A Concise Binomial Model for Nonlinear Creep-Fatigue Crack Growth Behavior at Elevated Temperatures

The creep-fatigue crack growth problem remains challenging since materials exhibit different linear and nonlinear behaviors depending on the environmental and loading conditions. In this paper, we systematically carried out a series of creep-fatigue crack growth experiments to evaluate the influence from temperature, stress ratio, and dwell time for the nickel-based superalloy GH4720Li. A transition from coupled fatigue-dominated fracture to creep-dominated fracture was observed with the increase of dwell time at 600 °C, while only the creep-dominated fracture existed at 700 °C, regardless of the dwell time. A concise binomial crack growth model was constructed on the basis of existing phenomenal models, where the linear terms are included to express the behavior under pure creep loading, and the nonlinear terms were introduced to represent the behavior near the fracture toughness and during the creep-fatigue interaction. Through the model implementation and validation of the proposed model, the correlation coefficient is higher than 0.9 on ten out of twelve sets of experimental data, revealing the accuracy of the proposed model. This work contributes to an enrichment of creep-fatigue crack growth data in the typical nickel-based superalloy at elevated temperatures and could be referable in the modeling for damage tolerance assessment of turbine disks.

Effects of ECAE process on the fatigue crack growth rate of copper metal

In this study, the effects of the number of passes performed by the Equal Channel Angular Extrusion as a severe plastic deformation process on copper metal's microstructure and mechanical properties, especially its resistance to fatigue crack growth, have been investigated. The experimental results show that as the number of processes passes increases, the copper metal grains become finer and as a result less stress is concentrated at the starting points of the fatigue fracture, which delays the fracture. For example, after performing 8 ECAE process passes, the threshold values of fatigue crack growth increases by 113.2% relative to the base metal. Moreover, as the grains become smaller, the number of grains and consequently the number of grain boundaries will increase and thus more obstacles will be placed in the way of crack growth. Also, the SEM images indicate that many fine and equiaxed dimples in processed copper become smaller as the number of passes increases. This shows that finer and more equiaxed grains will be obtained by repeating the ECAE process and thus repeating the occurrence of recrystallization. It was cleared that this process improves the mechanical properties of the copper other than the failure strain. However, by increasing the number of process passes, this problem can be significantly reduced. Highlights The fine grains considerably delay the fatigue fracture By ECAE process, the threshold value of fatigue crack growth increases by 113.2% All zones resulting from fatigue fracture are recognizable in fractured ECAE sample The SEM images indicate that a ductile failure has occurred in the tensile samples

Numerical Determination of Paris Law Constants for Carbon Steel Using a Two-Scale Model

For most engineering alloys, the long fatigue crack growth under a certain stress level can be described by the Paris law. The law provides a correlation between the fatigue crack growth rate (FCGR or da/dN), the range of stress intensity factor (ΔK), and the material constants C and m. A well-established test procedure is typically used to determine the Paris law constants C and m, considering standard specimens, notched and pre-cracked. Definition of all the details necessary to obtain feasible and comparable Paris law constants are covered by standards. However, these cost-expensive tests can be replaced by appropriate numerical calculations. In this respect, this paper deals with the numerical determination of Paris law constants for carbon steel using a two-scale model. A micro-model containing the microstructure of a material is generated using the Finite Element Method (FEM) to calculate the fatigue crack growth rate at a crack tip. The model is based on the Tanaka-Mura equation. On the other side, a macro-model serves for the calculation of the stress intensity factor. The analysis yields a relationship between the crack growth rates and the stress intensity factors for defined crack lengths which is then used to determine the Paris law constants.