D.W. Dean

Failure assessment diagram analysis of creep crack initiation in 316H stainless steel

In this work the time dependent failure assessment diagram (TDFAD) approach is applied to the study of crack initiation in Type 316H stainless steel, a material commonly used in high temperature applications. A TDFAD has been constructed for the steel at a temperature of 550 °C, and was found to be relatively insensitive to time. The TDFAD procedure is then applied to predict initiation times, at increments of creep crack growth Δa=0.2 and 0.5 mm, for tests on compact tension specimens and the results compared to experimentally determined values. It has been found that initiation time predictions are sensitive to the creep toughness values, and to the limit load (or reference stress) solution used. Conservative predictions of initiation times have been achieved through the use of the lower bound creep toughness values in conjunction with the plane strain limit load solution. The plane stress limit load solution has given conservative predictions for all bounds of creep toughness used.

Effect of thermal ageing on creep and oxidation behaviour of Type 316H stainless steel

The UK has unique experience in operating high temperature civil nuclear power systems, known as advanced gas cooled reactors (AGRs). One of the primary challenges for extending the lifetime of the AGR power stations is to understand the interaction that occurs between the AGR CO2 environment and creep-fatigue cracking behaviour. This is one of the life limiting degradation mechanisms for steel components within the reactor pressure vessel. This paper addresses the effect of thermal aging on material internal state that controls both the creep deformation and oxidation behaviour of Type 316H stainless steels when they are exposed at a simulated AGR environment. Experimental results from creep tests are discussed with respect to a multi-scale self-consistent model, while experimental results from oxidation tests are considered with respect to the application of measured short term data to predict the long term oxidation behaviour. Finally, the interaction between oxidation and creep and its impact on high temperature structural integrity of AGR nuclear systems are discussed.

The Influence of Plasticity on Crack Length Measurements Using the Potential Drop Technique

The potential drop (PD) technique is one of the most common methods for determining crack growth however, other factors can also change the resistance of the specimen, which may erroneously be interpreted as crack extension. In tough, ductile materials, plastic strain can cause a significant change in PD. This paper presents an experimental investigation which quantifies the apparent crack extension due to strain, prior to the onset of physical crack growth and considers ways to mitigate it. Compact Tension, C(T), and Single Edge Notch Tension, SEN(T), specimen geometries are considered with a range of crack lengths. The influence of probe location is also considered. The results identify apparent crack extensions of up to 1.0 mm in the absence of any physical crack extension. This can be reduced through careful selection of probe locations. Appropriate locations are suggested for the geometries considered. It is also shown that high constraint geometry can significantly reduce the influence of plasticity on PD.

The Influence of Inelastic Damage on Tensile Deformation and Creep Crack Growth Behaviour of Type 316H Stainless Steel

Inelastic deformation (combined plastic and creep strain) is known to have significant effects on the tensile and creep deformation behaviour of Type 316H stainless steel. In this work the influence of inelastic strain on the room temperature tensile behaviour of 316H has been examined. Plastic strain was introduced into the material by uniform pre-compression (PC) to 8% plastic strain at room temperature. In addition, creep strain was subsequently introduced into samples by performing uniaxial creep tests on the pre-compressed material. These creep tests were interrupted at various stages of life so that the influence of inelastic damage on the tensile response of the material could be examined. In addition, creep crack growth (CCG) tests have been performed on compact tension C(T) specimens made of 8% pre-compressed material at 550 °C and the results are compared to the existing data on PC material. The results from these tests have been discussed in terms of specimen constraint, initial crack length and loading effects on the CCG behaviour of the PC material.Copyright

Derivation of shakedown factors (Ks) for Type 316 and Type 321 stainless steel at high temperature

Abstract The shakedown factor, Ks, is defined in R5 Volume 2/3 as the factor applied to the minimum 0.2% proof stress, Sy, in order to obtain the material ratchet limit, in terms of the maximum stress level at which the materials exhibits cyclically stable behaviour. This experimentally derived factor is used in the R5 assessment procedure to perform simple checks for shakedown and also when a more detailed shakedown analysis is required. In addition, it is also used to determine the start-of-dwell stress as part of a creep-fatigue damage assessment. This paper presents the derivation of this shakedown factor, Ks, from a number of cyclic load-controlled tests, which were carried out on several casts of Types 316H and 321 steel between room temperature and 650°C. These tests involved subjecting uniaxial specimens to blocks of continuous load controlled cycles, the stress range remaining constant during each block, but then being increased until material cyclic instability was observed. Since the type of cycle was expected to be related to reactor trip or fault conditions, and only a few hundred were expected in the lifetime of the plant, the tests were based on blocks of 500 repeated cycles. Tensile tests were also carried out on the same materials in order to provide values of the 0.2% and 1% proof stress which are used to derive the Ks parameter. The Ks factor is sensitive to the strain rate used in the tensile tests. Finally, the technique used to evaluate the shakedown factor is discussed, along with possible modifications to the R5 methodology to improve the representation of stress-strain cycles at elevated temperatures.

