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Abstract There were usually two drawbacks in calculating the long-term strength through isochronal curves. One was that the inflection point of the isochronal curve was generally not so apparent leading to the determination of the long-term strength with uncertainties. The other was that the testing time was usually short with only few loading steps，so the test results were not accurate enough. Therefore, the creep test to the salt rock from Jianghan in Hubei province in this study was carried out for about five months long. There was an obvious inflection point on the isochronal curve plotted according the test data and the long-term strength was thus determined without uncertainty. The longer testing time and the more loading steps were proved to be the vital measure to guarantee the credibility of the test results. A new fractional viscoelastic-plastic creep model was established according to the experimental results together with the improvements in two aspects. The proposed model was proved to be simpler and more reasonable in terms of the process of derivation，the outcome of test data fitting and the results of theoretical analysis.
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Received: 11 July 2013
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2014, Vol. 33 
