Creep-fatigue Tests Research Articles

Research on Application of Supported Vector Machine to Creep-Fatigue Lifetime Prediction of P91 Steel

Creep-fatigue tests were carried out to study creep-fatigue property of P91 steel in many references. There are two kinds of loading control methods. One is controlled by stress, and the other is controlled by strain. In this study the creep-fatigue test data loading controlled by stress are summarized. Based on these data Supported Vector Machine(SVM) method is used to build up the models to predict the creep-fatigue lifetime of P91 steel. The results show SVM method can be used to predict the creep-fatigue life of P91 steel, and the value of insensitive factor is very important. When the value of insensitive factor is small and the training error of the model is small too, but the prediction ability decreases. On the contrary when insensitive factor is increased, the training error will become large, and the prediction ability will increase. So during the training, mean square error should not be regarded as the unique goal function.

Validation of a Phenomenological Lifetime Estimation Method with Biaxial Experiments at High Temperature

Modern 9-12%Cr steels are widely used for steam turbine components. For the design optimization and lifetime estimation of steam turbines, it is very important to investigate and describe the deformation as well as crack initiation behavior at critical location of steam turbine components under multiaxial service-type loading conditions. In this paper a phenomenological lifetime estimation method was validated by multiaxial loading. The applicability and reliability of this lifetime estimation model was confirmed by a recalculation of biaxial creep fatigue tests performed on cruciform specimens of rotor steel X12CrMoWVNbN10-1-1 successfully.

Creep-fatigue life assessment for three kinds of solders

This paper presents a creep-fatigue life evaluation for three kinds of solders of Sn–37Pb, Sn– 3.5Ag and Sn – 3.0Ag – 0.5Cu. Strain controlled push – pull creep-fatigue tests were performed for solid bar specimens under fast–fast, fast–slow, slow–fast and strain–hold strain waveforms and the effect of strain waveforms on creep-fatigue lives is discussed. Sn– 3.5Ag and Sn– 3.0Ag – 0.5Cu solders showed stronger creep-fatigue resistance compared to Sn –37Pb solder. The applicability of the linear damage rule, the strain range partitioning method and the grain boundary sliding model is discussed for predicting creep-fatigue lives of the three solders. The strain range partitioning method and the grain boundary sliding model satisfactorily predicted creep-fatigue lives within a small scatter.

Low cycle fatigue and creep-fatigue behavior of Ni-based alloy 230 at 850 °C

Strain-controlled low cycle fatigue (LCF) and creep-fatigue testing of Ni-based alloy 230 were carried out at 850°C. The material creep-fatigue life decreased compared with its low cycle fatigue life at the same total strain range. Longer hold time at peak tensile strain further reduced the material creep-fatigue life. Based on the electron backscatter diffraction, a novel material deformation characterization method was applied, which revealed that in low cycle fatigue testing as the total strain range increased, the deformation was segregated to grain boundaries since the test temperature was higher than the material equicohesive temperature and grain boundaries became weaker regions compared with grains. Creep-fatigue tests enhanced the localized deformation, resulting in material interior intergranular cracking, and accelerated material damage. Precipitation in alloy 230 helped slip dispersion, favorable for fatigue property, but grain boundary cellular precipitates formed after material exposure to the elevated temperature had a deleterious effect on the material low cycle fatigue and creep-fatigue property.

Experimental and modeling results of creep–fatigue life of Inconel 617 and Haynes 230 at 850 °C

Creep–fatigue testing of Ni-based superalloy Inconel 617 and Haynes 230 were conducted in the air at 850°C. Tests were performed with fully reversed axial strain control at a total strain range of 0.5%, 1.0% or 1.5% and hold time at maximum tensile strain for 3, 10 or 30min. In addition, two creep–fatigue life prediction methods, i.e. linear damage summation and frequency-modified tensile hysteresis energy modeling, were evaluated and compared with experimental results. Under all creep–fatigue tests, Haynes 230 performed better than Inconel 617. Compared to the low cycle fatigue life, the cycles to failure for both materials decreased under creep–fatigue test conditions. Longer hold time at maximum tensile strain would cause a further reduction in both material creep–fatigue life. The linear damage summation could predict the creep–fatigue life of Inconel 617 for limited test conditions, but considerably underestimated the creep–fatigue life of Haynes 230. In contrast, frequency-modified tensile hysteresis energy modeling showed promising creep–fatigue life prediction results for both materials.

