Influence of prior cyclic hardening on high temperature deformation and crack growth in type 316L (N) stainless steel

For power generating equipment subjected to cyclic loading at high temperature, crack growth could arise from the combinations of fatigue and creep processes. There is potential for the material to undergo hardening (or more generally changes of material state) as a consequence of cyclic loading. Results of an experimental study to examine the influence of prior cyclic hardening on subsequent creep deformation are presented for type 316L(N) stainless steel at 600°C. Experiments were also carried out to explore creep crack growth at constant load, and crack growth for intermittent cyclic loading. For the as-received material there is substantial primary creep (hardening) at constant load, while for the cyclically hardened material at constant load the creep curves show recovery, and increasing creep rate with increasing time. Specimens subjected to prior cyclic hardening were also used for a series of creep and creep-fatigue crack growth tests. These tests demonstrated that there was accelerated crack growth compared to crack growth in as-received material.     

Modelling tail effects in creep-crack initiation and growth for 316L stainless steel

For complex loading history (creep and fatigue) applied to engineering components, assessment procedures generally estimate the crack initiation and growth by using the summation of continuous fatigue and pure creep crack growth rates. This text deals with the pure creep correlation established in laboratory tests and applied to components subjected to creep-fatigue loading. The trend of the creep opening displacement history superimposed onto the crack progress is sufficient to predict what kind of tail effect will occur when plotting ? vs. C*. The exponent of this correlation is demonstrated to be very close to unity, whatever creep stage is concerned. The contribution of either the material behaviour or the crack extension to the ? -C* correlation is discussed.     

Scatter bands in creep and fatigue crack growth rates in high temperature plant materials data

As a part of the European Commission supported project BE 1702: ‘HIDA’ Creep, creep-fatigue and high temperature fatigue crack growth data for five high temperature plant steels were accessed from a number of published and unpublished sources. These large sets of data were reviewed, and re-analysed where necessary, and plotted in terms of various crack growth rate correlating parameters. Thus limits of scatter bands and mean and upper 95% confidence limit creep and fatigue crack growth correlations are proposed. The present work covers a wide range of variables such as test specimen geometries, sizes, loading conditions and temperatures. Therefore, the correlations proposed are considered universal. However, it is envisaged that these correlations will be refined in future by enlarging the database and exploring the effect of the variables described above. The five materials studied are AISI 316 stainless, 2.25CrlMo, P91, 1CrMoV (forged), and 1CrMoV (cast) steel.     

Analysis of fatigue delamination growth in flip-chip package

An incremental crack extension procedure is implemented for simulating the growth of an interface defect in an electronic flip-chip device subjected to fatigue temperature cycling. The distributions of fracture mechanics parameters including the strain energy release rate, the stress intensity factors and phase angles along the curvilinear front of an embedded corner defect on the interface of silicon die and underfill are estimated and substituted into a subcritical crack growth model to predict the evolution of the defect under cyclic loading condition. It is observed from the analysis that the corner defect is under crack faces contact condition during temperature cycling, and consequently, the delamination growth is under mode-II and mode-III driving forces. In addition, the crack growth rate is highest in the middle of the crack front, and the corner crack evolves from an initially concave front to a quarter-circular front under temperature cycling.     

R5 and British Standards defect assessment procedures

In this paper, a basic high temperature defect assessment procedure is first described. This is common to the Nuclear Electric R5 procedure and the British Standards Document PD6539. Then the paper provides some details of the PD6539 method and the R5 procedure. The latter procedure covers a number of situations beyond the scope of the basic procedure, such as crack growth in dissimilar metal welds and creep-fatigue crack growth, and these are discussed.     

CREEP-FATIGUE CRACK GROWTH BEHAVIOR IN 1Cr-1Mo-0.25V STEEL. PART II: CRACK GROWTH BEHAVIOR AND MODELS

Abstract— Creep-fatigue crack growth (CFCG) behavior of an ex-service 1Cr-1Mo-0.25V steel was investigated for hold times of 100 s, 15 min, and 8 h for a trapezoidal loading waveform at a temperature of 538°C. The correlation of the crack growth rate with ( C _t)_avg for various hold times was significantly improved when an appropriate estimation scheme, previously proposed by the authors, was used to estimate this creep fracture mechanics parameter for this material. Crack growth data under creep-fatigue conditions and analysis procedures used to reduce them are described in detail in this paper.     

