Creep-fatigue Regime Research Articles

Life prediction of P91 piping accounting for creep fatigue conditions

ABSTRACT To address the lack of available standard codes that deal with interactions between creep and fatigue, which is generally approached with a simple superposition law, the parameter associated with CFCG is introduced, under the hypothesis of partial creep strain reversal based on the creep reversal parameter . The TDFAD methodology used to predict initial crack growth during creep and creep-fatigue regime with P91 grade steel operating at 600°C is reviewed and integrated. This state-of-the-art crack tip parameter is introduced and accounts for small-scale creep, in the creep-fatigue crack propagation assessment.

Determination of the dominant failure mechanism of P92 steam piping subjected to daily operational cycle using finite element (FE) technique

In a bid to minimise the cost of electrical energy production through the reduction in the quantity of coal usage, the power generation companies resolved to operate different daily cycles characterized by peak and off-peak energy demand periods. These daily operational cycles have left the power plant components operating in a possible creep-fatigue regime. As such, earlier failure of the plant’s components such as the steam pipes due to creep, fatigue, or the combination of both failure modes became inevitable. This study employed finite element (FE) technique to determine the dominant failure mechanism and useful life of P92 power generation steam piping subjected to one of the daily operational cycles. The outcome of the study showed that the failure of the piping when subjected to the daily cycle is creep dominated, and failure due to fatigue or possible creep-fatigue interaction was impossible since the daily cycle was insufficient to induce the extensive plastic strain required for the initiation and propagation of fatigue failure. Hence, the best form of operating steam pipe/piping is steady-state, since subjecting them to daily cycles significantly reduce their useful life.

Cyclic and time-dependent crack growth mechanisms in Alloy 617 at 800 °C

Crack growth mechanisms for Alloy 617 at 800 °C were investigated in air with specific emphasis on the transition from cycle-dependent to time-dependent crack growth mechanisms in the creep-fatigue regime. Crack growth studies were conducted using compact tension samples, a load ratio of 0.5, and triangular 5 Hz, 0.33 Hz, and 0.05 Hz waveforms, a trapezoidal 0.05 Hz waveform with 17 s hold time, and sustained loading. Fatigue crack growth rates were relatively insensitive to changes in frequency and hold times in air up to ∆K ≈ 11.5 MPa√m for R = 0.5, i.e, Kmax = 23 MPa√m. Above this threshold, the onset of time dependent crack growth was observed via a creep void nucleation and coalescence mechanism for triangular and trapezoidal waveforms with a loading frequency of 0.05 Hz, and during sustained loading. An estimate of the threshold for stress assisted grain boundary oxidation (SAGBO) crack growth was calculated to be 23 MPa√m, and oxidized grain boundaries observed near the crack tip were mostly uncracked, suggesting the SAGBO threshold was not reached before the onset of the void nucleation mechanism. A comparison of results across all available studies suggests that a threshold-based transition from cycle- to time-dependent crack growth at 800 °C likely exists. However, the stress intensity factor does not maintain similitude to accurately define a threshold across studies. Thus, gaining an understanding of the crack tip stress states that define the various time dependent mechanisms should be considered in future work.

Characterizing crack growth behavior and damage evolution in P92 steel under creep-fatigue conditions

The present paper characterized crack growth behavior and damage evolution in P92 steel under creep-fatigue interaction conditions. Creep-fatigue crack growth tests were conducted at a constant load amplitude with various hold times. To reveal the role of constraint in creep-fatigue regime, specimens with different crack depths and thicknesses were used. Combination with a non-linear creep-fatigue interaction damage constitutive model, the crack growth and damage evolution behaviors were simulated using finite element method. Under creep-fatigue condition, time dependent crack growth rate increased as the duration period was reduced. This was attributed to the role of enhanced fatigue damage with the decreasing of the dwell time. Moreover, the deviation of crack growth rate with different dwell times became small as crack grew. With the crack length increasing, the creep damage level was improved regardless the dwell time, which may be due to the fact that the creep damage dominated crack growth in this stage. Furthermore, the crack growth rate increased as the crack depth became deep and the specimen thickness became large, as a result of the increased constraint level. A load-independent constraint parameter Q* was introduced to correlate the crack growth rate, which provided a good prediction for specimens under different constraint conditions.