A double edge notch specimen design for tension–compression fatigue crack growth testing

Compact, C(T), specimens are commonly used for fatigue and creep-fatigue crack growth testing under constant-load-amplitude conditions. The use of the standard C(T) specimens [ASTM] is limited to positive load ratios. They are also limited in the amount of crack growth data that can be developed at high stress intensity values due to accumulation of plastic and/or creep strains leading to ratcheting in the specimen. Testing under displacement control can potentially address these shortcomings of the load-controlled tests for which the C(T) geometry is unsuitable. A double edge notch tension–compression, DEN(T–C), specimen to perform displacement controlled creep-fatigue crack growth testing is developed and optimized with the help of finite element and boundary element analyses. The specimen has threaded cylindrical ends for applying the loads. The test section is rectangular with a half width, W, and half height, H, and a constant thickness, B. The height to width ratio, H/W, of the specimen must be kept low to avoid buckling during compressive loading while ensuring that the stress-intensity parameter values are not strongly dependent on the chosen H/W value. Simple optimization analysis shows that a H/W ratio of 1.2 is likely to best meet these requirements. However, only performing actual experiments will confirm whether this choice of H/W value is optimum.In the finite element analyses, the specimen is modeled using an effective height (heff) to width (W) ratio, heff/W, of 1.46 to account for the fillet regions on the two ends of the specimens. Accurate expressions for estimating the fracture mechanics crack tip parameters such as the stress intensity parameter, K, the crack mouth opening displacement (CMOD), and the load-line displacement (LLD) are developed over a wide range of crack sizes for the DEN(T–C) specimen. Expressions are also developed for crack size as a function of specimen compliance.

Microstructure evolution in a 2618 aluminium alloy during creep-fatigue tests

Abstract Microstructure changes in the 2618 aluminium alloy during creep-fatigue tests were studied. These tests simulate the conditions of the application of this alloy in devices for the exhaustion of hot gasses generated during fire in closed or difficultly accessible areas. Creep-fatigue tests result in high dislocation density in subgrains and narrow subgrain boundaries, in contrast to creep tests reported in our previous work where large subgrains were observed with relatively wide subgrain boundaries and relatively low dislocation density in grains. Extensive precipitation occurred with denuded (precipitate-free) zones along grain boundaries. The coherent S-phase (Al2CuMg) transformed into partially coherent needle-shaped S′ precipitates. Superior stress amplitude caused reduced lifetime and wider denuded zones. A model of the formation of denuded zones along (sub)grain boundaries was proposed.

The effect of recycled concrete aggregate and steel slag on the dynamic properties of asphalt mixtures

A high percentage of asphalt mixtures (in terms of weight and volume) are composed of aggregates; thus, new aggregates are needed to build and maintain roads. The use of waste material as aggregates in the pavement industry can prevent the accumulation of waste material and environmental pollution and can reduce primary production costs. Thus, the objective of the present study was to determine the mechanical properties of asphalt mixtures with waste concrete and steel slag aggregates. Six different asphalt mixtures containing three types of aggregate (dacite, recycled concrete and steel slag) were used to prepare Marshall specimens and to determine the optimal asphalt binder content. The mechanical characteristics of the mixtures were evaluated by Marshall stability, indirect tensile resilient modulus, dynamic creep and indirect tensile fatigue tests. The results indicated that the optimal mixture contained steel slag coarse aggregates and recycled concrete fine aggregates.

Effect of ratchet strain on fatigue and creep–fatigue strength of Mod.9Cr–1Mo steel

► Uniaxial fatigue and creep–fatigue tests with superimposed strain were performed. ► Variety of superimposed strain were applied as ratchet strain in the tests. ► Effect of superimposed strain on fatigue and creep–fatigue life is negligible. ► A cyclic softening character reducing the effect of superimposed strain. The effect of ratcheting deformation on fatigue and creep–fatigue life in Mod.9Cr–1Mo steel was investigated. Uniaxial fatigue and creep–fatigue testing with superimposed strain were performed to evaluate the effect of ratcheting deformation on the failure cycle. In a series of tests, a specific amount of superimposed strain was accumulated in each cycle. The accumulated strain as ratcheting deformation, cycles to reach the accumulated strain, and test temperatures were varied in the tests. In the fatigue tests with superimposed strain at 550 °C, slight reductions of failure lives were observed. All of the numbers of cycles to failure in the fatigue tests with superimposed strain were within a factor of 1.5 of that of the fatigue test without superimposed strain at 550 °C. The apparent relationship between failure cycles and testing parameters was not observed. In fatigue tests with superimposed strain at 550 °C, maximum mean stress was insignificant and generated in early cycles because Mod.9Cr–1Mo steel exhibits cyclic softening characteristics. It was assumed that suppression of mean stress generation by cyclic softening reduces the effect of ratcheting strain. Conversely, failure lives were increased by accumulated strain in the test conducted at 450 °C because of stress–strain hysteresis loop shrinkage caused by cyclic softening induced by the accumulated strain. In the creep–fatigue tests with superimposed strain, test results indicated that the accumulated stain was negligible. It was concluded that the effect of ratcheting deformation on fatigue and creep–fatigue life is negligible as long the parameters are within strain limits of the design and construction code for fast reactors (inelastic strain at surface: <2%, which is allowable maximum inelastic strain in code).

