Impression creep of lead
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The impression creep behavior of lead was investigated using a 100 jj,m diameter punch at ambient and elevated temperatures (433 K-563 K) under punching stresses of 6-70 MPa. The results were compared with the data obtained from conventional creep tests reported in the literature. Unlike the indentation creep test, the impression creep test showed a steady-state velocity after a short transient period when the flat-end cylindrical punch was pushed against the lead surface by a constant load. Both the temperature and stress dependences were comparable to those of the constant stress tensile creep tests under similar conditions. A master curve for lead was established by collecting data from the impression creep tests and the constant stress tensile creep tests. The indentation creep measurements for lead were included also. However, the indentation data depend on the load applied.
I. INTRODUCTION Impression creep measurements used to characterize the creep properties of materials have many advantages. An important one is that all the creep information can be collected from one sample, which not only greatly reduces the effort for sample preparation but also reduces the sample-to-sample variation in properties. Because the punch geometry is a flat-end cylinder, the plastic zone developed beneath the punch assumes a constant size when the punch penetrates into the material at a constant load. There is no tertiary stage observed in this test. It has been demonstrated that both the stress dependence and the temperature dependence of the steady-state impression velocity agree with the corresponding quantities for the creep rate measured from the conventional constant tensile or compression stress creep tests. 1-4 The strain rate in the conventional creep test relates to the ratio of impression velocity to the punch diameter used in the impression creep test. Empirically, there is a factor, around one third, which converts the impression punching stress to the conventional tensile or compression stress. These relations have been established numerically by finite element calculations5 and verified experimentally.1-6 Impression creep is also a method of characterizing local material properties. If the punch diameter is small, the impression technique can be used to obtain mechanical properties of a thin film, thin surface coatings, small second phase particles, and individual components in a composite. It can be used also to detect material inhomogeneities. A calculation based on the diffusion mechanism7 shows that the impression method is a possible technique to detect decohesion between a thin film and its substrate. J. Mater. Res., Vol. 9, No. 4, Apr 1994 http://journals.cambridge.org
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The punch diameters used to do most of the impression measurements reported in the literature range from 1 mm to 2 mm. Chu and Li6 carried out the impression creep on succinonitrile crystals and some of the results were obtained from a small punch of diameter 0.4 mm. We succeeded in assembling an impression creep
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