Measurement of fatigue crack growth in large compact tension specimens using an AC magnetic field method

Fatigue crack growth rate data are required in order to carry out a numerical analysis of the fatigue performance of complex structural components. These data are obtained by measuring crack growth in standard fracture mechanics specimens. A new method for measuring fatigue crack growth in compact tension specimens has been developed. The technique is based on the measurement of the surface magnetic fields produced when passing a high-frequency alternating current through the specimen. Fatigue crack growth data recorded using this method indicated an accuracy of ±0.02 mm when compared with optical measurements. The technique is suitable for computer-controlled operation and could easily be applied to other standard specimen geometries.     

Theoretical and numerical modelling of creep crack growth in a carbon-manganese steel

This paper presents an analytical and numerical study of time dependent crack growth at elevated temperatures. A triaxiality dependent damage model is used to represent the multiaxial creep ductility of the material and an analytical model to predict steady state crack growth in terms of the fracture parameter C^∗, designated the NSW-MOD model, is presented. This model is an enhancement of the earlier NSW model for creep crack growth as it accounts for the dependence of stress and strain on angular position around the crack tip. Elastic-creep and elastic-plastic-creep finite element analyses are performed for a cracked compact tension specimen and the crack propagation rate in the specimen is predicted. It is found that in general the NSW-MOD model gives an accurate estimate of the crack growth rate when compared to the finite element predictions and experimental data for a carbon-manganese steel. However, crack growth rates predicted from the finite element analysis at low values of C^∗ may be higher than those predicted by either the NSW or NSW-MOD model. This enhanced level of crack growth may be associated with the non-steady state conditions experienced at the crack tip.     

Damage and fracture of polyvinylidene fluoride (PVDF) at 20 °C: Experiments and modelling

Flexible oil pipelines are multilayered structures used for the transport of crude oil or natural gas from the seabed to the surface in offshore oil fields. Because of severe service conditions, composite structures made of metallic and polymeric layers must be used. PVDF is a good candidate as it accommodates tensile and flexural deformations and guarantees water tightness. The present paper focuses on the mechanical properties of PVDF at 20 °C. Several specimens with different geometries were tested: smooth specimens, notched specimens and cracked specimens under tension and bending. Fracture surfaces were examined to determine fracture mechanisms. Based on mechanical testing and microscopic observations, a modified Gurson-Tvergaard-Needleman model for semi-crystalline polymers is proposed. The model allows both the non-linear behavior and the cracking of polymer structures to be represented.     

Creep failure simulations of 316H at 550 °C: Part I – A method and validation

This paper proposes a method to simulate creep failure using finite element damage analysis. The 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 and, 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 316H at 550 °C. Effects of the mesh size and scatter in uniaxial ductility are also investigated.     

Effect of stress dependent creep ductility on creep crack growth behaviour of steels for wide range of C*

Abstract

In this work, the effect of stress dependent creep ductility on the creep crack growth (CCG) behaviour of steels has been investigated by finite element simulations based on ductility exhaustion damage model. The relationship between the transition region of creep ductility and the transition behaviour of CCG rate on da/dt-C* curves has been examined and the CCG life assessments of components and CCG resistance of materials for a wide range of C* were discussed. The results show that with increasing the transition region size of creep ductility, the transition C* region size on da/dt-C* curves increases. With moving transition region position of creep ductility to high stress region (increasing transition stress levels), the transition C* region on the da/dt-C* curves also moves to high C* region. Decreasing transition stress levels and transition region sizes of creep ductility and increasing the lower shelf and upper shelf creep ductility values can improve the CCG resistance of materials. If the extrapolation CCG rate data from the high C* region or from the transition C* region are used in life assessments of the components at low C* region, the non-conservative or excessive conservative results may be produced. Therefore, the CCG rate data should be obtained for a wide range of C* by long term laboratory tests or numerical predictions using the stress dependent creep ductility and model.     

