Creep Crack Growth Research Articles

The influence of pre-compression on the creep deformation and failure behaviour of Type 316H stainless steel

The influence of pre-compression to 8% plastic strain at room temperature has been examined on the tensile properties, uniaxial creep deformation, creep crack initiation and growth behaviour of Type 316H stainless steel at 550°C. Uniaxial creep and crack growth tests have been performed on the pre-compressed (PC) material and the results compared to existing long term (>15,000h) and short term test data on as-received (AR) (i.e. uncompressed) material. Pre-compression has been found to increase the materials subsequent yield stress in tension. Therefore the extent of non-linearity observed on the load–displacement curves of uniaxial creep rupture and crack growth tests on PC material is limited compared to AR material. In addition pre-compression causes a significant reduction in creep ductility and rupture time, although similar average and minimum creep strain rates are found in PC and AR materials. The creep crack growth (CCG) data on PC and AR materials have been characterised using the steady state creep C* parameter employing appropriate validity criteria and geometry dependent fracture mechanics parameter solutions. The CCG results are compared to the creep crack growth prediction models. Based on the creep properties, creep ductility and metallurgical observations of the fracture behaviour of the AR and PC materials, it has been shown that short term creep crack growth tests on PC material may be used to predict long term creep crack initiation (CCI) and CCG behaviour of AR material at 550°C.

Effect of residual stress on creep crack growth behavior in ASME P92 steel

Effect of residual stress on creep crack growth behavior in ASME P92 steel was investigated using compact tension (CT) specimen. Tensile residual stresses were generated ahead of crack tip by loading in compression beyond yield and then unloading. The maximum region of residual stress was obtained by numerical simulations which calculated the models with various notch radiuses and penetration loads. After pre-compression, creep crack growth tests were carried out on pre-compressed and original CT specimens to study the effect of residual stress. Furthermore, the corresponding approach on predicting creep crack growth behavior for components with residual stress was also investigated.

Difficulties in interpreting data from creep crack growth tests on type 316H weldments

Experimental creep crack growth data are generally obtained by following standard methods such as ASTM E1457-07 and subsequently characterised using the C* parameter. These data are then used in assessment procedures, such as R5, together with reference stress estimates of C* in the component, to predict creep crack growth behaviour. Some modifications to the ASTM E1457 creep crack growth testing and analysis methods have already been proposed following a previous analysis of data from long term creep crack growth tests on type 316H parent material. This paper reports the results of creep crack growth tests on type 316H heat affected zone material at 550°C using compact tension (CT) specimens manufactured from non-stress relieved thick section butt welds. It is shown that interpretation of the data from these weldment tests is complicated by both the discontinuous nature of the cracking process and the presence of significant residual stresses in the CT specimens. Further modifications to creep crack growth testing and analysis methods are proposed to address difficulties arising from the discontinuous nature of the cracking process, and further work is identified to investigate the influence of the residual stresses present in the specimens on the observed crack growth behaviour.

Creep Crack Growth Modeling of Low Alloy Steel using Artificial Neural Network

Prediction of crack growth under creep condition is prime requirement in order to avoid costly and time-consuming creep crack growth tests. To predict, in a reliable way, the growth of a major crack in a structural components operating at high temperatures, requires a fracture mechanics based approach. In this Study a novel technique, which uses Finite Element Method (FEM) together with Artificial Neural Networks (ANN) has been developed to predict the fracture mechanics parameter (C*) in a 1%Cr1%MoV low alloy rotor steel under wide range of loading and temperatures. After confirming the validity of the FEM model with experimental data, a collection of numerical and experimental data has been used for training the various neural networks models. Three networks have been used to simulate the process, the perceptron multilayer network with tangent transfer function that uses 9 neurons in the hidden layer, gives the best results. Finally, for validation three case studies at 538°C, 550°C and 594°C temperatures are employed. The proposed model has proved that a combinations of ANN and FEM simulation performs well in estimation of C* and it is a powerful designing tool for creep crack growth characterization.

Micromechanisms and a computational model of growth of low-temperature creep cracks in materials

We analyze and synthesize the results of investigations available from the literature on the micromechanisms of growth of creep cracks and, on the basis of this analysis, show that these cracks grow mainly by the initiation, increase, and coalescence of pores. This enables us to formulate a computational model for the evaluation of the period of subcritical growth of low-temperature creep cracks in structural materials. As a result, we obtain a kinetic equation of growth of these cracks in the form of dependence of the crack growth rate on the stress intensity factor. Together with the initial and the final conditions, this equation forms a computational model for the evaluation of the period of subcritical growth of lowtemperature creep cracks.

