scholarly journals Growth behavior of fatigue cracks in ultrafine grained Cu smooth specimens with a small hole

2015 ◽  
Author(s):  
Masahiro Goto ◽  
Kakeru Morita ◽  
Junichi Kitamura ◽  
Takaei Yamamoto ◽  
Masataka Baba ◽  
...  
Keyword(s):  
Fatigue Cracks ◽  
Maximum Shear Stress ◽  
High Stress ◽  
Crack Tip ◽  
Shear Plane ◽  
Growth Behavior ◽  
Shear Mode ◽  
Aspect Ratios ◽  
Ultrafine Grained ◽  
Loading Axis

In order to study the growth mechanism of fatigue cracks in ultrafine grained copper, stresscontrolled fatigue tests of round-bar specimens with a small blind hole as a crack starter were conducted. The hole was drilled on the surface where an intersection between the shear plane of the final ECAP processing and the specimen surface makes an angle of 45° or 90° with respect to the loading axis. At a low stress ( ? a = 90 MPa), the direction of crack paths was nearly perpendicular to the loading direction regardless of the location of the hole. Profile of crack face was examined, showing the aspect ratio (b/a) of b/a = 0.82. At a high stress ( ? a = 240 MPa), although the growth directions inclined 45° and 90° to the loading-axis were observed depending on the location of the drilling hole, crack faces in these cracks were extended along one set of maximum shear stress planes, corresponding to the final ECAP shear plane. The value of aspect ratios was b/a = 0.38 and 1.10 for the cracks with 45° and 90° inclined path directions, respectively. The role of deformation mode at the crack tip areas on crack growth behavior were discussed in terms of the mixed-mode stress intensity factor. The crack path formation at high stress amplitudes was affected by the in-plane shear-mode deformation at the crack tip.

Fatigue-induced damage and crack growth of Cu processed by ECAP

2015 ◽  
Vol 29 (06n07) ◽  
pp. 1540028 ◽  
Author(s):  
Masahiro Goto ◽  
Kakeru Morita ◽  
Jyunichi Kitamura ◽  
Masataka Baba ◽  
Seung-Zeon Han ◽  
...  
Keyword(s):  
Crack Growth ◽  
Fatigue Cracks ◽  
Shear Bands ◽  
Shear Plane ◽  
Surface Damage ◽  
Crack Growth Behavior ◽  
Angular Pressing ◽  
Slip Bands ◽  
Ultrafine Grained ◽  
Loading Axis

The fatigue-induced damage and crack growth behavior were studied on the ultrafine grained copper processed by equal channel angular pressing (ECAP). At high stresses, fatigue cracks were initiated at the shear bands (SBs) formed along the shear plane of the final ECAP. At low stresses, the grain coarsening occurred due to dynamic recrystallization. The slip bands were then formed inside these grains and subsequently served as an initiation sites for cracks. The direction of crack growth, either 45° or perpendicular to the loading axis, varied depending on the stress. The formation and growth mechanisms of fatigue crack are discussed based on the micrographic observation of surface damage.

Behavior of Fatigue Cracks Generated from a Small Artificial-Defect in Plain Specimen of Copper Processed by Equal Channel Angular Pressing

2018 ◽  
Vol 941 ◽  
pp. 614-619
Author(s):  
Masahiro Goto ◽  
Takashi Iwamura ◽  
Takaei Yamamoto ◽  
Seung Zeon Han ◽  
Junichi Kitamura ◽  
...  
Keyword(s):  
Equal Channel Angular Pressing ◽  
Crack Path ◽  
Fatigue Cracks ◽  
Fatigue Tests ◽  
Artificial Defect ◽  
Angular Pressing ◽  
Ultrafine Grained ◽  
Round Bar ◽  
Physical Background ◽  
Loading Axis

Fatigue tests of ultrafine-grained copper processed by equal channel angular pressing were conducted on the round-bar specimens with a small artificial-defect. The fatigue crack initiated from the defect at an early fatigue stage. After the crack initiation, the crack grew with a 45° inclination to the loading axis at stress amplitudes above 180 MPa. At the stress less than 160 MPa, however, the crack grew perpendicular to the loading axis. The physical background of deferent crack path directions between high-and low-stresses was discussed from the viewpoint of a morphological feature of damaged traces along the crack path.