Damage mechanics based predictions of creep crack growth in 316 stainless steel

This paper describes a novel modelling process for creep crack growth prediction of a 316 stainless steel using continuum damage mechanics, in conjunction with finite element (FE) analysis. A damage material behaviour model, proposed by Liu and Murakami [1], was used which is believed to have advantages in modelling components with cracks. The methods used to obtain the material properties in the multiaxial form of the creep damage and creep strain equations are described, based on uniaxial creep and creep crack growth test data obtained at 600 °C. Most of the material constants were obtained from uniaxial creep test data. However, a novel procedure was developed to determine the tri-axial stress state parameter in the damage model by use of creep crack growth data obtained from testing of compact tension (CT) specimens. The full set of material properties derived were then used to model the creep crack growth for a set of thumbnail crack specimen creep tests which were also tested at 600 °C. Excellent predictions have been achieved when comparing the predicted surface profiles to those obtained from experiments. The results obtained clearly show the validity and capability of the continuum damage modelling approach, which has been established, in modelling the creep crack growth for components with complex initial crack shapes.     

Prediction of creep crack growth properties of P91 parent and welded steel using remaining failure strain criteria

Old grades of creep resistant materials such as P11 and P22 have been studied in depth and data and prediction models are available for design and fitness for service assessment of creep rupture, creep crack growth, thermo-mechanical fatigue, etc. However, as the 9%Cr material is relatively new, there is relatively limited data available and understanding with respect to quantifying the effect of variables on life prediction of components fabricated from P91 is more difficult. Since grade P91 steel was introduced in the 1980s as enhanced ferritic steel, it has been used extensively in high temperature headers and steam piping systems in power generating plant. However, evidence from pre-mature weld failures in P91 steel suggests that design standards and guidelines may be non-conservative for P91 welded pressure vessels and piping. Incidences of cracking in P91 welds have been reported in times significantly less than 100,000 h leading to safety and reliability concerns worldwide. This paper provides a review and reanalysis of published information using properties quoted in codes of practice and from recent research data regarding the creep crack growth of P91 steel, and uses existing models to predict its behaviour. Particular areas where existing data are limited in the literature are highlighted. Creep crack growth life is predicted based on short-term uniaxial creep crack growth (CCG) data. Design and assessment challenges that remain in treating P91 weld failures are then addressed in light of the analysis.     

9%Cr heat resistant steels: Alloy design, microstructure evolution and creep response at 650 °C

In this work 9%Cr alloys were designed supported by computational thermodynamic methods. Two sets of alloys were produced: 9%Cr alloys with 0.1%C and 0.05%C and 9%Cr alloys containing ∼0.03% Ti with 0.1%C and 0.05%C (always wt%). Microstructure investigations showed good agreement with the predicted phases of the thermodynamic modeling. The volume fraction of precipitated M₂₃C₆ carbides is directly related to the carbon content of the alloys. For Ti-containing alloys the precipitation of nano-sized Ti-rich MX carbonitrides was observed. The microstructure evolution (sub-grain and particle size) during creep at 650 °C/100 MPa was investigated by STEM-HAADF. The sub-grain size evolution and the coarsening of precipitates (MX carbonitrides, M₂₃C₆ and Laves phase) were more pronounced for Ti-containing alloys. 9Cr alloys without Ti and with low carbon content presented the highest creep strength of all investigated alloys.     

Experimental investigation of near crack tip creep deformation in alloy 800 at 650^C

Using a novel, noncontacting experimental method, the values for creep fracture parameters C(t) and C* have been experimentally quantified from full field, time-dependent surface deformation measurements in the crack-tip region for the first time. Near-tip experimental deformation results were obtained for a stationary crack in alloy IN800 at 650^C and compared to predictions based on both far-field measurement and finite element analyses. Results indicate that (a) for short times, the C(t)- integral is time and path dependent; (b) for long times, the C(t)-integral approached a constant for all paths which is agreement with C* values obtained from far-field experimental data. Comparisons of experimental results with FEM solutions were illustrated by means of radial and angular variations, full-field contour plots, as well as line integration form. Results from these comparisons indicate that (c) the experimental strain data portrays trends similar to those observed in the FEM solutions; (d) experimental, near-tip strain values were different in magnitude from the predicted values for all angles, and (e) the transition time obtained from the tests was about 10 times longer than that calculated from a formula proposed by Riedel and Rice (1980), indicating that primary creep cannot be neglected for this material. This revised version was published online in August 2006 with corrections to the Cover Date.     

