Papers by Keyword: Fatigue Damage

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Authors: Martin Petrenec, Jaroslav Polák, Tomáš Šamořil, Jiří Dluhoš, Karel Obrtlík
Abstract: In-situ Low Cycle Fatigue test (LCF) at temperature 635 °C have been performed in SEM on flat specimen ofcast Inconel 713LC superalloy. The aim of the investigation was to studymechanisms of the fatigue damage during elastic-plastic cycling by theobservations of the characteristic surface relief evolution and theaccompanying internal dislocation structures. The selected locations on thesurface were systematically studied in-situ and documented by SEM and usingAFM. The surface relief in the first tensile half-cycle was formed by numerousslip steps on the primary slip planes (111). In the following compressionhalf-cycle additional opposite slip were formed. The relief was modified in thenext cycles but without forming additionally new slip traces in the primarysystem. The reorientation of two grains in the gauge area was measured usingEBSD. At the end of cyclic loading the relation between surface persistent slipmarkings and persistent slip bands in the interior of the material wasdocumented by TEM on lamella prepared by FIB. The early stages of extrusion andintrusion formation were documented. The damage mechanism evolution is closelyconnected with the cyclic strain localization to the persistent slip bands thatare also places of fatigue crack initiation.
Authors: Li Zhang, Si Chu Gong, Xu Dong Ma
Abstract: A law on the cumulative damage is presented basing on total strain energy induced as damage parameter to calculate the cumulative damage when the specimens of concrete subjected to fatigue loading.Then the maximum of critical cumulative damage and location of production are determined basing on the equation of cumulative fatigue damage combined with experimental result through using the finite element analysis and the critical plane method in fatigue analysis.The relation equation between the standardized critical total strain energy density and stress level is obtained by considering the impact of loading level.The fatigue life of specimens can be predicted by combining the equation of cumulative fatigue damage with the relation equation of damage and stress level and the prediction results coincide with experimental results very well.
Authors: Peng Gang Mu, Xiao Peng Wan, Mei Ying Zhao
Abstract: Fatigue damage of composites can be described by the residual stiffness and residual strength, and the same damage state can be described by the two mechanical parameters equivalently. Based on this assumption, a new pair of fatigue damage accumulation models are established to simulate fatigue behavior and predict the fatigue life of composites. Each of two equations contains three parameters and has the similar form, and the power function relationships between the two damage indices are constructed. The proposed model, combining with constant life diagrams and failure criteria are used to estimate the fatigue life of composites, and good agreement is observed between the present model and experimental results.
Authors: Peng Gang Mu, Xiao Peng Wan, Mei Ying Zhao
Abstract: Fatigue damage growth can be described by the gradual reduction of the stiffness and strength, and damage expressed by the two degradation methods are equivalent. According this assumption, a pair of cumulative fatigue damage models based on residual stiffness and residual strength is proposed, and then the connection between the two damage indices is established. The two damage models follow equation with the same form, but the parameters in them are not equal. Each of the two models contains three unknown parameters which have pertinent effect on the damage growth rates, in addition, the models also take into account of the effects of stress levels and stress ratios. By fitting the experimental data, it is observed that the parameters in damage functions obey the linear relation well, and the unknown constants are deduced with finite tests easily. Finally, the models are found to give a good description of fatigue damage evolutions of different stacking sequence for both of the stiffness and strength.
Authors: Manabu Enoki, Yozo Fujino, Elisa Sorrivi, S. Takayuki, Erasmo Viola
Abstract: In this study, a new wireless smart sensor for fatigue damage detection of a single steel bar is proposed. A modified stress intensity factor is used to introduce a master curve of fatigue crack growth. Acoustic Emission signals are also measured and incorporated in the procedure for the structural health monitoring.
Authors: Mohd Noor Baharin, Zulkifli Mohd Nopiah, Shahrum Abdullah, Mohd Jailani Mohd Nor
Abstract: This paper presents the comparative study on two types of the clustering technique for decomposing Variable Amplitude (VA) loadings signals based on its amplitude. These two techniques are used to recognize clusters or patterns of fatigue damaging events in the record which will bring aboutthe majority of fatigue damage. However, one of the problems that existswhencomparing which technique will produce better clusters is the fact thata clustering validation index isneeded. In this study, techniques that were used were theFuzzy C-means and C-means. At first, the VA data weresegmented using the Running Damage Extraction (RDE) technique. Then, each segment produced wasanalysed using the strain life approach and global statistical signal values. Finally, the accuracy of each clustering technique wasmeasured based on the OV coefficient index. From the study, the index shows that the Fuzzy C-means technique produced much better clusters rather than the C-mean clustering technique.
