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.

Simulating Residual Stresses Using a Modified Wedge-Loaded Compact Tension Specimen

2013 ◽  
Author(s):  
P. Kapadia ◽  
H. Zhou ◽  
C. M. Davies ◽  
R. C. Wimpory ◽  
K. M. Nikbin
Keyword(s):  
Stainless Steel ◽  
Residual Stresses ◽  
Crack Growth ◽  
Crack Tip ◽  
Compact Tension ◽  
Image Correlation ◽  
Internal Stresses ◽  
Fe Model ◽  
Creep Ductility ◽  
Crack Tip Plasticity

Residual stresses are induced in components when fabrication processes produce internal stresses or local deformation and cause accelerated creep damage and cracking during service at elevated temperatures. A method of inducing residual stresses in laboratory fracture specimens is proposed where an oversized wedge is inserted into the crack mouth of a compact tension, C(T), type specimen. In this way the extent of internal stresses can be controlled in order to minimise the level of crack tip plasticity which inherently reduces the remaining strain to failure. Numerical simulations of wedge insertion into specimens made of 316H austenitic stainless steel have been developed to calibrate the wedge insertion process. These models have been experimentally validated using surface strains measured during the wedge insertion, using Digital Image Correlation (DIC), and Neutron Diffraction (ND) measurements. The validated Finite Element (FE) model is used to determine the wedge insertion depth required to maximise the residual stresses without causing significant crack tip plasticity. The validated numerical simulation is used to determine the wedge insertion depths of further wedge-loaded C(T) specimens made from uniformly pre-compressed 316H stainless steel. The reduced creep ductility of this material increases the rate of crack growth under creep conditions. This method of inducing residual stresses with limited crack tip plasticity allows creep crack growth under simulated secondary loading conditions to be investigated without the influence of non-uniform creep ductility caused by work hardening.

Life Estimation of Cracked Stainless Steel Components Under Creep Conditions

1991 ◽  
Vol 113 (3) ◽  
pp. 303-306 ◽  
Author(s):  
V. M. Radhakrishnan ◽  
M. Kamaraj ◽  
V. V. Balasubramaniam
Keyword(s):  
Stainless Steel ◽  
Crack Growth ◽  
Nuclear Power ◽  
State Energy ◽  
Creep Crack Growth ◽  
Experimental Investigations ◽  
Plane Stress Condition ◽  
Line Integral ◽  
Energy Rate ◽  
Creep Crack

Experimental investigations have been carried out to study the creep crack growth in types 316, 308 Cb, and 304 L stainless steel in the temperature range of 873–1073 K under plane stress condition. Testings have been carried out with both the base metal and the welded composite joints, because such joints are commonly used in nuclear power industries. Among the various parameters tried to correlate the creep crack growth, the energy rate line integral has been found to give the best description of the crack growth rate. The steady-state energy rate line integral has been found to correlate well with the rupture time. Based on this observation, life estimations are presented for thin components containing various initial defect sizes.

Compressive Pre-Strain Effects on the Creep and Crack Growth Behaviour of 316H Stainless Steel

2010 ◽  
Author(s):  
C. M. Davies ◽  
David W. Dean ◽  
A. N. Mehmanparast ◽  
K. M. Nikbin
Keyword(s):  
Stainless Steel ◽  
Crack Growth ◽  
Creep Crack Growth ◽  
Compact Tension ◽  
Growth Behaviour ◽  
Creep Ductility ◽  
Data Set ◽  
Crack Growth Behaviour ◽  
Constraint Effects

The effects of compressive plastic pre-strain on the creep deformation and crack growth behaviour of Type 316H stainless steel have been examined. Creep crack growth (CCG) tests have been performed on compact tension specimens of material which had been uniformly pre-strained by 4% and 8% in compression at room temperature. The CCG behaviour of the pre-compressed material has been interpreted in terms of the creep fracture mechanics parameter C* and compared with that of a significant data set of as-received (un-compressed) specimens and with CCG models. All creep testing has been performed at a temperature of 550 °C. High CCG rates, for a given value of C* have been observed for the pre-compressed material, compared with those of as-received material and these data follow the same trends as the long-term CCG data for as-received material. These observations are explained in terms of specimen constraint effects and variations in creep ductility.

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.

Phased Array Ultrasonic Measurement of Fatigue Crack Growth Profiles in Stainless Steel Pipes

2006 ◽  
Vol 129 (4) ◽  
pp. 737-743 ◽  
Author(s):  
L. Satyarnarayan ◽  
D. M. Pukazhendhi ◽  
Krishnan Balasubramaniam ◽  
C. V. Krishnamurthy ◽  
D. S. Ramachandra Murthy
Keyword(s):  
Stainless Steel ◽  
Fatigue Crack ◽  
Crack Growth ◽  
Phased Array ◽  
Potential Drop ◽  
Outer Diameter ◽  
Steel Pipes ◽  
Bending Load ◽  
Load Arrangement ◽  
Beach Marks

This paper reports experimental sizing of fatigue crack profiles that are initiated from artificially made circumferential starter notches in stainless steel pipes of 169mm outer diameter and 14.33mm thickness, which were subjected to cyclic bending loads in a four point bending load arrangement using two nondestractive evaluation (NDE) methods: (a) phased array ultrasonic technique and (b) alternating current potential drop technique. The crack growth estimated using the two NDE techniques were compared with the beach marks that were present in the fracture surface. A simulation study using the ray tracing method was carried out to model the ultrasonic wave propagation in the test specimen, and the results were compared with the experimental results.

Numerical Simulation of Creep Crack Propagation for Austenitic Stainless Steel 0Cr18Ni9 at 550°C

2011 ◽  
Vol 230-232 ◽  
pp. 596-599
Author(s):  
Li Jie Chen ◽  
Zun Qun Gong ◽  
Qi Zhao
Keyword(s):  
Stainless Steel ◽  
Austenitic Stainless Steel ◽  
Crack Propagation ◽  
Crack Growth ◽  
Elevated Temperatures ◽  
Creep Crack Growth ◽  
Tensile Creep ◽  
Computational Error ◽  
Propagation Length ◽  
Creep Crack

First, tensile creep curve and creep propagation tests are conducted for austenitic stainless steel 0Cr18Ni9, i.e. 304 stainless steel at 550°C. The corresponding time hardening creep law is given for stresses ranging from 240 to 320 Mpa and the creep crack propagation length under a tension load of 10kN is measured by using QUESTAR long focus microscope system. Second, with the commercial finite element (FE) code ANSYS, the critical crack tip opening displacement (CTOD) is considered as crack propagation criterion to simulate the creep crack growth in the standard compact tension (CT) specimen. The FE predictions of the creep crack length in the primary and secondary stages are found to agree reasonably with the experimental results. The maximum computational error between the predictions and the experiment results is within 10%. Hence, the critical CTOD is a feasible criterion for crack growth simulations at elevated temperatures.

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