Crack In Residual Stress Field Research Articles

Finite element analysis of fretting fatigue properties of GH4169 superalloy considering the surface treatments

In the present work, the fretting fatigue properties of GH4169 superalloy at 650℃ were investigated through numerical simulation considering the influences of the surface treatments. As a detrimental phenomenon, fretting fatigue involves fatigue and wear simultaneously, where the coefficient of friction and residual stress fields induced by the surface coating or modification techniques may affect the fretting fatigue lifetimes by changing the complex stress state between the two contacting surfaces. The numerical solutions were implemented to compare the crack initiation angles and paths under different surface conditions. The simulation results indicated that fretting fatigue cracks in residual stress fields were tendentiously parallel to the surface resulting in crack retardation and debris peeling, as verified by experimental observations. Additionally, three propagation models were compared to study the effect of compressive loading on the driving force of crack propagation rates, especially where compressive residual stresses was present. The combination of the linear superposition model with the Harter-T method was proven to be more accurate in the fretting fatigue life prediction, which may provide a modeling basis for service life prediction of engineering components in many practical applications.

1215 残留応力場中のき裂へのModified Ritchie-Knott-Rice破壊基準の適用(OS8-6 材料力学と計算力学の新展開)

1215 残留応力場中のき裂へのModified Ritchie-Knott-Rice破壊基準の適用(OS8-6 材料力学と計算力学の新展開)

Measurement of Local Initiation, Early Growth and Retardation of Physically Short Fatigue Cracks Using Amended DCPD Method

Different experimental methods are being used in laboratories for automatic measurement of fatigue crack growth rates and threshold values. Such data belong to mechanical properties essential for an assessment of residual life of components and structures containing cracks of length more than several millimetres. However, if the material contains small crack-like defects like inclusions or pores, knowledge about local resistance against fatigue growth of physically short cracks becomes very important. Damage mechanisms are even more complicated in case of a presence of subsurface residual stress field, e.g. due to technological effects. Experimental difficulties connected with investigation of short fatigue crack growth (FCG) can be reduced in case of use of automatic indirect methods. The aim of the work described in this paper was to explore possibilities and limits of the use of DCPD (direct current potential drop) method for physically short crack measurement and to use the optimised method for an evaluation of local damage process of initiation and early growth of short fatigue crack in residual stress field induced by shot peening.

Fourier basis for the engineering assessment of cracks in residual stress fields

A theoretical basis is presented for determining the significance of a residual stress distribution of arbitrary shape on the crack tip stress intensity factor for a centre-cracked plate as a function of crack length. The Fourier series based approach enables one to increasingly add more spatial definition to the stress field and thereby determine the level of detailed knowledge of the residual stress required to make a reliable assessment of structural integrity. The approach is applied to examples of measured symmetric distributions of residual stresses in welded plates and used to determine the significance of residual stress length-scales in fracture mechanics analysis.

Investigation of stress intensity factor for overloaded holes and cold expanded holes

An understanding of how residual stresses due to overloads effect stress intensity factors for fatigue cracks at common stress concentrators, such as holes, is important. In this paper a weight function approach is used to determine stress intensity factors for cracks in residual stress fields, due to three types of overloads for holes in metallic plates with and without subsequent remote loading. The cases resulting in compressive residual stresses are remote tension overload and hole cold expansion. Also considered is the case of a tensile residual stress field due to a compressive overload. Initially generic cases for a D6ac steel plate are considered. Stress intensity solutions are given for different crack sizes for different levels of overload which produce yield zones of different sizes. For both remote overload cases it is shown that once the crack length is the same or larger than the initial yield zone, the stress intensity factors are the same as for the case without the initial overload. However, for the cold expanded hole case the beneficial reduction in stress intensity factor extends some distance outside the initial yield zone. Then the application of cold expansion to an Aluminium alloy fatigue test coupon representing a lower wing skin location in the C-130 aircraft is considered. Here the significant reduction in stress intensity factor is investigated. Some potential inaccuracies in the weight function method for compressive fields, due to possible crack closure are also discussed.

