CRACK GROWTH PREDICTIONS UNDER VARIABLE AMPLITUDE LOADING BASED ON LOW CYCLE FATIGUE DATA (original) (raw)
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Analytical models based on damage accumulation by cyclic plasticity have been developed to predict the fatigue crack growth da/dN vs. ∆ ∆ ∆ ∆K curve using ε ε ε εN parameters. The strain singularity of the idealized crack is avoided by modeling the crack as a notch and by shifting the origin of the HRR field from the crack tip to a point inside the crack, which is lo-calized by matching the HRR strain at the crack tip with the strain pre-dicted at that point by a strain concentration rule. The idea that the crack growth is caused by the sequential failure of volume elements ahead of the crack tip is extended to deal with the variable amplitude loading case. A good agreement between the crack growth predictions (by the direct integration of a damage function based solely on εN parameters) and the experiments was obtained for one structural material under vari-able amplitude load histories. Moreover, an Elber-type opening load concept can be introduced into the model, to separate the ...
A Non-Singular Cumulative Damage Model to Predict Fatigue Crack Growth under Service Loading
After dividing into three classes the mechanisms that can cause load sequence effects on fatigue crack growth depending on where they act in relation to the crack tip (before, at or after it), some results that cannot be explained by plasticity-induced crack closure are discussed. Then, two mechanisms are quantitatively proposed as viable options in these cases, crack bifurcation at the crack tip (studied elsewhere) and damage accumulation ahead of the crack tip. A model of this type is proposed where fatigue cracking is assumed caused by the sequential failure of volume elements or tiny ε ε ε εN specimens in front of the crack tip, calculated by damage accumulation concepts. The crack is treated as a sharp notch with a small but not zero radius, avoiding the physically unrealistic singularity at its tip. The crack stress concentration factor and a strain concentration rule are used to calculate the notch root strain and to shift the origin of a modified HRR field, resulting in a non-singular model of the strain distribution ahead of the crack tip. In this way, the damage caused by each load cycle, including the effects of residual stresses, can be calculated at each element ahead of the crack tip using the correct hysteresis loops caused by the loading. This proposed approach is first validated by comparing the measured with the predicted da/dN×∆ ∆ ∆ ∆K curves of three structural alloys. The predictions are made using only ε ε ε εN, toughness and threshold properties, since the model does not need any fitting constant. This idea is then extended to predict fatigue crack growth under variable amplitude loading, assuming that the width of the volume element broken at each cycle is equal to the region ahead of the crack tip that suffers damage beyond its critical value. The reasonable predictions of the measured fatigue crack growth behavior in steel specimens under service loads corroborate this simple and clear way to correlate da/dN and ε ε ε εN properties. 2 c 2 Yc c
A note on fatigue crack growth predictions based on damage accumulation ahead of the crack tip
Models are proposed to predict the fatigue crack growth (FCG) process using crack initiation properties and critical damage concepts. The crack is modelled as a sharp notch with a very small but finite tip radius to remove its singularity, using a strain concentration rule. In this way, the damage caused by each load cycle and the effects of residual stresses can be calculated at each element ahead of the crack tip using the correct hysteresis loops caused by the loading, without the need for adjustable parameters. A quite good agreement between the εN-based crack growth predictions and experiments is obtained both for constant and for variable amplitude load histories.
Fatigue crack growth predictions based on damage accumulation calculations ahead of the crack tip
Computational Materials Science, 2009
Fatigue Crack growth modeling Critical damage model Strain-life method Hutchinson-Rice-Rosengren field Crack growth algorithm a b s t r a c t Models are proposed to predict the fatigue crack growth (FCG) process using crack initiation properties and critical damage concepts. The crack is modeled as a sharp notch with a very small but finite tip radius to remove its singularity, using a strain concentration rule. In this way, the damage caused by each load cycle and the effects of residual stresses can be calculated at each element ahead of the crack tip using the hysteresis loops caused by the loading, without the need for adjustable parameters. A computational algorithm is introduced to calculate cycle-by-cycle crack growth using the proposed methodology. A quite good agreement between the eN-based crack growth predictions and experiments is obtained both for constant and for variable amplitude load histories.
Procedia Engineering, 2014
Reliable prediction of fatigue life of the structural components under variable amplitude loading requires accurate computation of the residual stresses ahead of crack tip. In present study, a novel method of estimation of linear elastic stress field using concept of fictitious notch rounding is presented. The stress field is later used for evaluation of residual stress distribution. Corrective stress intensity factor is calculated from the residual stress field using modified weight function method. Corrective residual stress intensity factor is used to find the effective maximum stress intensity factor and effective stress intensity range. The values of root radius of aluminium alloys computed using proposed method is found to be closely matching with the values available in literature. Numerical investigation has been carried out to predict remaining life of plate panel with and without considering load interaction effects.
