An Engineering Approach to the Prediction of Creep Crack Growth

1986 ◽  
Vol 108 (2) ◽  
pp. 186-191 ◽  
Author(s):  
K. M. Nikbin ◽  
D. J. Smith ◽  
G. A. Webster
Keyword(s):  
Crack Growth ◽  
Plane Strain ◽  
Plane Stress ◽  
Elevated Temperatures ◽  
Process Zone ◽  
Creep Crack Growth ◽  
Crack Tip ◽  
Creep Damage ◽  
Creep Ductility ◽  
Creep Crack

This paper is concerned with assessing the integrity of cracked engineering components which operate at elevated temperatures. Fracture mechanics parameters are discussed for describing creep crack growth. A model is presented for expressing growth rate in terms of creep damage accumulation in a process zone ahead of the crack tip. Correlations are made with a broad range of materials exhibiting a wide spread of creep ductilities. It is found that individual propagation rates can be predicted with reasonable accuracy from a knowledge only of the material uni-axial creep ductility. An engineering creep crack growth assessment diagram is proposed which is independent of material properties but which is sensitive to the state of stress at the crack tip. Approximate bounds are presented for plane stress and plane strain situations and it is shown that crack growth rates about fifty times faster are expected under plane strain conditions than when plane stress prevails.

Computational Modelling of High Temperature Steady State Crack Growth Using a Damage-Based Approach

2003 ◽  
Author(s):  
Masataka Yatomi ◽  
Noel P. O’Dowd ◽  
Kamran M. Nikbin
Keyword(s):  
Crack Growth ◽  
Plane Strain ◽  
Plane Stress ◽  
Computational Study ◽  
Creep Crack Growth ◽  
Computational Modelling ◽  
Crack Tip ◽  
Creep Damage ◽  
Creep Model ◽  
Manganese Steel

In this work a computational study of creep crack growth in a carbon manganese steel is presented. The constitutive behaviour of the steel is described by a power law creep model and the accumulation of creep damage is accounted for through the use of a well-established model for void growth in creeping materials. Two dimensional finite element analyses have been performed for a compact tension specimen and it has been found that the predicted crack growth rate under plane strain conditions approaches that under plane stress conditions at high C* levels. Furthermore it has been shown, both experimentally and numerically, that an increase in test temperature causes the convergence of the cracking rate to occur at higher values of C*. This trend may be explained by the influence of crack-tip plasticity, which reduces the relative difference in constraint between plane stress and plane strain conditions. The constraint effect has been quantified through the use of a two-parameter characterisation of the crack tip fields under creep conditions.

Prediction of creep crack growth from uniaxial creep data

1984 ◽  
Vol 396 (1810) ◽  
pp. 183-197 ◽  
Keyword(s):  
Crack Growth ◽  
Growth Rates ◽  
Process Zone ◽  
Creep Crack Growth ◽  
Creep Process ◽  
Uniaxial Tensile ◽  
Creep Data ◽  
Creep Ductility ◽  
Creep Crack ◽  
Crack Growth Rates

In this paper uniaxial tensile creep data are used in conjunction with fracture mechanics concepts to predict creep crack growth rates in materials having a wide range of creep ductilities. A model is proposed of creep damage accumulation in a process zone ahead of the crack tip. The model allows all stages of creep to be incorporated in an approximate manner and creep ductility to be stress and stress-state sensitive. Good agreement is obtained with experimental crack growth data on a range of low alloy steels, a stainless steel, an aluminium alloy and a nickel-base superalloy. It is found that cracking rate is insensitive to the creep process zone size but inversely proportional to creep ductility. Crack growth rates under plane strain conditions are shown to be about fifty times those for plane stress loading.

Finite Element Damage Analyses for Predictions of Creep Crack Growth

2010 ◽  
Author(s):  
Chang-Sik Oh ◽  
Nak-Hyun Kim ◽  
Sung-Hwan Min ◽  
Yun-Jae Kim
Keyword(s):  
Experimental Data ◽  
Finite Element ◽  
Crack Growth ◽  
Damage Mechanics ◽  
Creep Crack Growth ◽  
Creep Damage ◽  
Simulation Method ◽  
Creep Tests ◽  
Creep Ductility ◽  
Creep Crack

This paper provides the virtual simulation method for creep crack growth test, based on finite element (FE) analyses with damage mechanics. Creep tests of smooth bars are used to quantify the constants of creep constitutive equation. The reduction of area resulting from creep tests of smooth and notched bar is adopted as a measure of creep ductility under multiaxial stress conditions. The creep ductility exhaustion concept is adopted for calculating creep damage, which is defined as the ratio of creep strain to the multiaxial creep ductility. To simulate crack propagation, fully damaged elements are forced to have nearly zero stresses using user-defined subroutine UHARD in the general-purpose FE code, ABAQUS. The results from 2D or 3D FE analyses are compared with experimental data of creep crack growth. It is shown that the predictions obtained from this new method are in good agreement with experimental data.

