LCF Tests Research Articles

Pengaruh rasio regangan terhadap perilaku Llow Cycle Fatigue (LCF) paduan Al 7075-T7

The aluminium 7075 alloy is widely used in the aircraft industry for components on the front spar, stabilizer, frames that requires a high strength and low density ratio.The aerospace and automotive industries need to design for lightweight materials, where aluminum alloys are often used as the main component.Aluminum alloyshave good corrosion resistance properties. This material is used in a wide field not only for household appliances but also for industrial purposes, for example air-craft, car, marine and other constructions.The phenomenon of ductility decreasing due to pre-fatigue deformation, is the most significant for the combination of long pre-fatigue time, high humidity, and low strain rate.The process of making specimen shapes and dimensions were prepared according to ASTM E8 standard for the tensile specimens and ASTM E606 standardfor LCF test, respectively. Characterization of changes in material structures and the types of failure that occuredwerecarried out using OM, SEM and EDX.

Investigation of fracture behaviour and low cycle fatigue properties of cryorolled Al-Mg alloy

In this work influence of cryogenic forming on mechanical and fatigue properties are investigated for Al-Mg (AA 5754) alloy. The received alloy is subjected to rolling at liquid nitrogen temperature (cryorolling) for 40% thickness reduction. The material’s performance against crack growth for as received and cryorolled alloys are evaluated by fatigue crack growth (FCG) test at the stress ratios (R-Ratios) of 0.1, 0.2 and 0.5. Brittle fracture toughness (KIc) and ductile fracture toughness (JIc) are experimentally evaluated for both as received and cryorolled alloys. A series of uniaxial strain controlled low cycle fatigue tests (LCF) at room temperature are conducted to evaluate fatigue performances of the alloys. Significant improvements in yield and tensile strength are observed for cryorolled AA 5754. FCG tests exhibit lower value of the fatigue threshold and higher value of the fatigue life for cryorolled alloys. Increase in brittle fracture toughness (KIc) and decrease in ductile fracture toughness (JIc) value is observed for cryorolled alloys. Cryorolled alloys show mild hardening in the initial cycles followed by noticeable cyclic softening at all strain amplitudes during LCF tests. The hysteresis loops and fatigue lives have been investigated for both as received and cryorolled alloys. The extended finite element method (XFEM) has been utilized to simulate the elastic-plastic crack growth during fracture toughness evaluation. Heaviside function and asymptotic crack tip enrichment functions are used for modelling of cracks in compact tension and three point bend specimens. LCF behaviour of the material is modelled by finite element method (FEM) using Chaboche kinematic hardening model. The fracture surface morphologies obtained from different tests are studied by scanning electron microscopy (SEM).

Creep-fatigue design rules for cyclic softening steels

Based on existing lifetime and damage models a new approach is developed for the assessment of creep-fatigue lifetime of components made out of cyclic softening steel, in particular EUROFER97. For the application of the new creep-fatigue rules among others lifetime data from special tests, in particular creep tests on pre-fatigued specimens are required. Therefore, a test matrix is proposed and conducted for EUROFER97 in which the efforts are minimized by utilizing the Monkman-Grant relationship and its independence on pre-cyclic deformation and cyclic softening, respectively. To facilitate their implementation in existing design criteria the new creep-fatigue rules are simplified by introducing the cyclic softening stress factor with which the impact of cyclic softening on creep rupture time is considered. The assessment of creep-fatigue in LCF tests with hold-times performed on EUROFER97 using the new rules demonstrates their applicability to cyclic softening ferritic-martensitic steels without being overly conservative. A draft for implementing the simplified rules in the respective chapter of structural design code is elaborated in addition.

