Minimization of stress concentration factors in fatigue crack repairs

A numerical study is reported of a repair by flaw removal on a conventional welded joint. The repair profile is optimized with respect to the flaw and joint dimensions in order to minimize the resulting stress concentration factor (SCF). Two dimensional (edge repair) and three dimensional (surface repair) finite element analyses were made for the determination of SCF values and a graphical representation of results is presented. A relation between edge repair and surface repair is obtained and short and long repairs are defined. The weld geometry and repair orientation effects on SCF values are discussed. Finally, implications on using short and long repairs on fatigue initiation and inspection are presented.     

Correlation of fatigue crack propagation in polyethylene pipe specimens of different geometries

Correlation in mechanisms and kinetics of step-wise fatigue crack propagation in polyethylene pipe specimens of different geometries is studied experimentally. It is shown that crack propagation in a non-standard specimen cut from a real pipe and conserving the pipe geometry can be effectively simulated using a standard compact tension specimen. Good correlation in both kinetics of step-wise crack propagation and fractography between the specimens is achieved if experimental conditions are chosen to assure equal values of (a) stress intensity factor and (b) stress intensity factor gradient at the initial notch tips. These results extend previous technique of fatigue accelerating slow crack growth used to predict lifetime of polyethylene pipes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Modelling fatigue crack propagation in CT specimens

Although there are a great number of numerical studies focused on the numerical simulation of crack shape evolution, a deeper understanding is required concerning the numerical parameters and the mathematical modelling. Therefore, the objectives of the paper are the study of the influence of numerical parameters, particularly the radial size of crack front elements and the magnitude of individual crack extensions, the mathematical modelling of crack propagation regimes, and the linking of crack shape changes with K distribution. A relatively simple through-crack geometry, the CT specimen, was studied and the numerical model was validated with experimental results with a good agreement. The K distribution along crack front was found to be the driving force for shape variations. Shape variations were found to be one order of magnitude lower than K variations.     

Intergranular crack paths during fatigue in interstitial-free steels

Interstitial-free (IF) steels are known to exhibit intergranular (IG) faceting during fatigue under certain conditions. The presence of IG facets is often ascribed to either environmental effects or to grain boundary embrittlement. In many cases this attribution to environment or embrittlement is erroneous with the IG faceting actually arising from the intrinsic slip characteristics of body-centred cubic (bcc) alloys. This paper summarises the background to slip-induced intergranular fatigue and explores two issues of importance to users of IF steels; alloy conditions in which IG fatigue occurs and whether the fatigue performance in the presence of an IG crack path is lower than similar IF steels which exhibit a transgranular crack path. To explore this latter issue fatigue performance is presented as a function of yield strength.     

Simulation of simplified zigzag crack paths emerging during fatigue crack growth

When subjected to fatigue loading, microstructurally short cracks form zigzag shapes in single shear mechanisms due to nucleation, glide and annihilation of dislocations. Such crack progression is geometrically complicated and computationally expensive to model. This paper investigates the possibilities of efficiently modelling such growth behaviour by geometrical simplifications of the zigzag path. The crack path and the emerging plasticity was modelled by two different dislocation formulations and it was found that, irrespective of dislocation modelling technique, a valid geometrical approximation was to disregard the surface roughness except for the zigzag section closest to the crack tip.     

Cyclic fatigue crack growth tests on disc specimens

A ring has been designed and built in which four disc specimens containing bore cracks were subjected to fatigue conditions by continuous cycling between variable upper and lower speed limits. The behaviour of the individual cracks was found to be inconsistent. While in the majority of cases the crack growth rate was always less than that predicted by the expression da/dN = C(ΔK)ⁿ, where C and n constants which were found from crack growth tests on standard fracture toughness specimens, an important exception was found in which a pair of cracks grew slowly over a large number of cycles before growing at a very much faster rate than that predicted. Some cracks grew much more slowly than predicted and some did not grow at all, even though minimum values of ΔK were always greater than threshold values. The circumstances which govern the different types of crack growth rate behaviour are examined using finite element analysis. It appears that, for a given specimen geometry and rotational speed, the controlling influences are the number and relative lengths of the bore cracks.     

Evaluating fatigue crack propagation properties using miniature specimens

Standard fatigue crack propagation (FCP) test placed strict requirement on specimen size. FCP rate in subsize specimen was found to be slightly but consistently slower than that in the standard specimens. Based on a critical study on two aluminum alloys and two steels, we found the lower rates can be attributed to the plane stress state in the miniature specimens being different from that in the standard specimens. By taking account of crack closure, the miniature specimen data are brought in line with the standard specimen results and may serve as an upper bound estimate of FCP properties.     

