Semi-elliptical Fatigue Cracks Research Articles

Mode I stress intensity factors for semi-elliptical fatigue cracks in curved round bars

Some cyclically loaded components such as mooring chains can develop fatigue cracks in locations where the shape of the part is equivalent to that of a curved or bent round bar. Here we consider a semi-elliptical crack growing from the surface of a curved round bar. This geometry can for example represent a chain link segment with a crack located at its inner- or outer radius. The surface crack can be either almond shaped, sickle shaped or straight-fronted. Stress intensity factors (SIFs) over the fronts of such crack geometries are in the present work investigated for several elementary mode I stress distributions. Finite element analysis and linear elastic fracture mechanics methods are used to develop semi-analytical solutions for the SIF at any point on the crack front. Effects of relative bar curvature on numerical results are demonstrated. Relative to otherwise identical cracks in straight bars, SIFs for cracks in the curved bars considered here are found to differ by up to 8%. With an offshore mooring chain model as a case example, the estimation of SIFs for cracks in a complex residual stress field is furthermore demonstrated using a cubic polynomial stress approximation.

Estimation of the drill pipes residual resource under the multiaxial stress state

The object of research is the operation of drill pipes in a complex stress state. One of the most problematic places in this case is the process of fatigue, that is, the gradual accumulation of damage to the material of drill pipe under the influence of time-varying stresses. This leads to accidents, the elimination of which is accompanied by significant material and temporary losses.Based on the conducted critical analysis of scientific developments on this issue, it is noted that the use of kinetic diagrams of fatigue failure allows us to experimentally establish empirical parameters of fatigue of the material of drill pipe by laboratory research of small samples. And also to take into account the influence of the front shape of semi-elliptic and annular fatigue cracks on the residual resource. In addition, it is concluded that methods requiring additional attention, taking into account the combined effect on the kinetics of fatigue damage of normal and shear stresses.Therefore, in the course of the research, the positions of fracture mechanics were used. In particular, a feature of the proposed approach is the use in the formula for determining the stress intensity factor before the front of a semi-elliptical fatigue crack in the cross section of a tubular structure of equivalent stress, taking into account both the normal and shear component. This same stress, determined in accordance with the fourth theory of strength, is also used to calculate the critical crack depth.In order to assess the obtained results, a comparative calculation is made of the number of cycles of load change required to increase the depth of the specified crack from the initial to the critical value, not taking into account the value of the shear stress. It is established that the latter, with other unchanged conditions, can reduce the life of the drill pipe by up to two times.Thanks to the proposed method, it is possible to construct graphical dependencies of the fatigue life of drill pipes, taking into account the magnitude of the torque, other things being the same.

Numerical modeling of the semi-elliptical fatigue crack growth using damage accumulation approach

Numerical modeling of the semi-elliptical fatigue crack growth using damage accumulation approach

OS1528 エンジニアリングプラスチックPCの半楕円表面疲労き裂の進展挙動とそのシミュレーション

OS1528 エンジニアリングプラスチックPCの半楕円表面疲労き裂の進展挙動とそのシミュレーション

Application of the “GUCHI” Technique in Radiographic Testing for Flaw Sizing of Pipe

“GUCHI” (Geometric Unravel for Crack Height Image) technique is one of the stereo-radiography methods for flaw sizing in radiographic testing. The AN (Application of Nondestructive Testing techniques for industrial fields) committee in the JWES (The Japan Welding Engineering Society) carried out the experiment on this technique. The results were reported at FENDT 2002 and APCNDT 2003 and 2006 with data on the steel plates and pipes including fatigue cracks and EDM slits. The flaw sizing by RT-GUCHI was performed in a round robin test on 6 inches stainless steel pipe which had circumferential semi-elliptical fatigue cracks with 0.2t, 0.4t and 0.6t height at the OD surface and on plate including a fatigue crack. A trial for flaw sizing of SCC was also performed.This paper describes the fundamental of GUCHI technique, the discussion results of the accuracy for flaw sizing which was investigated before 2006 and the results of the round robin tests on test specimens with fatigue cracks.

Shear Mode Growth and Threshold of Small Fatigue Cracks in SUJ2 Bearing Steel

The shear mode propagation and threshold behaviors of small, semi-elliptical fatigue cracks were investigated in the JIS-SUJ2 bearing steel. Fully-reversed torsional loading was combined with a static axial compressive load to obtain crack growth in the longitudinal direction of cylindrical specimens. Non-propagating cracks smaller than ≅1mm in size were obtained (i) by decreasing the stress amplitude in tests using notched specimens, and (ii) by using smooth specimens in constant stress amplitude tests. The Mode II and Mode III threshold stress intensity factor ranges, ΔKIIth and ΔKIIIth, were estimated from the shapes and dimensions of non-propagating cracks. The threshold values decreased with a decrease in crack size, and there was no significant difference between ΔKIIth and ΔKIIIth. Wear on the crack faces was inferred by a powder and also by changes in microstructure in the wake of a crack. The ΔKIIth in the absence of interference of crack faces was estimated to be approximately 13MPa·m1/2 for a 1mm long crack.

Surface crack and cracks networks in biaxial fatigue

Semi-elliptical fatigue crack growth in 304 L stainless steel, under biaxial loading, was investigated. Compared to those of through-cracks under uniaxial loading, the growth rate of surface cracks is increased by a non-singular compressive stress and reduced by a tensile stress, when R = 0. Plasticity-induced crack closure under biaxial loading was investigated through 3D finite element simulations with node release. Roughness and phase-transformation-induced closure effects were also discussed. The interactions in two-directional crack networks under biaxial tension were investigated numerically. It appears that the presence of orthogonal cracks should not be ignored. The beneficial influence of interaction-induced mode-mixities was highlighted.