A synergistic creep fatigue failure model damage (case of the alloy Z5NCTA at 550 °C)

Creep-fatigue tests have been performed on nickel base alloy Z5NCTA 33-21 grade 1. Hold time varied from 0, 10, 30, 90, 300, 1440 and 10,080 min at two values of strain range. It was found that the number of cycles to failure N_R decreases with holding time t_m, according to a power law:

N_R=1.88×10³·(t_m)^-0.39.     

Creep behaviour and microstructural evolution in AISI 316LN + Nb steels at 650 °C

The paper deals with the effect of niobium in the wrought AISI 316LN steels on the long-term creep characteristics at 650 °C. Casts B and C contained 0.1 and 0.3 wt.%Nb, respectively. As a reference material the niobium free Cast A was used. Small additions of niobium to the AISI 316LN steel resulted in a significant reduction of the minimum creep rate and shortening of the tertiary creep stage. At time to rupture exceeding 10⁴ h the creep rupture strength of the niobium-bearing Casts B and C was slightly inferior to the Cast A. Two nitrides formed in the Casts B and C: Z-phase and M₆X. The minimum creep rate in niobium-bearing casts was favourably affected by precipitation of the Z-phase. The dimensional stability of Z-phase particles was very high, but niobium additions also accelerated the formation and coarsening of η-Laves and σ-phase. Coarse σ-phase particles at grain boundaries contributed significantly to the shortening of the tertiary creep stage.     

Effect of processing parameters on the evolution of dislocation density and sub-grain size of a 12%Cr heat resistant steel during creep at 650 °C

The influence of hot-deformation and tempering temperature on the microstructure evolution of a 12%Cr heat resistant steel during short-term creep at 80-250 MPa and 650 °C was investigated. Quantitative determination of dislocation density and sub-grain size in the initial microstructure and after creep was investigated by STEM-HAADF. A correlation between microstructure evolution and creep response is established. All crept samples showed a significant increase of sub-grain size and a reduction of dislocation density. Hot-deformed samples showed better creep strength than non hot-deformed samples due to homogenization of the microstructure. The tempering temperature affected the dislocation density and the sub-grain size evolution, influencing the creep behavior.     

Experimental investigation of specimen size effect on creep crack growth behavior in P92 steel welded joint

In order to clarify the effect of constraint induced by specimen size on creep crack growth behavior of P92 steel welded joint, creep crack tests were carried out on the compact tension specimens with thick thickness and thin thickness, the crack tip of which were located at different distinct zones of welded joint. Tested results revealed that even in thin thickness specimens, fine grained heat affected zone specimens exhibited a fast creep crack growth rate compared with other micro-zones specimens due to a low creep crack resistance and a high multistress state. The fractographies of these specimens exhibited an accelerated number of spherical particles that were caused by the coalescence of creep voids. Furthermore, the correlation of C^* with creep crack growth rate was dependent on specimen thickness. As the specimen thickness increased from 10 to 30 mm, the creep crack growth rate increased. This was due to the increase in constraint level ahead of crack tip during creep crack propagation.     

