Long-term Creep Behavior Research Articles

The effects of physical aging on the viscoelastic behavior of a thermoset polyester

The effect of physical aging on the viscoelastic behavior of a cross-linked polyester resin is reported. Tensile creep and dynamic mechanical analysis (DMA) tests yielded creep compliance (S(t)) and dynamic shear complex compliance ( J ∗(ω) ) of the polymer. Thermoreversible physical aging was found to have significant effects on the creep response of the polyester. Physical aging shift factors were determined at 60, 80, 100 and 120°C. Standard frequency-temperature superposition procedures were used to construct master curves for the dynamic mechanical data. The shift constants c 1 = (70) and c 2 = (450° C) determined from DMA data were used to construct a smooth master curve for creep data. The predictions of long-term behavior by means of master curve construction were found to agree with experimentally measured long-term creep behavior when differences in material aging were introduced.

Constrained cavity growth models of longitudinal creep deformation of oxide dispersion strengthened alloys

Two models of constrained cavity growth are developed to describe the long-term longitudinal creep behavior of nickel based oxide dispersion strengthened (ODS) alloys. For both models the rupture time is taken as the time for a transverse grain boundary to cavitate fully. A diffusive cavity growth law is assumed to govern cavitation. The applicability of the respective models is determined by the particular grain morphology achieved by thermal-mechanical processing. The first model assumes that longitudinal grain boundaries are unable to slide; hence displacements due to cavitation must be matched by displacements due to dislocation creep in adjoining grains. This model predicts a low stress exponent at the transition from single crystal to cavitation creep behavior, and higher stress exponents at stresses below this transition. Good agreement is found between the model predictions and creep data for MA 754 at 1000 and 1093 °C. A second model considers a grain morphology wherein longitudinal grain boundaries are able to slide by means of deformation of pockets of fine grains. Cavitation of transverse grain boundaries is thus controlled by grain boundary sliding. This model predicts a stress exponent of 1 at low stresses, and serves as an upper bound for the creep rate when a duplex grain morphology is present. Model predictions are in good agreement with creep data for a heat of MA 754 with a duplex grain morphology.

Creep of copper at cryogenic temperatures

Abstract The long-term creep behaviour of OFHC copper was investigated at 4.2 K and 77 K. At 77 K, steady-state creep was observed and the creep rates were tens of orders of magnitude higher than would be expected from extrapolations of ambient temperature creep data. The stress exponent obtained at 77 K is 2.2. The apparent activation energy at 77–90 K range is about 0.02 eV. At 4.2 K, transient creep was observed. TEM study of all crept specimens showed cell structures in support of the occurrence of substantial creep at cryogenic temperatures.

Long-term creep of copper at cryogenic temperatures

The equipment designed and used in this study basically consists of a constant load lever arm creep machine, a vacuum thermally isolated cryostat that is cooled by a low rate of cryogen flow, and a testing to transmit the load onto the specimen. Briefly, Figure i shows the tensile testing insert and the cryostat schematically. The tensile testing insert consists of the pull rod, the compression tube, and the specimen grips. The temperature of the specimen can be controlled with an accuracy of +l°K. The strain of the specimen was measured directly using a strain gage technique, which is capable of resolving +1 uc on the specimen. Typical creep results at 77K and 34.5 MPa initial applied stress are given in Figure 2. There is no indication of the exhaustion of creep strain when the test was terminated at i00 hours. Indeed, a steady-state creep behavior rather than a logarithmic creep claimed in another study of copper (8) was observed. The derivative of the creep curve, i.e., strain rate versus time, is also plotted in Figure 2. The onset of the steady-state region, as shown by the minimum creep rate, was determined to occur at 75 hours. The steady-state creep rate was 4 x 10 -11 sec -I. From these results, it can be concluded that long-term creep testing (about 100 hours) is necessary to study the creep behavior of materials at very low temperatures. With the current ability of conducting long-term cryogenic creep tests, more creep data will be generated to systematically investigate the long-term low temperature creep behavior of OFHC copper and other materials at the 77 K range and at 4.2K. Acknowledgments The authors wish to thank the International Copper Research Association (INCRA) and Dr. L.M. Schetky, the Technical Director, for supporting this program. We also are grate

The effect of orientation and sense of applied uniaxial stress on the morphology of coherent gamma prime precipitates in stress annealed nickel-base superalloy crystals

The coarsening of coherent γ′[Ni3(Al, Ti)] precipitates in single crystals of a representative nickel-base superalloy, Udimet-700, is shown to be affected by a uniaxial stress applied during annealing. Depending on the sense of the applied stress and its crystallographic orientation, stress annealing results in oriented cuboidal, plate, or parallelepiped shaped γ′ precipitates. A general thermodynamic analysis of the effect of stress annealing on precipitate morphology is presented that takes into account free energy changes due to changes in bulk precipitation strain, effective modulus, coherency strain energy, and the total interphase boundary area. The analysis correctly predicts the observed γ′ precipitate morphologies as a function of stress axis orientation, stress sense, the lattice misfit of the precipitate phase, and the elastic constants of the matrix and precipitate phases. The analysis also shows that stress induced morphological changes can be completely precluded, as may be desired to optimize mechanical behavior, only if the elastic constants of the matrix and precipitate phases are equal. Changes in morphology due to changes in bulk precipitation strain, which in the case of Udimet-700 is shown to be the dominant effect, can be eliminated by alloying for zero lattice misfit or, in single crystals, by stressing parallel to . Applications to long-term creep behavior and to the fabrication of composite structures are discussed.