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

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.Copyright

Specimen Orientation and Constraint Effects on the Creep Crack Growth Behaviour of 316H Stainless Steel

Pre-compression (PC) is found to have strong effects on the tensile, uniaxial creep rupture and creep crack growth (CCG) behaviour of type 316H stainless steel at 550 °C. In this work, blocks of 316H steel have been pre-compressed to 8% plastic strain at room temperature and compact tension, C(T), specimens are extracted from the pre-strained blocks with loading directions parallel and normal to the PC axis. The influence of specimen orientation and thickness on the CCG behaviour of the PC material is examined. The results are compared to short term and long term CCG behaviour of 316H steel at the same temperature. Higher CCG rates and shorter CCI times have been found in PC material with a loading direction normal to the PC axis compared to that parallel to the PC axis. These observations are discussed with respect to the microstructural effects.Copyright

Numerical Simulation of Residual Stresses Induced in Compact Tension Specimens Using Electron Beam Welding

In welded components residual stresses on the order of yield magnitude can exist, allowing creep damage and cracking to occur under secondary stresses at elevated temperatures. A method of inducing residual stresses in compact tension, C(T), specimens is proposed using Electron Beam (EB) welding, which is simulated using a sequential thermal-mechanical model. The thermal model has been verified by comparison to thermocouple measurements obtained from instrumented EB welding experiments on blocks made of ex-service Type 316H stainless steel. Residual stress measurements, obtained by the neutron diffraction technique, have also been used to verify the mechanical model. It has been found that in the proposed EB welding method plasticity is localised and limited to just a few millimetres away from the notch whilst at the same time exhibiting a near yield level residual stress at the crack tip. Thus this technique may allow the effects of residual stresses on creep crack growth to be investigated by the EB welding technique without material property changes due to crack tip plasticity influencing the results.© 2012 ASME

Experimental Determination of Elastic and Plastic LLD Rates During Creep Crack Growth Testing

Elastic and plastic load line displacement (LLD) rates are often ignored when analyzing Creep Crack Growth (CCG) tests due to difficulties in accurately determining their value for complex crack morphologies typical of creep. Instead, the total LLD rate is assumed to be entirely due to creep. This simplistic approach overestimates the crack tip characterizing parameter C* which is non-conservative. This paper presents a review of the current method of interpreting CCG test data in ASTM E1457 and proposes an improved approach which accounts for the elastic and plastic LLD rates. Estimations of the elastic and plastic LLD rate are obtained from a partial unload immediately after load-up and a full unload, at the end of the test, prior to final failure. Some finite element validation of this method is presented. Implementing this approach will facilitate more realistic CCG laws.

Predictions of Creep Crack Initiation Periods in Pre-Compressed 316H Stainless Steel

Pre-compression of 316H stainless steel significantly alters its tensile, uniaxial creep and crack growth behaviour. It has previously been shown that reliable and conservative creep crack initiation predictions can generally be obtained for as-received 316H stainless steel using a variety of prediction methods. Given the changes in material behaviour caused by pre-compression, this paper applies similar prediction methods to pre-compressed 316H stainless steel at 550°C.Several procedures are available for estimating creep crack initiation time periods. The suitability of a procedure depends on the availability of the necessary material data. The procedures considered in this paper include the use of the creep fracture mechanics parameter C*, the crack opening displacement concept, the sigma-d approach and the time dependent failure assessment diagram.Creep crack growth tests have been performed on compact tension specimens manufactured from 316H stainless steel which was uniformly pre-compressed by 4% and 8% at room temperature. For each test, the time for creep crack initiation to occur was recorded. Predicted creep crack initiation times have been compared with the experimentally determined values. Comparisons with as-received material are also included.For pre-compressed material, conservative creep crack initiation predictions were only consistently achieved using steady state creep crack growth rates predicted from C*. This is a significant difference to as-received material for which conservative predictions were generally obtained by a variety of methods. At this time, there is only a limited set of pre-compressed data making it difficult to draw firm conclusions about the appropriateness of the various creep crack initiation prediction methods. The differences in the results between the pre-compressed and as-received material do however highlight the need for further tests on pre-compressed material.Copyright