Predictions of Propagation Behavior of Semi-Elliptical Slant Surface Cracks in Residual Stress Fields Based on Simulations of the Partial Elastic Contact of Crack Surfaces

In residual stress fields, the so-called partial elastic contact of crack surfaces sometimes occurs, where a fatigue crack is closed at the crack mouth while it is open at the crack tip. The partial elastic contact has a significant effect on the lives of fatigue cracks. However, there are few studies on the partial elastic contact of crack surfaces for three-dimensional cracks. In this paper, the propagation paths and lives of semi-elliptical slant surface fatigue cracks are predicted. The finite element method was employed for these simulations. The residual stress was introduced by applying an equivalent nodal force in correspondence with initial strains. The results of the propagation paths and lives considering the partial elastic contact of crack surfaces differed from those without consideration of the elastic contact. In particular the estimated results of the crack propagation rate sometimes brought one-order difference. These results demonstrate the necessity of simulation considering the partial elastic contact of crack surfaces.

On the Stress Intensity Factor of Cracks in Residual Stress Field

The use of weight function technique in fatigue crack growth subjected to external cyclic loading and residual stress field has been questioned by several researchers in that the technique is unable to account for the residual stress redistribution during the crack growth. In this paper a center cracked tension specimen containing residual stresses was analyzed by finite element method. The crack growth was simulated by releasing the nodes ahead of crack tip in stepwise and the stress intensity factors induced by residual stresses at different crack lengths were estimated. The results from the numerical analysis are identical to the weight function solution, which demonstrates that the weight function technique can be used for the fatigue crack growth analysis in residual stress field, unless the residual stress distribution is disturbed by the plastic yield.

Predictions of Propagation Behavior of Semi-Elliptical Slant Surface Cracks in Residual Stress Fields Based on Simulations of the Partial Elastic Contact of Crack Surfaces

In residual stress fields, the so-called partial elastic contact of crack surfaces sometimes occurs, where a fatigue crack is closed at the crack mouth while it is open at the crack tip. The partial elastic contact has a significant effect on the lives of fatigue cracks. However there are few studies on the partial elastic contact of crack surfaces for three-dimensional cracks. In this paper, the propagation paths and lives of semi-elliptical slant surface fatigue cracks are predicted. The finite element method was employed for these simulations. The residual stress was introduced by applying an equivalent nodal force in correspondence to an initial strain to each finite element. The results of the propagation paths and lives of simulations considering the partial elastic contact of crack surfaces differed from those without the consideration of the elastic contact. Especially the estimated results of crack propagation rate sometimes brought one-order difference. These results demonstrate the necessity of simulation considering the partial elastic contact of crack surfaces.

Predictions of Propagation Behavior of Slant Corner Fatigue Cracks in Residual Stress Fields Based on Simulations of the Partial Elastic Contact of Crack Surfaces

In residual stress fields, the so-called partial elastic contact of crack surfaces sometimes occurs, where a fatigue crack is closed at the crack mouth while it is open at crack tip. The partial elastic contact has significant effect on the lives of fatigue cracks. However there are few studies on the partial elastic contact of crack surfaces for three-dimensional cracks. In this paper, the propagation paths and lives of slant corner fatigue cracks are predicted by using three-dimensional fatigue crack propagation simulations considering the partial elastic contact of crack surfaces. The finite element method was employed for the simulations. The crack propagation rates obtained by simulations considering the partial elastic contact of crack surfaces differed from those without considering the contact, demonstrating the necessity of simulation considering the partial elastic contact of crack surfaces. However the propagation paths and crack shapes estimated considering the partial elastic contact hardly differed from those without the consideration.

