Computation of Effective Fatigue Thresholds Based on a New Concept of Crack Closure

2006 ◽  
Vol 324-325 ◽  
pp. 803-806 ◽  
Author(s):  
Jana Horníková ◽  
Pavel Šandera ◽  
Jaroslav Pokluda
Keyword(s):  
Crack Closure ◽  
Loading Condition ◽  
Stress Ratio ◽  
Fatigue Cracks ◽  
Theoretical Concept ◽  
Fatigue Threshold ◽  
Threshold Values ◽  
Range Effect ◽  
Plain Strain ◽  
Good Agreement

A new theoretical concept of crack closure under plain strain was applied to assess the effective fatigue threshold under various loading condition for selected aluminium and titanium alloys of different microstructures. The concept is based on the long-range effect of geometrically necessary dislocations remaining in the wake of propagating fatigue cracks. Calculated threshold values FKeff,th for 7475 aluminium alloy are about 1.9 MPa.m1/2 (in vacuum) and 1 MPa.m1/2 (in air), and about 2.5 MPa.m1/2 and 3.3 MPa.m1/2 for c-titanium and Ti-2.5%Cu, respectively. All calculated values are nearly independent on both the microstructure and the applied stress ratio and they are in a good agreement with experimental data.

Microstructure-Aided Analysis of Fatigue Thresholds in Duplex Stainless Steel

2007 ◽  
Vol 567-568 ◽  
pp. 101-104 ◽  
Author(s):  
Jaroslav Pokluda ◽  
Tomáš Podrábský ◽  
Karel Slámečka ◽  
Simona Hutařová ◽  
Guo Cai Chai
Keyword(s):  
Fatigue Cracks ◽  
Fatigue Threshold ◽  
Threshold Region ◽  
Threshold Values ◽  
Duplex Microstructure ◽  
Martensitic Steels ◽  
High Fatigue ◽  
Crack Paths ◽  
Plasticity Induced Crack Closure ◽  
Good Agreement

High fatigue threshold values of duplex ferritic-martensitic steels are interpreted by using a unified model of roughness- and plasticity induced crack closure. Complex metallographical and fractographical analysis was performed in order to obtain characteristics of tortuous crack paths produced by crack deflection and branching mainly at austenite/ferrite interfaces. Calculated values of effective thresholds are in a good agreement with experimental data. The total level of extrinsic toughening (closure + shielding) induced by the duplex microstructure was determined to be as much as 70% of measured fatigue threshold values. This is the main reason for the high resistance to propagation of long fatigue cracks in the near-threshold region.

Assessment of Extrinsic Crack Tip Shielding in Austenitic Steel near Fatigue Threshold

2008 ◽  
Vol 385-387 ◽  
pp. 49-52 ◽  
Author(s):  
Jaroslav Pokluda ◽  
Yoshiyuki Kondo ◽  
Karel Slámečka ◽  
Pavel Šandera ◽  
Jana Horníková
Keyword(s):  
Austenitic Steel ◽  
Crack Closure ◽  
Plastic Zone Size ◽  
Fracture Morphology ◽  
Theoretical Concept ◽  
Austenitic Steels ◽  
Fatigue Threshold ◽  
Threshold Region ◽  
Crack Wake ◽  
Plain Strain

Asymmetric arrangements of stored crack-wake dislocations and low values of the size ratio SR, the plastic zone size / the characteristic microstructural distance, were found to be of basic importance for the shear misfit of crack flanks causing the roughness-induced crack closure in case of plain strain conditions. The crack wake dislocations produce also the plasticity induced crack closure as a result of a near-tip mismatch perpendicular to crack flanks. According to a recently published theoretical concept, an estimation of these extrinsic shielding effects in the threshold region of fatigue crack propagation was made for austenitic steel of Japan provenience. Related fatigue experiments were based on a standard load shedding technique associated with monitoring of the crack closure level. The surface roughness was analysed by means of the optical chromatography that enables a 3D reconstruction of fracture morphology. Calculated and measured effective threshold values of about 2.2 MPa.m1/2 are practically identical. Total levels of the extrinsic toughening induced by the austenitic microstructure are rather low when compared to those identified in ferritic- and ferritic-austenitic steels.

