Life Curves Research Articles

Fatigue Performance of Concrete with Pre-Cracks in Tension-Compression Cycles

A tension-compression cycle fatigue test was performed in order to study the fatigue property of C50 concrete with pre-cracks in cyclic loading. The stress ratio was-1 and the amplitude was 0.2 MPa ~1.30 MPa. The results show that the modified coefficient of fatigue strength is 0.198~0.265 and the infinite life fatigue strength is below 0.45MPa. While the log value of fatigue life is approximately linear with the amplitude of fatigue load stress, the discreteness of fatigue life, the particularity of concrete, has little to do with the amplitude. The S-N, P-N fatigue life curves and the constant fatigue life diagram of pre-crack concrete are obtained.

Excellent Cycle Performance of Pouch Cells at a High Rate Investigated By a Combination of Synchrotron Based X-Ray Diffraction and Absorption Spectroscopy

Lithium-ion batteries are widely developed for vehicle applications and energy storage systems owing to their high power density, long cycle life, and lack of memory effect. The cycling stability of a Li-ion battery plays an important role in its practical application, especially for use in electric vehicles, which is generally affected by the types of electrode materials [1], binder properties [2], and electrolyte compositions [3].Li-ion cells based on LiFePO4cathodes have been considered as one of the most promising candidates for large-scale application due to its low cost, low toxicity, good electrochemical and excellent thermal stability. However, the tendency for capacity degradation behaviour of LFP cells is still seen as a problem for practical application.The capacity fading mechanism of a laminated pouch type 1.0 Ah cell consisting of a graphite anode and an olivine LiFePO4 cathode during a long-term cycling test was studied in this report. The LiFePO4cathodes have a mole ratio of iron to phosphorus in the range of 0.93 to 0.99. As shown in Fig. 1, the LFP cell with a Fe/P ratio of 0.99 has the best cyclic performance at the charge/discharge rate of 2 C/5 C. The cycle life curves of the pouch cell show the number of cycles increases to 2269 cycles with the capacity retention (C.R.) of 80% as the Fe/P ratio increases from 0.93 to 0.99.To investigate the aging mechanism of the LiFePO4 cathode and graphite anode, changes in the crystalline structure through the charge/discharge reactions were analysed by the synchrotron based X-ray diffraction (XRD) experiments. After long-term cycling tests, the appearance of LiOH and LiOH．H2O phases in the cycled anodes reveals that some Li ion released from the LiFePO4 phase during the charge process did not come back to the FePO4 phase, as shown in Fig. 2. Due to fewer lithium ions, some of the cathode material lost electrochemical activity and this brought down the reversible capacity significantly. In addition, the anode with the Fe/P ratio of 0.95 after 506 cycles has relatively stronger peaks than those of the other samples in Fig. 2(b). It indicates that the decrease in the Fe concentration of stoichiometric LiFePO4resulted in a shortage of lithium ion for the cathode material, and consequently the capacity fading rate accelerated.In this study, a combination of X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and electrochemical testing are used to understand the physical and electrochemical characterizations of the graphite anode and the LiFePO4cathode with the different Fe/P mole ratios during long-term cycling tests.

Research and Implementation on Methods of Evaluating Lifetime of LED Light Source for LED Lighting System

This research adopts comprehensive evaluation to evaluate lifetime of LED light source for lighting system. By the method, under the condition of LED light source working outside, the situations of LED light source lifetime are analyzed with attenuation rate of illumination got from illumination data of LED light source in practical work. The real lifetime characteristics and real life curve of the LED light source are obtained eventually.

Monitoring the damage evolution of flexural fatigue in unidirectional carbon/carbon composites by electrical resistance change method

This paper reported simultaneous monitoring damage evolution of flexural fatigue in unidirectional carbon-fiber-reinforced carbon composites (C/C composites) by electrical resistance change (ERC) methods. The degree of irregularity in electrical resistance changes increased with stress levels increasing. The shapes of electrical resistance change rate–fatigue cycle curves can reflect stress levels and damage types of tested samples: sawtooth shapes reflected delamination at a higher stress level; and “peak” shapes reflected inner damages in one fiber bundle at the fatigue limit stress level. In addition, the similarity of initial electrical resistance–fatigue life curve and S–N curve was observed clearly. In summary, ERC methods can monitor the damage evolution and qualitatively estimate the fatigue life of unidirectional C/C composites.