New stress intensity factor solutions of a periodic array of cracks in residual stress field

Many important applications of crack mechanics involve self-equilibrating residual or thermal stress fields. For these types of problems, the traditional fracture mechanics approach based on the superposition principle has ignored the effect of crack surface contact when the crack-tip propagates into the residual compressive region. Contact between the crack faces and the wedging action are responsible for subsequent crack-tip reopening, which often leads to a much larger mode I stress intensity factor. In this study, an analytical approach is used to study the effect of crack face contact for a period array of collinear cracks embedded in several typical residual stress fields. It is found that the nonlinear contact between crack surfaces dominates the cracking behavior in residual/thermal stress fields, which is responsible for crack coalescence.

되풀이 소성영역 크기를 이용한 피로 균열 닫힘 거동의 유한요소해석

An elastic-plastic finite element analysis is performed to investigate detailed closure behaviour of<br/> fatigue cracks in residual stress fields and the numerical results are compared with experimental results. The finite element analysis performed under plane stress using contact elements can predict fatigue crack closure behaviour. The mesh of constant element size along crack surface can not predict the opening level of fatigue crack. Specially, the mesh of element sizes depending upon the reversed plastic zone size included the effect of crack opening point can precisely predict the opening level. By using the concept of the mesh of element sizes depending upon the reversed plastic zone size included the effect of crack opening point, the opening level of fatigue crack can be determined very well.<br/>

Contact Analysis for Collinear Multiple Cracks in Residual Stress Field

A method is proposed for analyzing stress intensity factors and crack profiles for collinear multiple cracks perpendicular to welded joints in an infinite plate. Using the basic solution of a single crack and taking unknown density of fictitious tractions, Fredholm integral equations and algebraic equations are formulated based upon traction-free conditions and crack face displacements, respectively. These equations are solved simultaneously, considering the contact effect of crack surfaces. Using the derived density of fictitious tractions, the stress intensity factors and displacements of multiple cracks are determined. Some numerical examples are analyzed.

The evaluation of stress intensity factors for plane cracks in residual stress fields

A reliable, efficient method is described for modelling plane cracks in arbitary residual stress fields, using the technique of distributed dislocations. This allows correctly for re-distribution of residual stresses as the crack grows. Problems where crack closure occur are discussed, and implications for solution by finite element procedures are inferred and confirmed by comparison.

Proposed integration of notch-strain and fatigue crack-growth analyses

A model for the analysis of the growth of fatigue cracks in residual stress fields is developed by integrating notch-strain analysis techniques with linear-elastic fracture-mechanics-based crack-growth analysis techniques. The method by which elastic-plastic response stress gradients are estimated is presented along with a discussion of the use of those gradients in a fatigue crack-growth analysis. The modeling of crack-growth acceleration and retardation effects and their use after the stress response calculations are discussed. And in conclusion, the results of a newly developed computer program, which incorporates the combined techniques, are compared with crack-growth data from an in-service U.S. Air Force aircraft.

An overview of the mechanics of fracture and fatigue in the presence of residual stress

Many engineering components contain sharp crack-like defects, or they may develop such defects early in their lifetime: welded components and those subjected to fretting or aggressive chemical or thermal environments are obvious examples. When components containing such defects are subjected to cyclic (fatigue) loading the cracks may extend during each loading cycle until they reach a critical length, causing failure of the component. The presence of residual stresses may dramatically alter the fatigue lifetime. In this paper the basic concepts of Linear Elastic Failure Mechanics, fatigue crack growth and residual stress are reviewed in order to demonstrate that the amalgamation of these concepts is logical and consistent. The importance of both geometry and loading in the determination of stress intensity factor ( K) is indicated, as is the validity of the super-position technique as applied to cracks in residual-stress fields. In the general area of fatigue-crack growth, it is noted that the correction of K to account for residual stresses is important for the determination of both stress intensity range and R (the ratio of the minimum to the maximum value of K during a loading cycle). Such information may be employed in predicting the form of standard S— N curves for welded details containing residual stress. The requirement for a test specimen in the investigation of fatigue-crack growth is noted, and suggestions made for a point loaded, ring-type standard test specimen.