Quantification and Prediction of Residual Stresses in Creep Crack Growth Specimens

2014 ◽  
Vol 777 ◽  
pp. 25-30 ◽  
Author(s):  
Ali Mehmanparast ◽  
Catrin M. Davies ◽  
Kamran Nikbin
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Creep Crack Growth ◽  
Crack Tip ◽  
Creep Damage ◽  
Compact Tension ◽  
Life Assessment ◽  
Creep Crack ◽  
Fatigue Crack Growth Test

An important issue to be considered in the life assessment of power plant components is the effects of prior creep damage on subsequent fatigue crack growth and fracture behavior. To examine these effects, creep damage has been introduced into 316H stainless steel material by interrupting creep crack growth (CCG) tests on compact tension, C(T), specimens at 550 °C. During the CCG tests, the specimen is loaded in tension, crept and unloaded after a small amount of crack extension. This process introduces compressive residual stress fields at the crack tip, which may subsequently affect the fatigue crack growth test results. In this work, neutron diffraction (ND) measurements have been conducted on interrupted CCG test specimens, which contain creep damage local to the crack tip, and the results are compared to predictions obtained from finite element (FE) simulations. Reasonable agreement has been found between the FE predictions and ND measurements.

Modelling Crack Growth in the Life Assessment of Elevated Temperature Components

2007 ◽  
Vol 348-349 ◽  
pp. 709-712
Author(s):  
Kamran M. Nikbin
Keyword(s):  
Fracture Mechanics ◽  
Crack Growth ◽  
Plane Strain ◽  
Elevated Temperatures ◽  
Creep Crack Growth ◽  
Crack Propagation Rate ◽  
Life Assessment ◽  
Manganese Steel ◽  
Growth Data ◽  
Creep Crack

Modelling of Creep Crack Growth (CCG) using analytical and numerical methods is relevant to life assessment procedures of components operating at elevated temperatures. This paper compares an analytical crack prediction and a numerical based virtual CCG technique used in fracture mechanics components with sample experimental results. Two approaches are presented. First the well developed strain exhaustion model called the NSW and the modified NSW-MOD models which predict plane stress/strain bound crack initiation and growth rates for engineering alloys and the second a damage-based approach used to numerically predict the crack propagation rate in Finite Element models of fracture mechanics specimens. The results from both methods are correlated against an independently determined C* parameter. As an example the NSW and the extended NSW-MOD strain exhaustion models are applied to compare to the experimental data and FE predictions for two steels at Carbon-Manganese steel tested at 360 oC and a weld 316H stainless steel at 550 oC. For values of C* within the limits of the present creep crack growth data presented the plane strain crack growth rate predicted from the numerical analysis is found to be less conservative than the plane strain NSW model but more conservative than plane strain NSW-MOD model.

Numerical Prediction of Constraint Effect of Creep Crack Growth

2009 ◽  
Author(s):  
Masataka Yatomi ◽  
Kamran M. Nikbin
Keyword(s):  
Crack Growth ◽  
Plane Strain ◽  
Plane Stress ◽  
Creep Crack Growth ◽  
Constraint Effect ◽  
Three Point Bending ◽  
Creep Crack ◽  
Reference Stress ◽  
Edge Notch ◽  
Stress And Strain Rate

This paper presents the effect of constraint on creep crack growth (CCG) using FE analysis based on the stress and strain rate state at the crack tip. The comparison is made by modelling C(T) specimen tests under plane stress and plane strain conditions using creep properties of three different steels (C-Mn steel, P91 steel, and 316H austenitic steel). In addition, in order to examine the constraint effect on CCG due to geometry single edge notch specimen (SENT), centre cracked tension specimen (CCT) and three point bending (3PB) specimen have also been analysed. In all cases it is found that when the reference stress under plane strain conditions is higher than the yield stress, there is little difference between CCG rates under plane stress and plane strain.

Prediction of Creep Crack Growth for Modified 9Cr-1Mo at 600°C

2012 ◽  
Author(s):  
Nak Hyun Kim ◽  
Yun Jae Kim ◽  
Woo Gon Kim ◽  
Hyeong Yeon Lee
Keyword(s):  
Prediction Model ◽  
Crack Growth ◽  
Damage Model ◽  
Creep Crack Growth ◽  
Creep Damage ◽  
Compact Tension ◽  
Compact Tension Specimen ◽  
Element Analysis ◽  
Creep Ductility ◽  
Creep Crack

This paper introduce theoretical creep crack growth prediction model and provides experimental validation of the approach for simulating creep crack growth using finite element analysis method, recently proposed by the authors. The FE creep damage model is based on the creep ductility exhaustion concept, and incremental damage is defined by the ratio of incremental creep strain and multi-axial creep ductility. A simple linear damage summation rule is applied. When accumulated damage becomes unity, element stresses are reduced to zero to simulate progressive crack growth. For validation, simulated results are compared with experimental data for a compact tension specimen of modified 9Cr-1Mo at 600°C under various loading levels. The simulated results agree well with experimental C*-da/dt data. The test data are also compared with theoretical CCG prediction model.

