Electron Microfractography Applied to Large Pipe Bending Fatigue Tests

1968 ◽  
Vol 26 ◽  
pp. 424-425
Author(s):  
Ursula E. Wolff
Keyword(s):  
Crack Propagation ◽  
Wall Thickness ◽  
Nuclear Reactor ◽  
Fatigue Cracking ◽  
Crack Propagation Rate ◽  
Propagation Rate ◽  
Practical Aspect ◽  
Fatigue Tests ◽  
Study Program ◽  
Bending Fatigue

Among the different causes of pipe failures, fatigue cracking has to be considered in any design program. Since the integrity of nuclear reactor piping is of particular importance, an AEC pipe study program includes a task to assess quantitatively the crack propagation rate through pipes for cracks originating at surface flaws. This problem has been attacked theoretically by the use of fracture mechanics, and practically by striation measurements on fracture surfaces produced by full-scale bending fatigue tests. This paper deals with the practical aspect of applying electron microfractography to such fractures as a means to determine crack propagation rates in pipes of different dimensions and materials, and with different flaw locations.Six-inch diameter pipes with a wall thickness of 7.5, 10.8, or 18 mm were notched inside or outside and failed in reverse bending fatigue by growing a crack through the wall thickness. The crack front was marked at intervals by reducing the strain amplitude for a number of cycles.

Corrosion Fatigue Mechanisms and Fracture Mechanics Based Modelling for Subsea Pipeline Steels

2017 ◽  
Author(s):  
Ankang Cheng ◽  
Nian-Zhong Chen
Keyword(s):  
Fracture Mechanics ◽  
Crack Propagation ◽  
Corrosion Fatigue ◽  
Fatigue Cracking ◽  
Crack Propagation Rate ◽  
Propagation Rate ◽  
Propagation Time ◽  
Loading Frequency ◽  
Pipeline Steels ◽  
Driving Mechanisms

Subsea structures such as pipelines are vulnerable to environment-assisted crackings (EACs). As a type of EAC, corrosion fatigue (CF) is almost inevitable. For such a process, stress corrosion (SC) and hydrogen-assisted cracking (HAC) are the two mainly driving mechanisms. And it was further pointed out that slip dissolution (SD) and hydrogen embrittlement (HE) should be responsible for SC and HAC respectively. Based on such a fact, a two-component physical model for estimating the CF crack propagation rate was proposed. The proposed model was built in a frame of fracture mechanics integrated with a dissolution model for C-Mn steel and a newly established model by the authors accounting for the influence from HE upon crack propagation. The overall CF crack propagation rate is the aggregate of the two rates predicted by the two sub-individual models, and then the crack propagation time is calculated accordingly. The model has been proven to be capable of capturing the features of HE influenced fatigue cracking behaviour as well as taking mechanical factors such as the loading frequency and stress ratio into account by comparison with the experimental data of X42 and X65 pipeline steels.

scholarly journals Fatigue of thin periodic triangular lattice plates

2019 ◽  
Vol 300 ◽  
pp. 03002 ◽  
Author(s):  
Yifan Li ◽  
Anton Shterenlikht ◽  
Martyn Pavier ◽  
Harry Coules
Keyword(s):  
Fatigue Crack ◽  
Crack Propagation ◽  
Triangular Lattice ◽  
Crack Propagation Rate ◽  
Propagation Rate ◽  
Fatigue Tests ◽  
Fatigue Properties ◽  
Fatigue Crack Propagation Rate ◽  
Stress And Strain ◽  
Effective Stress Intensity Factor

A method for predicting the fatigue life of triangular lattices is proposed in this paper by considering fatigue properties of single lattice struts. Fatigue tests of different sizes of lattice plates of aluminium alloy, and tests of single struts with different maximum fluctuating loads, have been conducted to validate this method. It is found that the struts in a triangular lattice break near to strut intersections, where stress and strain concentrations occur. Similar crack propagation paths were observed in different lattice plate specimens: the cracks grew at a 30° angle to the initial edge crack in the upper half of lattice plate. The mixed-mode fatigue crack propagation rate was also studied and expressed using an effective stress intensity factor. A size effect on the crack growth rate of triangular lattice plates was also observed: a fatigue crack will propagate slightly quicker in larger triangular plates than in smaller ones.

Effect of Periodic Overloads on Fatigue Crack Propagation in Aluminium Alloys

2006 ◽  
Vol 519-521 ◽  
pp. 1065-1070
Author(s):  
J. Heidemann ◽  
J. Albrecht ◽  
G. Lütjering
Keyword(s):  
Fatigue Crack ◽  
Crack Propagation ◽  
Fatigue Crack Propagation ◽  
Crack Closure ◽  
Crack Propagation Rate ◽  
Propagation Rate ◽  
Fatigue Tests ◽  
Propagation Mechanism ◽  
Constant Amplitude ◽  
R Ratio

