A finite-element creep analysis of a center-crack specimen was carried out under small-scale to extensive creep conditions. The crack was assumed to be stationary. Two constitutive models were used, one consisting of elastic-power-law creep and the other of elastic-power-law creep plus rate-independent power-law strain hardening. The mechanics basis of the Ct parameters, which has been proposed for correlating creep crack growth behavior under conditions ranging from small-scale to extensive creep, is explored. Also, an earlier discrepancy in this test specimen between analytically predicted deflection rates and experimental results is investigated. The authors conclude that in small-scale creep, Ct does not characterize the instantaneous crack-tip singular stress field, but accurately reflects the compliance change due to the development of the creep zone around the crack tip. The experimental results, which show higher displacement rates than predicted, cannot be explained by inclusion of time- and rate-independent plasticity but may possibly be due to primary creep.
techniques for detecting crack tip cavitation damage including electron metallography, synchrotron x-ray tomography, digital microradiography and small angle neutron scattering (SANS) were also evaluated. The study led to the following important conclusions. • In the presence of large scale cavitation damage and crack branching, time rate of creep crack growth (da/dt) i does not correlate with Ct or with C*. On the other hand, when the cavitation damage is constrained, da/dt is uniquely characterized by C t. • Area fraction of grain boundary cavit_ted, Ac, appears to be the single damage parameter for characterizing the extent of cavitation damage ahead of crack tips. The distribution of Ac ahead of the crack tip is a complex function of loading wave form and cycle time.
The effect of hold time on the high temperature notched fatigue strength of the CMSX-4 single crystal superalloy has been experimentally investigated. Notched specimens with orientation 〈001〉 have been tested at 950°C in load control in the low-cycle fatigue (LCF) life regime both without and with dwell. For the same applied stress range (net-section stress), the effect of hold time results in an appreciable reduction of fatigue life. Several scanning electron microscope (SEM) observations of the failed specimens have been made in order to associate the actual failure (crack initiation) location to the correct crystallographic orientation and to the critical location in the notch as given by suitable multiaxial fatigue parameters. Moreover, SEM observations reveal the change of the failure mode from pure fatigue to fatigue/creep conditions. In order to model the material behavior in the notch, stress-strain analysis of the notched specimens was performed by an elastic-plastic finite-element method (FEM) procedure; the parameters describing the material behavior of the single crystal superalloy have been identified by comparison with experimentally determined stress-strain fatigue behavior and creep data of smooth specimens. A suitable model for the assessment of the reduction of fatigue life in creep-fatigue conditions is proposed.
MASTER _, DIS"r'Rt_ON OF: Tilts I_OC, UM'ENT IS UNLIMITED techniques for detecting crack tip cavitation damage including electron metallography, synchrotron x-ray tomography, digital microradiography and small angle neutron scattering (SANS) were also evaluated. The study led to the following important conclusions. • In the presence of large scale cavitation damage and crack branching, time rate of creep crack growth (da/dt) i does not correlate with Ct or with C*. On the other hand, when the cavitation damage is constrained, da/dt is uniquely characterized by C t. • Area fraction of grain boundary cavit_ted, Ac, appears to be the single damage parameter for characterizing the extent of cavitation damage ahead of crack tips. The distribution of Ac ahead of the crack tip is a complex function of loading wave form and cycle time.
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