Creep-fatigue crack growth in austenitic stainless steel centre cracked plates at 650°C. Part II: Defect assessment according to the A16 document

In this paper, a defect assessment according to the simplified methods proposed in the French A16 document is presented. These methods allow one to estimate fracture mechanic parameters such as δK or C* based on a reference stress approach. The overall objective of the study is to evaluate the ability of such methods to predict creep-fatigue crack growth obtained in a large component containing a semi- elliptical surface notch. The experiment consists of a wide austenitic stainless steel centre cracked plate heated up to a temperature of 650°C and subjected to more than 3000 bending cycles with a one hour holdtime at the maximum load of the cycle. The analysis according to the A16 document together with the material properties necessary for the analysis are detailed in the paper. The experimental crack growth rates are modelled both on the surface and through the thickness of the plate using a linear summation of cyclic and creep contributions. The applicability of two models, consisting for the first one in re-initialising the creep law at the beginning of the holdtime and for the second one in accounting for creep history is investigated. The results predicted with the first model are found to be very conservative whilst those obtained using the second model are found to be in good agreement with the experimental measurements.     

Thermomechanics of brittle fracture

On the basis of the thermodynamics of thermoelastic deformation we propose an energy condition of the Griffith type for brittle fracture of solids under single loading. An analysis is given of the proposed condition under the plane stressed (strained) state for the two known models of an isolated defect. In the first model, stresses on the external (distant) surface of a solid with a defect are prescribed the same as in a similar solid without a defect. In the second model, displacements are prescribed on the external surface of a solid with a defect, which correspond to the load applied to the solid, but before the defect or crack was formed. Stresses on the surface of the defect in both models are equal to zero. It is shown that the first model in the isothermal case of deformation leads to the Griffith condition. The second model meets the energy condition of the Griffith type from which, under additional assumptions concerning the shape of the defect, and from conditions of isotropy and convexity, we obtain a curve of fracture (macroscopic criterion of fracture) in an ellipse form in the space of principal stresses. The orientation of the crack was determined. Coefficients of the curve of fracture obviously depend upon elastic constants, temperature, linear coefficient of thermal expansion and crack dimensions.     

Biaxial tension fatigue response of concrete

Concrete structures such as rigid airport pavements are subjected to repeated high-amplitude loads resulting from passing aircraft. The resulting stress-state in concrete is a biaxial combination of compression and tension. It is of interest to understand the response of plain concrete to such loading conditions, which will enable development of realistic material models for implementation in mechanistic pavement design procedures.The objective of this work is to characterize the quasi-static and low-cycle fatigue response of concrete subjected to biaxial stresses in the biaxial tension region, where the principal tensile stress is larger than or equal in magnitude when compared with the principal compressive stress. An experimental investigation of material behavior in the biaxial tension region is conducted. The experimental setup consists of the following test configurations: (a) notched concrete beams tested in three-point bend configuration, and (b) hollow concrete cylinders subjected to torsion.Failure of concrete in the biaxial tension region is shown to be a local phenomenon under quasi-static and fatigue loading, wherein the specimen fails owing to a single crack. The crack propagation is studied using the equivalent elastic crack concept. It is observed that the crack growth rate in constant amplitude fatigue loading exhibits a two-phase process: a deceleration phase followed by an acceleration stage. The crack growth in the acceleration stage is shown to follow Paris law. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the considered biaxial fatigue response.     

Thermomechanics of brittle fracture

On the basis of the thermodynamics of thermoelastic deformation we propose an energy condition of the Griffith type for brittle fracture of solids under single loading. An analysis is given of the proposed condition under the plane stressed (strained) state for the two known models of an isolated defect. In the first model, stresses on the external (distant) surface of a solid with a defect are prescribed the same as in a similar solid without a defect. In the second model, displacements are prescribed on the external surface of a solid with a defect, which correspond to the load applied to the solid, but before the defect or crack was formed. Stresses on the surface of the defect in both models are equal to zero. It is shown that the first model in the isothermal case of deformation leads to the Griffith condition. The second model meets the energy condition of the Griffith type from which, under additional assumptions concerning the shape of the defect, and from conditions of isotropy and convexity, we obtain a curve of fracture (macroscopic criterion of fracture) in an ellipse form in the space of principal stresses. The orientation of the crack was determined. Coefficients of the curve of fracture obviously depend upon elastic constants, temperature, linear coefficient of thermal expansion and crack dimensions.     

