Fatigue Crack Closure Levels Research Articles

Closure measurement and analysis for small cracks from natural discontinuities in an aluminium alloy

Some materials such as 7050-T7451 and β-annealed Ti–6Al–4V exhibit very rough fatigue crack surfaces which suggests that roughness induced crack closure might be a significant issue. The present work examined a range of data for 7050-T7451 material which included experimental results under constant amplitude and variable amplitude/spectrum loading. Crack closure was either measured directly, or the effects were inferred through results from previous fractographic investigations. Analyses using an updated and improved version of the FASTRAN crack growth code combined with the experimental data were used to identify a new approach to correlate closure levels and crack growth behaviour. The new approach includes direct compliance based crack opening measurements to correlate the baseline constant amplitude properties and then using a cycle-by-cycle strip yield model with realistic constraint factors to model crack growth under spectrum loading. The new approach is consistent with fundamental differences in fatigue crack closure levels under constant and variable amplitude/spectrum loading which have been identified at the individual cycle by cycle level down to very small crack sizes. The new approach is shown to work very well for a range of cases considered here, and it provides a very useful insight into this complex behaviour.

Fatigue Crack Growth Fundamentals in Shape Memory Alloys

In this study, based on a regression of the crack tip displacements, the stress intensity range in fatigue is quantitatively determined for the shape memory alloy Ni2FeGa. The results are compared to the calculated stress intensity ranges with a micro-mechanical analysis accounting for the transformation-induced tractions. The effective stress intensity ranges obtained with both methods are in close agreement. Also, the fatigue crack closure levels were measured as 30 % of the maximum load using virtual extensometers along the crack flanks. This result is also in close agreement with the regression and micro-mechanical modeling findings. The current work pointed to the importance of elastic moduli changes and the residual transformation strains playing a role in the fatigue crack growth behavior. Additional simulations are conducted for two other important shape memory alloys, NiTi and CuZnAl, where the reductions in stress intensity range were found to be lower than Ni2FeGa.

Improvement of fatigue crack growth resistance by controlled overaging in 2024‐T3 aluminium alloy

ABSTRACTA controlled overaging process is proposed to increase region II fatigue crack propagation resistance of 2024‐T3 aluminium alloy. Overaging was achieved by subjecting the material from the initial T3 state to heat treatment at specific aging temperatures resulting in substantial reduction in hardness. Fatigue crack growth tests were subsequently performed in the intermediate ΔΚ region to assess the influence of aging treatment on fatigue crack propagation rate. The experimental results showed that overaging at high temperatures enhances the fatigue crack growth resistance of the material with regard to initial T3 state. Fatigue crack growth rates were found to decrease with increasing overaging temperature. Cyclic stress strain tests were performed to assess the impact of the performed overaging on cyclic behaviour. The results revealed that cyclic strain hardening is enhanced in the overaged material, contributing to increased fatigue crack closure levels.

PRIZE FOR BEST PAPER VOLUME 27

Fatigue & Fracture of Engineering Materials & StructuresVolume 28, Issue 6 p. 575-575 Free Access PRIZE FOR BEST PAPER VOLUME 27 First published: 05 May 2005 https://doi.org/10.1111/j.1460-2695.2005.00911.x Announcement in the International Journal: Fatigue and Fracture of Engineering Materials and Structures AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract has been awarded to Fatigue crack propagation in cast duplex stainless steels: thermal aging and microstructural effects V. Calonne, A. F. Gourges and A. Pineau Fatigue Fract. Engng. Mater. Struct. , 27 (1), pp . 31-44 Abstract The ferrite phase of cast duplex stainless steels becomes embrittled after thermal ageing, leading to a significant decrease in fracture properties. In the present paper, the influence of ageing and solidification structure on the fatigue crack growth rates (FCGRs) and on the fatigue crack growth mechanisms in a cast duplex stainless steel is studied. FCGRs measured at room temperature increase slightly after ageing at 400 °C, due to ferrite cleavage and to the resulting irregular shape of the crack front. The crack propagates without any preferential path by successive ruptures of ferrite and austenite phases. The macroscopic crack propagation plane depends on the crystallographic orientation of the ferrite grain. Secondary cracks can appear due to the complex solidification structure. This in turn influences the FCGR. The fatigue crack closure level decreases with increasing ageing. This can be explained by a decrease in the kinematic cyclic hardening of these materials. Volume28, Issue6June 2005Pages 575-575 RelatedInformation

Fatigue crack propagation in cast duplex stainless steels: thermal ageing and microstructural effects

ABSTRACTThe ferrite phase of cast duplex stainless steels becomes embrittled after thermal ageing, leading to a significant decrease in fracture properties. In the present paper, the influence of ageing and solidification structure on the fatigue crack growth rates (FCGRs) and on the fatigue crack growth mechanisms in a cast duplex stainless steel is studied. FCGRs measured at room temperature increase slightly after ageing at 400 °C, due to ferrite cleavage and to the resulting irregular shape of the crack front. The crack propagates without any preferential path by successive ruptures of ferrite and austenite phases. The macroscopic crack propagation plane depends on the crystallographic orientation of the ferrite grain. Secondary cracks can appear due to the complex solidification structure. This in turn influences the FCGR. The fatigue crack closure level decreases with increasing ageing. This can be explained by a decrease in the kinematic cyclic hardening of these materials.

The influence of cross‐sectional thickness on fatigue crack growth

For thin structures, fatigue crack growth rates may vary with the structure's thickness for a given stress intensity factor range. This effect is mainly due to the change in the nature of the plastic deformation when the plastic zone size becomes comparable with, or greater than, the cross‐sectional thickness. Variations in the constraint affect both the crack tip plastic blunting behaviour as well as the fatigue crack closure level. Approximate expressions are constructed for the constraint factor based on asymptotic values and numerical results, which are shown to correlate well with finite element results. It is demonstrated that the present results not only permit predictions of the specimen thickness effects on fatigue crack propagation under spectrum loading, but also eliminate the need to determine the constraint factor by curve‐fitting crack growth data.

EXPERIMENTAL AND ANALYTICAL ESTIMATES OF FATIGUE CRACK CLOSURE IN AN ALUMINIUM‐COPPER ALLOY PART II: A FINITE ELEMENT ANALYSIS

Abstract— A numerical investigation of the fatigue crack closure phenomenon has been performed by an elastic‐plastic finite element analysis. Computer software was developed to consider many aspects affecting plasticity‐induced crack closure. Linear and power‐law hardening models are considered in the finite element analysis. The paper presents results from the study carried out on compact tension (CT) coupons at various crack lengths corresponding to different loading conditions. Finally the results of the analysis are compared with the experimental estimates of fatigue crack closure levels obtained from laser interferometry, scanning‐electron and transmission‐electron fractography presented in Part I of this paper on identical specimens.

Change of fatigue crack closure level with gauge location along crack line

Change of fatigue crack closure level with gauge location along crack line

Crack closure at positive stresses during random load fatigue crack growth

Fatigue crack closure at positive stresses during broad band, random load fatigue crack growth, has been investigated. Tests were performed on centre-notched specimens made from an aluminium alloy (B.S. 2L 71). The closure stress was found to be dependent upon the material thickness, the mode of fracture and the stress ratio Q. Under plane strain fracture mode conditions it was found that the closure stress for 22 mm thick specimens was greater than that found with 6 mm thick specimens. This difference in closure behaviour produced a slower fatigue crack growth rate for the thicker specimens. In addition it was found that for a given r.m.s. stress the fatigue crack closure level only impinged upon the dynamic range at low mean stress levels.