A Comparison of Crack-Tip Field Parameters for Large and Small Fatigue Cracks

1986 ◽  
Vol 108 (3) ◽  
pp. 206-213 ◽  
Author(s):  
K. S. Chan ◽  
J. Lankford ◽  
D. L. Davidson
Keyword(s):  
Crack Growth ◽  
Fatigue Cracks ◽  
Crack Tip ◽  
Integral Approach ◽  
Crack Tip Field ◽  
Cyclic Plastic Zone ◽  
Small Cracks ◽  
Experimental Strain ◽  
Path Dependent ◽  
Crack Growth Rates

A comparison of the elastic-plastic crack-tip fields of large and small fatigue cracks has been made using the ΔJ-integral approach. Using experimental strain and displacement range measurements obtained by means of the stereoimaging technique, the ΔJ-integral has been computed for an aluminum alloy containing either large or small fatigue cracks, by performing line-contour integration along a variety of rectangular paths around the crack tip. These calculations reveal that for both large and small cracks the ΔJ-integral is path-dependent, and that the value of ΔJ increases with decreasing distance from the crack tip. Using an average local ΔJ or one estimated from the CTOD, the enhanced crack growth rates associated with small fatigue cracks can be explained on the basis of the large average ΔJ-integral within the cyclic plastic zone. The effect of crack closure on the computed local ΔJ (or ΔK) and crack growth is discussed.

Experimental Determination of Crack Driving Forces for Small Cracks

2016 ◽  
Vol 258 ◽  
pp. 495-500
Author(s):  
Angelika Brueckner-Foit ◽  
Pascal Pitz ◽  
Phillip Grahlmann ◽  
Frank Zeismann
Keyword(s):  
Stress Intensity ◽  
Driving Forces ◽  
Fatigue Cracks ◽  
Crack Tip ◽  
Image Correlation ◽  
Mode Ii ◽  
Crack Tip Field ◽  
Small Cracks ◽  
Experimental Values

The crack tip field of small fatigue cracks was measured using digital image correlation DIC. For this purpose, smooth specimen were fatigued until a certain amount of damage was visible on the surface. This specimens were then placed in a micro-tensile device in the SEM. The grey value patterns obtained at two different load levels were analyzed with DIC. Fitting the Williams series for the crack tip field to the corresponding DIC-displacement field in the crack tip area yielded the mode-I, and the mode-II stress intensity factors together with the T-stresses. It was found that the experimental values of the stress intensity factor were comparatively high with pronounced mode-II contributions.

Effect of Constraint Induced by Specimen Geometries on Crack Growth Behavior Under Creep-Fatigue Interaction

2018 ◽  
Author(s):  
Lei Zhao ◽  
Lianyong Xu
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Growth Rates ◽  
Crack Tip ◽  
Crack Growth Behavior ◽  
Growth Behaviour ◽  
Crack Growth Behaviour ◽  
Creep Fatigue ◽  
Crack Growth Rates

Creep-fatigue interaction would accelerate the crack growth behaviour and change the crack growth mode, which is different from that presenting in pure creep or fatigue regimes. In addition, the constraint ahead of crack tip affects the relationship between crack growth rate and fracture mechanics and thus affects the accuracy of the life prediction for high-temperature components containing defects. In this study, to reveal the role of constraint caused by various specimen geometries in the creep-fatigue regime, five different types of cracked specimens (including C-ring in tension CST, compact tension CT, single notch tension SENT, single notch bend SENB, middle tension MT) were employed. The crack growth and damage evolution behaviours were simulated using finite element method based on a non-linear creep-fatigue interaction damage model considering creep damage, fatigue damage and interaction damage. The expression of (Ct)avg for different specimen geometries were given. Then, the variation of crack growth behaviour with various specimen geometries under creep-fatigue conditions were analysed. CT and CST showed the highest crack growth rates, which were ten times as the lowest crack growth rates in MT. This revealed that distinctions in specimen geometry influenced the in-plane constraint level ahead of crack tip. Furthermore, a load-independent constraint parameter Q* was introduced to correlate the crack growth rate. The sequence of crack growth rate at a given value of (Ct)avg was same to the reduction of Q*, which shown a linear relation in log-log curve.

Plastic Strain Distribution at the Tips of Propagating Fatigue Cracks

1976 ◽  
Vol 98 (1) ◽  
pp. 24-29 ◽  
Author(s):  
D. L. Davidson ◽  
J. Lankford
Keyword(s):  
Aluminum Alloy ◽  
Fatigue Crack ◽  
Plastic Strain ◽  
Plastic Zone ◽  
Strain Distribution ◽  
Dimensionless Parameter ◽  
Fatigue Cracks ◽  
Crack Tip ◽  
Cyclic Plastic Zone ◽  
Plastic Zones

The techniques of selected area electron channeling and positive replica examination have been used to study the plastic zones attending fatigue crack propagation in 304 SS, 6061-T6 aluminum alloy, and Fe-3Si steel. These observations allowed the strain distribution at the crack tip to be determined. The results indicate that the concepts of a monotonic and a cyclic plastic zone are essentially correct, with the strains at demarcation between these two zones being 3 to 6 percent. Strain distribution varies as r−1/2 in the cyclic zone and as ln r in the monotonic plastic zone. The strain distributions for all materials studied may be made approximately coincident by using a dimensionless parameter related to distance from the crack tip.