Fatigue-Life Predictions Including the Effects of Hold Time and Multiaxial Loads on Crack-Coalescence Behavior

Two aspects of crack-coalescence behavior are reported. The first concerns a regime frequently referred to in the literature as creep-fatigue interactions but which in this paper is essentially a time-dependent, fatigue-failure process. The second relates to crack coalescence under a wide range of different multiaxial stress-strain states. In the framework of the first approach, a fatigue-crack growth model is derived based on experimental observations during high-temperature, high-strain, reversed-bend, hold-time tests on AISI 316 stainless steel. Essential features of these tests are the compressive and the tensile 60-min hold periods on different surfaces, which induce, respectively, transgranular-short and intergranular-long cracks. The latter, more damaging cracks involve the coalescence of numerous short cracks to form a dominant Stage II crack that leads to failure. Then, in the framework of the second approach, the crack-coalescence model is advanced to predict the fatigue lifetimes for multiaxial, variable amplitude, proportional loading of a medium carbon steel commonly used to manufacture engineering components. It is shown that under high strain fatigue conditions the models used for the calculations of lifetime must necessarily involve crack-coalescence behavior if unsafe lifetime predictions are to be avoided.     

R-CODE software for the support of calculations required by the R5 and R6 procedures

The R6 Procedure and the ‘Assessment procedure for the high temperature response of structures’, R5, are well known and continue to be developed by Nuclear Electric Ltd in the UK. This paper looks at the software that has been developed to help undertake the calculations required by these assessment procedures. R-CODE version 3.00 computerises creep crack growth and incorporates simplified creep-fatigue crack growth calculations. The program may also be used to perform assessments according to PD6539 and PD6493 or BS7910. Full allowance can be made for incubation and redistribution periods. Continuum damage may be evaluated in terms of life fraction or ductility exhaustion methods. The program covers all the Categories, Options and Failure Assessment Diagrams of R6, for defining stability under short-term loadings when used for low temperature assessments.     

Book Reviews

‘Early’ creep-fatigue crack growth rates have been measured in complex-cycle large single edge notched bend feature-specimen tests on a 1¼ CrMoV turbine casting steel at 550°C. Crack propagation rates initially accelerate with increasing distance below the stress concentration to a peak value. The depth at which this maximum occurs depends on the notch geometry and the magnitude of any superimposed primary loading. ‘Early’ creep-fatigue crack growth rates are dependent on crack size, notch root strain range and any creep damage accumulated due to primary and secondary loading.     

Kriech-Ermüdungsverhalten der Titanlegierung IMI 834 bei 600 °C

Creep-Fatigue Behaviour of the Titanium Alloy IMI 834 at 600 °C In the present study the creep-fatigue behaviour of the Titanium alloy IMI 834 at 600°C was investigated. A comparison of the crack initiation life behaviour and of the crack propagation as caused by different types of complex creep-fatigue cycles (with hold times into tension and/or into compression direction and with different loading rates into tension and/or into compression direction) showed, that a slow increase of the loadings into tension reduced the life and increased the crack velocity more than hold times at the maximum load. Furthermore, there existed environmental influences. On the basis of the experimental investigations the prediction capability of convenient crack initiation life prediction methods was evaluated. It turned out that the prediction capability of the Strain Range Partitioning Method could be improved if it was frequency modified. The prediction capability of the Frequency Modification Method could also be improved, if mean stresses in the cycles were explicitly accounted for. In the short and long crack stage the propagation behaviour could be correlated well if the effective cyclic J-Integral was used. This is of importance for damage tolerance considerations. Because the strains and the stresses at the crack tip are most important for the crack propagation behaviour, they were analysed on the basis of the Finite Element Method. It was found that the strains and stresses differed for different types of creep-fatigue cycles.     

