Studies On Three-dimensional Fracture And Fatigue Crack Growth Simulation Of Curve Cracked Structures

The conception of damage tolerance design has been widely recognized as the scientific thought to ensure security and economical efficiency of aircrafts. Based on the theory of fracture mechanics, the two core tasks of damage tolerance design are residual strength assessment and prediction of fatigue crack growth for cracked structure. At present, the theoretical basis and technical platform of damage tolerance design are two-dimensional (2D) fracture mechanics and commercial CAE software, which bring two major difficulties: first, it usually has to do the most conservative assessment due to lack of appropriate theoretical basis for non-standard cracked structures in the framework of 2D fracture theory; second, the assessment of residual strength and fatigue crack growth simulation is involved due to the complicated operations for building CAE model of cracked structure. For these two issues, the theories and modelling technics of part-through curve cracks are studied systematically and deeply. The main progresses are as follows:(1) The CAE sub-models of typical three-dimensional (3D) cracks are established and gathered to a sub-model database. Then the 3D crack auto-building-model technique is developed to insert crack sub-model to geometric or finite element model. On this basis, a parameterized post-processing module is established. Accordingly, the entire process from model building to post-processing can be entirely controlled in the program files, which provides technical support for residual strength assessment and fatigue crack growth simulation of complex 3D cracked structure.(2) A conception of equivalent thickness is proposed based on the three-dimensional constraint factor. This conception establishes an equivalent relationship between a point on curve crack front line and straight-through crack tip in the mid plane of plate. Based on the equivalence, the material fracture parameters or data obtained from straight-through cracked plates can be reasonably used to assess residual strength, crack opening stress, crack propagation life and other key issues. This conception bridges the gap between standard specimens in laboratory to practical complex structures.(3) Based on the equivalent thickness conception and 3D fracture theory, a two-parameter fracture criterion of non-through curve cracked specimen is proposed which unifies the fracture assessment of non-through curve cracked specimens and standard straight-through cracked plates. This criterion can be widely used in fracture assessments for various part-through curve cracked structures, and build a theoretical foundation of residual strength assessment for complex 3D cracked structures.(4) Based on the equivalent thickness conception and the strip yield model, a crack closure model of part-through curve cracked specimen is proposed. This model can be used to calculate the opening stresses and effective stress intensity factor ranges along non-through curve crack front line. Therefore, the crack growth life and crack shape evolution can be predicted.(5) The values of retardation factors in three typical spectrum loading fatigue crack growth models (Wheeler model, Willenborg/Chang model and equivalent damage model) in plane stress, plane strain and 3D stress constraint state are systematically studied and compared. It is found that the influence of 3D stress state on plastic zone size in front of crack tip will further significantly affect the overload retardation or reverse acceleration. When the plastic zone models are used to predict spectrum loading fatigue crack growth, it is necessary to accurately calculate the 3D constraint factor in front of crack tip, and then the accurate plastic zone size and crack propagation rate can be obtained.Based on the above studies, the proposed methods, techniques and models are used to damage tolerance analyses of specific engineering structures. It is shown that the current research results provide a viable solution for the major scientific and technological problems in damage tolerance design.