Life Prediction of IN738LC Considering Creep Damage under Low Cycle Fatigue

As growing attention on greenhouse gas reduction, developing an eco-friendly power generation system of power plants and enhancing its efficiency has been actively researched. In order to increase the efficiency, the operating temperature has been raised, and hence the application of Ni-base super-alloy has increased to a core component of the power generation system like a turbine blade, a rotor and a boiler. Especially, turbine blades are exposed to high-temperature and high-pressure, and they bear mechanical loads due to the centrifugal force of the high-speed rotation. Thus, the durability of the blade material should be tested under high temperature conditions. In high temperature LCF tests, damage occurs in specimens because of creep. Hence, the aim of this study is to estimate the life of LCF precisely by considering the creep damage in specimens generated during the LCF test. We conducted the LCF tests to propose a new life prediction method considering the fatigue and creep damage. Finally, this study demonstrates the reliability of the prediction method by comparing the predicted life with the actual fatigue life. Through this, we expect to predict the fatigue life more accurately by predicting the fatigue life under different fatigue strain frequencies considering the creep damage.

Ratcheting behavior of Eurofer97 at 550 °C

The components in reactor are usually exposed to cyclic loading caused by frequent startups, shutdown and load fluctuations which lead to ratcheting effect. Eurofer97 is a structural material candidate for in-vessel components of future fusion reactor. To investigate the ratcheting behavior of Eurofer97, uniaxial stress-controlled cyclic loading tests are performed at 550 °C. Ratcheting rates under various loading conditions, including various peak stresses, stress ratios and stress rates are determined to build the database of Eurofer97 for future application in the blankets of fusion reactor. An already developed unified visco-plastic constitutive model for mod.9Cr-1Mo FM steels is further applied to adapt the ratcheting behavior of Eurofer97. The developed model describes both cyclic softening in strain-controlled LCF tests and its impact on the ratcheting behavior of mod.9Cr-1Mo FM steel fairly good, at both room temperature and 550 °C. Verifying whether the current model can also describe the ratcheting behavior of Eurofer97 is a further aim of the work reported here.

The Attempt of the Low-Cycle Fatigue Life Description of Chosen Creep-Resistant Steels Under Mechanical and Thermal Interactions

AbstractThe study focuses on the problem of determination of low-cycle fatigue properties for the chosen group of creep-resistant steels used in the power and chemical industries. It tries to find the parameter which would describe well the fatigue life and take into account mechanical loads and temperature. The results of LCF tests have been presented in the paper. New parameter P has been introduced. This parameter joins a plastic strain range, a stress range and temperature. The fatigue life has been predicted versus parameter P. The comparison of the predicted and observed values of fatigue life shows the agreement between these values. The method of fatigue life prediction formulated in this way is expected to describe the behavior of materials under thermo-mechanical fatigue.

Experimental and numerical characterization of low cycle fatigue and creep fatigue behaviour of P92 steel welded joint

AbstractLow cycle fatigue (LCF) and creep fatigue interaction (CFI) behaviour of P92 steel welded joint were investigated experimentally and numerically. Strain‐controlled LCF tests at different strain amplitudes and CFI tests at different peak strain holding time were conducted. Evolutions of cyclic stress response, mean stress, and creep strain during cycling were described, in which the influence of strain amplitude and holding time were investigated. A specific heat treatment process was proposed to get the homogenous simulated material of fine grain region and coarse grain region in the heat affected zone. Material parameters of parent material, fine grain heat affected zone, coarse grain heat affected zone, and weld metal in the unified viscoplasticity model were then determined and validated. To predict the LCF and CFI behaviour of welded joint, 3‐dimensional unified viscoplasticity model with a modified isotropic variable was compiled into ABAQUS UMAT. The comparison between the predicted and experimental result under LCF and CFI loadings showed that the simulation results were reasonable and agreed with the experimental data well.

Accelerated material data generation for viscoplastic material models based on complex LCF and incremental creep tests

Viscoplastic material models are quite important for realistic characterization of deformation behaviour of high-temperature components. However, their material parameter adjustment is expensive and usually requires extensive material testing. The high-temperature deformation behaviour of cast iron GJS X SiMo 4.1 was determined by accelerated complex material testing methods in order to establish a cost-effective identification of viscoplastic material model parameters. Incremental creep tests with mixed-load and temperature steps, together with relaxation and stepped-tensile tests, were carried out in order to determine the stress-dependent creep strain rate at characteristic temperatures and stresses. The cyclic plastic deformation behaviour was studied with complex low-cycle fatigue (C-LCF) evaluations at varying strain rates, amplitudes and temperatures. Thereafter, the results were employed to identify material model parameters of a Chaboche-type viscoplastic material model, which was applied to represent the cyclic deformation and creep behaviour with a correct representation of strain-rate effects at low strain rates.