Elevated temperature low cycle fatigue of grey cast iron used for automotive brake discs

This paper evaluates the fatigue life properties of low carbon grey cast iron (EN-GJL-250), which is widely used for automotive brake discs. Although several authors have examined mechanical and fatigue properties at room temperatures, there has been a lack of such data regarding brake discs operating temperatures. The tension, compression and low cycle fatigue properties were examined at room temperature (RT) and at brake discs’ working temperatures: 500°C, 600°C and 700°C. The microstructure of the material was documented and analysed. Tensile stress–strain curves, cyclic hardening/softening curves, stress–strain hysteresis loops, and fatigue life curves were obtained for all the above-mentioned temperatures. It was concluded, that Young’s modulus is comparable with both tension and compression, but yield its strength and ultimate strength are approximately twice as great in compression than in tension. All the mechanical properties remained quite stable until 500°C, where at 700°C all deteriorated drastically. During fatigue testing, the samples endured at 500°C on average at around 50% of cycles at room temperature. Similar to other materials’ properties, the cycles to failure have dropped significantly at 700°C.

Weibull modeling of the probabilistic S–N curves for rolling contact fatigue

A Weibull modeling of the probabilistic stress–life (S–N) curves is proposed for rolling contact fatigue (RCF). Shape, scale and location parameters of Weibull distribution, the normal life confidence parameters and the exponent of S–N relation are all deduced from set of test data. Life confidence assessment is incorporated into the probabilistic curves for common applied case of limited samples. Applicability of the present modeling is checked by describing the test data of G20CrNi2Mo bearing steel. Results real that two-parameter (2-P) and three-parameter (3-P) Weibull distributions are acceptable models. But bigger difference is observed in high reliability district. The 3-P one has smaller standard deviation of lives and this deviation is constricted with increasing of stress levels, which is consistency with results of most classical fatigue experiments. While the 2-P modeling fails to well characterizing the test data. This indicates that an appropriate distribution determination is a primary task for RCF analysis. And the 3-P Weibull is a better modeling for the present test data.

Fatigue assessment of thin‐welded joints with pronounced terminations

ABSTRACTThe present paper reviews and contains simple modifications to extend the applicability of global and local fatigue concepts to thin‐welded specimens with pronounced weld terminations (starts/ends). Experimental fatigue data reported by Kaffenberger and Vormwald have been used for the validation of the proposed procedures. Special attention is paid to the notch strain concept. The material state is explicitly considered by means of corresponding strain–life curves in conjunction with PSWT, PMorrow or the experimental mean stress sensitivity factors proposed by Olivier. The accuracy of Neuber's sharp and mild notch formulas, Seeger–Beste's formulation and Glinka's strain energy density rule within the context of the notch strain approach has been investigated. Simple rules accounting for stress relaxation effects have been additionally proposed and implemented into the calculations. Comparison of experimental with theoretical fatigue lives validates the accuracy and robustness of the proposed procedures to calculate fatigue design S–N curves.

Notch stress and fracture mechanics based assessment of fatigue of seam weld ends under shear loading

ABSTRACTWelding processes often lead to pronounced weld termination points (start/end), which are prone to fatigue events such as crack initiation. The fatigue of weld ends under shear loading, especially in thin sheet structures, is not sufficiently explored yet. In the present investigation, the real geometry of welds was obtained by scanning the three‐dimensional surface, especially of weld ends. Cryogenic breaking of the weld opened the view to the weld root geometry and especially to the critical location, where the weld root migrates to a weld toe at the weld end. In the experimental part of this research, fatigue testing of thin sheet plane and cylindrical specimens as well as large components containing weld ends was performed. Based on the data, weld end life curves have been obtained. The notch stress concept and a fracture mechanics based approach were applied for describing the results of fatigue tests providing sufficient accuracy.

Numerical and experimental study on the optimization of overload parameters for the increase of fatigue life

Fatigue life and crack growth retardation due to periodic tensile overloads, which are superposed on constant amplitude cycles, have been investigated in the present study. In the numerical analyses, the Walker equation with the generalized Willenborg model has been used. The analyses and experiments have been performed on C(T) specimens made of 7075 aluminium. The periodic overloads have been induced at two different periodicities and the effect of overload ratio and overload periodicity on the fatigue life has been studied. The results of the experiments and analyses reveal that the normalized fatigue life versus overload ratio curve has a maximum point, which indicates that there are greater retardation effects in the intermediate overload ratio values. Also, the numerical and experimental results indicate that at the occurrence ratios (the ratio of the number of overload cycles to the number of constant-amplitude cycles between overloads) of 1/50 and 1/500, the maximum interaction occurs at the overload ratios of 1.5 and 2.0, respectively.

