Theory And Test Methods For Cyclic And Quasi-Static Crack Growth

The basic research of structural integrity in the limit strength, the residual life and the initiation failure for service components containing defects under the quasi-static and cyclic loading is often concerned by engineering and academic areas. Under some special or extreme environments, however, such as high temperature, corrosion, irradiation, vacuum and others, related mechanical properties of material cannot be directly and efficiently obtained even in the laboratory. It is difficult to analyze the structural integrity of components containing defects without the basic data for mechanical properties of materials. Considering the damage correlation for the representative volume element (RVE) of material, the study of evaluation theory about mechanical properties of material is thoroughly and comprehensively developed. Because the evaluation theory is very significant in the analysis of structural integrity of components containing defects. In the present study, four main works that are the correlations between the critical damage law of axial/torsional low cycle fatigue and the mode-Ⅰ/mode-Ⅱ fatigue crack growth rate, between the critical damage law of axial low cycle fatigue and the evolution of non-penetrating mode-Ⅰ structural fatigue crack growth, for the estimating J integral resistance (JR) curve based on the load separation technique of the front face compact tension specimen, are studied in more detail. The main works are introduced as follows:1. For the mode-I crack, the cycle stress and strain field near the crack tip is obtained by considering the Rice’s plastic superposition theory under the small scale yield and two kinds of stress and stain fields (HRR field and RKE field) under the remote uniaxial tensile loading. Because the crack closure phenomenon exists near the mode-Ⅰ crack tip under the remote tensile-compressive cyclic loading, the effective cycle stress and strain field and the effective cycle plastic zone are calculated by considering the crack closure factor. Using two kinds of effective cycle stress and strain fields and two types of fatigue failure criterions (the plastic strain energy density and the Miner’s linear damage accumulation) in the range of the effective cycle plastic zone, the theory model of mode-Ⅰ fatigue crack growth rate (Shi-Cai model used for life assessment of mode-Ⅰ crack) is established based on the critical damage law of axial low cycle fatigue. Comparing fifteen engineering material’s experimental data for fatigue crack growth rate with model’s prediction results, the accuracy of Shi-Cai model is discussed and analyzed in more detail from three aspects that are the crack closure under the tensile-compressive cyclic loading, two types of fatigue failure criterions and two kinds of effective cycle stress and strain fields.2. About thin plates containing the non-penetrating mode-I structural crack, the theory model of the residual life and the shape evolution law is developed by considering the Shi-Cai model, the stress intensity factor (SIF) of ahead of the non-penetrating mode-Ⅰ structural crack tip and the non-self-similar principle of the non-penetrating mode-Ⅰ structural fatigue cracking. Based on three kinds of material’s experimental data for non-penetrating structural fatigue crack growth, the accuracy of theory model for thin plates containing the non-penetrating mode-Ⅰ structural fatigue cracking is discussed in more detail. Considering 7075-T6 Aluminum alloy, in addition, the shape evolution law for thin plates containing five kinds of non-penetrating mode-Ⅰ structural cracks under the remote cycle tension, bend, and tension-bend loading is thoroughly studied. Results show that the final shape of structural cracking depends only on the type of remote cyclic loading and has nothing to do with the initial flaw.3. For the mode-Ⅱ crack, the cycle stress and strain field near the crack tip is obtained by considering the Rice’s plastic superposition theory under the small scale yield and the HRR field under the remote shear loading. Because the crack surface interference phenomenon exists near the mode-Ⅱ crack tip under the remote shear cyclic loading, the effective cycle stress and strain field and the effective plastic zone are estimated by introducing the modified parameter Msli in the vicinity of the mode-Ⅱ crack tip. Using the effective cycle stress and strain field and two kinds of fatigue failure criterions (the plastic strain energy density and the Miner’s linear damage accumulation) in the range of effective cycle plastic zone, the theory model of mode-Ⅱ fatigue crack growth rate (Shi-Cai model used for life assessment of mode-Ⅱ crack) is established based on the critical damage law of torsional low cycle fatigue. Comparing two kinds of Aluminum alloy’s (2024-T351 and 7075-T6) experimental data for fatigue crack growth rate with model’s prediction results, the accuracy of Shi-Cai model is verified in the present study. The Shi-Cai model’s prediction results for the upper and lower offset 50% are shown in the present study, and the experimental data for two kinds of Aluminum alloy materials are contained within the range.4. Based on the energy principle of elastic-plastic fracture mechanics, the relationship for the geometry factor between the load line compact tension specimen and the front face compact tension specimen is established. And, the load separation technique of the front face compact tension specimen is proposed to obtaining the J integral resistance (JR) curve. Considering the standard load line compact tension (LLCT) specimen, the single edge bend (SEB) specimen, and the front face compact tension (FFCT) specimen developed in the present study, the analysis software for J-load separation theory (J-LST) is completed by using the visual basic (VB) language. The calculated results of the J-LST and the standard method are compared and discussed. Using the fracture theory of the front face compact tension specimen, A508-Ⅲ steel’s JR curves in two directions, "S" and "X", are obtained based on the 1/2T FFCT specimen. Considering the relationship for the crack opening displacement (COD) between FFCT specimen and LLCT specimen, in addition, a new method by adopting the iterative method for analyzing the experimental data (load vs displacement) of FFCT specimen subjected to the monotonic tensile load is established to obtain the fracture toughness.