A method for corrosion-fatigue life prediction

It is well recognized that environment has significant effects on the failure of cyclically loaded members/structures Existing experimental data indicates that fatigue life is much shorter in corrosive environment that in more inert environment such as a dry air or vacuum. This paper presents a method and strategy to predict/estimate life under corrosion-fatigue. A corrosion fatigue factor kcorr is defined as the ratio of the fully-reversed stress amplitude in air, (a)air, over that in corrosive environment, (a)corr, for a given fatigue life in terms of a number of cycles to failure, Nf, i.e. kcorr = (a)air/(a)corr at the same Nf. The corrosion fatigue factor resembles the widely used fatigue notch factor kf. The proposed strategy requires the S-N curve in air and the corresponding kcorr factor. Experimental data for three materials, namely 7075-T651, 6161-T561 and 4140 steel tested in laboratory air and 3.5% of NaCl solution were used to illustrate and validate the proposed method. A fairly good agreement is demonstrated in terms of the correlation among air and corrosion-fatigue data

Understanding small crack effects on failure & threshold diagrams

It is well recognized that crack length has significant effects on both the monotonic failure stress and cyclic threshold stress conditions. Existing experimental data indicates that LEFM predicts correctly the failure and threshold stresses only for relatively long cracks. For small/short cracks, the LEFM overestimates both the failure and threshold stresses. Usually, an argument has been made that LEFM is violated in terms of a crack length with respect to the crack-tip plastic zone that results in overestimating the failure stress for small cracks. On the other hand, the Kitagawa diagram has been used to experimentally illustrate the connection between the observed threshold stresses for very small cracks and predictions from LEFM analysis. To understand the failure stress diagram for small/short cracks, we introduce a plasticity correction in calculation of the effective critical stress intensity factor (KC eff). Earlier analysis showed that Irwin’s correction of KC eff underestimates whereas the conventional Dugdale - Barenblatt yield-strip model overestimates the effects of the crack-tip plasticity on the failure stress for relatively small/short cracks. The rationale of a plasticity correction utilized in the failure stress diagram is extended to understand the Kitagawa diagram in terms of the observed threshold stress versus a crack size. This plasticity correction also explains the associated physics behind experimental observations in contrast to El Haddad’s phenomenological curve fitting approach. Both type of diagrams demonstrate an analogy which governs interrelation between stress, crack/defect size and fracture mechanics analysis. Using the existing data from the literature, the proposed analysis of small cracks effects is discussed with relevance to monotonic and cyclic loading conditions, under inert and corrosion environments

Unusual behavior of long cracks at low dk: Marci effect

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Use of an inverse life plot for fatigue endurance/limit estimation

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A cloud-based fatigue analysis tool with interactive Neuber’s master curve

A cloud-based fatigue analysis and life prediction tool is presented. Users around the globe may access it via Internet by means of multiple platforms such as desktop and laptop computers, tablets and/or smart-phones. It is an intuitive educational software, aimed also to assist designers in the pre-prototyping stage in fatigue life estimation for smooth or notched parts subjected to constant amplitude, block loading, and spectrum loading histories. For spectrum amplitude loading a dedicated spectrum software package is provided, which is essential for a potential clean-up and/or desired modifications of a raw spectrum data. Subsequently, a rainflow method is utilized and the corresponding hysteresis loops at the notch-root or critical “hot spot” location are determined and plotted. For notched components, an interactive Neuber’s master curve is utilized and discussed. It is shown, that the Neubers’s master curve is only material dependent and is applicable for both monotonic and cyclic loading situations

Unusual Fatigue Crack Growth Behavior of Long Cracks at Low Stress Intensity Factor Ranges

In this article, we characterize and review the unusual lack of threshold in fatigue crack growth (FCG) behavior for some alloys at low values of stress intensity factor ranges ΔK and its implications to damage-tolerant design approaches. This unusual behavior was first observed by Marci in 1996 in IMI 834 alloy. Conventional applications of linear elastic fracture mechanics to FCG analysis at constant R-ratio (or Kmax) assumes that (da/dN) decreases monotonically with decreasing ΔK and approaches the threshold value of ΔKth with (da/dN) ≤ 10−7 mm/cycle for a given R (or Kmax). However, instead of ΔK threshold behavior, some materials exhibit plateau or acceleration in da/dN rate with decreasing ΔK for long cracks tested in both constant R and Kmax conditions. This unusual (da/dN)-ΔK behavior is only observed experimentally but not understood and represents a challenge to scientists and engineers to model the safe fatigue life prediction of structures under low amplitude vibrating loads

Non-linear models for assessing the fatigue crack behaviour under cyclic biaxial loading in a cruciform specimen

This work addresses FEM-based numerical simulations of fatigue crack propagation in a cruciform specimen under biaxial loading and small scale yielding (SSY) conditions. Three non-linear models are used as fatigue crack propagation laws. The cruciform specimens are made of aluminium alloy D16T and modelled with non-linear material properties. In the experimental tests, starting from a surface flow mechanically created in the centre of the cruciform specimen, biaxial loading conditions are applied to create various pre-cracked configurations that correspond to different values of initial crack aspect ratios. In the numerical simulations, the material behaviour is modelled with a low hardening bilinear law. Each simulation is carried out with one of the established non-linear laws. At the end of the fatigue crack propagation simulations, several crack aspect ratios are obtained. Finally, these aspect ratios are compared with the corresponding experimental tests available in the literature

An Interactive Web-based Tool for Fatigue Analysis and Life Prediction

AbstractThis paper presents an interactive web-based tool for fatigue analysis and life prediction methodology of smooth and notched components. Users around the globe may access it via Internet by means of multiple platforms such as desktop and laptop computers, tablets and/or smart-phones. In particular, the users with a limited fatigue background would benefit from “on-the-fly” fatigue learning experience. It makes a self-explanatory and frustration free web-based education software, which allows users to learn the fatigue fundamentals while expanding their knowledge on modern fatigue analysis methods. The tool is aimed to assist a designer in fatigue analysis and file predictions of parts subjected to constant amplitude, block loading, and variable loading histories. For variable amplitude loading a dedicated spectrum software package is provided, which is essential for a potential clean-up and/or desired modifications of a raw spectrum data. Subsequently, a rainflow method is utilized and the corresponding hysteresis loops at the notch-root or critical location are determined and plotted. In analyses, three convenient and innovative “master curves” techniques are used, namely for Neuber's rule and also for stress- and strain-based fatigue life approaches