Creep crack growth in phosphorus alloyed oxygen free copper

Creep crack growth (CCG) has been studied in phosphorus-alloyed oxygen-free copper (Cu-OFP) at 22, 75, 175, and 215°C with compact tension (CT) specimens. At 175 and 215°C, the cracks grew about 10mm before final instantaneous failure. In contrast, there was no visible crack growth at 22 and 75°C. Strongly deformed grains were observed adjacent to the cracks at 175 and 215°C. Intergranular creep cavities were found around the cracks. At 22 and 75°C, deformed grains and some cavities as well as microcracks were observed close to the crack tip.A model for crack propagation based on creep damage formation has been formulated to interpret the test results. Rupture criteria based on both creep ductility exhaustion and grain boundary cavitation were taken into account. The contribution from the ductility exhaustion to the creep damage dominated at the lower two test temperatures whereas the contribution from grain boundary cavitation at the higher test temperatures. The model can describe the influence of temperature on the observed creep crack propagation. It can also account for the observed cavitation in a qualitative way.

Crack growth rate and crack path in adhesively bonded joints: Comparison of creep, fatigue and fracture

The quality of an adhesive–adherend combination is often judged by whether the failure surface or crack path is cohesive or interfacial. The relationship between crack path and test method was examined by comparing the performance of individual adhesive–adherend combinations (six) in quasi-static fracture, mixed-mode fatigue to the threshold, and creep crack growth. In mixed-mode loading, crack paths in creep were similar to those in quasi-static fracture even though the crack growth rates in creep were much smaller. This was attributed to similar crack-tip plastic zone sizes in the toughened epoxy adhesive during creep and quasi-static fracture. Both creep and threshold fatigue tests produced interfacial failure when the test environment created condensed moisture at the crack tip and the loading was mixed-mode. Interfacial failure was also evident in room-temperature dry environments in mixed-mode fatigue at slow crack growth rates near the threshold, but was not seen in either creep or quasi-static fracture under mixed-mode loading in dry air. This increased sensitivity of near-threshold, mixed-mode fatigue to surface properties was caused by the proximity of the crack path to the interface as a result of a smaller plastic zone size. Therefore, the interfacial or cohesive failure of an adhesive system can be a function of the test being conducted and may not be an absolute indicator of joint quality.

Effects of Heat Treatment on the Creep Crack Growth Behavior in Al/Al-4wt%Cu Functionally Graded Material

A conventional hot-pressing method was used to produce Al/Al-4wt%Cu functionally graded material (FGM). Heat treatment, which included solid solution treatment (T4) and aging treatment (T6), was carried out on hot-pressed (F) specimens. The creep crack growth tests were performed under constant loading of the creep-testing machines. The distribution of copper composition was investigated by line analysis via electron probe microanalysis. Fracture morphology and creep crack paths were studied by scanning electron microscopy. During heat treatment, the thickness of the graded transition layer increased due to copper composition redistribution. Creep crack growth retardation was found when crack propagated from the graded transition region to the Al-4wt%Cu layer. Greater improvement in creep crack growth resistance was achieved by the T4 and T6 states of Al/Al-4wt%Cu FGM. For T4 and T6 state specimens, the micro-cracks and crack kinking in the transition region were observed, which prevented creep crack growth.

Simplified estimates of the creep crack growth parameter C(t) under primary/secondary stresses using the enhanced reference stress method

This paper describes simplified methods to estimate the fracture mechanics parameter, C(t), related to creep crack growth rates in non-steady state creep conditions produced by primary or secondary stresses. The methods proposed incorporate effects from initial plasticity and redistribution during a short period after a loading dwell starts, in addition to the estimate of the steady state creep crack growth parameter C∗ by the enhanced reference stress method. The methods have been validated by performing finite element elastic–plastic creep analyses of a circumferentially cracked cylinder subjected to load-controlled tension or thermal loads.

The effect of residual stress on polymer pipe lifetime

This paper presents a quick methodology to assess the lifetime of an internally and externally pressurized polymer pipe using linear elastic fracture mechanics. The non-homogenous distribution of residual stress induced by production technology is incorporated into the numerical model of the cracked pipe. The stress intensity factor of the axially oriented crack growing from the inner surface through the pipe wall is estimated and combined with experimentally determined creep crack growth kinetics used for lifetime assessment. Major parameters affecting the pipe’s lifetime are identified and discussed. A simplified equation for estimation of the stress intensity factor value, removing the need for complicated finite element simulation, is then proposed. The combination of the creep crack growth tests and the approach presented can be a very powerful tool for reliable lifetime estimation of plastic pipes.

