The fracture mechanics similitude concept: questions concerning its application to the behavior of short fatigue cracks

1983 ◽  
Vol 57 (2) ◽  
pp. L27-L30 ◽  
Author(s):  
R.O. Ritchie ◽  
S. Suresh
Keyword(s):  
Fracture Mechanics ◽  
Fatigue Cracks

Correlations Between Advanced Nondestructive Evaluation Methods and Fracture Mechanics Parameters

1980 ◽  
Vol 102 (1) ◽  
pp. 50-55 ◽  
Author(s):  
C. M. Teller ◽  
J. R. Barton ◽  
G. A. Matzkanin ◽  
F. N. Kusenberger ◽  
R. E. Beissner
Keyword(s):  
Surface Wave ◽  
Fracture Mechanics ◽  
Nondestructive Evaluation ◽  
Surface Waves ◽  
Fatigue Cracks ◽  
Small Surface ◽  
Magnetic Perturbation ◽  
Hinge Model ◽  
Pulse Echo ◽  
Aisi 4340

Investigations were conducted on surface entering fatigue cracks, 0.13 to 1.27 mm (0.005 to 0.05 in.) long, in rod-type tensile specimens of HY 180 and AISI 4340 steels using surface wave ultrasonics and magnetic perturbation. Pulse-echo, surface wave ultrasonic results indicate that at 10 MHz the minimum flaw size detectable in HY 180 specimens is two to three times larger than that in AISI 4340. Also, fatigue cracks as long as 0.76 mm (0.030 in.) in HY 180 specimens could not be reliably detected with 10 MHz surface waves unless load was applied to open the crack. Extensive magnetic perturbation measurements on AISI 4340 suggest a “hinge” model for the opening of small surface entering fatigue cracks.

Fatigue Life Predictions of Smooth and Notched Specimens Based on Fracture Mechanics

1981 ◽  
Vol 103 (2) ◽  
pp. 91-96 ◽  
Author(s):  
M. H. El Haddad ◽  
T. H. Topper ◽  
T. N. Topper
Keyword(s):  
Fatigue Life ◽  
Fracture Mechanics ◽  
Intensity Factor ◽  
Crack Growth ◽  
Fatigue Cracks ◽  
Propagation Model ◽  
Actual Length ◽  
Notched Specimens ◽  
Material Condition ◽  
Length Constant

An elastic plastic fracture mechanics solution for short fatigue cracks in smooth and notched specimens is presented which admits plasticity by replacing the conventional stress term with a strain term and accounts for the propagation of very short cracks by the introduction of an effective crack length which is equal to the actual length increased by length l0, the length constant l0 is characteristic of the material and material condition and is calculated from the smooth specimen endurance limit and the long crack threshold stress intensity. Crack growth results for cracks in both elastic and plastic strain fields of notched specimens when interpreted in terms of this strain based intensity factor showed excellent agreement with elastic long crack data. This intensity factor when combined with a propagation model that includes all stages of crack growth also successfully predicted the total fatigue life of the smooth and notched specimens studied here. The predicted propagation life of elliptical and circular notched specimens is in all cases within 50 percent of the actual fatigue lives.

Critical Strain Energy Density along the Curved Front of the Growing Fatigue Crack

2005 ◽  
Vol 482 ◽  
pp. 307-310
Author(s):  
Tomáš Denk ◽  
Vladislav Oliva ◽  
Aleš Materna
Keyword(s):  
Fracture Mechanics ◽  
Fatigue Cracks ◽  
Anomalous Behavior ◽  
Fatigue Threshold ◽  
Stress Constraint ◽  
Short Cracks ◽  
T Stress ◽  
Residual Fatigue ◽  
Two Parameter ◽  
Different Levels

A two-parameter constraint-based fracture mechanics approach is used to explain the effect of the constraint on the apparently anomalous behavior of short fatigue cracks. The different levels of stress constraint are quantified by the T-stress, and microstructurally as well as mechanically short cracks are discussed. Short cracks generally behave more sensitively to the constraint than the long ones. It is shown that in most cases, the existence of short cracks goes hand in hand with an intrinsic loss of the constraint, which contributes to a decrease of their fatigue threshold values and accelerates their growth. In this paper, the above effect is quantified and conclusions concerning the applicability of the fracture mechanics parameters and approaches to the estimation of the residual fatigue life of structures are discussed.

