Physically short crack propagation in metals during high cycle fatigue (original) (raw)

Short fatigue crack behaviour under low cycle fatigue regime

International Journal of Fatigue, 2017

This paper addresses the important issue of the description of short fatigue crack behaviour. It is typical for these cracks that they propagate under large scale yielding conditions at the crack tip, which means that the non-linear fracture mechanics has to be applied. This paper presents results of experiments designed to measure the short crack growth rates in five different materials-316L steel, Eurofer 97 steel, ODS Eurofer steel, Duplex 2205 steel and Al 6082 alloy. The crack growth rates of these materials are described using different fracture mechanics parametersthe stress intensity factor, the J-integral and the plastic part of the J-integral. These approaches are evaluated and compared. The comparison revealed that the plastic part of the J-integral is the parameter governing the short crack growth rate in large scale yielding conditions. Moreover, crack growth rate data from all the tested materials measured at various loading levels lies on a unique curve. This remarkable observation suggests that the crack growth rate is determined by the extent of energy spent to plastic deformation, irrespective of the other materials properties.

Theoretical analysis on the behaviour of short fatigue cracks

International Journal of Fatigue, 2002

Based on the Navarro-Rios model, the boundaries of short crack growth are predicted. Theoretical analysis reveals that the extent of short crack growth regime is principally governed by the relation between the fatigue limit and the cyclic yield stress of the material. Materials with low values of s FL /s cy show extensive short crack behaviour, while in material with high values of s FL /s cy short crack behaviour is very limited. The results of the analysis are validated with experimental data. 

A general model for crack growth from initial defect in Very-High-Cycle Fatigue

Procedia Structural Integrity

During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.

008 Short crack and long crack propagation in metals based on damage mechanics concepts

Cp2012, 2013

The fatigue assessment of metallic structural components under uniaxial cyclic loading is traditionally tackled through experimental methods such as the S-N curve approach. For more complex variable stress states, such as multiaxial stress histories, the fatigue safety can be analysed by employing a physics-based damage mechanics approach. On the other hand, fatigue failure can be recognized as the result of a stable crack propagation up to a critical condition and, in this context, the availability of suitable laws to properly describe and quantify the crack propagation is a crucial aspect. In the present paper, a fatigue crack propagation law for both short (Low-Cycle-Fatigue) and long crack regime (High-Cycle-Fatigue) is discussed based on damage mechanics concepts. Fatigue crack growth law and damage mechanics approach are compared in order to determine both the damage value according to a given fatigue crack growth (FCG) law and the crack length associated to a given mechanical damage of the fatigued material. Such two methods are shown to be different formulations of the same physics-based approach to fatigue phenomena.

Fatigue crack initiation at a notch

International Journal of Fatigue, 2011

A short crack approach has been developed to determine the fatigue crack initiation life at a notch tip, in which the initiation life is assumed to correspond to the period during which an initial crack, equivalent to a microscopic defect, grows to attain a detectable size. This approach is compared to conventional local strain approach and it is shown that the short crack approach gives acceptable predictions as compared to experiments, in the 2024 T351 alloy. In the 7449 alloy, a different approach, using marker-band technique has been applied to determine the short crack growth kinetics at a notch. The advantages and disadvantages of the conventional strain based approach and the short crack model are discussed.

The transition between small and long fatigue crack behavior and its relation to microstructure

International Journal of Fatigue, 2008

The transition point from small to long crack behavior is experimentally studied in a single phase aluminum alloy. It is shown that scatter decreases until reaching a steady state value for long crack growth. This point is defined as the transition from micro-structurally small to long crack growth and is shown to correspond to the point when the growing crack front intersects approximately 15 grains. This transition point is experimentally validated from fatigue crack growth data both from single, corner micro-cracks and multi-site micro-cracks on a smooth surface.

A model for fatigue crack propagation

Engineering Fracture Mechanics, 1975

Ab&act-A model for fatigue crack propagation is presented which incorporates low cycle fatigue, mechanical proper&s and a microstructurally-associated process zone. Comparison of the model to publirhea data for 4340 (hard and soft), a s&es of TRIP steels, TiiAL4V, 20%T6 aed 300 ppdc mamgiag steel shows good agrccmcnt.

A fatigue crack propagation model

Engineering Fracture Mechanics, 1984

A model for fatigue crack propagation has been developed which incorporates mechanical, cyclic and fatigue properties as well as a length parameter. The latter can be associated with the microstructure of the material. The fatigue failure criterion is based on a measure of the dissipated plastic strain energy. This model predicts crack propagation at low and intermediate AK values, i.e. stage I crack growth rate as well as that of the stage II. A number of crack growth rate models proposed earlier, are shown to be particular cases of the one developed herein. Predictions of the model are in good agreement with the experimental data. The required data for predicting the crack growth rate, can be found in standard material handbooks where fatigue properties are listed.

Critical review of the fatigue growth of short cracks

Engineering Fracture Mechanics, 1986

Retirement for cause (RFC) has become a popular design/analysis philosophy because it facilitates continued use of components which would otherwise be retired based on a safe life philosophy. RFC permits this continued service on the basis that service-induced damage tracked by periodic inspections will not develop to a critical size within a future operating interval. Successful implementation of RFC requires fracture mechanics technology to predict in-service growth behavior. Recent observations suggest that nonconservative estimates of crack growth rate (and therefore inspection interval) and critical flaw size arise when conventional linear elastic fracture mechanics (LEFM) is applied to predict the growth of physically small cracks. This paper summarizes the results of an extensive limited-circulation critical review of the phenomenology and mechanics of short crack growth with a view to identifying when this anomalous growth makes RFC analyses untenable in terms of LEFM. Factors which control the growth of short cracks are enumerated. It is shown for unnotched samples that the apparent effect may be traced to microcrack closure and the metallurgical. mechanical, and environmental transients which develop in the transition from initiation to steady-state growth. For notch samples the anomalous growth of cracks is traced to the inelastic action that develops a displacement-controlled notch field, which, contrary to LEFM analysis, dominates crack extension. Mechanics analyses relevant to characterizing the growth of short cracks are discussed. A crack tip opening displacement criterion is indicated to be appropriate.

A New Three-Parameter Model for Predicting Fatigue Crack Initiation Life

Journal of Materials Engineering and Performance, 2011

Based on the idea that the fatigue damage is caused by the cyclic damage strain, a concept of the critical damage quantity is introduced and a new three-parameter model is developed. The model contains three material performance parameters, i.e., the fatigue ductility coefficient, the fatigue ductility exponent, and the theoretical strain endurance limit. The fatigue ductility coefficient reflects the existence of the critical damage quantity. The fatigue ductility exponent shows the damage resistance ability of the material. And the theoretical strain endurance limit represents the existence of the critical cyclic strain. By using the proposed model, the fatigue crack initiation life of metallic materials can be predicted.