An Extension of the Monkman-Grant Model for the Prediction of the Creep Rupture Time Using Small Punch Tests

Creep tests on a small scale have the potential to be used without significant removal of material or in areas where the available material is limited. In this paper an extension of the modified Monkman-Grant model for the prediction of the creep rupture time using Small Punch Creep Tests (SPCT) is investigated. This test basically consists of punching, under constant load, a small size specimen (10x10x0.5 mm) with the ends fixed. For this purpose an AZ31B magnesium alloy, taking a test temperature of 150 A(0)C, has been selected. The Monkman-Grant relation is a predictive model, initially developed for uniaxial creep tests, which can be used to predict the rupture time of tests which have been interrupted once secondary stage of creep has been reached. The proposed extension of the Monkman-Grant model for SPCT is based on the definition of the Minimum Relative Punch Displacement Rate. The experimental techniques and data analysis, involving small punch testing, are explained in detail. The proposed predictive model allows the test times of small punch testing to be reduced and it can be directly applied to predict the failure time from an interrupted test at the time when the Minimum Relative Punch Displacement Rate is reached. Good correlations are obtained by comparing the failure time from the proposed Monkman-Grant extension with the experimental failure time.