Experimental investigation of hot-work tool steels performances under the creep-fatigue regime

In the present research, an innovative testing method, specifically developed to characterize the tool steels under creep-fatigue conditions, was carried out on a TQ1 (X36CrMoV5-2) hot-work tool steel in cooperation with Constellium R&D center. The experimental campaign consisted of different testing conditions, and most of the specimens were nitrided to account for the specific surface state of the tools. Tests were performed on a 100 kN MTS fatigue machine equipped with a heating furnace. A creep-fatigue loading type was applied to the specimens, i.e., a cyclic load with a dwell time, in order to properly reproduce the conditions acting on extrusion tools and dies. Under a constant temperature of 520 °C, the effects of four different load levels and two different values of dwell times were evaluated. In addition, selected test conditions were replicated with specimens not nitrided with the aim to evaluate and quantify the influence of the superficial treatment. Final results were presented in terms of fatigue diagram of the TQ1 steel and compared to the performances of the H11 tool steel tested in a previous research by the same authors.

Predicting failure modes in creep and creep-fatigue crack growth using a random grain/grain boundary idealised microstructure meshing system

An idealised microstructure mesh model combined with a novel creep and creep-fatigue damage accumulation model was employed to simulate crack growth behaviour under creep/fatigue conditions for a modified 9Cr-1Mo steel. The influence of microstructures on the crack growth behaviour was studied using a random grains separated by idealised grain boundaries. For accurately representing the damage accumulation in creep-fatigue regime, a non-linear damage model was employed. In this case, creep damage was determined by multiaxial ductility exhaustion approach and fatigue damage was reliant on maximum stress and plastic range ahead of crack tip per loading cycle. When creep dominated, cracks mainly propagated along grain boundaries in steady crack growth stage. As an exception, the crack growth in the tertiary crack growth stage gradually changed from intergranular to mixed model and finally transgranular fracture. In contrast, in creep-fatigue regime, the crack growth behaviour was greatly reliant on the dwell time. For short duration period, the crack mainly propagated in a transgranular manner. But as the dwell time increased to greater than 600s, the creep intergranular fracture dominated once again. Furthermore, the grain size gradient had little influence on the crack growth model and only affected the fracture life in creep and creep-fatigue regimes.

A novel creep–fatigue interaction damage model with the stress effect to simulate the creep–fatigue crack growth behavior

In this paper, a novel creep–fatigue interaction damage model was proposed to evaluate damage evolution and crack growth behavior in creep–fatigue regime. The model incorporated the maximum stress effect into the low cycle fatigue damage accumulation and utilized the non-linear summation approach to consider the creep–fatigue interacted effect. The model could predict the relation between (da/dt)avg against (Ct)avg in creep–fatigue interacted environment and demonstrate the crack growth behavior under pure fatigue condition, which respectively matched well the corresponding experimental results. In addition, the crack growth rate was greatly dependent on the dwell time in creep–fatigue regime. The crack growth rate (da/dt)avg was increased with decreasing the hold time at the same (Ct)avg values. During crack growth process, the creep damage accounted for a larger portion compared with the fatigue damage. Furthermore, in creep–fatigue regime, the effect of the initial crack depth on crack growth behavior was dependent on the hold time while the applied initial stress factor range had slight effect on the crack growth behavior.

Dynamic mechanical behavior of polymer modified bitumen

The dynamic mechanical behaviour of bitumen (BIT) modified with styrene/butadiene/styrene block copolymer (SBS) were investigated. Dynamic mechanical analysis (DMA) were performed in the temperature range –80 to 60 °C. The primary viscoelastic functions were determined at the traffic frequency, 5 Hz. The BIT+SBS blends were investigated in creep fatigue regime at temperature 10, 20, 30, 40 and 50 °C. Dynamic mechanical behaviour of BIT+SBS blends depends on their morphological characteristics, number of phases, phase compositions and phase content in blend, as well as time and temperature. The curves of primary viscoelastic functions, storage modulus (E′), loss modulus (E′′) and loss tangent (tg δ) vs. temperature of polymer modified bitumen differ from unmodified bitumen and indicate the presence of the swollen polybutadiene and polystyrene phases in bitumen phase. The curve E′ vs. temperature of polymer modified bitumen show the rubbery plateau. With the increment of SBS content the rubbery plateau is shifted to high temperatures. At the constant load the creep values of BIT-SBS blends increase and those of creep modulus decrease over a period of time. These effect are more pronounced in samples with higer content of SBS. The time-temperature correspondence principle was applied to create master curves for the reference temperature 10 °C for the creep modulus of BIT + SBS blends. The data were analysed using WLF and Arrhenius equations.

