Micro/macro-crack growth due to creep-fatigue dependency on time-temperature material behavior

The implicit character of micro-structural degradation is determined by specifying the time history of crack growth caused by creep-fatigue interaction at high temperature. A dual scale micro/macro-equivalent crack growth model is used to illustrate the underlying principle of multiscaling which can be applied equally well to nano/micro. A series of dual scale models can be connected to formulate triple or quadruple scale models. Temperature and time-dependent thermo-mechanical material properties are developed to dictate the design time history of creep-fatigue cracking that can serve as the master curve for health monitoring. In contrast to the conventional procedure of problem/solution approach by specifying the time- and temperature-dependent material properties as a priori, the desired solution is then defined for a class of anticipated loadings. A scheme for matching the loading history with the damage evolution is then obtained. The results depend on the initial crack size and the extent of creep in proportion to fatigue damage. The path dependent nature of damage is demonstrated by showing the range of the pertinent parameters that control the final destruction of the material. A possible scenario of 20 yr of life span for the 38Cr2Mo2VA ultra-high strength steel is used to develop the evolution of the micro-structural degradation. Three micro/macro-parameters mu(center dot), d(center dot) and sigma(center dot) are used to exhibit the time-dependent variation of the material, geometry and load effects. They are necessary to reflect the scale transitory behavior of creep-fatigue damage. Once the algorithm is developed, the material can be tailor made to match the behavior. That is a different life span of the same material would alter the time behavior of mu(center dot), d(center dot) and sigma(center dot) and hence the micro-structural degradation history. The one-to-one correspondence of the material micro-structure degradation history with that of damage by cracking is the essence of path dependency. Numerical results and graphs are obtained to demonstrate how the inherently implicit material micro-structure parameters can be evaluated from the uniaxial bulk material properties at the macroscopic scale. The combined behavior of creep and fatigue can be exhibited by specifying the parameter xi with reference to the initial defect size an. Large xi (0.90 and 0.85) gives critical crack size a(cr) = 11-14 mm (at t < 20 yr) for a(0) about 1.3 mm. For small xi (0.05 and 0.15), there results critical a(cr) = 6-7 mm (at t < 20 yr) for a(0) about 0.7-0.8 mm. The initial crack is estimated to increase its length by an order of magnitude before triggering global to the instability. This also applies xi approximate to 0.5 where creep interacts severely with fatigue. Fine tuning of a(cr) and a(0) can be made to meet the condition of t = 20 yr. Trade off among load, material and geometric parameters are quantified such that the optimum conditions can be determined for the desired life qualified by the initial-final defect sizes. The scenario assumed in this work is indicative of the capability of the methodology. The initial-final defect sizes can be varied by re-designing the time-temperature material specifications. To reiterate, the uniqueness of solution requires the end result to match with the initial conditions for a given problem. This basic requirement has been accomplished by the dual scale micro/macro-crack growth model for creep and fatigue. (C) 2008 Elsevier Ltd. All rights reserved.