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The recently developed theory for isotropic material failure is briefly outlined. Many steps and stages of the verification process for the theory are fully documented so that it can be used with assurance in a wide variety of directions. Then, the failure theory is used to develop a new measure or index for the degree of ductility of material failure in any state of stress. This new failure property is named the ductility number, Nd. It is rigorously derived, intuitively compatible, and very easy to use. Applications and interpretations of Nd are given. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, an analytical study is carried out on the work‐hardening, elastic‐plastic stress distributions in a cracked body under antiplane shear deformation. A modified Ramberg‐Osgood law is introduced to describe the material behaviour, and stress and strain fields are derived in closed form. Compared with the conventional Ramberg‐Osgood formulation, the new law includes the effect of a new parameter, κ, which allows the transition from the ideally elastic behaviour (low stress regime) to the power law behaviour (large stress regime) to be controlled, thus providing 1 more degree of freedom to better fit the actual behaviour of engineering materials. A discussion is carried out on the features of stresses and strains close to and far away from the crack tip. [ABSTRACT FROM AUTHOR]

This paper investigates the ability of the USAF characteristic K approach to represent the growth of cracks from small naturally occurring material discontinuities. The examples discussed in this paper are associated with two different load spectra. The first case studied involves crack growth in AA7050-T7451 under a combat aircraft load spectrum. The second case studied the crack growth in AA7050-T7451 under a civil aircraft load spectrum. It is shown that using the root mean square value of ΔK in a load block to characterize the stress intensity factor together with the NASGRO equation, with the constants determined from constant amplitude tests on long cracks, enables the growth of small cracks under these two different complex load spectra to be reasonably accurately represented. [ABSTRACT FROM AUTHOR]

The present work studied the ultrasonic fatigue fracture of the partially pre-alloyed Fe-Cu-Mn-C powder metallurgy steel fabricated by die-wall lubricated warm compaction. The microstructure and ultrasonic fatigue fracture behaviour were analyzed. A mathematical model was established to solve the fatigue crack growth process. Results indicate that the Fe-2Cu-0.5Mn-1C sintered material has good mechanial properties. The fatigue crack initiates at large pore cluster near the surface of sample, and the ultrasonic fatigue striations have been observed near crack source area. The crack source area is divided into several small regions: the micro fatigue source area, crack growth area and fast fracture area. In addition, if the material breaks at 107 times of cycles loading in fatigue tests, the crack \stabe growth stage accounts for about 8.3×105 cycles, which is only a small part of the whole ultrasonic fatigue fracture process. The cycle times of crack stabe growth stage is inverse proportion to the square of the stress. [ABSTRACT FROM AUTHOR]

The strain energy density (SED) averaged over a material dependent control volume proved to control fracture and fatigue behaviour of different materials in many engineering problems. A method to rapidly estimate the averaged SED at the tip of cracks under in-plane mixed mode I + II loading has been recently proposed. It is based on the peak stresses evaluated from finite element (FE) analyses, according to the peak stress method (PSM). Coarse finite element meshes automatically obtained from the free mesh generation algorithm available in a commercial FE code can be adopted. In the present paper Ansys® software has been adopted. The aim of the present contribution is to extend the FE approach to cracks subjected to out-of-plane mixed mode I + III loading. The advantage of the proposed approach is two-fold: there is no need of mesh refinement in the close neighbourhood of the points of singularity, so that coarse FE meshes can be adopted; moreover, geometrical modeling the control volume in FE models is no longer necessary. Bars weakened by circumferential outer cracks subjected to out-of-plane mixed mode (I + III) loading have been analysed taking into consideration different crack lengths, mode mixities and average sizes of the adopted FE meshes. A comparison between approximate values of the averaged SED according to the peak stress approach and accurate values calculated directly from the FE strain energy by adopting very refined FE meshes has been successfully performed within a range of applicability. [ABSTRACT FROM AUTHOR]

It is important to evaluate microscopic stress distributions on the order of the grain size to clarify the deformation and fracture behavior of structural materials. In this study, the microscopic stress distribution of coarse-grained austenitic stainless steel during tensile testing was measured using a method that employs cross-correlation of the electron backscatter diffraction (EBSD) pattern, and the effect of the reference point selection on the measured stress was investigated. First, parameters that characterize the quality of the EBSD pattern measurement—namely, the image quality (IQ), confidence index (CI), and Fit—and the parameter that corresponds to the extent of the plastic deformation—the kernel average misorientation (KAM)—were used to determine the reference point. It was confirmed that the value of the microscopic stress measured using the EBSD-based approach is relative to the value of the stress state of the reference point. Second, multiple reference points were determined within a grain by considering only the location, and the stress distribution in the grain was evaluated using each of the reference points. The deviation of the measured stress was calculated from the averaged value for all the measured results using a certain reference point, and its relationship to parameters such as the IQ, CI, Fit, and KAM of the reference point used was investigated. It was demonstrated that the choice of parameters did not significantly affect the evaluation of the relative stress distribution within a grain. The results of this study therefore show that the selection of the reference point does not affect the measured stress distribution, as long as the evaluation is limited to the relative stress within a grain for the selected reference point. [ABSTRACT FROM AUTHOR]