A Numerical Modelling Approach for Time-Dependent Deformation of Hot Forming Tools under the Creep-Fatigue Regime

The present study was aimed at predicting the time-dependent deformation of tools used in hot forming applications subjected to the creep-fatigue regime. An excessive accumulated plastic deformation is configured as one of the three main causes of premature failure of tools in these critical applications and it is accumulated cycle by cycle without evident marks leading to noncompliant products. With the aim of predicting this accumulated deformation, a novel procedure was developed, presented, and applied to the extrusion process as an example. A time-hardening primary creep law was used and novel regression equations for the law’s coefficients were developed to account not only for the induced stress-temperature state but also for the dwell-time value, which is determined by the selected set of process parameters and die design. The procedure was validated against experimental data both on a small-scale extrusion die at different stress, temperature, load states, and for different geometries and on an industrial extrusion die which was discarded due to the excessive plastic deformation after 64 cycles. A numerical-experimental good agreement was achieved.

Compatibilization of Thermoplastic Polyurethane and Polypropylene with a SEBS Compatibilizer

The effect of styrene-ethylene/buthylene-styrene triblock copolymer (SEBS) on the thermal and rheological properties of thermoplastic polyurethane/polypropylene (TPU/PP) blends was investigated. For the selection of polymer materials and polymer blends for various fields of applications the stability of materials under constant deformation are very important. The blends were therefore characterized by measuring secondary viscoelastic functions creep, recovery and creep modulus using dynamic mechanical analysis (DMA) in the creep fatigue regime. The master curves at the reference temperature of 25°C were created by time-temperature correspondence (TTC) principle. The correlation of the creep modulus with time, temperature and addition of compatibilizer was discussed. The differential scanning calorimetry (DSC) results indicated that the addition of SEBS as a compatibilizer in TPU/PP blends increases glass transition temperature (Tg) and decreases crystallinity (χc). SEBS block copolymer acts as an efficient compatibilizer for TPU/PP blends.

Crack growth in the creep-fatigue regime under constrained loading of thin sheet combustor alloys

Damage tolerant lifing methodologies are an essential requirement to support the safe and reliable operation of combustors in the modern gas turbine aero-engine. These static components experience a wide range of temperature superimposed upon a complex stress field resulting from their geometrical form. Features such as seam welds, injector ports and cooling hole arrays could potentially initiate and subsequently interact with fatigue cracks, while the thin sheet structure of the combustion annulus locally imparts plane stress dominant conditions. However, on the macro-scale the stress system is highly constrained, ensuring that cracks usually arrest before achieving a critical size, thereby avoiding failures. These service observations must now be supplemented by a detailed scientific understanding of fatigue behaviour in combustor alloys under representative conditions.The technical challenge of evaluating crack growth rates in thin sheet laboratory scale specimens under strain controlled fatigue at high temperatures is addressed. The alloy system of interest was the nickel based superalloy Haynes 230. A matrix of novel fatigue crack growth tests is reported at temperatures ranging from 816°C to 982°C under a variety of strain ratios (εmin/εmax) with crack growth monitored via an automated pulsed DCPD system. Contemporaneous measurements of constitutive behaviour will demonstrate the significant degree of stress relaxation that occurs under these constrained conditions, continually modifying the crack tip driving force. This was best quantified by the corresponding stress ratio, which generally fell throughout the period of the test and under dwell loading cycles often resulted in a highly negative value (e.g. stress ratio lower than minus fifteen) incorporating a minimal tensile stress contribution. Crack growth can clearly continue under such highly compressive regimes. The contribution from creep damage mechanisms will be illustrated via post test fractographic and microstructural examination.

Dissolution of Fine γ’ Precipitates of MC2 Ni-Based Single-Crystal Superalloy in Creep-Fatigue Regime

The dissolution kinetics of an ultra-fine γ’ precipitation occurring in the γ matrix between the standard secondary precipitates of MC2 Ni-based single crystal superalloy was investigated. Creep-fatigue experiments at 1050°C including an overheating at 1200°C were performed on <111> oriented specimens to study the effects of fine γ’ particles on the plastic deformation. During these experiments, a decrease of the plastic deformation rate was observed just after the temperature peak. This hardening effect disappears once the fine γ’ precipitates had been dissolved. A mean time for this hyperfine precipitation dissolution could then be highlighted. Based on both simple binary diffusion and complex diffusion analysis, the mean time for the dissolution of the fine γ’ precipitates is analyzed and compared to the experimental ones. It is shown that considering only a simple binary diffusion is not sufficient and it should be considered a more complex diffusive analysis involving additional interplays.