On the fatigue crack growth prediction under variable amplitude loading
Computational and experimental analysis of damaged materials 2007, 2007
During the last decades, numerous papers have been published on fatigue life and fatigue crack growth prediction under variable amplitude loading. The fatigue crack growth prediction models are fracture mechanics based models that have been developed to support the damage tolerance concepts in metallic structures.
Singular and non-singular approaches for predicting fatigue crack growth behavior
International Journal of Fatigue, 2005
In this work, three classes of mechanisms that can cause load sequence effects on fatigue crack growth are discussed: mechanisms acting before, at or after the crack tip. After reviewing the crack closure idea, which is based on what happens behind the crack tip, quantitative models are proposed to predict the effects at the crack tip due to crack bifurcation. To predict the behavior ahead of the crack tip, a damage accumulation model is proposed. In this model, fatigue cracking is assumed caused by the sequential failure of volume elements or tiny 3N specimens in front of the crack tip, calculated by damage accumulation concepts. The crack is treated as a sharp notch with a small, but not zero radius, avoiding the physically unrealistic singularity at its tip. The crack stress concentration factor and a strain concentration rule are used to calculate the notch root strain and to shift the origin of a modified HRR field, resulting in a non-singular model of the strain distribution ahead of the crack tip. In this way, the damage caused by each load cycle, including the effects of residual stresses, can be calculated at each element ahead of the crack tip using the correct hysteresis loops caused by the loading. The proposed approach is experimentally validated and extended to predict fatigue crack growth under variable amplitude loading, assuming that the width of the volume element broken at each cycle is equal to the region ahead of the crack tip that suffers damage beyond its critical value. The reasonable predictions of the measured fatigue crack growth behavior in steel specimens under service loads corroborate this simple and clear way to correlate da/dN and 3N properties. q
A model to quantify fatigue crack growth by cyclic damage accumulation calcula.PDF
Elber's hypothesis that K eff can be assumed as the driving force for fatigue crack growth (FCG) is the basis for strip-yield models widely used to predict fatigue lives under variable amplitude loads, although it does not explain all load sequence effects observed in practice. To verify if these models are indeed intrinsically better, the mechanics of a typical strip-yield model is used to predict FCG rates based both on Elber's ideas and on the alternative view that FCG is instead due to damage accumulation induced by the cyclic strain history ahead of the crack tip, which does not need or use K eff ideas. The main purpose here is to predict FCG using the cyclic strains induced by the plastic displacements calculated by strip-yield procedures, assuming there are strain limits associated both the with the FCG threshold and with the material toughness. Despite based on conflicting principles, both models can reproduce quite well FCG data, a somewhat surprising result that deserves to be carefully analyzed.
On the theoretical modeling of fatigue crack growth
Although fatigue is by far the most common mode of failure of structural materials, mech-anistic understanding is still lacking. For example, the fundamental Paris law which relates the crack growth rate to stress-intensity factor range is still phenomenological and no reliable mechanistic model has been established for a given material or class of materials despite numerous investigations over a half a century. This work is an attempt to theoretically model fatigue crack propagation induced by alternating crack-tip plastic blunting and re-sharpening in the mid-range of growth rates on the basis of inputs from experiments that measure macroscopic material behavior, e.g ., response to uniaxial cycling loading. In particular, we attempt to predict Paris law behavior by accounting for the material consti-tutive behavior in response to cyclic loading by modeling crack advance solely in terms of the underlying plastic dissipation. We obtain the steady-state condition for crack growth based on plastic dissipation, numerically using finite element analysis, which involves a methodology to address plastic closure upon unloading. For a given stress-intensity range, we calculate the crack propagation rate from the steady-state condition through each cycle of loading and unloading of a cracked compact-tension specimen, without resorting to any specific criterion for crack advance. Published by Elsevier Ltd.
Crack Growth Model For Estimating The Fatigue Life Under Variable Loading
2011
This paper examines the fatigue short and long cracks behaviour in 2024 T 4 aluminum alloy under rotating bending loading and stress ratio R = -1. In the short cracks region, cracks grow initially at a fast rate but deceleration occurs quickly and, depending on the stress level, they either arrest or are temporarily halted at a critical length. This critical length is shown to conincide with the value of the microstructure parameter, grain size diameter, An empirical model which describes short and long cracks rates is developed and is seen in good agreement with the experimental observations in this alloy. Comparison of the empirical model lives results with cumulative fatigue results has shown encouraging experimentally agreement while liner rule gave a non – reasonable prediction.