The influence of variable amplitude loading on fatigue crack propagation was investigated for two high purity versions of the alloy Al 2024 in sheet form, one with fine equiaxed grains, and the other with coarse elongated grains. Fatigue tests on center cracked specimens were conducted in vacuum at constant amplitude (R-ratio of 0.1) and with periodically applied single tensile overloads with an overload ratio of 1.5. The number of intermittent baseline cycles between consecutive overloads was varied (n=100 and n=10.000). Detailed fractographic investigations were carried out for the identification of changes in the fracture surfaces due to the overloads. Crack closure measurements were performed in all cases. The results revealed a strong influence of the overloads on the crack propagation rate. Whether overloads are retarding or accelerating the fatigue crack propagation depends on the crack propagation mechanism at constant amplitude loading and the number of intermittent baseline cycles. For n=100 retardation occurred for the fine grained alloy exhibiting homogeneous slip at constant amplitude while acceleration was observed for the alloy with coarse elongated grains showing pronounced slip band fracture at constant amplitude. For n=10.000, the formation of steps parallel to the direction of crack propagation by overloads is assumed to be the reason for the observed increase in fatigue crack propagation resistance resulting in retardation for both alloys compared to constant amplitude and n=100. The influence of crack closure on the overload effects was minor. This was verified by additional tests at R=0.5.

scholarly journals Mode I Crack Propagation Experimental Analysis of Adhesive Bonded Joints Comprising Glass Fibre Composite Material under Impact and Constant Amplitude Fatigue Loading

Materials ◽  
2021 ◽  
Vol 14 (16) ◽  
pp. 4380
Author(s):  
Alirio Andres Bautista Villamil ◽  
Juan Pablo Casas Rodriguez ◽  
Alicia Porras Holguin ◽  
Maribel Silva Barrera
Keyword(s):  
Crack Propagation ◽  
Structural Integrity ◽  
Crack Propagation Rate ◽  
Fatigue Loading ◽  
Propagation Rate ◽  
Operating Conditions ◽  
Impact Testing ◽  
Mode I ◽  
Mode I Crack ◽  
Constant Amplitude

The T-90 Calima is a low-wing monoplane aircraft. Its structure is mainly composed of different components of composite materials, which are mainly bonded by using adhesive joints of different thicknesses. The T-90 Calima is a trainer aircraft; thus, adverse operating conditions such as hard landings, which cause impact loads, may affect the structural integrity of aircrafts. As a result, in this study, the mode I crack propagation rate of a typical adhesive joint of the aircraft is estimated under impact and constant amplitude fatigue loading. To this end, effects of adhesive thickness on the mechanical performance of the joint under quasistatic loading conditions, impact and constant amplitude fatigue in double cantilever beam (DCB) specimens are experimentally investigated. Cyclic impact is induced using a drop-weight impact testing machine to obtain the crack propagation rate (da/dN) as a function of the maximum strain energy release rate (GImax) diagram; likewise, this diagram is also obtained under constant amplitude fatigue, and both diagrams are compared to determine the effect of each type of loading on the structural integrity of the joint. Results reveal that the crack propagation rate under impact fatigue is three orders of magnitude greater than that under constant amplitude fatigue.

scholarly journals Crack Propagation Behavior of a Ni-Based Single-Crystal Superalloy during In Situ SEM Tensile Test at 1000 °C

Crystals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 1047
Author(s):  
Wenxiang Jiang ◽  
Xiaoyi Ren ◽  
Jinghao Zhao ◽  
Jianli Zhou ◽  
Jinyao Ma ◽  
...  
Keyword(s):  
Crack Propagation ◽  
Tensile Test ◽  
Single Crystal ◽  
Crack Propagation Rate ◽  
Propagation Rate ◽  
Single Crystal Superalloy ◽  
Propagation Path ◽  
Propagation Behavior ◽  
Crack Propagation Behavior

An in situ scanning electron microscope (SEM) tensile test for Ni-based single-crystal superalloy was carried out at 1000 °C. The stress displacement was obtained, and the yield strength and tensile strength of the superalloy were 699 MPa and 826 MPa, respectively. The crack propagation process, consisting of Model I crack and crystallographic shearing crack, was determined. More interestingly, the crack propagation path and rate affected by eutectics was directly observed and counted. Results show that the coalescence of the primary crack and second microcrack at the interface of a γ/γ′ matrix and eutectics would make the crack propagation rate increase from 0.3 μm/s to 0.4 μm/s. On the other hand, crack deflection decreased the rate to 0.05 μm/s. Moreover, movement of dislocations in front of the crack was also analyzed to explain the different crack propagation behavior in the superalloy.

Test Stand for Analysis of Fatigue Crack Propagation under Bending with Torsion

2008 ◽  
Vol 144 ◽  
pp. 90-93 ◽  
Author(s):  
Grzegorz Gasiak ◽  
Grzegorz Robak
Keyword(s):  
Fatigue Crack ◽  
Crack Propagation ◽  
Fatigue Crack Propagation ◽  
Digital Camera ◽  
Crack Propagation Rate ◽  
Propagation Rate ◽  
Test Stand ◽  
Crack Development ◽  
Fatigue Crack Propagation Rate ◽  
Fatigue Crack Development

The paper presents a test stand for measurements of fatigue crack propagation. The stand includes a fatigue machine MZGS–100 and a device for registration of the crack length. The test stand is equipped with a stereoscopic microscope with fluent magnification of 7× – 67.5×. The microscope can be applied for observation of fatigue crack development. The microscope is also equipped with a digital camera, which enables continuous observation of fatigue crack development on the computer monitor and it is not necessary to stop the machine. The test results obtained at this stand can be used for determination of fatigue life and fatigue crack propagation rate.

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