Blunt Crack Initiation and its Transition to Sharp Cracks in Pipeline Steel in Near-Neutral pH Solution

2012 ◽  
Author(s):  
B. Fang ◽  
R. L. Eadie ◽  
M. Elboujdaini
Keyword(s):  
Crack Growth ◽  
Pipeline Steel ◽  
High Stress ◽  
Crack Tip ◽  
Crack Mouth ◽  
Blunt Crack ◽  
Neutral Ph ◽  
Aspect Ratios ◽  
Risk Of Failure ◽  
Mouth Width

This paper reviews our research into pipeline stress corrosion cracking (SCC) in near-neutral pH (NNpH) environment to help understand the mechanisms on pit-to-crack transition and early growth to contribute to pipeline integrity management so that the risk of failure can be avoid or reduced. Pitted specimens by using two different techniques (passivation/immersion and electrochemical methods) were cyclically loaded in NNpH environment sparged with 5% CO2 / balance N2 gas mixture at high stress ratios (minimum stress/maximum stress), low strain rates and low frequencies which are close to the operational pipelines in the field. Blunt cracks initiation was seen first and associated with the pit geometry, and most of the blunt cracks were observed to have initiated from the corrosion pits that had the pit depth to surface width aspect ratios greater than 0.5. The blunt crack growth was engendered by anodic dissolution, which was facilitated by stress. So it was called as stress facilitated dissolution crack growth. These blunt cracks had considerably large crack tip width to crack mouth width aspect ratios, and the majority were below 0.5 to 0.6 mm deep, and considered dormant. Once cracks surpassed the critical value, around 0.5 to 0.6 mm, the cracks would be reactivated and the crack tip width to crack mouth width ratios became significantly smaller. Meanwhile, more hydrogen would be trapped in the plastic zones. Thus, hydrogen would play an important role in the crack propagation. So in this stage, cracks tended to become sharp and the mechanism was referred to hydrogen facilitated cracking. The observations from the field can be interpreted very well by using the proposed models. It was proposed that two different mechanisms are responsible for the early-stage crack growth and sharp cracks be removed to reduce the risk of failure in pipelines.

Relationship between Grain Growth and Formation of Fracture Surface of Ultrafine Grained Cu in High-Cycle Fatigue

2016 ◽  
Vol 713 ◽  
pp. 147-150
Author(s):  
M. Goto ◽  
Seung Zeon Han ◽  
Takaei Yamamoto ◽  
J. Kitamura ◽  
Kamil Kusno ◽  
...  
Keyword(s):  
Fracture Surface ◽  
High Cycle Fatigue ◽  
Crack Tip ◽  
Plastic Zone Size ◽  
Fatigue Tests ◽  
Growth Behavior ◽  
Cycle Fatigue ◽  
Zone Size ◽  
Angular Pressing ◽  
Ultrafine Grained

Fatigue tests were conducted on specimens of ultrafine grained copper produced by equal channel angular pressing. The growth behavior of a fatal crack was monitored successively. The morphological features of the crack growth paths and fracture surfaces were influenced by variation of grain size and damaged areas of the crack tip, depending on the stage of progress of the fatigue damage. The change in fracture surface was discussed by considering the interrelation between the reversible plastic zone size at the crack tip and the microstructure evolved during cyclic stressing.

Low-Cycle Fatigue Behaviour of Gas Turbine Alloys

1974 ◽  
Vol 188 (1) ◽  
pp. 321-328 ◽  
Author(s):  
W. J. Evans ◽  
G. P. Tilly
Keyword(s):  
Low Cycle Fatigue ◽  
Fatigue Cracks ◽  
High Stress ◽  
Cyclic Creep ◽  
Fatigue Curve ◽  
Fatigue Behaviour ◽  
Material Surface ◽  
Tension Stress ◽  
Cycle Fatigue ◽  
Stress Tests

The low-cycle fatigue characteristics of an 11 per cent chromium steel, two nickel alloys and two titanium alloys have been studied in the range 20° to 500°C. For repeated-tension stress tests on all the materials, there was a sharp break in the stress-endurance curve between 103 and 104 cycles. The high stress failures were attributed to cyclic creep contributing to the development of internal cavities. At lower stresses, failures occurred through the growth of fatigue cracks initiated at the material surface. The whole fatigue curve could be represented by an expression developed from linear damage assumptions. Data for different temperatures and types of stress concentration were correlated by expressing stress as a fraction of the static strength. Repeated-tensile strain cycling data were represented on a stress-endurance diagram and it was shown that they correlated with push-pull stress cycles at high stresses and repeated-tension at low stresses. In general, the compressive phase tended to accentuate cyclic creep so that ductile failures occurred at proportionally lower stresses. Changes in frequency from 1 to 100 cycle/min were shown to have no significant effect on low-cycle fatigue behaviour.