Fatigue Crack Growth Properties of 16MnR and 316L Steels at High Temperature

2005 ◽  
Author(s):  
Zengliang Gao ◽  
Weiming Sun ◽  
Weiya Jin ◽  
Ying Wang ◽  
Fang Zhang
Keyword(s):  
Fatigue Crack ◽  
High Temperature ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Growth Rates ◽  
Fatigue Cracks ◽  
Pressure Vessels ◽  
Effect Of Temperature ◽  
Growth Properties ◽  
Crack Growth Rates

Fatigue failures often take place in high temperature pressure vessels and equipment because of fluctuation of pressure and temperature. Fatigue crack growth properties of materials at high temperatures are very important for safety assessment of high temperature equipment. A series of fatigue crack growth tests were carried out, and fatigue crack growth rates were determined at 25∼500°C for typical steels 316L and 16MnR. The laws of fatigue crack growth of two materials at different temperatures and the effect of temperature on fatigue crack growth rates were studied. The results show that the crack growth rates increase with temperature for 316L steel. Both the exponent n and constant C for Paris law change with temperature. The fatigue cracks of 16MnR propagate at 150 °C and 300 °C more slowly than at room temperature and 425 °C. The fatigue crack growth rate at 425 °C is the highest for temperature range of 25–425 °C.

scholarly journals Microstructurally small fatigue crack growth rates in aluminium alloys for developing improved predictive models

2018 ◽  
Vol 165 ◽  
pp. 13004
Author(s):  
Madeleine Burchill ◽  
Simon Barter ◽  
Lok Hin Chan ◽  
Michael Jones
Keyword(s):  
Fatigue Crack ◽  
Crack Growth ◽  
Growth Rates ◽  
Small Crack ◽  
Fatigue Crack Nucleation ◽  
Standard Material ◽  
Growth Phenomenon ◽  
Small Cracks ◽  
Small Crack Growth ◽  
Crack Growth Rates

The fatigue or durability life of a few critical structural metallic components often sets the safe and/or economic useful life of a military airframe. In the case of aluminium airframe components, growth rates, at or soon after fatigue crack nucleation are being driven by near threshold local cyclic stress intensities and thus are very low. Standard crack growth rate data is usually generated from large cracks, and therefore do not represent the growth of small cracks (typically <1mm). Discussed here is an innovative test and analysis technique to measure the growth rates of small cracks growing as the result of stress intensities just above the cyclic growth threshold. Using post-test quantitative fractographic examination of fatigue crack surfaces from a series of 7XXX test coupons, crack growth rates and observations of related growth phenomenon in the threshold region have been made. To better predict small crack growth rates under a range of aircraft loading spectra a method by which standard material data models could be adapted is proposed. Early results suggest that for small cracks this method could be useful in informing engineers on the relative severity of various spectra and leading to more accurate predictions of small crack growth rates which can dominate the fatigue life of airframe components.

The Growth Rate of Semi-Elliptic Fatigue Cracks in Thick-Walled Cylinders

1983 ◽  
Vol 22 ◽  
Author(s):  
W. A. Lees ◽  
P. S. J. Crofton
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Internal Pressure ◽  
Crack Growth ◽  
Fatigue Cracks ◽  
Low Alloy Steel ◽  
Linear Elastic ◽  
Fatigue Data ◽  
Elastic Fracture ◽  
Crack Growth Rates

ABSTRACTThe rate of growth of fatigue cracks originating at the bore and at the outside surface of thick-walled low alloy steel cylinders has been measured for cylinders subjected to fluctuating internal pressure.Analysis of the results using linear elastic fracture mechanics relationships shows that crack growth rates found in air adequately predict the behaviour of cracks growing from the outside surface of a cylinder.Fatigue cracks growing from the bore of a cylinder subjected to fluctuating internal pressure are found to advance at a consistently higher rate than that predicted from crack growth rate tests carried out in air.These findings are discussed in relation to the proposed adoption of existent strain-life fatigue data for pressure vessel design.