Numerical predictions and experimental measurements of residual stresses in fatigue crack growth specimens

Controlling macro residual stress fields in a material while preserving a desired microstructure is often a challenging proposition. Processing techniques which induce or reduce residual stresses often also alter microstructural characteristics of the material through thermo-mechanical processes. A novel mechanical technique able to generate controlled residual stresses was developed. The method is based on a pin compression approach, and was used to produce well-controlled magnitudes and distributions of residual stresses in rectangular coupons and compact tension specimens typically used in fatigue crack growth testing. Residual stresses created through this method were first computationally modeled with finite element analysis, and then experimentally reproduced with various levels of pin compression. The magnitudes and distributions of residual stresses in experimental specimens were independently assessed with fracture mechanics methods and good correspondence was found between residual stresses produced using the pin compression and processing techniques. Fatigue crack growth data generated from specimens with low residual stresses, high residual stresses resulting from processing, and high residual stresses introduced through the new pin compression technique were compared and validated. The developed method is proposed to facilitate the acquisition and analysis of fatigue crack growth data generated in residual stresses, validate residual stress corrective models, and verify fatigue crack growth simulations and life predictions in the presence of residual stresses.     

Accounting for crack tip cyclic plasticity and creep reversal in estimating (Ct)avg during creep‐fatigue crack growth

Creep‐fatigue crack growth (C‐FCG) rates in a P91 steel at 625°C were correlated as the average time rate of crack growth during hold time, (da/dt)_avg , with (C_t)_avg. At 60‐second hold time, the rates were lower than for 600‐second hold time. At 600‐second hold time, the crack growth rates converged on to the creep crack growth rate (CCGR) trend. Thus, the CCGR trend represents the upper bound for time‐dependent crack growth rates in P91 materials. The analytical expressions based on considering just the elastic and secondary creep deformation rates overestimated the magnitudes of (C_t)_avg by as much as a factor of 10 for the 600‐second hold time tests. After accounting for the effects of cyclic plasticity during unloading, and accounting for only partial reversal of creep strains accumulated during hold time, the estimates of (C_t)_avg compared well with the measured values. C_R represents the extent of crack tip creep strain reversal, and t_pl is the time required for the crack tip creep zone during the hold time to become equivalent in size to the cyclic plastic zone in terms of stress carried by that region. Together, these parameters accurately account for the effects of crack tip cyclic plasticity on the magnitude of (C_t)_avg. Both t_pl and C_R depend on material properties, and the latter also depends on the hold time. A parameter ? is introduced that is dependent only on material properties and from which C_R can be estimated for a given hold time. t_pl and ? can be reported as part of the test results from C‐FCG testing.     

Viscoplastic behavior of a FeCrAl alloy for high temperature steam electrolysis (HTSE) sealing applications between 700 °C and 900 °C

High temperature steam electrolysis (HTSE) is one of the most promising technologies for the industrial production of hydrogen. However one of the remaining problems lies in sealing at high temperature. The reference solution is based on glass seals which presents several drawbacks. That explains why metallic seals are under development. The expected seal will be submitted to creep under low stresses between 700 °C and 900 °C, possibly involving complex loading and thermal history. The candidate material investigated in this work is a FeCrAl (OC404, Sandvik) supplied as a 0.3 mm thick sheet. The ability of this material to develop a protective layer of alumina was studied first, as well as grain size growth during thermal ageing. Creep and tensile tests were performed between 700 °C and 900 °C to determine its mechanical properties. This database was used to propose and identify an elasto-viscoplastic behavior for the material. Creep was described by the Sellars-Tegart law. This law was then used to simulate and predict creep indentation tests performed in the same range of temperatures.     

