Lattice continuum and diffusional creep

Diffusional creep is characterized by growth/disappearance of lattice planes at the crystal boundaries that serve as sources/sinks of vacancies, and by diffusion of vacancies. The lattice continuum theory developed here represents a natural and intuitive framework for the analysis of diffusion in crystals and lattice growth/loss at the boundaries. The formulation includes the definition of the Lagrangian reference configuration for the newly created lattice, the transport theorem and the definition of the creep rate tensor for a polycrystal as a piecewise uniform, discontinuous field. The values associated with each crystalline grain are related to the normal diffusional flux at grain boundaries. The governing equations for Nabarro–Herring creep are derived with coupled diffusion and elasticity with compositional eigenstrain. Both, bulk diffusional dissipation and boundary dissipation accompanying vacancy nucleation and absorption, are considered, but the latter is found to be negligible. For periodic arrangements of grains, diffusion formally decouples from elasticity but at the cost of a complicated boundary condition. The equilibrium of deviatorically stressed polycrystals is impossible without inclusion of interface energies. The secondary creep rate estimates correspond to the standard Nabarro–Herring model, and the volumetric creep is small. The initial (primary) creep rate is estimated to be much larger than the secondary creep rate.

Brittle Creep Failure, Critical Behavior, and Time-to-Failure Prediction of Concrete under Uniaxial Compression

Understanding the time-dependent brittle deformation behavior of concrete as a main building material is fundamental for the lifetime prediction and engineering design. Herein, we present the experimental measures of brittle creep failure, critical behavior, and the dependence of time-to-failure, on the secondary creep rate of concrete under sustained uniaxial compression. A complete evolution process of creep failure is achieved. Three typical creep stages are observed, including the primary (decelerating), secondary (steady state creep regime), and tertiary creep (accelerating creep) stages. The time-to-failure shows sample-specificity although all samples exhibit a similar creep process. All specimens exhibit a critical power-law behavior with an exponent of −0.51 ± 0.06, approximately equal to the theoretical value of −1/2. All samples have a long-term secondary stage characterized by a constant strain rate that dominates the lifetime of a sample. The average creep rate expressed by the total creep strain over the lifetime (tf-t0) for each specimen shows a power-law dependence on the secondary creep rate with an exponent of −1. This could provide a clue to the prediction of the time-to-failure of concrete, based on the monitoring of the creep behavior at the steady stage.

Creep Strength of Chromium-Containing Conventional and ODS Steels in Oxygen-Controlled Pb at 650°C

Two martensitic conventional steels, T91 (modified 9Cr1Mo) and P92 (NF616), and two ODS steels with 12 and 14 mass.% chromium content were subjected to creep-rupture tests in stagnant oxygen-controlled Pb with co = 10−6 mass.% at 650°C. The 9Cr conventional steels were tested in liquid metal at 75–200 MPa, while the stress for the tests of ODS steels were chosen in the range of 190–400 MPa. Reference tests were carried out in stagnant air under similar loading. Both ODS steels show obvious change in secondary creep rate and failure appearance at 330–350 MPa, while the martensitic steel P92 tested in Pb at 75 MPa (tR = 13,090h) shows a significant drop in creep strength accompanying with a change from ductile to brittle fracture and a considerably reduced necking. Effects of microstructure evolution during the exposure at the elevated temperature and risk of LME due to a direct contact to liquid metal are discussed.

An Experimental Study on Creep Behaviour of 2Cr11NiMoVNbNB Steel

The creep experiments of 2Cr11NiMoVNbNB steel were carried out at 550oC and 600oC. The creep damage laws of this material are discussed. An increase of the stress exponent with decreasing temperature was found experimentally and a modified secondary creep rate equation including in creep threshold stress was derived. Creep damage analyses are made under constant stress and under relaxed stress with creep threshold stress. The experimental results show that the damage value calculated with creep time is almost equal to that calculated with the creep rate, and an iteration method of calculating is proposed.

A streamline solution for rigid laterally loaded piles in permafrost

A streamline solution for the design of laterally loaded rigid piles in permafrost is presented. The proposed method relies on a power law to describe the rate dependence of permafrost creep response. It describes the soil movement with a kinematically admissible velocity field and estimates the overall reaction at a given pile section with the bound theorem for a creeping material. The approach is valid only for a secondary creep rate and a stationary state of stress. Key words: pile, lateral load, velocity field, secondary creep rate.

On Time-Temperature Parameters for Correlation of Creep-Rupture Data in Stainless Steel Weldments

The Manson-Haferd, Larson-Miller, and Orr-Sherby-Dorn time-temperature parameters were applied to creep-rupture data obtained from testing two batches of austenitic stainless steel weldments. It was found that none of these correlated the data satisfactorily. A new parameter, based on a modification of one proposed originally by Manson and by Goldhoff and Sherby, was found to adequately correlate the data. The Minimum-Commitment, Station-Function Approach of Manson and Ensign was also applied, the results of which supported those obtained from the analysis made using the parameters listed above. Finally, from the relationship between rupture-time and secondary creep-rate, it is suggested that the form of the rupture data may be useful in predicting the physical basis for creep.

Post Creep Microstructures of Ti-6A12Sn4Zrf3Mo Alloy

It has been reported (1,2) that in highly textured titanium alloys the secondary creep rate in the direction normal to major concentration of basal poles at elevated temperatures is much greater than that of the c-direction. This phenomenon can be attributed to two major reasons: the first is that Youngs Modulus in the c-direction is approximately 25% higher than in the a-direc- tion; the second is that if an alpha grain is oriented in the c-direction with respect to the applied stress, the Schmid factor for prismatic planes is zero, and thus the slip on the major slip systems is extremely restricted. In the present study, two sets of creep specimens have been prepared from a highly textured Ti-6A12Sn4Zr6Mo alloy plate; one set is parallel to the rolling direction and the other set is parallel to the long transverse direction (major concentration of c-poles).