Microstructural Aspects of Fatigue Parameters of Lead-Free Sn-Zn Solders with Various Zn Content

Abstract The study includes the results of research conducted on selected lead-free binary solder alloys designed for operation at high temperatures. The results of qualitative and quantitative metallographic examinations of SnZn alloys with various Zn content are presented. The quantitative microstructure analysis was carried out using a combinatorial method based on phase quanta theory, per which any microstructure can be treated as an array of elements disposed in the matrix material. Fatigue tests were also performed using the capabilities of a modified version of the LCF method hereinafter referred to in short as MLCF, which is particularly useful in the estimation of mechanical parameters when there are difficulties in obtaining many samples normally required for the LCF test. The fatigue life of alloys was analyzed in the context of their microstructure. It has been shown that the mechanical properties are improved with the Zn content increasing in the alloy. However, the best properties were obtained in the alloy with a chemical composition close to the eutectic system, when the Zn-rich precipitates showed the most preferred morphological characteristics. At higher content of Zn, a strong structural notch was formed in the alloy because of the formation in the microstructure of a large amount of the needle-like Zn-rich precipitates deteriorating the mechanical characteristics. Thus, the results obtained during previous own studies, which in the field of mechanical testing were based on static tensile test only, have been confirmed. It is interesting to note that during fatigue testing, both significant strengthening and weakening of the examined material can be expected. The results of fatigue tests performed on SnZn alloys have proved that in this case the material was softened.

Low Cycle Fatigue behavior of AA7075 with surface gradient structure produced by Ultrasonic Shot Peening

Effect of ultrasonic shot peening (USSP) for different durations, with graded surface structures, was studied on low cycle fatigue behavior of the aluminium alloy 7075 (AA7075) at room temperature. LCF tests were conducted at different total strain amplitudes. Surface microstructures of the USSP treated specimens were characterized using X-ray diffraction and transmission-electron microscopy. No phase change was found due to USSP. Surface region of the ultrasonically shot peened sample was found to develop nanosize grains of 16-20 nm. Significant improvement was observed in LCF life of the specimen subjected to USSP for 180 seconds. However, LCF life of the sample USSPed for long duration of 300 seconds was reduced. The higher fatigue life resulting from USSP for 180 seconds, may be attributed to increased resistance of the USSPed sample against fatigue crack initiation, due to grain refinement in the surface region.

Characterization and modeling of the ratcheting behavior of the ferritic–martensitic steel P91

The ratcheting behavior of the 9%-Cr-1%-Mo ferritic–martensitic (FM) steel P91 is investigated by uniaxial strain- and stress-controlled cyclic loading tests at room temperature and 550 °C. Ratcheting rates under multiple loading conditions are measured to build the database of P91 for the further application in generation IV fission reactors. The unconventional asymmetry of stress under strain-controlled tests at room temperature predicts the non-zero ratcheting with zero mean stress, which is approved in the stress-controlled tests. A unified viscoplastic deformation model for RAFM steels is further modified to adapt the ratcheting behavior of P91. The current model describes cyclic softening in strain-controlled LCF tests very well. However it strongly overestimates the uniaxial ratcheting rates in stress-controlled tests, due to application of the Armstrong–Frederick dynamic recovery rule. Based on further analysis of back stresses (BS), a new rule for dynamic recovery is designed to fit the ratcheting rates under multiple loading conditions.