Fitness for service of welded components under creep and creep–fatigue loading

The welded materials are prone to damage particularly in heat-affected zones in base metal as well as in fusion line and weld metal. Therefore the treatment of welded components operating in the creep and creep–fatigue regime is of immediate industrial interest. It needs to be addressed by a methodology that will be developed and incorporated into design codes such as ASME, as well as in Fitness-for-Service assessment procedures. Assessment of welded components for Fitness-for-Service is addressed in fragments in various procedures. This is due to the complex microstructures of weldments that include base metal, heat-affected zone and weld metal. The R5 high temperature assessment procedure, produced by BE in the UK, formalized the methods for base metals loaded in static and creep–fatigue with sections describing the treatment of welds. Subsequently, procedures for assessing defects at high temperature, similar to those in R5, have been included in British Standards, the French A16 procedure and in API 579. Section 8 of the recent European thematic network FITNET Fitness-for Service procedure which is incorporated into R5, specifies methods for assessing defects in structures operating at high temperatures and subject to creep and creep–fatigue loading conditions. The reliability of the structural assessments following the codes depends strongly on availability of reliable data required as input data. Most of the data obtained to date on high temperature materials has been reported on creep crack growth (CCG), ignoring the creep crack initiation (CCI) where as creep–fatigue data is scarce. Recent European and international collaborative effort included EC projects CRETE and ECCC, and ESIS TC11, WG on High Temperature Testing of Welds concentrated on CCI as well as CCG testing and analysis of industrial specimens and assessment. This has led to development of a Code of Practice for High Temperature Testing of Weldments via Int. Institute of Welding (IIW) to be presented to ISO for standardization. The present paper gives an overview of the high temperature assessment procedures with recent developments in procedures. Experimental and analytical test results are presented following the codes and assessment procedures. The experimental work on deformation and crack behavior of similar welds of ferritic steel P22, studied at 550 °C are reported. P22 materials are commonly used by power generation and chemical processing industrial applications since couple of decades.

A cohesive zone model for fatigue and creep–fatigue crack growth in single crystal superalloys

A numerical analysis using cohesive zone model under cyclic loading is proposed to develop a coupled predictive approach of crack growth in single crystal. The process of material damage during fatigue crack growth is described using an irreversible cohesive zone model, which governs the separation of the crack flanks and eventually leads to the formation of free surfaces. The cohesive zone element is modeled to accumulate fatigue damage during loadings and no damage during unloadings. This paper presents the damage model and its application in the study of the crack growth for precracked specimens. The use of cohesive zone approach is validated through a convergence study. Then, a general procedure of parameters calibration is presented in pure fatigue crack growth. In the last section, an extension of the cohesive zone model is presented in the case of creep–fatigue regime at high temperature. The model showed its capability to predict with a good agreement the crack growth in the case of complex loading and complex specimen geometries.

Effect of styrene/ethylene/butylene/styrene block copolymer on the dynamic mechanical behaviour and processability of high-impact polystyrene

Abstract Blends of high-impact polystyrene (PS-HI) and styrene/ethylene/butylene/ styrene block copolymer (SEBS) were investigated to determine the effects of rubber on the polymer properties. The processing behaviour of the PS-HI + SEBS blends was analyzed and the dynamic mechanical behaviour of the processed blends was examined. The rheological behaviour of the prepared blends during processing was followed by measuring the torque, output and melt pressure in a twin extruder. Dynamical mechanical analysis was performed in the temperature range of -150 to 160°C. The blends were also investigated in the creep fatigue regime and relaxations were determined at 25 - 65°C. Master curves for the reference temperature 25°C were created using the time-temperature superposition principle. With increasing SEBS content there was not significant increase of the torque value for blends below a rubber content of 70 wt.-% with respect to neat PSHI. An influence of SEBS content on the increase of the apparent viscosity was observed above 40 wt.-% of SEBS. The curves of storage modulus E’, loss modulus E’’ and loss tangent tan δ vs. temperature are affected by lowering the hard phase (PS) content: tan δ of the soft phase ethylene/butylene (EB) increases, while tan δ for the hard phase and storage modulus decrease. All samples exhibit a single glass transition of the hard phase. At constant load the creep of PS-HI, SEBS and PS-HI + SEBS blends increases and the creep modulus decreases over a period of time for all samples examined. These effects are more pronounced in samples with lower hard phase content. The energy-time-temperature correspondence principle was applied to create master curves for the reference temperature 25°C for the creep modulus of PS-HI, SEBS and PS-HI + SEBS blends. Microscopic morphology results confirm the main conclusions obtained from the processing and dynamic mechanical behaviour of the blends. The phase-separated microstructure is more pronounced in PS-HI + SEBS blends. The rise in fracture energy with SEBS introduction appears in the whiteness of the SEM microphotographs, first as white crests, and in PS-HI + SEBS blends with higher SEBS content as globular structures.