크리프와 반복 피로하중에 의한 폴리에틸렌의 실시간 구조 변화

일정 하중에 지속적으로 노출되는 고분자의 장기간 사용을 위해서는 재료의 수명을 평가할 수 있는 가속화된 시험 방법이 필요하다. 반복 피로하중 시험법은 이러한 방법들 중 하나로 많은 관심을 받고 있다. 본 연구에서는 X-선 회절법을 이용하여 고밀도 폴리에틸렌의 반복 피로하중에 의한 구조적 변화와 크리프 변형을 비교하고자 하였다. 이를 위하여 별도의 인장시험기를 제조, X-선 회절기에 부착하여 장시간 변형 과정을 성공적으로 관찰하였다. 그 결과 크리프와 반복 피로하중 사이의 거시적이고 뚜렷한 차이에도 불구하고 결정화도, 결정크기 및 면간거리와 같은 결정의 미세구조는 두 방법에서 거의 동일하게 관찰되었다. 그러나 항복점 전(BYP), 항복점(YP) 그리고 항복점 후(AYP)로 각각 변형시킨 후 시험한 시료의 경우 AYP와 다른 두 시료간 뚜렷한 구조적 차이를 확인할 수 있었다. Long-term performance of polymer under constant sustained load has been the main research focus, which created a need for the accelerated test method providing proper lifetime assessment. Cycling fatigue loading is one of the accelerated test method and has been of great interest. Microstructure change of high density polyethylene under cyclic fatigue loading and creep was examined utilizing a tensile device specially designed for creep and fatigue test and also can be attachable to the X-ray diffractometer. In this way, the crystal morphology change of polyethylene under creep and cyclic fatigue load was successfully monitored and compared. Despite the marked differences in macroscopic deformation between the creep and cyclic fatigue tests, crystal morphology such as crystallinity, crystal size, and <TEX>$d$</TEX>-spacing was as nearly identical between the two test cases. Specimens pre-deformed to different strains, i.e., before yield point (BYP), at yield point (YP) and after yield point (AYP), however, showed markedly different changes in crystal morphology, especially between AYP and the other two specimens.

Effect of Pre-Strain and Ratcheting Strain on Fatigue and Creep-Fatigue Lives in Mod.9Cr-1Mo Steel

The effect of ratcheting deformation and pre-strain on fatigue and creep-fatigue life in Mod.9Cr-1Mo steel was investigated. Uni-axial fatigue and creep-fatigue tests with pre-strain and progressive strain were performed to evaluate the effect of pre-strain and ratcheting strain on the failure cycle. In a series of tests, a specific amount of progressive strain was accumulated in each cycle. The accumulated strain as ratcheting deformation and cycles to reach the accumulated strain were varied in the tests. In the fatigue tests with pre-strain, failure lives were not declined. In the fatigue tests with progressive strain, slight reductions of failure lives were observed, however, they were within a factor of 1.5 of the failure life in normal fatigue test. The apparent relationship between failure cycles and testing parameters were not observed. Analyses of the test results indicated that the effect of ratcheting strain on the fatigue failure life is negligible as long as the parameters are within the strain limit of FR code (strains at surface less than 2%). In the creep-fatigue tests with progressive strain, the same conclusion of the fatigue tests was obtained. In both kinds of tests, maximum mean stresses during the tests were insignificant and/or generated in early cycle in the tests, and this character is considered as a reason of that the effect of ratcheting deformation on the fatigue and creep-fatigue lives are insignificant.

Creep-fatigue life assessment for three kinds of solders

This paper presents a creep-fatigue life evaluation for three kinds of solders of Sn–37Pb, Sn– 3.5Ag and Sn – 3.0Ag – 0.5Cu. Strain controlled push – pull creep-fatigue tests were performed for solid bar specimens under fast–fast, fast–slow, slow–fast and strain–hold strain waveforms and the effect of strain waveforms on creep-fatigue lives is discussed. Sn– 3.5Ag and Sn– 3.0Ag – 0.5Cu solders showed stronger creep-fatigue resistance compared to Sn –37Pb solder. The applicability of the linear damage rule, the strain range partitioning method and the grain boundary sliding model is discussed for predicting creep-fatigue lives of the three solders. The strain range partitioning method and the grain boundary sliding model satisfactorily predicted creep-fatigue lives within a small scatter.