An investigation of effects of welding residual stresses on creep crack growth for a low alloy butt weld

R5 is an assessment procedure for the high temperature response of structures and R5 Volume 7 specifies the creep crack growth assessment for low alloy welds. In power plants, almost all welds operating in creeping regime have been post weld heat treated (PWHT). However, low levels of welding residual stresses may still exist. It is known that the welding residual stresses in weldments can be difficult to estimate, as their magnitude and distribution depend on materials, geometry, weld procedure and heat treatment.For high temperature low alloy steel, CrMoV, butt welds, R5 Volume 7 suggests that a bending welding residual stress of 60 MPa should be assumed for a circumferential defect following a post weld heat treatment. For conservatism, the relaxation of this low level of the welding residual stress in a service component operating at high temperatures for a long period time has been discounted in R5. As the actual welding residual stresses can be difficult to estimate, it is necessary to carry out a detailed investigation for the effects of the welding residual stresses on the creep crack growth and a number of associated crack growth parameters.In this paper, for a typical low alloy steel butt weld in ½Cr½Mo¼V (‘½CMV’) pipework with 2¼Cr1Mo weld metal under a given set of internal pressure, temperature and system moment, a series of creep crack growth analyses have been conducted using the R5 Volume 7 assessment procedure, assuming the welding residual stresses of 10 MPa–100 MPa and three different crack initiation times. A postulated service-initiated internal circumferential defect (semi-elliptical surface-breaking surface defect) of 2.3 mm deep by 15 mm long has been used.The investigation results are presented by a series of graphs which show the effects of the welding residual stresses and different crack initiation times on the creep crack growth for the weld studied. After a detailed discussion, conclusions can be drawn.

Material pre-conditioning effects on the creep behaviour of 316H stainless steel

Material pre-conditioning by, for example, pre-strain through component bending and welding is known to alter the creep deformation and creep crack growth (CCG) behaviour of 316H stainless steel. Experimental test data on the creep deformation and crack growth behaviour of 316H weldment compact tension specimens at 550 °C, where the starter defect was introduced into the heat affected zone (HAZ), have been compared to those of obtained from similar specimens manufactured from parent material, which had been subjected to 8% compressive plastic pre-strain at room temperature. Similar degrees of accelerated cracking behaviour compared to parent material, for given values of C*, were exhibited in both 316H HAZ and pre-compressed parent materials. This acceleration has been attributed to the influence of material hardening effects and the reduction of creep ductility in the pre-conditioned materials. These results are discussed in terms of the potential for using material pre-conditioning to assist in predicting the long term cracking behaviour of high temperature 316H stainless steel plant components from shorter term laboratory CCG tests.

Time-dependent crack growth behavior for a SMAW weldment of Gr. 91 steel

This paper presents the experimental results on creep crack growth (CCG) behavior for BM, WM and HAZ in the weldment of Gr.91 steel, which was prepared by a shield metal arc weld (SMAW) process. A series of CCG data was obtained by the creep and creep crack growth tests under different applied loads for BM, WM and HAZ in Gr. 91 welded joint at 600 °C. The CCG behaviors were characterized by the empirical equation of the da/dt vs. C∗ fracture parameter, and the CCG laws for the BM, WM and HAZ were constructed and compared, respectively. Results showed that for a given value of C∗, the WM and HAZ were almost equal, but they were about two-times faster in the CCGR curves than the BM. This reason is that the CCGRs were closely attributed to their creep strength and creep strain rates. It is thus supposed that creep cracks will dominantly initiate in the HAZ, because lower creep strength existed in the HAZ compared with BM.

Creep failure simulations of 316H at 550 °C: Part II – Effects of specimen geometry and loading mode

In this paper, the FE damage analysis method, proposed in Part I, is applied to simulate creep crack growth in six different types of cracked specimens of 316H at 550°C. Comparison with experimental results shows that simulated creep crack growth rates agree well with experimental data. Shortcomings of the present method to predict creep crack initiation is briefly discussed.

Deterministic and probabilistic creep–fatigue–oxidation crack growth modeling

Fatigue, creep, oxidation or their combinations have long been recognized as the principal mechanisms in many high-temperature failures in power plant components, turbine engines, and exhaust systems in vehicles. Depending on the specific materials and loading conditions and temperature, the role of each damage mechanism may change significantly, ranging from independent development to competing and combined creep–fatigue, fatigue–oxidation, and creep–fatigue–oxidation. In this paper a new linear superposition theory is proposed to model the cycle-dependent and time-dependent creep–fatigue–oxidation crack growth phenomena. The model can be reduced to creep–fatigue and fatigue–oxidation crack growth models previously developed by the authors as well as, under some assumptions, the current widely used linear superposition theory. The limits of the current superposition theory and the advantages of the new theory are clearly demonstrated with several worked examples. A general probabilistic analysis procedure is also proposed by introducing the uncertainties of parameters in fatigue, creep, and oxidation crack growth laws with the help of the Monte Carlo simulation.

