Measurement of Crack Growth Rates By use of the SEM

1980 ◽  
Vol 38 ◽  
pp. 138-139
Author(s):  
R. Brown
Keyword(s):  
Crack Growth ◽  
Surface Crack ◽  
Fatigue Testing ◽  
Electrical Potential ◽  
Potential Drop ◽  
In Situ Observation ◽  
Growth Data ◽  
Short Cracks ◽  
Optical Examination ◽  
Number Of Cycles

Often, measurement of the growth of short cracks with the number of cycles is required during fatigue testing. Optical examination is limited to useful magnification of ×1000 and the distinction between slip and a short crack is extremely difficult to make. Electrical potential drop measurement is not sufficiently sensitive to determine the start, on a microscopic level, of a short single crack. In situ observation of fatigue in the SEM presents enviromental problems. However plastic replicas taken from a smooth specimen surface during fatigue can be imaged in the SEM to produce extremely accurate short surface crack measurements. The initiation of a crack can be very accurately determined and surface crack lengths as short as 2μm can be measured. The technique allows a comparison between macroscopic crack growth data and data produced from short cracks, 2 to 300μm in length. As example of the process has been carried out for Ti-6A1-4V.

Results From Environmentally-Assisted Short Crack Fatigue Testing on Austenitic Stainless Steels

2020 ◽  
Author(s):  
Ben Coult ◽  
Adam Griffiths ◽  
Jack Beswick ◽  
Peter Gill ◽  
Norman Platts ◽  
...  
Keyword(s):  
Crack Growth ◽  
Fatigue Testing ◽  
Crack Nucleation ◽  
Austenitic Stainless Steels ◽  
Short Crack ◽  
In Situ Monitoring ◽  
Growth Data ◽  
Short Cracks ◽  
Prediction Approach ◽  
Total Life

Abstract The effect of environment on fatigue life is currently assessed using methods (such as NUREG/CR-6909) that may be excessively conservative when applied to plant components and loading transients. To reduce this conservatism, the ASME WG-EFEM has proposed the development of an improved assessment methodology for environmental fatigue based on a Total Life Prediction approach that would be adequately, but not excessively, conservative. Such an approach necessitates the development of analytical methods for the various stages of crack nucleation, short crack growth and long crack growth. Hence, there is a requirement to undertake testing within the short crack growth regime that would bridge the gap between fatigue nucleation and long crack growth (Paris Law) enabling better prediction of total life measured by fatigue endurance. A test methodology has been developed by Wood to enable short crack growth testing with in-situ monitoring using DCPD. Testing has been undertaken in both high temperature air (300°C) and simulated end-of-cycle primary water chemistry at 300°C on cold-worked stainless steel specimens, which were subject to a range of load ratios and rise times. FEA modelling has been undertaken to determine the effective stress intensity factors applied under the loading conditions based on the specific material properties. This paper presents the results from a testing program conducted with EPRI, to measure fatigue crack growth data for short cracks from 0.15 mm to 1.0 mm. Crack growth rates have been compared to those predicted in ASME, Code Case N-809 and results from material specific in-house testing to assist the understanding of the behaviour of mechanically short cracks.

The Influence of Creep Strain on Crack Length Measurements Using the Potential Drop

2015 ◽  
Author(s):  
K. M. Tarnowski ◽  
C. M. Davies ◽  
K. M. Nikbin ◽  
D. W. Dean
Keyword(s):  
Stainless Steel ◽  
Crack Growth ◽  
Material Properties ◽  
Crack Extension ◽  
Electrical Potential ◽  
Creep Crack Growth ◽  
Potential Drop ◽  
Fe Model ◽  
Creep Properties ◽  
Length Measurements

