External Semi-elliptical Surface Crack Research Articles

Mixed mode surface crack growth in aluminium alloys under complex stress state

In this study fatigue crack growth tests were carried out on hollow cylindrical specimens, made of D16T and B95AT aluminum alloys (analogue of 2024 and 7075 aluminum), with initial external semi-elliptical surface cracks and undergoing complex stress state. The crack growth behavior of aforementioned materials was studied under cyclic axial tension, pure torsion and combined tension+torsion fatigue loading. Optical microscope measurements and the crack mouth opening displacement (CMOD) method were respectively used to monitor crack length and calculate crack depth. The experimental crack front positions were highlighted by using a beach mark procedure during the tests. The stress strain field along the crack front of semi-elliptical cracks in the cylindrical hollow specimens was assessed by Finite Element Method (FEM) analysis. The stress intensity factors (SIFs) were calculated by the 3D M-integral approach and the distributions of equivalent elastic SIFs along the crack front were used for crack growth rate assessment under mixed mode conditions. As a result, the fracture resistance parameters of aluminum alloys under complex stress state were presented. The simulation of cracks propagation turned out to be consistent with experimental results.

Reliability assessment of cracked pipes subjected to creep-fatigue loading

The reliability analysis of defective pipes, which operate under creep-fatigue loading conditions has been studied. An austenitic stainless steel pipe containing an axially external semi-elliptical surface crack, subjected to variable internal pressure at elevated temperature has been considered as the benchmark problem. Different parameters in material behavior, geometry parameters, and operational condition have been assumed as random variables in the probabilistic assessments. The reliability of the cracked pipes have been evaluated at various operation times, using different reliability methods. Sensitivity analyses have been conducted to investigate the importance measure of the random variables involved in the problem. Moreover, the effects of reducing the scattering of the random variables on the variations of the reliability index, β, has been examined.

Elastic and plastic stress intensity factor in specimen of aluminum alloys under tension and bending in the temperature range

The elastic and plastic stress intensity factors (SIF) are studied through computations for two aluminum alloys in the temperature range. Subject for studies is central notched specimens with external semi-elliptical surface crack. Wide range of relative crack depth and aspect ratio is studied. The calculations are carried out under uniaxial tension and a three-point bending for aluminum alloys D16 and B95 at -60°С, 20°С and 250°С. The elastic and plastic stress intensity factor distributions along the curvilinear semi-elliptic crack front are obtained as a function of the relative sizes of defects, the loading type and temperature. Fundamental differences in the behavior of the elastic and plastic SIFs for surface cracks are established at high, room and low temperatures. The distributions of elastic SIF are the same at different temperatures for both materials. Contrary to that, the plastic SIF is sensitive to the plastic properties of the materials. The plastic SIF gradually increases by increasing the temperature. The plastic SIF, which is sensitive to the constraint effects and elastic-plastic material properties, is attractive as the self-dependent unified parameter for characterization of the material fracture resistance properties.

Effect of temperature on the growth of fatigue surface cracks in aluminum alloys

To understand the effect of temperature on the rate of fatigue crack growth in D16T and B95AT aluminum alloys, fatigue crack growth experiments using a constant load amplitude were carried out on hollow cylindrical specimens with induced external semi-elliptical surface cracks. The rates of fatigue crack growth in the specimens were determined as a function of temperature, ranging from low (−60 °C) to elevated (+250 °C), and were analyzed as a function of the elastic and plastic stress intensity factors (SIF). The process of numerical calculations includes the analysis of the elastic constraint parameters in the form of the non-singular T-stress and TZ –factor, as well as the elastic-plastic constraint parameters in the form of the local stress triaxiality h and In-factors for the various tested materials at the different tested temperatures. The interpretation of the fatigue crack growth data in terms of the plastic SIFs results in a single general curve with a small scatter band over a wide range of temperatures.

Effect of different environmental conditions on surface crack growth in aluminum alloys

Fatigue surface crack growth is studied through experiments and computations for aluminum alloys D16T and B95AT (analogue of 2024 and 7075 aluminum). Subjects for studies are cylindrical hollow specimens with external semi-elliptical surface crack. The variation of fatigue crack growth rate and surface crack paths behavior was studied under cyclic loading for different environmental conditions. Uniaxial tension tests were carried out at low (-60°C), room (+23°C) and high (+250°C) temperature. For the same specimen configuration and the different crack front position as a function of cyclic loading and temperatures conditions the distributions of governing parameter of the elastic-plastic stress fields in the form of In-factor along various crack fronts was determined from numerical calculations. This governing parameter was used as the foundation of the elastic-plastic stress intensity factor (SIF). Both elastic and plastic SIF approach was applied to the fatigue crack growth rate interpretation. It is found that there is a steady relationship between the crack growth rate and the plastic SIF in the form of general curve within a relatively narrow scatter band for all tested specimens at different temperatures.

