Abstract
Torsional creep tests of a mild steel were carried out under varying stress levels at 500°C by using thin walled tubular test pieces. In the present test conditions under constant stress, buckling occurs within the range of primary creep, where the relation between strain γ and time t is expressed by Andrade's equation, γ=γ0+βtn, like as in tensile creep tests. For varying stress tests, stress level is varied to the lower stress of τu=-12.8, -6.4, 0kg/mm2 and is maintained for 60min after the upper stress τ0=12.8kg/mm2 for 60min, and the cycle is repeated. The instantaneous strain, creep rate, and total creep are greater when the previous stresses are higher in the reverse direction. Accordingly, the creep resistance is increasing with test time, and yet the increase is smaller when the stress returns to the constant τ0 from higher τu. The instantaneous recovery of strain in releasing of stresses is constant if the stresses were in the same height, and is not varied by the number of cycles. The rate of recovery strain under τu=0, i.e. during the rest of load, is in linear relation with time when the both are shown by logarithmic coordinate, and is not affected by the number of cycles.
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