Effect and mechanism of out-of-plane constraint on creep crack growth behavior of a Cr–Mo–V steel

The effect and mechanism of out-of-plane constraint on creep crack growth rates were experimentally investigated. The results show that the out-of-plane constraint effect on creep crack growth rates is related to C∗-integral levels. In low C∗-integral region, with increasing out-of-constraint, there is a change in creep fracture mode from ductile to brittle intergranular fracture, which leads to the increase in creep crack growth rates. In middle and high C∗-integral regions, the constraint basically has no effect or slight effect on the creep crack growth rates. In creep life assessments, the effects of constraints and C∗-integral levels on the creep crack growth rates should be considered.

Microstructural Degradation in Power Plant Steels and Life Assessment of Power Plant Components

Extensive creep testing was carried out on 1Cr1Mo¼V low alloy forging and casing steels in as received (normalized and tempered) and aged condition. Both the steels exhibited considerable secondary and tertiary stages in the creep tests conducted in the temperature range between 813 and 873 K (540 °C and 600 °C). The primary stage, though present, was rather negligible in higher temperature ranges. Casting steel showed wedge type cavities which grew as cracks along the grain boundaries. In the case of forged steel, the voids were elliptical and flat which developed during the tertiary stage of creep deformation. Tertiary creep deformation and creep ductility of the two steels investigated have been analyzed based on the type of voids developed during the tertiary stage. Based on detailed microstructural investigations using scanning and transmission electron microscope, prevailing damage mechanisms such as structural transformation, particle coarsening, grain boundary thickening have been identi ied and applied for remaining life assessment of the two steels. Hardness and other mechanical properties were observed to decrease slightly on aging; both for rotor forging and casing casting steel. The softening occurred due to dissolution of M3C and Mo2C carbides and coagulation of others resulting in reduced creep strength. Gradual fall of creep strength at intermediate aging times was due to recovery in ferrite, gradual depletion of solid solution carbides from the ferrite matrix, metastability and transitional character of precipitated carbides. Creep crack growth studies have also been carried out on both the steels. Life assessment calculations have also been carried out using creep crack growth methodology.

Simulations of creep crack growth in 316 stainless steel using a novel creep-damage model

The paper provides an advanced creep model which gives reasonable descriptions of the effects of damage and stress state from micromechanics viewpoint. A modified creep ductility exhaustion approach is employed to calculate the creep damage. Based on the proposed creep-damage model, numerical analyses of creep crack growth are conducted with a failure simulation technique. The simulated results are compared favorably with available experimental data for compact tension and thumbnail crack specimens for 316 stainless steel tested at 600°C. The comparisons show the excellent capability of the proposed model in predicting the crack growth rate and progressive crack profiles.

高Cr鋼溶接継手材の初期クリープき裂成長速度特性とき裂発生寿命評価

高Cr鋼溶接継手材の初期クリープき裂成長速度特性とき裂発生寿命評価

Creep/Fatigue Crack Growth Testing, Modelling and Component Life Assessment of Welds

Abstract Crack growth and fracture predictions are used in high temperature equipment operating in the creep and creep/fatigue range by assessing crack initiation and growth from an existing defect. These procedures are relevant to fossil, nuclear power generation, chemical and aerospace industries. There are three key factors which determine a successful methodology for remaining life assessment of engineering components. The first is good testing techniques; the second is the development of appropriate and accurate correlating parameters to treat the results in a the unified and verifiable manner in order to produce ‘benchmark’ material crack growth properties and the third is an accurate modelling procedure for life assessment methods. This presentation considers the first two aspects and identifies future trends and improvements in the developments of standardisations in creep and creep/fatigue crack growth testing. Under the auspices of the Versailles Agreement on Materials and Standards (VAMAS) committee a Code of Practice on creep crack growth of components has been developed. The procedure identifies methods of testing for non-standard, welded feature test components in VAMAS TWA31 Technical Working Area on Creep and Creep/Fatigue Crack Growth of weldments containing residual stress. The overview highlights the important points in these pre-standardisation collaborative efforts by presenting the methods of analyses and example of their application to feature type components.