Fatigue Crack Growth in Ultrafine-Grained Copper Obtained by ECAP

2014 ◽  
Vol 891-892 ◽  
pp. 1099-1104 ◽  
Author(s):  
Mandana Arzaghi ◽  
Christine Sarrazin-Baudoux ◽  
Jean Petit
Keyword(s):  
Crack Growth ◽  
Growth Rates ◽  
Fatigue Cracks ◽  
Deformation Texture ◽  
Coarse Grained ◽  
Fine Grained ◽  
Angular Pressing ◽  
Ultrafine Grained ◽  
Crack Growth Rates ◽  
Threshold Regime

The propagation of long fatigue cracks in ultra fine-grained (UFG) copper obtained by equal channel angular pressing (ECAP) is investigated in the mid ΔK range and in the near threshold regime. The crack growth rates in UFG copper are substantially faster than in coarse-grained (CG) copper. A huge influence of environment is observed, with growth rates faster of more than two orders of magnitude in air compared to vacuum. The crack growth mechanisms are discussed on the basis of microfractographic observations and the deformation texture.

scholarly journals Approximation of the crack-tip field in fatigue cracks in bridge steel specimens: DIC analysis of different constraint levels

2019 ◽  
Vol 13 (49) ◽  
pp. 97-106
Author(s):  
Stanislav Seitl ◽  
Petr Miarka ◽  
Vladimir Ruzicka ◽  
Lucie Malikova ◽  
Alejandro Cruces ◽  
...  
Keyword(s):  
Fatigue Cracks ◽  
Crack Tip ◽  
Crack Tip Field ◽  
Bridge Steel

Prediction of Environmentally Assisted Cracking on Gas and Liquid Pipelines

2006 ◽  
Author(s):  
J. Been ◽  
R. Eadie ◽  
R. Sutherby
Keyword(s):  
Crack Growth ◽  
Growth Rates ◽  
Pressure Fluctuations ◽  
Low Frequencies ◽  
Small Pressure ◽  
Environmentally Assisted Cracking ◽  
Pressure Time ◽  
Growing Crack ◽  
Small Cracks ◽  
Crack Growth Rates

A model has been developed to predict crack growth on pipelines from environmentally assisted cracking in near-neutral pH environments (often-termed low-pH stress corrosion cracking (SCC)). The model is based on the results of cyclic loading experiments and is used in conjunction with pressure time variations in the pipeline determined from the operating SCADA records to predict the growth of an assumed existing crack in the pipe. The crack grows through different crack growth regimes, which are determined by the size of the pressure variations and the instantaneous crack dimensions. For a growing crack that experiences relatively high pressure fluctuations, as often encountered on liquid lines, reasonable crack growth predictions were made based on corrosion fatigue. An approach based on crack tip strain rate appears more suitable for the prediction of crack growth of small cracks and for cracks on gas lines with small pressure fluctuations. The model is designed so that the effect of stress intensifiers (like the long seam weld crown) that are often associated with these failures can be included. The model can be used in its present format for prioritizing inspections on both gas and liquid pipelines. Whereas predicted crack growth rates compare favorably with rates measured in the field, further work is required to incorporate additional mechanical and environmental effects, in particular to improve the prediction of small crack growth rates. Low crack velocities may be possible in the presence of small pressure fluctuations and low frequencies, but they may be less probable.

Steady fatigue crack growth: micromechanism and mathematical modeling

2018 ◽  
Vol 84 (11) ◽  
pp. 52-69 ◽  
Author(s):  
N. V. Tumanov
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Front ◽  
Fatigue Cracks ◽  
Crack Tip ◽  
Tensile Load ◽  
Metallic Materials ◽  
Fatigue Striations ◽  
Shaped Crack

A universal energy-intensive micromechanism of periodic splitting-rupture (PSR) is revealed which proceeds at the front of the fatigue cracks in metallic materials, providing their steady growth, forming T-shaped crack tip and striated microrelief of the fracture surface. The PSR micromechanism is caused by a critical (prior to fracture) fragmentated structure formed in the area of the crack front where the material is subjected to multiple and increasing plastic deformation. This universal prefracture structure is a final stage of the evolution of the deformational structures emerged in front of the fatigue crack at the stage of stable crack growth in metallic materials with different initial structural states. This is responsible for universality of PSR micromechanism and fatigue striations. Fatigue striations are the traces of extending crack front with T-shaped tip formed during brittle transverse microsplitting along the overstressed boundaries of critical fragmentated structure. Based on 3D finite element modeling of the stress-strain state in front of the cracks with T-shaped tip, it is established that the value and the location of maximum of normalized in-plain stresses (acting in front of crack tip in the plane of crack along the normal to its front) are close or coincide for the cracks of different configuration and different types of tensile load under condition that splitting in the T-shaped crack tip is considerably less than the crack length. Taking into account the PSR micromechanism and asymptotic stress distribution in front of T-shaped crack tip the physically based mathematical model for steady fatigue crack growth is developed along with the techniques for prediction of steady fatigue crack growth in full-scale components under simple and complex loading cycles.

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