Stress Intensity Factors for Embedded, Surface, and Corner Cracks in Finite-Thickness Plates Subjected to Tensile Loading

The aim of this study is to obtain the stress intensity factor (SIF) along the crack front of elliptical cracks located in finite-thickness plates subjected to imposed displacement or applied tensile load, for different crack geometries (relative depths and aspect ratios) and crack configurations (embedded, surface, and corner). The SIF was calculated from the J-integral, obtained by the finite element method. The results show how the SIF grows with the increase in the relative crack depth and with the decrease in the aspect ratio, with the corner crack being the most dangerous configuration and the embedded crack the most favorable configuration. By increasing the plate length, the SIF rises when the plate is under imposed displacement and decreases when the plate is subjected to applied tensile load, both cases tending towards the same SIF curve.

Research on the Fatigue of Small Impulse Turbine Blade Based on the Numerical Simulation and Experimental Tests

Reusable spacecraft is increasingly attracting researchers’ attention. However, the experimental investigations on the turbine blade of the rocket engine are rarely published. Thus, the fatigue of a small impulse rocket turbine blade is explored in the current work. First, the specimen and the electrode of electrical discharge machining are carefully designed. Then, the electrical discharge machining is used to machine the specimen. To study the fatigue properties, the finite element analyses are separately performed on the blade model and the specimen. Based on the numerical results, a fatigue test is carried out to reproduce the most vulnerable position. Finally, the microstructural structures of the specimen are detected using the scanning electron microscope (SEM). Results show that (1) different from the aviation field, the specimen is unable to be machined with the welding method because it destroys the crucial details and the mechanical properties; (2) the maximum plastic strain is present at the leading edge close to the hub, at which a 760 μm corner crack appears at the 10113th fatigue cycle. This work provides a feasible method of using the EDM process to machine specimen for the small impulse turbine blade.

Application of J2 Integral Method in Calculating Complicated Stress Intensity Factors on Three-dimensional Components

In this paper, a technique to determine complicated stress intensity factors on three-dimensional components, which based on the conservation law and the elementary mechanics is proposed, it only needs the geometric relationship between multiple singular stress fields from the crack section, and obtaind the relationship between the stress at different crack tips. In the expression of the stress intensity factor K, K is proportional to the stress σ at the crack tip, and we can get the supplementary equation of between different stress fields K according to the ratio of the stress at the crack tip, then use the J-integral method to calculate the stress intensity factors of different stress fields. In order to verify the feasibility of this method, a cracked R-fluted shells model was constructed. Under the action of the bending moment, the corner crack propagation is simulated through the reserved corner crack, and two crack tips with different stress fields are generated during the simulation. The experimental result indicates that the proposed method is effective for cracked R-fluted shells. It is also shown that the method has universal applicability for solving complex stress intensity factors on three-dimensional components.

The Effect of a Three-Dimensional Quarter-Circle Corner Crack on the Stress Intensity Factors of a Nonaligned Semi-Elliptical Surface Crack in an Semi-Infinite Solid Under Uniaxial Tension

