Comparison of creep crack growth rate in heat affected zone of welded joint for 9%Cr ferritic heat resistant steel based on C, dδ/dt, K and Q parameters

Creep crack growth behavior is very sensitive to the materials’ micro-structures such as the heat affected zone of a weld joint. This is a main issue to be clarified for 9%Cr ferritic heat resistant steel for their application in structural components. In this paper, high temperature creep crack growth tests were conducted on CT specimens with cracks in the heat affected zone of weld joints of W added 9%Cr ferritic heat resistant steel, ASME grade P92. The creep crack growth behavior in the heat affected zone of welded joint was investigated using the Q^∗ concept following which the algorithm of predicting the life of creep crack growth has been proposed. Furthermore, three-dimensional elastic-plastic creep FEM analyses were conducted and the effect of stress multiaxiality of welded joint on creep crack growth rate was discussed as compared with that of base metal.     

Interface diffusion-induced creep and stress relaxation in unidirectional metal matrix composites under biaxial loading

Analytical solutions are developed for interface diffusion-induced creep and stress relaxation in unidirectional metal matrix composites under biaxial transverse loading. The driving force for the interface diffusion is the normal stress acting on the interface, which is obtained from rigorous Eshelby inclusion theory. The solutions are a function of the applied stress, volume fraction and radius of the reinforced-fiber, the modulus ratio between the fiber and the matrix, specially, exhibit a strong dependence of creep rate and stress relaxation behavior on the biaxial stress ratio. Moreover, the solution for the interface stress presented in this study also gives some insight into the relationship between the interface diffusion and interface slip. For the application of the solutions in the realistic composites, the scale effect is taken into account by detailed finite element analysis based on a unit cell model.     

Study on effects of solar radiation and rain on shrinkage, shrinkage cracking and creep of concrete

In this paper, the effects of actual environmental actions on shrinkage, creep and shrinkage cracking of concrete are studied comprehensively. Prismatic specimens of plain concrete were exposed to three sets of artificial outdoor conditions with or without solar radiation and rain to examine the shrinkage. For the purpose of studying shrinkage cracking behavior, prismatic concrete specimens with reinforcing steel were also subjected to the above conditions at the same time. The shrinkage behavior is described focusing on the effects of solar radiation and rain based on the moisture loss. The significant environment actions to induce shrinkage cracks are investigated from viewpoints of the amount of the shrinkage and the tensile strength. Finally, specific compressive creep behavior according to solar radiation and rainfall is discussed. It is found that rain can greatly inhibit the progresses of concrete shrinkage and creep while solar radiation is likely to promote shrinkage cracking and creep.     

Sigmoidal creep of Ni3(Al, Ta)

Incremental creep tests have been used to explore the time-dependent plastic behavior of single-slip oriented Ni₃(Al, Ta) at low temperatures in the anomalous flow regime. For selected incremental creep experiments at 20 and 100 °C, it was discovered that Ni₃Al exhibited sigmoidal creep, where there is a significant time delay before the plastic strain rate accelerates to a maximum value during a creep experiment. Several of the factors that affect the sigmoidal creep response have been identified. The origin of sigmoidal creep is accounted for using a simple model of work hardening in Ni₃Al, where the acceleration of the creep rate is a direct result of the annihilation of the existing dislocation substructure.     

Continuous Relaxation Spectrum for Concrete Creep and its Incorporation into Microplane Model M4

Efficient numerical finite-element analysis of creeping concrete structures requires the use of Kelvin or Maxwell chain models, which are most conveniently identified from a continuous retardation or relaxation spectrum, the spectrum in turn being determined from the given compliance or relaxation function. The method of doing that within the context of solidification theory for creep with aging was previously worked out by Ba?ant and Xi in 1995 but only for the case of a continuous retardation spectrum based on the Kelvin chain. The present paper is motivated by the need to incorporate concrete creep into the recently published Microplane Model M4 for nonlinear triaxial behavior of concrete, including tensile fracturing and behavior under compression. In that context, the Maxwell chain is more effective than the Kelvin chain, because of the kinematic constraint of the microplanes used in M4. The paper shows how to determine the continuous relaxation spectrum for the Maxwell chain, based on the solidification theory for aging creep of concrete. An extension to the more recent microprestress-solidification theory is also outlined and numerical examples are presented.     

Failure analysis of a boiler tube in USC coal power plant

This paper presents failure analysis of final superheater tube in ultra-supercritical (USC) coal power plant. Visual inspection was performed to find out the characteristics of fracture of the as-received material. And the micro-structural changes such as grain growth and carbide coarsening was examined by scanning electron microscope. Detailed microscopic studies were made to find out the behavior of the scale exfoliation on the waterside of tubes. From those investigations, the creep rupture may be caused by the softened structure induced by carbide coarsening and accelerated by the metal temperature increase by the impediment of heat transfer due to voids.     

Cold bending of advanced ferritic steels: ASTM grades T23, T91, T92

The metallurgical background of advanced ferritic steels must be considered during every stage of fabrication, including forming operations, such as cold bending, because they can negatively affect the high-temperature properties of the material when not performed properly.     

Effect of long-term stress and temperature exposure on the fracture toughness of Ti-62222 alloy

Fracture toughness tests were conducted on a Ti-62222 (titanium alloy) sheet being considered for use in high temperature aircraft applications in the as received condition and after exposing the pre-cracked specimens to a sustained stress intensity, K, level between 55 and 60.5 MPa for 200 h at 350°C. It was concluded that the fracture toughness does not degrade as a result of exposure to high temperature and the K levels in this material. The tensile strength in the exposed condition also remained the same as in the as received condition.     

Deformation and life assessment of high temperature materials under creep fatigue loading

The knowledge of mechanical long term behaviour under static and cyclic loading for high temperature components requires methodologies for life assessment in order to employ the full potential of materials. A phenomenological life time prediction concept which was developed for multi‐stage creep fatigue loading demonstrates the applicability of rules for synthesis of stress strain path and relaxation including an internal stress concept, as well as mean stress effects. Further, a creep fatigue interaction concept which was also developed covers a wide range of creep dominant loading as well as fatigue dominant loading. Service‐type experiments conducted at different strain rates and hold times for verification purposes demonstrate the acceptability of life prediction method for variation of conventional 1 %Cr‐steels as well as modern high chromium 9‐10 %Cr‐steels. Generally, the service life of components is influenced by multi‐axial behaviour. Multi‐axial experiments with e.g. notched specimens and with cruciform specimens accompanied by advanced methods for calculation of stress strain path and life time prediction stress conditions are of future interest.     

A micromechanics study of competing mechanisms for creep fracture of zirconium diboride polycrystals

A micromechanics model was developed to simulate creep fracture of ceramics at high temperatures and material properties pertinent to zirconium diboride (ZrB₂) were adopted in the simulation. Creep fracture is a process of nucleation, growth, and coalescence of cavities along the grain boundaries in a localized and inhomogeneous manner. Based on the grain boundary cavitation process, creep fracture can be categorized into cavity nucleation-controlled and cavity growth-controlled processes. On the other hand, based on the deformation mechanism, the separation between two adjacent grain boundaries can be categorized into diffusion-controlled and creep-controlled mechanisms. In this study, a parametric study was performed to examine the effects of applied stress, cavity nucleation parameter, grain boundary diffusivity, and applied strain rate on cavity nucleation-controlled versus growth-controlled process as well as diffusion-controlled vs. creep-controlled mechanism during creep fracture of ZrB₂.