CONTINUOUS SEM OBSERVATIONS OF CREEP-FATIGUE DAMAGE PROCESSES

In order to examine the relation between damage evolution and changes in microstructure, e.g. from creep cavities, surface micro-cracks and dislocation structures at high temperature, strain controlled creep-fatigue tests were performed and interrupted at several damage levels on Types 304 and 316 stainless steels. The creep-fatigue tests on Type 304 stainless steel at a low strain level were conducted in a high-temperature fatigue testing machine combined with a scanning electron microscope, and the micro-crack initiation and growth behaviour were continuously observed to clarify the damage extension mechanism. It was found that even though many cavities were initiated and grew on the internal grain boundaries of the specimens during the strain-controlled tests, the failure life was governed by the propagation of surface cracks. On the other hand, micro-cracks of about the order of one grain size were initiated mainly along grain boundaries normal to the loading axis under low stress creep-fatigue, and the crack propagation rate of the micro-cracks was slow and random due to the nature of the microstructures. The micro-cracks gradually opened in the loading direction with increasing number of cycles and coalescence contributed to growth.     

SHORT CRACK COALESCENCE AND GROWTH IN 316 STAINLESS STEEL SUBJECTED TO CYCLIC AND TIME DEPENDENT DEFORMATION

Abstract— A study has been undertaken into short crack growth behaviour of AISI type 316 stainless steel under creep-fatigue conditions at 550°C within the high strain range of 0.9 to 2.5% and including a 60 min hold-time. During the high-temperature, reverse-bending tests, surface crack initiation and growth on both the tensile-hold and the compressive-hold sides of circular-section specimens were monitored by means of a plastic replication technique. Detailed analysis revealed that under creep-fatigue conditions, the initiation and growth behaviour of many individual cracks and their subsequent coalescence to form a major Stage II (tensile) crack was the dominant feature in the failure process. A life prediction model is proposed which incorporates the process of short crack coalescence. Satisfactory predictions of creep-fatigue lifetimes are derived from the model.     

Effect of circular holes on the ratchet limit and crack tip plastic strain range in a centre cracked plate

In this paper a centre cracked plate subjected to cyclic tensile loading and cyclic bending moment is considered. The effect of circular holes drilled in the region of the crack tip on the ratchet limit and crack tip plastic strain range is studied. Direct evaluation of the ratchet limit and crack tip plastic strain range is solved by employing the new Linear Matching Method (LMM). Parametric studies involving hole diameter and location are investigated. The optimum hole location for reducing the crack tip plastic strain range with the least reduction in ratchet limit is identified, and located at a distance 10% of the semi-crack length from the crack tip on the side opposite the ligament for both cyclic tensile loading and cyclic bending moment cases. It is also observed that the optimum location is independent of the hole size for both cyclic loading cases.     

FATIGUE CRACK GROWTH BEHAVIOUR OF A ZINC-ALUMINIUM ALLOY AT HIGH HOMOLOGOUS TEMPERATURES

Abstract— The fatigue crack growth behaviour and crack closure response of a zinc base die casting alloy at high homologous temperature were studied. The crack growth rate was both frequency and temperature dependent. The frequency dependence of crack growth rate, which has been commonly attributed to creep-fatigue interaction, can be rationalized by the crack closure phenomenon. The temperature dependence is contrary to that observed in other materials and cannot be simply explained in terms of the interaction between creep and fatigue damage. The effect of a single tensile overload on the crack growth behaviour at high homologous temperatures has also been investigated.     

A French guideline for defect assessment and leak before break analysis

A large program was launched in France in order to develop defect assessment procedures and leak-before-break methods (LBB) applicable for the design and for operating FBR plants. As a result of the collaboration between CEA, EdF and Novatome, a French guideline A16 has been produced as a first step in order to produce further documents in RCC-MR for design and for in-service inspection. This paper presents the main items developed in this guide considering low and high temperature: ? fatigue or creep-fatigue crack initiation based on the σ_d approach calculating stress and strain at a distance d=50 μm from the crack tip.

? fatigue crack growth based on da/dN-δK_eff relationship with a δK_eff derived from a simplified estimation of δJ by reference stress.

? creep-fatigue crack growth adding the fatigue crack growth and the creep crack growth during the hold time derived from a simplified evaluation of C*,

? ductile tearing based on simplified estimation of J (reference stress method) and plastic collapse of the ligament with and without holdtime,

? leak-before-break procedure.

Some detailed examples explaining the use of this A16 guide are presented.