The Fitness-for-Service (FFS) approach requires the evaluation of the mutual impact of nonaligned, multiple cracks on each other. As such, initially one must resolve whether existing, nonaligned, parallel cracks in the structure should be treated as merged or as separate, multiple cracks for FFS evaluation. Criteria and standards found in existing literature on how to deal with multiple, nonaligned cracks are very source dependent, and those guidelines are often developed from on-site, service inspections without exact and methodical substantiation. Based on this determination, the authors previously reported on the impact of an embedded crack on an edge crack using a two-dimensional model, and, more recently using a three-dimensional (3D) model, on the impact of a semicircular surface crack on a quarter-circle corner crack. However, actual crack shapes identified using nondestructive techniques are 3D in nature, normally not semicircular, and their impact are of mutual importance. Thus, the stress intensity factor (SIF) distribution along the semi-elliptical surface crack is as significant as the SIF distribution of the corner crack in the application of FFS standards. Therefore, nonaligned cracks with varied arrangements and shapes and the SIFs along their crack fronts are considered crucial in order to obtain more practical information on the application of rules provided in FFS codes. In this study, over 330 different cases are solved and the behavior of the SIF distribution along a 3D semi-elliptic nonaligned surface crack is assessed when affected by a quarter-circle corner crack of various geometries in an infinitely large solid. For a given geometry of a quarter-circle corner crack, a detailed examination of the corner crack's impact on the 3D SIFs of the surface crack is carried out as a function of the surface crack's ellipticity, and the horizontal (H) and vertical (S) separation distances between the two cracks. The analysis was replicated for various arrangements of separation distances S and H. The results from this study are considered noteworthy to the understanding of the relation between the criteria and standards in FFS community and the consequence of their application in engineering practice. The results demonstrate that the 3D SIFs along the crack front of the semi-elliptical surface crack can be affected profoundly by the presence of the quarter-circle corner crack. The corner crack's existence may amplify or diminish the SIF of the surface crack for those points of the semi-elliptic surface crack front that approach the closest quarter-circle corner crack tip. Furthermore, when the two cracks are overlapped, the behavior of the SIF distribution as a function of separation distance is different in the vertical direction than in the horizontal direction due to a process called shielding. As the separation distances between the cracks increase in either direction, there is a separation distance after which the cracks can be treated as separate cracks, and, this distance is dependent on the relative crack lengths.

The Influence of a Non-Aligned Semi-Elliptical Embedded Crack on the Stress Intensity Factor Distribution Along the Front of a Quarter-Circle Corner Crack in a Semi-Infinite Plate Under Pure Bending

Fitness-for-Service codes require whether non-aligned cracks be treated as coalesced or separate multiple cracks. The authors previously reported on the effect between an corner and an embedded parallel crack in 2-D and in 3-D scenarios subject to tensile loading. Since realistic crack configurations detected using non-destructive methods are generally 3-D in nature, the study of 3-D effect under different loading types is deemed necessary in order to obtain more practical guidance. In this study, we investigate stress intensity factors (SIFs) along the crack front of a quarter-circle corner crack when affected by a semi-elliptic surface crack in a semi-infinite large solid under pure bending. While keeping constant the geometry of the quarter-circle corner crack, the SIFs along its front are studied for a wide range of geometrical configurations of the surface crack by varying its ellipticity b1/a1 = 0.1∼1; the relative crack size of the two parallel cracks a1/a2 = 1/3∼2; the normalized vertical gap, H/a2 = 0.4∼2; and the normalized horizontal gap, S/a2 = −0.5∼2 between the two cracks on using linear elastic fracture mechanics (LEFM). The results from this study are collectively significant to the understanding of the correlation between the criteria and standards in Fitness-for-Service community and the consequence of their usage in engineering practice.

Investigation Into Crack Closure Effects for Fatigue Crack Growth Under Negative R Conditions

Fatigue crack growth laws are typically dependent on the ratio between minimum and maximum Stress Intensity Factor (SIF), referred to as the load ratio (R). When part of the SIF range is compressive (and hence R is negative) the amount of growth for a given SIF range is reduced due to crack closure effects. Methods for capturing the effect of crack closure were presented in a previous PVP paper [1]. These methods are based around defining a scaling factor (q0) which is dependent on R and applied to the SIF range before calculated growth. Equations were provided for both best fit and bounding q0 factors. This paper presents a comparison between these methods and results of testing. The specimens used were square cross-section bars and were made from Type 304L stainless steel with an initial corner crack. A range of load ranges and R ratios (including some positive R values) were used and the testing was undertaken at 250°C in both air and a simulated PWR environment. The growth rate observed in the tests was used to derive the effective q0 factor observed in each stage of the testing. These values were then compared with the q0 methods that are used in actual defect tolerance calculations. The results agreed very closely with the derived best estimate q0 curves, with no discernible difference between the air and water results.