On Effective Stress Range Factor in Fatigue

1980 ◽  
Vol 102 (1) ◽  
pp. 147-152 ◽  
Author(s):  
K. M. Lal ◽  
S. B. L. Garg ◽  
I. Le May
Keyword(s):  
Fatigue Crack ◽  
Crack Closure ◽  
Effective Stress ◽  
Stress Ratio ◽  
Stress Amplitude ◽  
Simplified Model ◽  
Strain Hardening Exponent ◽  
Stress Range ◽  
Stress Cycles ◽  
Good Agreement

A detailed analysis of crack closure during fatigue crack propagation has been made using a simplified model. In addition, experimental work has been conducted using eddy current methods to determine crack closure. It was found that the effective stress range factor, U, depends on the applied stress range, stress ratio, yield strength and strain hardening exponent of the material, and that its value stabilizes in about 10 stress cycles for constant stress amplitude loading. Experimental and predicted results were seen to be in good agreement.

scholarly journals Study of Influence of Width to Thickness Ratio in Sheet Metals on Bendability under Ambient and Superimposed Hydrostatic Pressure

2021 ◽  
Vol 2 (3) ◽  
pp. 542-558
Author(s):  
Mohammadmehdi Shahzamanian ◽  
David Lloyd ◽  
Amir Partovi ◽  
Peidong Wu
Keyword(s):  
Hydrostatic Pressure ◽  
Stress Ratio ◽  
Fracture Strain ◽  
Stress Triaxiality ◽  
Volumetric Strain ◽  
Thickness Ratio ◽  
Sheet Metals ◽  
The Finite Element Method ◽  
Bending Strain ◽  
Good Agreement

The effect of the width to thickness ratio on the bendability of sheet metal is investigated using the finite element method (FEM) employing the Gurson–Tvergaard–Needleman (GTN) model. Strain path changes in the sheet with change in the width/thickness ratio. It is shown that bendability and fracture strain increase significantly by decrease in the width/thickness ratio. The stress state is almost uniaxial when the stress ratio (α) is close to zero for narrow sheets. Stress ratio is nothing but the major stress to minor stress ratio. This delays the growth and coalescence of micro-voids as the volumetric strain and stress triaxiality (pressure/effective stress) decrease. On the other hand, ductility decreases with increase in α for wider sheets. Fracture bending strain is calculated and, as expected, it increases with decrease in the width/thickness ratio. Furthermore, a brief study is performed to understand the effect of superimposed hydrostatic pressure on fracture strain for various sheet metals with different width/thickness ratios. It is found that the superimposed hydrostatic pressure increases the ductility, and that the effect of the width/thickness ratio in metals on ductility is as significant as the effect of superimposed hydrostatic pressure. Numerical results are found to be in good agreement with experimental observations.

scholarly journals Fatigue Crack Growth Behaviour and Role of Roughness-Induced Crack Closure in CP Ti: Stress Amplitude Dependence

Metals ◽  
2021 ◽  
Vol 11 (10) ◽  
pp. 1656
Author(s):  
Mansur Ahmed ◽  
Md. Saiful Islam ◽  
Shuo Yin ◽  
Richard Coull ◽  
Dariusz Rozumek
Keyword(s):  
Fatigue Crack ◽  
Crack Propagation ◽  
Crack Growth ◽  
Crack Closure ◽  
Fatigue Cracks ◽  
Propagation Mechanism ◽  
Amplitude Dependence ◽  
Crack Growth Behaviour ◽  
Crack Propagation Mechanism ◽  
Cp Ti

This paper investigated the fatigue crack propagation mechanism of CP Ti at various stress amplitudes (175, 200, 227 MPa). One single crack at 175 MPa and three main cracks via sub-crack coalescence at 227 MPa were found to be responsible for fatigue failure. Crack deflection and crack branching that cause roughness-induced crack closure (RICC) appeared at all studied stress amplitudes; hence, RICC at various stages of crack propagation (100, 300 and 500 µm) could be quantitatively calculated. Noticeably, a lower RICC at higher stress amplitudes (227 MPa) for fatigue cracks longer than 100 µm was found than for those at 175 MPa. This caused the variation in crack growth rates in the studied conditions.

scholarly journals The Effect of Technological Treatments and Loads on the Service Life of Fillet Welds

2020 ◽  
pp. 21-25
Author(s):  
Janusz Lewandowski ◽  
Dariusz Rozumek
Keyword(s):  
Service Life ◽  
Stress Ratio ◽  
Fatigue Cracks ◽  
Constant Stress ◽  
Fillet Welds ◽  
Test Results ◽  
Technological Treatment ◽  
Technological Treatments ◽  
Made In

The paper presents test results concerning the service life of fillet welds made in steel S355. In addition, the article discusses the initiation and growth of fatigue cracks in specimens subjected to bending with torsion. The tests were performed in relation to constant stress ratio R = -1 and 0. The results presented in the article take into account the effect of the technological treatment on the service life of the specimens. The tests revealed longer service life of the specimens not subjected to the technological treatment, both when R = -1 and 0.

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