One of the most common methods for estimating crack extension in the laboratory is electrical potential drop (PD). A key limitation of this technique is that it is sensitive to strains at the crack tip as well as crack extension. When producing J-R curves the onset of crack growth may be identified from a point of inflection on a plot of PD vs. CMOD. For creep crack growth (CCG) tests however, the effects of strain are often ignored. This paper investigates whether a similar method may be applied to CCG testing. A single CCG test was performed on type 316H stainless steel and a point of inflection, similar to that observed during J-R curve testing was identified. A finite element (FE) based approach was used to investigate this phenomenon further. A 3D sequentially-coupled structural-electrical FE model was used to reproduce the experimental PD vs. CMOD plot up to the point of inflection. The model was capable of predicting the general relationship between strain and PD. It predicted the magnitude of the change in PD to within 30%. A simplified 2D FE model was then used to perform a parametric study to investigate whether a similar trend may be expected for a range of materials. Power law tensile and creep properties were investigated with stress exponents of 1, 3 and 10. The results confirm that a point of inflection should be observable for the range of material properties considered.

Mechanical Factors Affecting Stress Corrosion Crack Growth Rates in Buried Pipelines

2000 ◽  
Author(s):  
S. B. Lambert ◽  
J. A. Beavers ◽  
B. Delanty ◽  
R. Sutherby ◽  
A. Plumtree
Keyword(s):  
Crack Growth ◽  
Growth Rates ◽  
Electrical Potential ◽  
Potential Drop ◽  
Maximum Load ◽  
Load Test ◽  
Factors Affecting ◽  
Transgranular Crack ◽  
Groundwater Environment ◽  
Crack Growth Rates

Over the past several years, investigations have been carried out into the rate of crack growth in pipeline steels in simulated, near-neutral pH, groundwater environment (NS4 solution). Pre-cracked specimens were subject to constant amplitude loading under various frequencies, maximum loads and R-ratios (minimum/maximum load). Test times varied from about 20 to 400 days. Transgranular crack features, similar to those found in service, have been observed. The extent of crack growth was monitored using either electrical potential drop or detailed metallographic examinations at two laboratories. The resulting crack growth rates from both labs are consistent with a superposition model based on a summation of fatigue (Paris Law) and static (SCC) crack growth rates. Differences between the results at the two laboratories are discussed.

scholarly journals Simulation of fatigue small crack growth in additive manufactured Ti–6Al–4V material

2020 ◽  
Vol 32 (6) ◽  
pp. 1745-1761 ◽  
Author(s):  
A. Gupta ◽  
W. Sun ◽  
C. J. Bennett
Keyword(s):  
Cyclic Loading ◽  
Crack Growth ◽  
Prediction Method ◽  
Small Crack ◽  
Test Results ◽  
Growth Data ◽  
Growth Behaviour ◽  
Crack Growth Behaviour ◽  
Small Crack Growth ◽  
Number Of Cycles

Abstract Additive manufacturing (AM) offers design freedom and ability to fabricate parts of complex shapes which are not often possible with the conventional methods of manufacturing. In an AM part, even with optimum build parameters, a complete elimination of defects is not possible and this makes it hard to fully deploy the AM technology to build load bearing parts operating under cyclic loading conditions. Many of these defects are < 1 mm in size and are categorised as ‘small cracks’. Local interaction of cracks with microstructural features and closure effects at the wake of the crack tip are some of the factors which make the growth behaviour of small and long cracks different. A crack growth life prediction method, which effectively considers the small crack growth behaviour, has been discussed in this paper. This proposed method includes a detailed finite element-based crack growth simulation using the ANSYS SMART fracture technology. The lifing calculations utilise the modified NASGRO equation and small crack growth data which was obtained from the published long crack growth data, corrected for closure effects. The predicted stress versus number of cycles curves were compared against the fatigue test results for the AM specimens in Ti–6Al–4V material. A good correlation between the predictions and test results suggests that the proposed method can be used to assess the small crack growth life of AM parts where the fatigue effects of cyclic loading can be quite significant.