Applications of Probabilistic Fracture Mechanics to Offshore Structures

1988 ◽  
Vol 41 (2) ◽  
pp. 61-84 ◽  
Author(s):  
Finn Kirkemo
Keyword(s):  
Fracture Mechanics ◽  
Model Updating ◽  
Fatigue Cracks ◽  
Limit State ◽  
Steel Structures ◽  
Local Stress ◽  
Offshore Structures ◽  
And Performance ◽  
Fatigue Limit State ◽  
Life Predictions

For offshore structures the fatigue limit state is governing the structural dimensions of several members and joint connections. Safety against fatigue failure is achieved through a combination of design requirements and performance of in-service inspections with repair of detected fatigue cracks. A review of uncertainties involved in fatigue life predictions by fracture mechanics is presented with particular reference to steel structures. Sources of uncertainties considered are: environmental conditions, hydrodynamic loading, global structural analysis, local stress calculation at fatigue sensitive points, and fatigue crack growth modeling by fracture mechanics. A probabilistic model using the fracture mechanics in probabilistic form is presented. This model accounts for uncertainties in loading, initial and critical defect sizes, material parameters, and in the uncertainty related to computation of the stress intensity factor. Failure probabilities are computed by first-order reliability methods and sensitivity factors are determined. Model updating based on in-service inspection results is formulated. Uncertainties with respect to detecting a crack and to correctly sizing a crack are included. Experience on application of the analysis method is presented.

Analysis of Fretting Crack Propagation Behavior with X-FEM Method

2012 ◽  
Vol 157-158 ◽  
pp. 1162-1166
Author(s):  
Wen Li ◽  
Ri Dong Liao ◽  
Li Tao Liu ◽  
Zheng Xing Zuo
Keyword(s):  
Fracture Mechanics ◽  
Crack Propagation ◽  
Crack Nucleation ◽  
Contact Region ◽  
Fatigue Cracks ◽  
Fretting Fatigue ◽  
Extended Finite Element ◽  
Propagation Behavior ◽  
Fem Method ◽  
Bulk Stress

Fretting fatigue cracks always initial at the tralling of contact region, because the stresses in the vicinity of the contact zone exhibit steep gradients. A fracture mechanics approach is usually used to estimate fretting fatigue propagation life. In this paper, extended finite element method combined with fracture mechanics is used to study fretting crack propagation behaviors. The computation results reveal that fretting crack nucleation is mainly decided by fretting, and the cycle bulk stress is the main reason for crack propagation. Also the X-FEM exhibits merits in fretting fatigue problem.

The Potential for Non-Conservative Results From the Probabilistic Fracture Mechanics Analyses Is Assessed Based on the Collected Observation of Fatigue Cracks in Offshore Steel Structures

2017 ◽  
Author(s):  
Ole Tom Vårdal
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
Structural Integrity ◽  
Fatigue Cracks ◽  
Steel Structures ◽  
Lessons Learned ◽  
Probabilistic Methods ◽  
Growth Potential

In structural integrity management, it is essential to know the fatigue crack growth potential. The lessons learned from use of refined fatigue analyses, fracture mechanics and probabilistic methods for platforms in-service are presented. For ageing offshore units of semi-submersible design, the inspection history of more than 20 000 NDT inspections and detection of close to 1000 fatigue cracks, are used in this study. These experience data are used to assess the potential for Non-conservative estimate for the fatigue crack growth potential.

Fatigue Modeling of Friction-Stir-Welded (FSW) Butt-Joints for Aerospace Applications

2019 ◽  
Author(s):  
Muhammad A. Wahab ◽  
Vinay Raghuram
Keyword(s):  
Fatigue Life ◽  
Aluminum Alloys ◽  
Fracture Mechanics ◽  
Crack Propagation ◽  
Fatigue Cracks ◽  
Strength Criterion ◽  
Fatigue Properties ◽  
Interface Element ◽  
Friction Stir ◽  
Stress Ratios

Abstract Among the recent research Friction-Stir-Welding (FSW) has been adopted worldwide as one of the dominant processes for welding lightweight aerospace Aluminum alloys. Al-2195 which is one of the new generation Aluminum alloys has been used in the external tank of the space shuttles. Aerospace fabricators are continuously pursuing FSW-technologies in its efforts to advance fabrication of the external tanks of the space shuttles. The future launch vehicles with reusable mandates require the structures to have excellent fatigue properties and improved fatigue lives. The butt-welded specimens of Al-2195 and Al-2219 are fatigue tested according to ASTM-E647. The effects of stress ratios, use of corrosion preventive compound (CPC), and the applications of periodic overloading on fatigue lives are investigated in this study. Scanning-electron-microscopy (SEM) is used to examine the criticality of the failure surfaces and the different modes of crack propagation that could have been initiated into the materials. It is found that fatigue life increases with the increase in stress ratio, and results show an increase in fatigue life ranging over 30% with the use of CPC, and the fatigue life increases even further with periodic overloading; whereas crack-closure phenomenon predominates the fatigue fracture. Fracture mechanics analysis and crack similitude was modified for fatigue cracks by Paris. Numerical studies using FEA has produced a model for fatigue life prediction scheme for these structures, where a novel strategy of the interface element technique with critical bonding strength criterion for formation of new fracture surfaces has been used to model fatigue crack propagation lives. The linear elastic fracture mechanics stress intensity factor is calculated using FEA and the fatigue life predictions made using this method are within 10–20% of the experimental fatigue life data obtained. This method overcomes the limitation of the traditional node-release scheme and closely matches the physics of the crack propagation.

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