Investigation on Stress and Strain Singularity in LEFM

2006 ◽  
Vol 306-308 ◽  
pp. 31-36
Author(s):  
Zheng Yang ◽  
Wanlin Guo ◽  
Quan Liang Liu
Keyword(s):  
High Stress ◽  
Crack Tip ◽  
Plane Stress State ◽  
Stress And Strain ◽  
Linear Elastic ◽  
Geometrical Nonlinear ◽  
Maximum Crack ◽  
Elastic Fracture ◽  
Plain Strain ◽  
Strain Singularity

Stress and strain singularity at crack-tip is the characteristic of Linear Elastic Fracture Mechanics (LEFM). However, the stress, strain and strain energy at crack-tip may be infinite promoting conflicts with linear elastic hypothesis. It is indicated that the geometrical nonlinear near the crack-tip should not be neglected for linear elastic materials. In fact, the crack-tip blunts under high stress and strain, and the singularity vanishes due to the deformation of crack surface when loading. The stress at crack-tip may still be very high even though the singularity vanishes. The low bound of maximum crack-tip stress is the modulus of elastic in plane stress state, while in plain strain state, it is greater than the modulus of elastic, and will increase with the Poisson’s ratio.

Features Testing of Stud Material Under Low Constraint Ductile Tearing

2017 ◽  
Author(s):  
P. James ◽  
M. Jackson ◽  
P. Birkett ◽  
C. Madew
Keyword(s):  
Structural Integrity ◽  
High Stress ◽  
Crack Tip ◽  
Pressure Vessels ◽  
Materials Testing ◽  
Screw Thread ◽  
Surface Point ◽  
Defect Tolerance ◽  
Material Response ◽  
Crack Tip Constraint

Defect tolerance assessments are carried out to support the demonstration of structural integrity for high integrity components such as nuclear reactor pressure vessels. These assessments often consider surface-breaking defects and assess Stress Intensity Factors (SIFs) at both the surface and deepest points. This can be problematic when there is a high stress at the surface, for example due to the stress concentration at the root of a screw thread. In the past this has led to the development of complex and costly 3D finite element analyses to calculate more accurate SIFs, and still resulting in small apparent limiting defect sizes based on initiation at the surface point. Analysis has been carried out along with supporting materials testing, to demonstrate that the increased SIF at the surface point is offset by a reduction in crack-tip constraint, such that the material exhibits a higher apparent fracture toughness. This enables a more simplistic assessment which reduces the effective SIF at the surface such that only the SIF at the deepest point needs to be considered. This then leads to larger calculated limiting defect sizes. This in turn leads to a more robust demonstration of structural integrity, as the limiting defect sizes are consistent with the capability of non-destructive examination techniques. The high SIF at the surface location, and the concomitant reduction in crack-tip constraint, meant that it was not possible to demonstrate the material response with conventional tests, such as those using shallow-notched bend specimens. Instead it was necessary to develop modified specimens in which semielliptical defects were introduced into a geometry which replicated the notch acuity at the root of a screw thread. These feature tests were used to demonstrate the principle, prior to testing with more conventional specimens to fit more accurately the parameters required to represent the material response in a defect tolerance assessment. Margins in defect tolerance assessments are usually measured against the initiation of tearing, even though the final failure for the material may occur at a higher load following stable crack extension. This work measured and assessed the benefit of reduced crack-tip constraint on both the point of initiation and on the development of the tearing resistance curve. This demonstrated that the effect of constraint was valid with tearing for this material and that there was additional margin available beyond the onset of tearing. The feature test geometry also provided evidence of the tearing behaviour at the surface and deepest points of a surrogate component under representative loading. This paper provides an overview of the range of tests performed and the post-test interpretation performed in order to provide the R6 α and k constraint parameters.

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