The relationship between observed fatigue damage and life estimation models

Observations of the surface of laboratory specimens subjected to axial and torsional fatigue loadings has resulted in the identification of three damage fatigue phenomena: crack nucleation, shear crack growth, and tensile crack growth. Material, microstructure, state of stress/strain, and loading amplitude all influence which of the three types of fatigue damage occurs during a dominant fatigue life fraction. Fatigue damage maps are employed to summarize the experimental observations. Appropriate bulk stress/strain damage parameters are suggested to model fatigue damage for the dominant fatigue life fraction. Extension of the damage map concept to more complex loadings is presented

A CRITICAL PLANE APPROACH TO MULTIAXIAL FATIGUE DAMAGE INCLUDING OUT‐OF‐PHASE LOADING

Abstract— A modification to Brown and Miller\u27s critical plane approach is proposed to predict multiaxial fatigue life under both in‐phase and out‐of‐phase loading conditions. The components of this modified parameter consist of the maximum shear strain amplitude and the maximum normal stress on the maximum shear strain amplitude plane. Additional cyclic hardening developed during out‐of‐phase loading is included in the normal stress term. Also, the mathematical formulation of this new parameter is such that variable amplitude loading can be accommodated. Experimental results from tubular specimens made of 1045 HR steel under in‐phase and 90° out‐of‐phase axial‐torsional straining using both sinusoidal and trapezoidal wave forms were correlated within a factor of about two employing this approach. Available Inconel 718 axial‐torsional data including mean strain histories were also satisfactorily correlated using the aforementioned parameter. Copyright © 1988, Wiley Blackwell. All rights reserve

Multiaxial cyclic deformation and non-proportional hardening employing discriminating load paths

Some novel discriminating multiaxial cyclic strain paths with incremental and random sequences were used to investigate cyclic deformation behavior of materials with low and high sensitivity to non-proportional loadings. Tubular specimens made of 1050 QT steel with no non-proportional hardening and 304L stainless steel with significant non-proportional hardening were used. 1050 QT steel was found to exhibit very similar behavior under various multiaxial loading paths, whereas significant effects of loading sequence were observed for 304L stainless steel. In-phase cycles with a random sequence of axial-torsion cycles on an equivalent strain circle were found to cause cyclic hardening levels similar to 90° out-of-phase loading of 304L stainless steel. In contrast, straining with a small increment of axial-torsion on an equivalent strain circle results in higher stress than for in-phase loading of 304L stainless steel, but the level of hardening is lower than for 90° out-of-phase loading. Tanaka\u27s non-proportionality parameter coupled with a Armstrong-Fredrick incremental plasticity model, and Kanazawa et al.\u27s empirical formulation as a representative of such empirical models were used to predict the stabilized stress response of the two materials under variable amplitude axial-torsion strain paths. Consistent results between experimental observations and predictions were obtained by employing the Tanaka\u27s non-proportionality parameter. In contrast, the empirical model resulted in significant over-prediction of stresses for 304L stainless steel. © 2010 Elsevier Ltd. All rights reserved

Multiaxial fatigue evaluation using discriminating strain paths

Fatigue life and available cycle counting methodologies based on the critical plane approach are examined under discriminating axial-torsion strain paths with random and incremental changes in straining direction. Fatigue lives for quenched and tempered 1050 steel with no non-proportional hardening were found to be more sensitive to non-proportionality of loadings as compared to 304L stainless steel with significant non-proportional hardening. Proportional or in-phase axial-torsion cycles with different axial to shear strain ratios within an equivalent strain circle when applied in a random sequence resulted in significant additional hardening for 304L stainless steel, similar to the non-proportional cyclic hardening observed in 90° out-of-phase loading. In contrast, when such cycles are applied with a gradual increment of the axial to shear strain ratio, the stress response of 304L stainless steel is closer to that observed for in-phase loading. However, the sequence of loading did not significantly affect fatigue life for either material. Experimentally observed failure planes for all strain paths were in very good agreements with predicted failure planes based on the Fatemi-Socie critical plane parameter. Finally, fatigue lives for both materials under various strain paths were predicted satisfactorily employing Fatemi-Socie parameter, Palmgren-Miner linear damage rule, and either Bannantine-Socie or Wang-Brown cycle counting method. © 2010 Elsevier Ltd. All rights reserved

Static and cyclic fatigue failure at high temperature in ceramics containing grain boundary viscous phase part ii modelling

In Ceramic marterials, the crack surface behind the crack tip is extensively bridged by adhesive viscous ligaments at high homologous temperatures. The extent of crack tip shielding from the externally applied stress by these viscous ligaments depends on material parameters, loading condition and loading types. Here a mechanistic model has been developed based on grain size and the width and viscosity of the grain boundary viscous layer to predict the ratio of lifetime under static loading to cyclic loading with various types of loading waveforms. A single dimensionless scalar parameter, 'I', has been identified as a critical parameter to determine whether the cyclic loading will be equally damaging or less damaging than the corresponding static loading at high homologous temperature. Model predictions agree with experimental results of various authors.Energy Department 93/1

Fatigue crack growth behaviour of tubular aluminium specimens with a circular hole under axial and torsion loadings

Experimental results were obtained from fatigue tests of thin-walled 7075-T6 and 2024-T3 aluminium tubular specimens with a circular hole. The loading conditions included axial, torsion, combined in-phase and out-of-phase axial-torsion, and axial with intermittent torsion cycles. Crack nucleation was observed on planes of maximum shear stress for both notched and smooth (un-notched) specimens. Macroscopic crack growth in notched specimens occurred along planes experiencing the maximum range of nominal principal stress (i.e. mode I crack growth) whereas smooth specimen cracks grew on maximum shear planes. This is explained in terms of different damage evolution mechanisms in smooth and notched specimens. Cracks grew faster in torsion compared to axial loading, as well as in in-phase loading compared to out-of-phase loading. This is attributed to the T-stress in torsion tests and crack branching with a torturous crack path in out-of-phase tests. Intermittent torsion cycles decreased fatigue life and accelerated axial crack growth rate. These effects and the differences in crack growth behaviour between different loadings are discussed. © 2014 Elsevier Ltd

Notched fatigue behavior under multiaxial stress states

Most engineering components and structures contain stress concentrations, such as notches. The state of stress at such concentrations is typically multiaxial due to the notch geometry, and/or multiaxiality of the loading. Significant portions of the fatigue life of notched members are usually spent in crack initiation (crack formation and microscopic growth) and macroscopic crack growth. Synergistic complexity of combined stress and stress concentration has been evaluated in a limited number of studies. Available experimental evidence suggests the current life estimation and fatigue damage analysis techniques commonly used may not be capable of accurate predictions for such complex and yet highly practical conditions. This paper investigates notched fatigue behavior under multiaxial loads using aluminum alloys. Many effects involved in such loading conditions are included. These include the effects of stress state (axial, torsion, combined axial-torsion), geometry condition (smooth versus notched), and damage evolution stage (nucleation and micro-crack growth versus long crack growth). © (2014) Trans Tech Publications, Switzerland