Low Cycle Fatigue behavior of SMAW welded Alloy28 superaustenitic stainless steel at room temperature

This paper focused on the study of Low Cycle Fatigue of welded joints of superaustenitic (Alloy28) stainless steels. Chemical composition and microstructure investigation of Base Metal (BM) and Weld Metal (WM) were identified. The results showed that both of composition is fully austenitic with a dendritic microstructure in the WM. Low cycle fatigue tests at different strain levels were performed on Base Metal (BM) and Welded Joint (WJ) specimens with a strain ratio Rε=−1. The results indicated that the fatigue life of welded joints is lower than the base metal. This is mainly due to the low ductility of the Welded Metal (WM) and the presence of welding defects. Simultaneously, Scanning Electron Microscope (SEM) observations of fractured specimens show that WJ have brittle behavior compared to BM with the presence of several welding defects especially in the crack initiation site. An estimation of the crack growth rate during LCF tests of BM and WJ was performed using distance between striations. The results showed that the crack initiation stage is shorter in the case of WJ compared to BM because of the presence of welding defects in WJ specimens.

Thermo Mechanical Fatigue Testing of GTD 111 Superalloy for Use in Gas Turbine Blades

Design of Turbo machinery is complex and efficiency is directly related to material performance, material selection is of prime importance. Temperature limitations are the most crucial limiting factors to gas turbine efficiencies. This paper presents the life of GTD 111 applied to gas turbine blade based on LCF and TMF test results. The LCF tests were conducted under various strain ranges based on gas turbine operating conditions. In addition, IP (in-phase) and OP (out of-phase) TMF tests were conducted under various strain ranges. The paper will focus light on above issues and each plays an important role within the Gas Turbine Material literature and ultimately influences on planning and development practices. It is expected that this comprehensive contribution will be very beneficial to everyone involved or interested in Gas Turbines.

Understanding the “blue spot”

Abstract During hot component fatigue tests there have been two cases of low life crack initiation of gas turbine rotating parts manufactured from the Titanium alloy Ti-6246. Both exhibited a small (~ 0.1 mm) elliptical ‘blue spot’ at the origin. Through validated striation count work and fracture mechanics it was established that fatigue had propagated with a near-nil initiation life. Early investigation suggested that the ‘blue spot’ was possibly a region of stage 1 fatigue growth, and was therefore a material behaviour concern with potential implications for service. During an investigation of a later cracking incident in this alloy, subsequently shown to have resulted from stress-corrosion cracking (SCC), near-identical fractographic characteristics to that seen in the “blue spot” were found that subtly differentiated it from stage 1 fatigue. Also, similar ‘blue spots’ have since been identified on Ti6246 Laboratory hot LCF test specimens and found to have been due to contamination by NaCl, through the application of focussed long-term EDX examination and other novel chemical analyses techniques. By the application of those techniques, fractography, and comparison against these specimens, Rolls-Royce and Imperial College London have collaborated to show that the original two component ‘blue spots’ were subtle examples of NaCl-related Hot Salt Stress-Corrosion Cracking (HSSCC). Such cracking has not been found to occur in service components, due to air pressure within the engine, and the effect is therefore confined to Laboratory and component tests at near-atmospheric pressure or below.

Austenitic high interstitial steels vs. CoCrMo – Comparison of fatigue behavior

The favourable combination of strength and ductility as well as excellent corrosion resistance of CrNiCN-austenitic steels is the reason for their use in biomedical engineering. However, Ni was attributed to cause allergies within a certain number of patients. Thus Ni-free alternatives like CoCrMo- and Ti-base alloys are often preferred. Other possible alternatives might be austenitic high nitrogen steels (AHNS) and austenitic high interstitial steels (AHIS). The aim of this study is to compare the fatigue properties of Ni-free austenitic CrMn(Mo)CN-steels with 0.85–1.07wt% C+N, CoCrMo-alloy with up to 0.25wt% C and conventional CrNiCN-steels with 0.01–0.42wt% C+N.The comparison was conducted using total strain controlled axial LCF and HCF fatigue tests at room temperature on solution-annealed samples. After mechanical testing the samples were investigated by means of SEM including EBSD and TEM in order to analyse the distribution of plastic strains.In contrast to the CrNiCN-steels, which show cyclic hardening before the generation of the first crack, the CrMn(Mo)CN as well as the CoCrMo-alloys only showed cyclic softening. Still within the HCF regime the Co-base alloys showed the best fatigue behavior in terms of numbers of cycles to fracture Nf for a given total strain amplitude, while the Ni-free CrMn(Mo)CN-steels were either as good or slightly worse. The results indicate a distinct correlation between slip behavior, stacking fault energy and the amount of interstitials in solid solution.