Influence of surface treated fly ash with coupling agent on asphalt mixture moisture damage

Moisture damage is of great negative impacts on asphalt pavement. It exacerbates the service performance (e.g., rutting, cracking) for asphalt mixture under traffic load. This paper describes the influence of surface treated fly ash with silane coupling agent on asphalt mixture moisture damage. The filler combined with coupling agent and fly ash was named compound fly ash modifier (CFAM). Primarily the properties of CFAM were characterized by Scanning Electron Microscopy and Fourier Transform Infrared Spectroscopy. As filler, its effects on moisture sensitivity of asphalt mixture were evaluated by modified Lottman test, indirect tensile stiffness modulus test, static creep and indirect tensile fatigue test. It is shown that the asphalt mixture with CFAM has excellent moisture sensitivity in terms of higher indirect tensile strength and tensile strength ratio during frost–thaw process. In addition, after specific moisture damage treatments, CFAM modified asphalt mixtures show better stiffness modulus, resistance to permanent deformation and fatigue life. It is concluded that CFAM can improve the moisture sensitivity of asphalt mixture effectively.

Comparison of permanent deformation and fatigue resistance of hot mix asphalts prepared with the same performance grade binders

In this study, the volumetric properties, Marshall stabilities, the indirect tensile stiffness moduli, and the fatigue and permanent deformation strengths of hot mix asphalts prepared with the same performance grade binders in accordance with Superpave method at optimum binder contents were comparatively investigated. In addition, the effect of bitumen modification using SBS on the mechanical characteristics of the mixtures was evaluated. Dynamic creep and indirect tensile fatigue tests were conducted at two distinct stress levels and three different loading periods. As the binder tests, it was determined that the modified bitumen obtained by adding of 3% SBS by weight into the B160/220 binder yielded the same level of performance with the B70/100 bitumen. It was also found that using 3% SBS by weight for the purpose of bitumen modification enhanced the desired properties of the mixture to a significant degree. It was observed that the best performance belonged to the mixtures containing B70/100 bitumen, while the worst performance was found for the mixtures made from B160/220 bitumen. Additionally, although the mixtures prepared by the same performance grade binders in accordance with the Superpave system were expected to behave similarly, it was found that they differed considerably with respect to their stiffness, tensile strength and strength against permanent deformation.

Characterizations of Creep-fatigue Crack Initiation and Growth Life for P92 using Circular Notched Round Bar Specimen

Abstract A testing method under creep-fatigue interaction conditions has been proposed as the ASTM standard E2714-09. In accordance with the ASTM standard, round bar smooth specimens have been recommended as a testing specimen. Additionally, creep-fatigue crack initiation and growth life are usually defined as the attainment of a specific rate of decrease in the maximum tensile stress or the modulus of elasticity ratios. However, these criterions of crack initiation and growth life on ASTM standard are not characterized in terms of physical crack size. In this study, the creep-fatigue crack initiation and growth tests for P92 were conducted using circular notched round bar specimens. An attempt is made to measure the crack length during the tests by the direct current potential drop method, which results in the establishment of a high accuracy crack length measurement. Additionally, the life of creep-fatigue crack growth was characterized by linear cumulative damage law combined with the Q*concept which has been proposed as fracture parameter to describe the life of creep crack growth.

Evaluation of the Testing and Analysis Methods in ASTM E2760-10 Creep-Fatigue Crack Growth Testing Standard for a Range of Steels

Abstract The development of the ASTM E2760-10, “Creep/Fatigue Crack Growth Testing” standard has initiated a phase of testing, analysis, and round robin initiatives associated with high temperature cyclic loading crack growth tests. Creep and fatigue are two complex independent mechanisms which may assist each other to drive the crack. A simplification of the macro creep/fatigue crack growth response of structures is therefore required. Using a linear cumulative damage method, as proposed in ASTM E2760, and other codes of practice, data for a range of steels are analysed. Limited crack growth data, using test methods similar to ASTM E2760, from previous collaborative projects for a range of steels are presented and analysed to identify the difficulties in the process. Creep-fatigue crack growth tests at frequencies ranging from 10 to 0.001 Hz are examined to consider the testing methodologies and in order to assess the effect of data scatter, dwell times, and the interaction region between creep crack growth (CCG) and fatigue crack growth mechanisms. Given the limited data available and the level of scatter, it can still be shown that the linear cumulative summation of static and high frequency data may be sufficient to predict creep/fatigue interaction both in terms of C* and ΔK, and the level of creep ductility will dictate appropriateness of the correlating parameter employed. Finally, in order to show that the level of data scatter can be dealt with in a predictive manner, the crack initiation and growth rates are also analysed using the NSW CCG model.