Dynamic mechanical study of thermoplastic polyurethane/ polypropylene blends

Abstract Thermoplastic polyurethanes (TPU) are an important class of elastomers that found many novel and specialized applications where high mechanical and chemical performances are prerequisites. They are known for their good mechanical strength, wear and tear resistance, and low-temperature elasticity. The aim of this work is to study the viscoelastic behaviour of TPU, polypropylene (PP) and their blends using dynamic mechanical analysis. Blends of TPU+PP as well as samples of pure TPU and PP were prepared using a laboratory twin-screw extruder. Primary and secondary viscoelastic functions were determined. Primary viscoelastic functions, viz. storage modulus, loss modulus and loss tangent, were evaluated in the temperature range -100 to 250°C. The secondary viscoelastic functions creep, recovery and creep modulus were investigated in the creepfatigue regime at 25 - 65°C. A master curve at the reference temperature 25°C for the creep modulus of TPU, PP and TPU+PP blends was created by applying the time-temperature correspondence principle. The obtained results are discussed.

Dynamic mechanical behavior of polymer modified bitumen

The dynamic mechanical behaviour of bitumen (BIT) modified with styrene/butadiene/styrene block copolymer (SBS) were investigated. Dynamic mechanical analysis (DMA) were performed in the temperature range -80 to 60 °C. The primary viscoelastic functions were determined at the traffic frequency, 5 Hz. The BIT+SBS blends were investigated in creep fatigue regime at temperature 10, 20, 30, 40 and 50 °C. Dynamic mechanical behaviour of BIT+SBS blends depends on their morphological characteristics, number of phases, phase compositions and phase content in blend, as well as time and temperature. The curves of primary visco-elastic functions, storage modulus (E′), loss modulus (E″) and loss tangent (tg δ) vs. temperature of polymer modified bitumen differ from unmodified bitumen and indicate the presence of the swollen polybutadiene and polystyrene phases in bitumen phase. The curve E′ vs. temperature of polymer modified bitumen show the rubbery plateau. With the increment of SBS content the rubbery plateau is shifted to high temperatures. At the constant load the creep values of BIT-SBS blends increase and those of creep modulus decrease over a period of time. These effect are more pronounced in samples with higer content of SBS. The time-temperature correspondence principle was applied to create master curves for the reference temperature 10 °C for the creep modulus of BIT + SBS blends. The data were analysed using WLF and Arrhenius equations.

The SRM Lifetime Prediction Rule for the Creep-Fatigue Interaction Regime Supported by a Suitable Constitutive Model

The application of conventional lifetime prediction methods requires a certain knowledge about the stress-strain response of a material to the applied load. This should preferably be simulated by an appropriate constitutive model. The empirical rule of strain rate modified linear accumulation of creep damage (the SRM rule) for the lifetime prediction of metallic materials in the creep-fatigue interaction regime has been supported by Chaboche's viscoplastic constitutive model. An inherent criterion for the validity of a given prediction has been defined. The application of the proposed method and its ability to predict life expectancies in various cyclic loading conditions, different from those fatigue tests required for the calibration of the SRM rule, has been shown using the IN 738 LC superalloy as an example.

Estimation of crack growth in the creep range during plastic cyclic loading

Crack growth investigations were performed on the creep-resistant steel 13 CrMo 4 4 in the fatigue and the creep fatigue regime, especially regarding the influence of creep damage on crack growth. To this effect, 2% creep strain was applied to the material at a temperature of 560°C. The crack propagation rate was determined as a function of the specimen shape, temperature, test frequency and hold times. In the case of compact tension (CT-)specimens, creep pretreatment does not affect crack growth. For center-cracked tension (CCT-)specimens, however, the creep pretreatment results in a considerable increase in the crack propagation rate. Hold times of 90 minutes at maximum loading cause an increase in da/dN due to further cavity nucleation. The hold time at which cavity nucleation might occur is evaluated. The dependency on frequency of crack growth may be evaluated by means of a linear superposition of creep and fatigue crack growth. The transition frequency above which pure fatigue crack growth occurs is calculated and the regimes of fatigue, creep and creep—fatigue interaction with environmental influences are characterized.