ABSTRACT Three-dimensional (3D) finite element analyses are carried out on single-edge bend [SE(B)] specimens for which the J-integral resistance curves ( J-R curves) have been experimentally determined to develop the constraint-corrected J-R curves for the X80 grade pipe steel. The constraint parameters considered in this study include QHRR, QSSY, Q SSY_ m, QLM, Q BM1, Q BM2, A2, h and Tz. The constraint-corrected J-R curves were developed on the basis of the constraint parameters obtained from finite element analysis and experimentally determined J-R curves associated with deeply cracked and medium-cracked SE(B) specimens and validated against shallow-cracked SE(B) specimens. The analysis results indicate that all the constraint parameters considered in this study except QHRR, QSSY, Q SSY_ m and QLM lead to reasonably accurate constraint-corrected J-R curves if the crack extensions are relatively small (≤0.7 mm). For larger crack extensions (≤1.5 mm), the Q BM1-based constraint-corrected J-R curve leads to the most accurate predictions of J among all the constraint parameters considered. [ABSTRACT FROM AUTHOR]

ABSTRACT The crack growth retardation is studied theoretically by drilling two symmetric holes at appropriate locations along the crack flanks. A fatigue crack growth code for two-dimensional elastic problems is developed to approve the effectiveness of this method. The crack growth retardation was examined using a parametric study both on the arrangements of the flank holes and the diameters of holes. The numerical results reveal that for some cases, the presence of flank holes significantly decreases the stress concentration around the crack tip and its stress intensity factor. The best positions for crack flank holes to provide the highest crack growth retardation are shown to be where the line connecting the two centres of holes passes the crack tip. A larger hole diameter and a closer distance between the flank holes and the crack tip result in lower crack growth rate and higher fatigue life. [ABSTRACT FROM AUTHOR]

ABSTRACT Load-controlled three-point bending fatigue tests were conducted on API X80 pipeline steel to investigate the effects of stress ratio and specimen orientation on the fatigue crack growth behaviour. Because of the high strength and toughness of X80 steel, crack growth rate was measured and plotted versus Δ J with stress ratio. The fatigue crack length is longer in the transverse direction, whereas the fatigue crack growth rates are nearly the same in different orientations. Finally, a new fatigue crack growth model was proposed. The effective J-integral range was modified by Δ Jp in order to correlate crack closure effect due to large-scale yield of crack tip. The model was proved to fit well for fatigue crack growth rate of API X80 at various stress ratios of R > 0. [ABSTRACT FROM AUTHOR]

Magneto acoustic emission (MAE) is a magnetic nondestructive testing (NDT) method that has been used in different fields for 30 years. MAE is a promising method for the early characterization of damage and the evaluation of the residual stress of ferromagnetic materials. However, this technique is still in its early stage and requires further development. The mechanism and influence factors of MAE are still under investigation. Quantitative NDT is difficult because of the lack of robust theoretical bases and models. In this study, we investigated the influence factors of MAE signals systematically and established a mathematical model to describe these influences. The special design of the specimen and the precise control of experimental conditions are the key points for obtaining reasonable experimental results and for developing the model. A methodology for stress assessment was developed on the basis of the proposed model and was verified by using the pure bending test. Results show that stresses within a measureable depth of 4 mm can be evaluated and that the maximum testing error is 30 %. [ABSTRACT FROM AUTHOR]

The size effect on the tensile strength of concrete is investigated experimentally for the case of equi-biaxial tension. Tests of tensile strength under uniaxial tension were carried out for comparison using four-point bend beams. For measuring the biaxial tensile strength, the ASTM C1550 test and the biaxial flexure test were examined. To study the size effect, unreinforced circular plates of three different sizes are tested, with 13 specimens per size. The size effect on the equi-biaxial tensile strength is found to be stronger than it is on the uniaxial tensile strength, and to exhibit the characteristics of the deterministic Type I size effect. The detailed experimental procedure and the results are reported in this paper. Under the assumption that a distinct continuous crack develops only after the peak load, the approximate law of size effect is derived from the stress redistribution due to a boundary layer of cracking. The analysis leads to a deterministic Type I size effect. [ABSTRACT FROM AUTHOR]

The cracks in the material are in varied forms, such as bend, cross and branch. The numerical and experimental methods were applied to study the fracture problem of the branch crack under compression, considered the influence of different branch crack inclination and length to the stress intensity factors and the wing crack propagation. The results show that the increase of the crack inclination stress intensity factor K1 decreases, and the stress intensity factors increase with the crack length increases, the branch crack with a large length may inhibit the smaller one. The sandstone specimens containing open branch cracks or kink cracks subjected by uniaxial compressive loads were used to analysis the wing crack propagation, the experimental results find that the forms of the destruction of the sandstone specimen are mainly type I wing cracks, and its distance from the crack tip decreases with the increase of branch crack inclination; the bigger branch crack inclination, the smaller wing crack initial angle but the bigger wing crack initial load. The comparison was also made between open and closed cracks, wing crack initial angle of the open crack is greater than the closed one's, but its wing crack initial load is just the opposite. [ABSTRACT FROM AUTHOR]

When multiple cracks approach one another, the stress intensity factor and J-integral value change due to the interaction of the stress field. Since the changes in these parameters are not always conservative in structural reliability evaluations, the interaction between multiple cracks should be taken into account. Section XI of the ASME Boiler and Pressure Vessel Code provides a flaw characterization rule for interacting multiple cracks. In Section XI, adjacent cracks are replaced with a coalesced single crack when the distance between the cracks is less than half of the crack depth. However, the criterion for the offset distance is given as an absolute value, although the magnitude of the interaction depends on the crack size. In the current study, an alternative criterion for the offset distance was examined. Linear-elastic and elastic-plastic analyses were performed for interacting semicircular and semi-elliptical surface cracks by the finite element method under a tensile or bending load. The change in the stress intensity factors and J-integral values due to the relative spacing of cracks was investigated. Based on the relationship between the magnitude of the interaction and the relative position of the cracks, the allowable ctriterion for the offset distance was discussed. [ABSTRACT FROM AUTHOR]