Authors: Kyung Seop Han, Kwang Hwan Oh
Abstract: A series of laboratory investigations concerned about fatigue assessment with acoustic emission method was presented. Fatigue aspects including cumulative fatigue damage, fatigue crack growth and creep-fatigue interaction were considered. As a basic approach, residual strength and acoustic emission characteristics of fatigue damaged materials were considered from the nominal stress-life (S-N) viewpoint. Acquired signal indicated that counts emission quantity can be a good measure of cumulated fatigue damage. In the fatigue crack growth approach, interrelationship between acoustic emission parameter and stress intensity factor was examined with different stress level and crack length. Experimental results were somewhat scattered since sensitive characteristics of acoustic emission method. However, their empirical relation indicated that counts rate correlated with fracture mechanics parameter. Finally, acoustic emission application was extended to the creep-fatigue interaction at elevated temperature. Emission response under each damage mode was compared and characterized. Based on these characteristics, creep-fatigue interaction was evaluated by use of acoustic emission parameter. Overall investigations concluded acoustic emission is very effective tool of fatigue assessment.
Authors: Rosen T. Tenchev, Brian G. Falzon
Abstract: Experimental static and fatigue tension-tension tests were carried out on 5HS/RTM6 composite intact coupons and coupons incorporating adhesively-bonded (FM300-2) stepped flush joints. The results show that the adhesive joint, which is widely used in repairs, significantly reduces the static strength as well as the fatigue life of the composite. Both, the static and the fatigue failure of the ‘repaired’ coupons occur at the adhesive joint and involve crack initiation and propagation. The latter is modelled using interface finite elements based on the decohezive zone approach. The material degradation in the interface constitutive law is described by a damage variable, which can evolve due to the applied loads as well as the number of fatigue cycles. The fatigue formulation, based on a published model, is adapted to fit the framework of the pseudotransient formulation that is used as a numerical tool to overcome convergence difficulties. The fatigue model requires three material parameters. Numerical tests show that a single set of these parameters can be used to recover, very accurately, the experimental S-N relationship. Sensitivity studies show that the results are not mesh dependent.
Authors: Chao Jiang, Xiang Lin Gu, Wei Zou
Abstract: This paper investigates the accelerating effect of fatigue damage on carbonation process through an experimental study. A total of 30 prism specimens, categorized by A or B according to the cement type used, were cast in this experiment program. Mid-span deflection-and beam bottom strain-based damage indexes were proposed to estimate fatigue damage states for type A and B specimens, respectively. After static and fatigue failure tests for 10 specimens, 16 specimens were subjected to fatigue loading up to the prescribed damage states. Subsequently, these fatigue-damaged specimens and four intact specimens were put together into a chamber filled with high concentration CO2 gas, and carbonation depths were measured after 14, 28, and 49 days. Carbonation tests showed that fatigue damage could increase the mean carbonation depth by 150% and that the regressed linear expression between the normalized mean carbonation depth and fatigue damage index for each type specimen agreed well with experimental data. Normal distribution models for carbonation depths in fatigue-damaged concrete were proposed and found to be comparable with the realistic frequency distribution histograms.
Authors: Stefano Beretta, Mauro Filippini, Luca Patriarca, Silvia Sabbadini
Abstract: In this work, a Ti-48Al-2Cr-2Nb alloy obtained with a additive manufacturing technique by electron beam melting (EBM) has been examined by conducting high cycle fatigue tests both with plain specimens and with specimens with artificially introduced defects with the objective of studying the growth behavior of small cracks. A consistent model for predicting the fatigue endurance strength of specimens with artificial defects is proposed, based on the Kitagawa diagram and taking into account of the presence of inherent microstructural features of the studied intermetallic alloy. Thus, the origin of fatigue failures due to intermetallic phases and orientation of lamellar colonies was investigated by means of micromechanical analysis through the use of high-resolution Digital Image Correlation (DIC). The local strain heterogeneities were measured out of the load frame by means of an optical microscope at high magnifications. The strain maps were then overlaid with the images of the microstructure and detailed analyses were performed to investigate the features of the microstructure where high local strain heterogeneities arise. High local residual plastic strains were measured inside lamellar colonies, which are detected as the precursor to fatigue crack initiation. The measure of the residual strains also provides further information on the role of the intermetallic phases on the fatigue behaviour of γ-TiAl alloys.
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