425 SUS304 鋼中非貫通疲労き裂近傍における塑性誘起マルテンサイト変態 : 応力比の影響

The present study investigates plasticity-induced martensitic transformation around a part-through crack in an austenitic stainless steel SUS304 fatigued at room temperature in air. Distributions of α' phase volume fraction ξ_<α'> around a semi-elliptical fatigue crack were measured with ferrite scope. The results were compared with the distributions of vertical magnetic flux density B_z above the crack in the specimen magnetized by a strong electromagnet. It was revealed that the B_z distributions reflected the α' phase distributions around the crack : i.e., the distance between the points where B_z reached the maximum and the minimum values B_<zmax> and B_<zmin> had linear correlations with surface crack length 2α. The B_<zmax> and the B_<zmin> values also showed linear relations with maximum stress intensity factors at the surface tip K_<amax> and at the depth position K_<bmax>. Each relation, i.e., 2α vs. 2l, B_<zmax> vs. K_<amax>, etc. can be represented by a single straight line regardless of R-ratio.

Reliability of Welded Structures Containing Fatigue Cracks

Abstract This study investigates the reliability of a cracked fillet welded T-joint typically found in offshore structures. A formulation for the aspect ratio (a/c) of a propagating semi-elliptical fatigue crack located at the toe of the weld is developed using Newman and Raju’s stress intensity factor for a cracked flat plate in conjunction with a weld magnification factor. The reliability in terms of fatigue lifetime is then calculated using the aspect ratio and Paris’s law of crack propagation with both fracture toughness and elastic-plastic failure criteria. The variation in crack aspect ratio in the T-joint is compared to that in a cracked flat plate, and examples are provided of reliability calculations for tension and bending loads.

An investigation of crack closure and the propagation of semi-elliptical fatigue cracks in Q1N (HY80) pressure vessel steel

The results of an experimental investigation of the effect of crack closure on the propagation of semi-elliptical fatigue cracks are presented. Load-shedding fatigue threshold tests were carried out at stress ratios of 0.2, 0.35, 0.5 and 0.7. Crack closure was measured at the surface and depth positions using backface strain gauges, near-tip gauges, and a clip gauge. Differences between the surface and depth growth behaviour are explained by considerations of the effects of the transition from plane stress conditions at the surface to plane strain conditions at the depth. The effects of stress ratios are attributed largely to differences in the crack opening displacement, which result in asperities coming into contact to induce roughness-induced crack closure.

Analysis of acoustic emission signals from friction of edges of a semielliptical fatigue crack

Analysis of acoustic emission signals from friction of edges of a semielliptical fatigue crack

The propagation of non-coplanar semielliptical fatigue cracks: Soboyejo, W.O. and Knott, J.F. Fatigue Fract. Eng. Mater. Struct. 1991 14 (1), 37–49

Reverse torsional fatigue tests conducted on aluminum alloy 2024-T351 samples exposed to argon, hydrogen and humid air environments showed that hydrogen absorption promotes multiple crack initiation and alters the fracture mode. The change in fracture mode was attributed to hydrogen-induced local overaging and interfacial weakening near the crack tip. The humid air environments produced hydrogen at high fugacities because of the reaction between water vapor and aluminum and caused greater effects on the fatigue life than did exposure to hydrogen gas at 1 atm. The local softening which resulted from overaging caused large decreases in the fatigue life.

A numerical investigation of the interaction and coalescence of twin coplanar semi-elliptical fatigue cracks

The interaction and coalescence of twin coplanar semi-elliptical cracks under both tension and bending loadings are investigated by numerical methods. Crack advance is computed on a step-by-step basis from the Paris equation using stress intensity factors calculated by the finite element method. Simplified computations are also performed in which coalescing cracks are recharacterized as single cracks. The fatigue crack growth curves predicted by the numerical methods are compared with experimental results.

Eddy-current signal analysis and inversion for semielliptical surface cracks

A scalar potential formulation of the δZ formula for the change in impedance of an eddy-current probe caused by a surface-breaking flaw is developed. The resulting formula is evaluated using a finite-difference method, which permits calculation of δZ for semielliptical flaws. The numerical results are checked by comparing calculations for rectangular-shaped flaws to previous calculations using an analytical solution for this geometry. Theoretical results are then verified by comparison with measurements on semielliptical fatigue cracks and EDM notches in aluminum alloy specimens using air-core eddy-current probes. An inversion method that compares features of the flaw profile, obtained by scanning the eddy-current probe along the length of the flaw, to a theoretical inversion chart (McFetridge chart) is demonstrated using the experimental data.

Growth characteristics for fatigue cracks embedded in fastener hole walls

A unique method for producing either single or double fatigue cracks embedded in fastener hole walls is described. The method repeatedly produces small semielliptical fatigue cracks and is valuable for basic experimental evaluations dealing with crack growth from fastener holes. Test specimens were prepared from 7075-T651 aluminum alloy plate for collecting constant amplitude embedded crack growth data for open and filled hoies without load transfer. Fractographic data are presented to show crack front shape and growth rates. Crack shape data correlated well with similar data derived from naturally induced embedded cracks. Analytical predictions were made for the same cases evaluated experimentally. Good correlations between the analytical and experimental results were obtained.