Integrable K-solutions for common fatigue crack growth specimens

Alternative stress intensity factor solutions are developed for several common fatigue crack propagation specimens. Unlike the K-solutions presently employed, the K-solutions developed herein allow direct integration of the Paris fatigue crack growth rate power law while retaining the same degree of accuracy.     

Plastic deformation effects and the rate of fatigue crack growth in stress concentration zones

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 3, pp. 71–74, May–June, 1992.     

Technical note High-cycle fatigue crack initiation and propagation behaviour of high-strength sprin steel wires

An attempt has been made to characterize high-cycle fatigue behaviour of high-strength spring steel wire by means of an ultrasonic fatigue test and analytical techniques. Two kinds of induction-tempered ultra-high-strength spring steel wire of 6.5 mm in diameter with a tensile strength of 1800 MPa were used in this investigation.
The fatigue strength of the steel wires between 10⁶ and 10⁹ cycles was determined at a load ratio R = −1. The experimental results show that fatigue rupture can occur beyond 10⁷ cycles. For Cr–V spring wire, the stress–life ( S – N ) curve becomes horizontal at a maximum stress of 800 MPa after 10⁶ cycles, but the S – N curve of the Cr–Si steel continues to drop at a high number of cycles (>10⁶ cycles) and does not exhibit a fatigue limit, which is more correctly described by a fatigue strength at a given number of cycles. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviour have been examined. Experimental and analytical techniques were developed to better understand and predict high-cycle fatigue life in terms of crack initiation and propagation. The results show that the portion of fatigue life attributed to crack initiation is more than 90% in the high-cycle regime for the steels studied in this investigation.     

Residual stress effects in fatigue crack growth

The paper describes the influence of residual stresses on the propagation of fatigue cracks in low-alloy steels used in the construction of North Sea Oil platforms. The results show faster crack growth in thicker plates and the development of secondary fatigue cracks in the residual stress field. Characteristics of fatigue crack growth are discussed.     

Probabilistic analysis of fatigue crack propagation in finite size specimens

By taking into account the effect due to the finiteness of the specimen's size, stochastic fatigue crack propagation processes are investigated on the basis of a Markov approximation method. Since the limiting property proved by Koiter causes a particular kind of singularity on the governing equation, the so-called death point must be introduced into the range of finite crack size. In the present study, the Fokker-Planck equation in a generalized sense is derived for such a particular case, and the influence of the specimen's size on the statistical properties of the residual life of the component is clarified by the use of numerical calculations.     

A crack opening stress equation for fatigue crack growth

International Journal of Fracture -     

Variation of fractographic appearance for different microstructures in welded joints having the same fatigue crack propagation properties

Fatigue fracture surfaces were examined with a scanning electron microscope to investigate the influence of the different microstructure between weld metal and heat affected zone. The specimens were centre-cracked type transverse butt welded joints. The relationship between macroscopic fatigue crack propagation rate and the stress intensity factor range is the same in spite of the difference in microstructure for both materials. It is shown that the fractographic appearance changes with microstructure even in the very low growth rate region near fatigue threshold. This suggests that fractographic appearance is not necessarily a guide to the rate of fatigue crack growth.     

Short fatigue crack growth from a blunt notch in plate specimens

Short crack growth behavior from a notch including crack closure and load ratio effects was investigated. Experiments and analyses were carried out using four-point bending specimens made of SAE 1045 steel, using a blunt notch keyhole specimen geometry. The lower the load ratio, the more notch effect on short crack growth behavior was observed. Short cracks in the notch affected zone had higher growth rates than long cracks. After the crack grew out of the notch effect field, short crack growth rates merged with the long crack growth rates. Several parameters were used to correlate the short crack growth rates including stress intensity factor range, effective stress intensity factor range, and stress intensity factor range based on notch root stress.     

Prediction of 3D small fatigue crack propagation in shot-peened specimens

Shot peening has been widely applied in industrial design to improve fatigue durability of high loaded machine components. The compressive residual stress induced by shot peening is in general assumed to be responsible for the improvement of material fatigue strength. In the present work a cyclic cohesive zone model is extended to analyze three-dimensional fatigue crack growth in shot-peened specimens. Fatigue crack growth behaviors in both unpeened and peened specimens are investigated using 3D finite element analysis. The parameters of the cohesive zone model have been identified in 2D unpeened specimens and are applied to predict peened specimens directly. The results indicate that shot peening strongly affects crack initiation time and crack profiles, but has little effect on crack propagation rate. It implies that the shot peening will hardly change Paris’ law used for the damage tolerant design.