant plastic deformation (even at nearly zero applied load), which cannot easily be distinguished from elastic deformation. In contrast, while the flat-bottom cylindrical punch is useless for hardness testing, it is well suited for measuring the modulus of elasticity because its contact area is constant and, consequently, the applied stress is simply proportional to the applied load. Hence, the modulus of elasticity can be determined at every point on the load-versus-displacement curve. Also, if the applied load is limited to below the value corresponding to the contact stress at the onset of plastic deformation, the deformation can be relied upon to be elastic over a complete loading/unloading cycle, making it unnecessary to subtract the effects of plastic deformation.
This work was done by Jeffrey I. Eldridge of Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4-8
21000 Brookpark Road
Cleveland Ohio 44135.
Refer to LEW-17412.
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