Surface flaws behavior under tension, bending and biaxial cyclic loading

Fatigue surface crack growth and in-plane and out-of-plane constraint effects are studied through experiments and computations for the aluminum alloy D16T. A tension/bending central notched plate and cruciform specimens under different biaxial loadings with external semi-elliptical surface cracks are studied. The variation of the fatigue crack growth rate and surface crack paths is studied under cyclic tension, bending and biaxial tension–compression loading. For the experimental surface crack paths in the tested specimens, the T-stress, out-of-plane Tz factor, local triaxiality parameter h and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of the In-integral are calculated as a function of the aspect ratio by finite element analysis to characterize the constraint effects along the semi-elliptical crack front. The plastic stress intensity factor approach is applied to the fatigue crack growth on the free surface, as well as at the deepest point of the semi-elliptical surface crack front, of the tested tension/bending plate and cruciform specimens. From the results, characteristics of the fatigue surface crack growth rate as a function of the loading conditions are established.

Surface and through thickness crack growth in cruciform specimens subjected to biaxial loading

Fatigue surface crack growth are studied through experiments and computations for aluminum alloy D16T. Subjects for studies are cruciform specimens under different biaxial loading with external semi-elliptical surface crack. The variation of fatigue crack growth rate and surface crack paths behaviour was studied under cyclic uniaxial tension, equal biaxial tension and biaxial tension-compression loading. For the experimental surface crack paths in tested specimens the T-stress, the out-of-plane TZ factor, the local triaxiality parameter h and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral were calculated as a function of aspect ratio by finite element analysis to characterization of the constraint effects along semi-elliptical crack front. Both elastic and plastic stress intensity factor approach was applied to the fatigue crack growth on the free surface of the cruciform specimens as well as in the deepest point of the semi-elliptical surface crack front. As result principal particularities of the fatigue surface crack growth rate as a function of loading conditions were established. The experimental and numerical results of the present study provide an opportunity to explore the suggestion that crack growth rate may be represented by the plastic stress intensity factor, rather than the magnitude of the elastic SIF alone.

Surface crack growth subject to bending and biaxial tension-compression

Fatigue surface crack growth and the in-plane and out-of-plane constraint effects are studied through experiments and computations for aluminium alloy D16T. Subjects for studies are cruciform specimens under different biaxial loading and bending central notched specimens with external semi-elliptical surface crack. Both the optical microscope measurements and the crack opening displacement (COD) method are used to monitor and calculate both crack depth and crack length during the tests. The variation of crack growth rate and surface crack paths behaviour is studied under cyclic pure bending and biaxial tension-compression fatigue loading. This work is centered on the relations between crack size on the free surface of specimen considered configurations, COD and aspect ratio under different fatigue loading conditions. For the experimental surface crack paths in tested specimens the T-stress, the local triaxiality parameter h, the out-of-plane TZ factor and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral were calculated as a function of aspect ratio by finite element analysis to characterization of the constraint effects along semi-elliptical crack front. The plastic stress intensity factor approach is applied to the fatigue crack growth on the free surface of the tested bending and cruciform specimens as well as the deepest point of the semi-elliptical surface crack front. As result fatigue surface crack paths or crack front positions as a function of accumulated number of cycle of loading are obtained.

Boundary element analysis on interaction of external surface crack and embedded crack in a pressurized cylinder

This paper analyzes the interaction of external semi-elliptical surface crack and embedded elliptical crack in a longitudinal plane of a pressurized cylinder. No numerical solution appears to be available in the literature so far. The analyzed ratios of the crack depth to crack length ( b S/ a S and b E/ a E) are 0.25 and 1.0; the distance between both cracks ( s 1/ b min) ranges from 0.5 to 2.0. In total 24 crack configurations are analyzed and the stress intensity factors along the crack front are presented. The numerical results show that the interaction makes the stress intensity factors along the crack fronts increased for both the surface crack and the embedded crack, compared with the single surface crack and single embedded crack. For the given crack sizes, the stress intensity factors for both the surface crack and embedded crack increase with the decrease of the distance between two cracks. The numerical analyses are carried out by the hybrid boundary element method.

Stress intensity factors for external semielliptical surface cracks in pressurized thick-walled cylinders using the hybrid boundary element method

The purpose of this paper is to present a comprehensive range of results for mode I stress intensity factors of external surface cracks in internally pressurized thick-walled cylinders. The hybrid boundary element method which is reviewed is used to calculate the stress intensity factors. The analysed ratios of crack depth to wall thickness ranged from 0·2 to 0·8; the ratios of crack depth to crack length ranged from 0·25 to 1·0; the ratios of wall thickness to cylinder radius were 0·5, 1·0 and 2·0. For crack configurations t R i = 0·5 , b a = 1·0 , b t = 0·6 and 0·8, the maximum difference between Atluri and Kathiresan's results obtained by the displacement hybrid finite element method and the present one is about 15·0%.