Validation Methodology of Crack Growth Measurement Using Potential Drop Method on SENB Specimens

2013 ◽  
Author(s):  
Marcio Ribeiro Antunes ◽  
Tiago Renck ◽  
Carlos Fortis Kwietniewski ◽  
Bruno Diehl ◽  
Eliakin Abreu ◽  
...  
Keyword(s):  
Crack Growth ◽  
Corrosion Fatigue ◽  
Oil And Gas ◽  
Potential Drop ◽  
High Water ◽  
Propagation Path ◽  
Growth Data ◽  
Corrosive Media ◽  
Drop Method ◽  
Growth Measurement

The Brazilian pre-salt oil and gas discoveries brought technical challenges as impressive as the reserves themselves. Besides the concerns with exploration, the oil contamination with CO2, H2S and chloride enriched seawater combined with critical cyclic loads due to the relative movement of the production vessel and high water depths imposes an environment chemically and mechanically aggressive. Suitable materials to work on such harsh conditions are few and one should consider the use of special materials, such as supermartensitic and superduplex stainless steels. Although the corrosion and mechanical properties of these materials are well established, still additional crack growth data in specific environments should be provide to the subsea equipment designer. Indeed, due to the combination of cyclic loading and corrosive ambient the corrosion fatigue phenomenon is a major concern. In order to evaluate the effect of oil contaminants on the corrosion fatigue resistance of candidate materials, one should provide methods for crack growth measurement other than the use of crack gauges since those can not be used in chemically aggressive solutions. The present work aims to validate the potential drop crack growth measurement method comparing the results obtained by this technique with those produced by crack gauges on SEN(B) (Single Edge Notch Bending) specimens in air. This validation effort is essential because the ASTM E647 standard only consider the use of C(T) (Compact Tension) specimens which actually does not represents the real cracks propagation path in crucial subsea equipment, such as risers, drill pipes et cetera, that is through the wall thickness. The results produced by the two tested methodologies have an excellent agreement which makes reliable the use of the potential drop method as an alternative to monitor and measure crack growth in corrosive media.

Fatigue Crack Growth and Coalescence Algorithm Starting from Multiple Surface Cracks

2014 ◽  
Vol 891-892 ◽  
pp. 1003-1008 ◽  
Author(s):  
John Hock Lye Pang ◽  
You Xiang Chew
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Surface Crack ◽  
Crack Depth ◽  
Surface Cracks ◽  
Growth Data ◽  
Aspect Ratios ◽  
Weld Toe ◽  
Fatigue Crack Propagation Life

Fatigue crack growth and propagation analysis in welded joints have to deal with the complexity of modeling multiple weld toe surface cracks originating from weld toes. Fitness-For-Service (FFS) assessments for weld toe surface cracks employ a fracture mechanics and Paris Law approach to predict the fatigue crack propagation life of a semi-elliptical surface crack (SESC) to failure. A fatigue crack growth algorithm for assessing multiple surface crack growth, coalescence and propagation life was initially validated with previuously report crack growth data for a fillet shoulder specimen. Next a parametric study for single, double, and triple SESCs located along the weld toe line of a fillet weld was investigated with three starting crack depth sizes (0.1mm, 0.5mm, 1.0mm) coupled with three different crack aspect ratios (a/c = 1.0, a/c = 0.5 and 0.25) giving a total of 27 cases studied.

Analysis and application of fatigue crack growth data

1975 ◽  
Vol 10 (4) ◽  
pp. 242-250 ◽  
Author(s):  
L P Pook
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Fatigue Testing ◽  
Fatigue Behaviour ◽  
Metal Fatigue ◽  
Load History ◽  
Growth Data ◽  
Engineering Problems ◽  
Long Time

The phenomenon of metal fatigue has been studied for a long time, but it has only relatively recently been appreciated that most structures, particularly welded joints, contain crack-like flaws, so that virtually the whole fatigue life is occupied by fatigue crack growth. The fracture mechanics concept of stress intensity factor has proved particularly convenient for the analysis of fatigue crack growth data in a form which can be applied directly to engineering problems, and its use has led to a much better understanding of the fatigue behaviour of structures. The effects of interaction between different load levels are not yet fully understood; this is not necessarily a serious drawback as servo-hydraulic fatigue testing equipment permits the application of virtually any load history to structures or fatigue crack growth specimens.

Sign in / Sign up

Export Citation Format

Share Document