In-plane and out-of-plane constraint effects on creep crack growth rate in Cr–Mo–V steel for wide range of C*

Abstract

In this work, the stress dependent creep ductility and strain rate model have been implemented in a ductility exhaustion based damage model and the creep crack growth (CCG) rates of a Cr–Mo–V steel in compact tension (C(T)) and middle tension (M(T)) specimens with different thicknesses and crack depths have been simulated over a wide range of C*. The effects of in-plane and out-of-plane constraints on CCG rates are examined. The results show that the in-plane and out-of-plane constraint effects on CCG rate are more pronounced for the high constraint specimen geometry (C(T)), while such effects are less significant for low constraint specimen geometry (M(T)). The constraint effects on CCG rates mainly occur in low and transition C* regions and the CCG rate increases with increasing in-plane and out-of-plane constraints. There exists interaction between in-plane and out-of-plane constraint in terms of their effects on CCG rate. The higher in-plane constraint strengthens the out-of-plane constraint effect on CCG rate and higher out-of-plane constraint also strengthens the in-plane constraint effect on CCG rate. The constraint effects on creep crack growth behaviour for a wide range of C* mainly arise from the interaction of crack-tip stress states and stress dependent creep ductility of the steel in different C* levels.     

Effects of side‐groove depth on creep crack‐tip constraint and creep crack growth rate in C(T) specimens

The effects of side‐groove depth on creep crack‐tip constraint and creep crack growth (CCG) rate in C(T) specimens have been quantitatively studied. The results indicate that with increasing side‐groove depth, the constraint level and CCG rate increase and constraint distribution along crack front (specimen thickness) becomes more uniform. The constraint and CCG rate of thinner specimen are more sensitive to side‐groove depth. Two new creep constraint parameters (namely R^* and A_c) both can quantify constraint levels of the specimens with and without side‐grooves, and the quantitative correlations of CCG rate with constraint have been established. The mechanism of the side‐groove depth effect on the CCG rate has also been analyzed.     

Characterisation and simulation of the creep behaviour of Nicrofer 6025HT wire material at 650 °C

Thermal efficiency of combined cycle power plants can be improved by increasing temperature and pressure in the steam turbine. Since typical power plant materials have presently reached their operation limit with higher steam temperature, the application of a new cooling system could reduce the material temperature to tolerable conditions. For this purpose, a new sandwich structure was developed comprising a woven wire mesh interlayer between two plane sheets. Cooling steam is fed into the interlayer where it can flow without severe losses. This sandwich structure is applied to the steam turbine casing as a wall cladding.Due to the combination of constant overpressure of cooling steam and high temperature exposure of hot steam, the structures are stressed parallel and perpendicular to the intermediate layer primarily by creep loads. To simulate the creep behaviour via the finite element method the exact knowledge of the creep behaviour of the constituents, the wire and the sheet, is essential. Therefore, creep tests at 650 °C on wire material, manufactured from the nickel base alloy Nicrofer 6025HT, were carried out to determine constitutive equations. The creep behaviour was simulated on the basis of the concept of the internal backstress, which was implemented in an adequate user subroutine of the commercial FEM-software Abaqus.     

Numerical investigation of creep crack growth in cross‐weld CT specimens. Part II: influence of specimen size

A numerical investigation of the influence of specimen size on creep crack growth in cross‐weld CT specimens with material properties of 2.25Cr1Mo at 550 °C is performed. A three‐dimensional large strain and large displacement finite element study is carried out, where the material properties and specimen size are varied under constant load for a total of eight different configurations. The load level is chosen such that the stress intensity factor becomes 20 MPa √m regardless of specimen size. The creep crack growth rate is calculated using a creep ductility‐based damage model, in which the creep strain rate ahead of the crack tip perpendicular to the crack plane is integrated taking the degree of constraint into account. Although the constraint ahead of the crack tip is higher for the larger specimens, the results show that the creep crack growth (CCG) rate is higher for the smaller specimens than for the larger ones. This is due to much higher creep strain rates ahead of the crack tip for the smaller specimens. If, on the other hand, the CCG rate is evaluated under a constant C * condition, the creep crack growth rate is found to be higher for the larger specimens, except when the crack is located in a HAZ embedded in a material with a lower minimum creep strain rate; then, the creep crack growth rate is predicted to be higher for the smaller specimen. In view of these results, it is obvious that the size effect needs to be considered in assessments of defected welded components using results from CCG testing of cross‐weld CT specimens.