The Influence of Corrosion Damage on Low Cycle Fatigue Life of Reinforcing Steel Bars S400

In this study the influence of corrosion and seismic load (low-cycle fatigue LCF) in the mechanical performance of reinforcing steel bars of S400 grade with 10 mm nominal diameter was investigated. There took place 140 tensile, LCF and salt spray tests which performed on reinforcing bars in different conditions. The results show that the corrosion level and surface conditions are the main parameters which affect to the low-cycle fatigue life of reinforcing bars. Moreover, through a non-linear regression analysis of the experimental data, a model of predicting the expectancy life of the corroded rebars was conducted. This prediction was based on two models: the first model was about an imposed of (total) strain amplitudes (eα) and the second model on predicting the strength degradation per cycle of fatigue in correlation with the plastic strain amplitudes (ep). Both the experimental study and the prediction modeling conducted for the same steel grade S400 with and without ribs. The model prediction of non-linear regression analysis, show a good agreement with the observed experimental results and adequately confirmed the experimental results showing that from the first levels of corrosion, the degradation of their life expectancy was obvious as well the rebars without ribs (smoothed) which present more advanced mechanical behavior and life expectancy against to the respective ribbed rebars.

Rotor Design Optimisation through Low Cycle Fatigue Testing

The current work aims at modifying the rotor geometry to improve the alternator performance. For that purpose, the fatigue behaviour of the material needs to be defined. Experimental tests were conducted on the Fe-3Si steel constituting the rotor. It is shown that this material hardens under cyclic loadings and TEM observations revealed unusual damage mechanisms. From the experimental LCF tests, the material parameters needed for FEA and the fatigue resistance law have been defined. The FEA results were validated on smooth and notch samples. Finally, the rotor design is modified which enables a light-weighting of the magnet or an increase of the torque value depending on the considered rotor and purpose.

Influence of Surface Morphology on the Fatigue Behavior of Metastable Austenitic Steel

Using a low temperature turning process with carbon dioxide cooling in the cutting zone a variation of the morphology at the specimen surfaces of the metastable austenitic steel AISI 347 was realized. In LCF and HCF fatigue tests at ambient temperature and 300 °C the influence of the surface morphology on the cyclic deformation behavior and fatigue life was investigated by the measurement of stress-strain hysteresis. An additional magnetic measurement allows the characterization of the phase transformation from paramagnetic austenite in ferromagnetic α´-martensite during the turning processes and during cyclic loading. The surface morphology was studied in detail by SEM and x-ray investigations.

Effect of graphite degradation on the LCF properties of nodular cast iron

The effect of degraded microstructure on the tensile and LCF properties was studied on a casing of a turbocharger, which exhibited locally irregular graphite formation. The tensile tests showed that the ductility values of the zone with degraded graphite decreased strongly compared to the zone with normal graphite, while the strength values decreased only slightly. Based on these results and a LCF reference curve for nodular cast iron with normal graphite structure, a synthetic LCF curve was generated for the material condition with degraded graphite structure. LCF tests on specimens with irregular graphite structure confirmed the predicted LCF behaviour quite good.

Creep-fatigue interactions in equiaxed and single crystal Ni-base superalloys

Ni-base superalloys are employed as structural materials for the most critical hot gas path components of gas turbines. The current market requirement is to cycle the machine every day, providing energy when it is most needed. It is therefore important to understand how creep and fatigue damages interact in these components. Starting from a significant knowledge base of mechanical and microstructural behaviour established from standard tests of the equiaxed and single crystal superalloys, creep-fatigue tests have been performed to evaluate how the two damage conditions develop together. The creep-fatigue testing conditions represent the maximum temperature and strain at the critical locations in real components, while the position of hold-time has been varied from tensile to compressive to understand the effect on reduction in crack initiation endurance with respect to standard LCF tests and on the microstructural mechanisms. The experimental test results have been explained in terms of microstructural evolution and they have been correlated to that observed at critical locations in real components.