In situ observations on creep fatigue fracture behavior of Sn–4Ag/Cu solder joints

The creep fatigue behavior of Sn–Ag/Cu solder joints was investigated in this study. Low-cycle creep fatigue tests were conducted on an in situ tensile stage, the deformation morphologies were observed by scanning electron microscopy and the microstructure of the solder was characterized using an electron back-scattered diffraction system. The results show that the creep fatigue process can be divided into three stages: a strain hardening stage, a steady deformation stage and an accelerating fracture stage. During the initial few cycles the strain increases rapidly because the solder is soft. After strain hardening of the solder becomes saturated, the strain increases approximately linearly with increasing cycles, strain concentration occurs in the solder close to the solder/Cu 6Sn 5 interface and initial microcracks are generated. When the microcracks evolve into long cracks, the failure accelerates and the specimens fracture at the interface shortly thereafter. The creep fatigue strain is contributed by plastic deformation and creep of the solder. Grain subdivision occurs in the solder when the plastic strain reaches a threshold, and then grain rotation takes place in the newly formed grains to accommodate further straining. Dislocation climb is the major creep mechanism, and grain boundary sliding can also be promoted after subdivision.

Creep and Environmental Effects on the High Temperature Creep-Fatigue Behavior of Alloy 617

Abstract Alloy 617 is the leading candidate material for an intermediate heat exchanger (IHX) of the very high temperature reactor (VHTR), expected to have an outlet temperature as high as 950°C. Acceptance of Alloy 617 in Section III of the ASME Code for nuclear construction requires a detailed understanding of the creep-fatigue behavior. Strain-controlled low cycle fatigue (LCF) tests including hold times up to 1800 s at maximum tensile strain were conducted at 950°C and a total strain range of 0.3 % in air and in a simulated VHTR impure helium. For similar test conditions, the creep-fatigue behavior was globally similar in both environments. The fatigue resistance decreased when a hold time was added at peak tensile strain, consistent with the observed change in fracture mode from transgranular to intergranular. Further increases in the tensile hold time, beyond 180 s, were not detrimental to the creep-fatigue resistance. The influence of oxidation on the fatigue crack initiation and propagation is discussed. Bulk damage in the form of grain boundary cracking was present in the interior of the creep-fatigue specimens. This bulk cracking was quantified and found to be similar for hold times of up to 1800 s consistent with the saturation in failure lives.

Potential of recycled fine aggregates powder as filler in asphalt mixture

The main objective of this study is to investigate the potential of using recycled fine aggregates powder (RFAP) as filler in asphalt mixtures. RFAP is a by-product of the production of Recycled Concrete Aggregate (RCA). The disposal of RFAP may cause many serious environmental problems. It is necessary to find potential usage to avoid these problems. In this study, RFAP is used as filler in asphalt mixture by totally replacing conventional limestone powder (LP). Firstly, the properties of RFAP is studied by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and X-ray fluorescence spectroscopy (XRF). Then the indirect tensile strength and modulus tests, three-point bending tests, dynamic creep tests and fatigue tests are conducted to compare the properties between asphalt mixtures with RFAP and traditional asphalt mixture with LP. Results indicate that RFAP can improve the properties of asphalt mixture, such as including water sensitivity and fatigue resistance. However, it may cause a little decrease of the low-temperature performance. All the results indicate that the RFAP can be used in asphalt pavement, especially in hot regions.

Analysis of creep ductility in a ferritic martensitic steel using a hybrid strain energy technique

Creep rupture data for a modified 9% Cr 1% Mo ferritic martensitic steel was statistically analysed to provide a description of a ductility exhaustion approach for the evaluation of creep damage that occurs in creep–fatigue dwells. Energy approaches have been previously used; however, these require the use of the creep–fatigue test data themselves to identify the constants. This paper describes an approach based on a hybrid strain energy density formulation that uses creep rupture data and hence aligned with time fraction or classical ductility approaches. These current methods, however, tend to be overly conservative. Ductility models also tend to be statistically problematic when fitting to some datasets, particularly when stress and temperature are highly correlated. The method developed here was found to give the highest correlation for the tested models and was statistically robust for collinearity and heteroskedastic behaviour.