Creep Crack Growth Predictions in 316H Steel Over a Wide Range of Stresses and Temperatures

2014 ◽  
Author(s):  
A. Mehmanparast ◽  
C. M. Davies ◽  
K. M. Nikbin ◽  
G. A. Webster
Keyword(s):  
Crack Growth ◽  
Elevated Temperatures ◽  
Prediction Models ◽  
Creep Crack Growth ◽  
Life Assessment ◽  
Creep Ductility ◽  
Crack Growth Behaviour ◽  
Creep Crack ◽  
Uniaxial Creep ◽  
Wide Range

The prediction of the creep crack growth (CCG) behaviour in engineering materials is of great importance in the life assessment of power plant components. The conventional technique to predict CCG is to employ uniaxial creep properties and appropriate damage models in finite element (FE) simulations or analytical CCG prediction models. Uniaxial creep trends for Type 316H SS have been recently estimated for a wide range of stresses and temperatures in [1] and FE CCG predictions have been made at 550 °C and validated through comparison with the experimental data. In this paper, FE CCG predictions using the developed uniaxial creep trends for a wide range of stresses and temperatures are presented and the results are compared with the predicted CCG trend at 550 °C and also with the analytical constant creep ductility NSW CCG prediction models. The results from FE predictions are discussed in terms of the temperature effects on the creep deformation and crack growth behaviour of components operating at elevated temperatures.

A Microstructural Study of Compressive Plastic Pre-Strain Effects on Creep Damage Behaviour of Type 316H Stainless Steel

2011 ◽  
Author(s):  
Ali N. Mehmanparast ◽  
Catrin M. Davies ◽  
Mahmoud Ardakani ◽  
Kamran M. Nikbin
Keyword(s):  
Stainless Steel ◽  
Crack Growth ◽  
Mechanical Behaviour ◽  
Creep Crack Growth ◽  
Creep Damage ◽  
Tensile Creep ◽  
Compression Process ◽  
Creep Ductility ◽  
Crack Growth Behaviour ◽  
Creep Crack

Compressive plastic pre-strain induced at room temperature in type 316H stainless steel, significantly influences the tensile, creep deformation and crack growth behaviour of the material. It is known that the material is hardened after pre-strain to 8% plastic strain and thus exhibits little or no plasticity during loading of uniaxial or creep crack growth (CCG) tests. In addition pre-compression (PC) has been found to reduce the creep rupture time, creep ductility and accelerate creep crack growth rates compared to as-received (AR) (i.e. uncompressed) material. In order to understand pre-straining effects on mechanical behaviour of 316H, optical and scanning electron microscopy (SEM) studies have been performed on uncompressed and 8% pre-compressed material. Samples have been examined in three orientations (i.e. parallel and perpendicular to the pre-compression direction). Furthermore, the influence of cold pre-compression on local creep damage formation ahead of the crack tip on interrupted CCG tests on AR and PC material has been studied. The results are discussed in terms of intergranular and transgranular damage caused by the compression process and the importance of microstructural changes on the mechanical behaviour of the material in long term tests.

Estimation of Creep Crack Growth Properties Using Circumferential Notched Round Bar Specimen for 12CrWCoB Rotor Steel

2005 ◽  
Vol 297-300 ◽  
pp. 397-402
Author(s):  
Je Chang Ha ◽  
Joon Hyun Lee ◽  
Masaaki Tabuchi ◽  
A.Toshimitsu Yokobori Jr.
Keyword(s):  
Crack Growth ◽  
Axial Stress ◽  
Creep Crack Growth ◽  
Turbine Rotor ◽  
Rotor Steel ◽  
Creep Ductility ◽  
Crack Growth Behaviour ◽  
Creep Crack ◽  
Round Bar ◽  
Growth Properties

Most heat resisting materials in structural components are used under multi-axial stress conditions and under such conditions ductile materials often exhibit brittle manner and low creep ductility at elevated temperature. Creep crack initiation and growth properties are also affected by multi-axial stress and it is important to evaluate these effects when laboratory data are applied to structural components. Creep crack growth tests using circumferential notched round bar specimens are a simple method to investigate multi-axial stress effects without using complicated test facilities. Creep crack growth tests have been performed using a 12CrWCoB turbine rotor steel. In order to investigate the effects of multi-axial stress on creep crack growth properties, the tests were conducted for various notch depths at 650°C. The circumferential notched round bar specimen showed brittle crack growth behaviour under multi-axial stress conditions. Creep crack growth rate was characterized in terms of the C* parameter. A 12CrWCoB turbine rotor steel has been tested using circumferential notched round bar specimens with different multi-axiality. Circumferential notched round bar specimens show increased brittle creep crack growth behaviour due to the multi-axial stress condition. Creep crack growth properties could be predicted by allowing for the decrease of creep ductility under multi-axial conditions.

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