High cycle fatigue under reversed bending of sandwich panels with GFRP skins and polyurethane foam core

The effect of fully reversed loading (R=−1) on high cycle fatigue performance of sandwich panels composed of polyurethane foam core and Glass Fiber Reinforced Polymer (GFRP) skins is studied and compared to that of similar panels tested under fully unloaded (R=0) conditions. Fatigue life curves are established and compared based on maximum loads of 30–70% of the ultimate monotonic strength (Pult). It was shown that panels consistently fail in shear of the foam core. The fatigue life reduces significantly at R=−1, to about 10% of that at R=0. In order to achieve at least 2 million cycles – the commonly acceptable fatigue life in structural engineering – the maximum service loads should be limited to 30% and 45% of Pult, respectively, for the cases of R=−1 and R=0. It is estimated that the threshold loading levels at infinite fatigue life are 23% and 37% of Pult, for the cases of R=−1 and R=0, respectively. By the end of fatigue life, up to 25% reduction in stiffness occurs. The transition between high and low cycle fatigue occurs between 200 and 5000 cycles. A 3-D Haigh diagram is also established for design purposes.

Analyzing Remaining Life Non-Probability Reliability and Parameter Sensitivity for Pipeline Subjected to Corrosion

This paper aims to study the problem of residual life prediction and non-probabilistic reliability analysis for pipeline subjected to corrosion. Based on corrosion pipe bursting equation, the remaining life non-probabilistic reliability analysis model of corrosion pipeline with interval variables was established. The sequential quadratic programming method has been used for solving the non-probabilistic reliability index of corrosion pipes under different service years. Then the relationship curve of the service life and the remaining life reliability of the pipeline corrosion were given. Using the finite difference method, the parameters sensitivity analysis of corrosion piping reliability was performed.

Tool Life and Flank Wear of Physical Vapour Deposited TiAlN Multilayer Coated Carbide End Mill Inserts when Machining AISI D2 Hardened Steel under Dry Cutting Condition

This study investigates the tool wear of PVD TiAlN multilayer coated end mill inserts when machining AISI D2 hardened steel in dry conditions. The experiment tests consisting three levels of cutting speed, Vc (80, 100, 120 m/min) and three levels of radial depth of cut, ae (3, 4, 5 mm) was used for the study. The PVD TiAlN coated carbide insert performs satisfactory under the conditions tested, as reasonable tool life is recorded. By linking the machining tests and tool life curves, the wear behaviour of the coated insert was described. It is observed that wear of the tool mainly occurred on the flank wear. Tool life decreasing and volume material removal (VMR) increasing as cutting speed and radial depth of cut increased due to higher temperature generated and contact area immerse, respectively. Tool failure modes and wear mechanisms were examined at various cutting parameters. Built-up edge (BUE), groove and micro-chipping were found to be the predominant tool failure modes for cutting tools. The highest volume material removal, VMR attained was 3750 mm3, meanwhile the higest tool life, TL was 9.69 min.The combination of cutting conditions that gave the best response for different components of tool wear, tool life and surface integrity.

Fatigue life estimation of FBGA memory device under vibration

This paper predicts the fatigue life of fine-pitch ball grid array (FBGA) solder joints in memory devices due to harmonic excitation through experiments and finite element analysis. Finite element models of the memory device with simplified solder joints and with detailed solder joints were developed as a global model and a local model, respectively. A global-local modeling technique was used in the finite element simulation to calculate the stress magnitude of solder joints in the memory device under vibration. Stress versus life (S-N) curve was generated for the memory devices under various vibration levels to derive the fatigue constants of solder material. The fatigue life of the memory device was then determined by using the Basquin equation and Miner’s rule. It was experimentally verified that the predicted fatigue life of the memory device under cumulative damage conditions matches the experimental results within reasonable accuracy.

LCF Behaviour of Ultrafine Grained 301LN Stainless Steel

Cyclic strain behaviour of metastable austenitic 301LN steel with different grain sizes – coarse (14μm) and ultrafine (1.4μm) grained – produced by the special thermo-mechanical treatment based on the martensite-to-austenite reversion was investigated. Low-cycle fatigue (LCF) tests were conducted on flat specimens at room temperature in fully push-pull mode under strain control with constant strain rate of 2×10–3 s–1 and constant total strain amplitude ranging from 0.4% to 0.9%. LCF behaviour was characterised by cyclic hardening/softening curves and fatigue life curves. After completion of fatigue tests surface fatigue damage was studied using scanning electron microscopy (SEM). Ferritoscope was adopted to reveal the destabilization of austenitic structure and evaluate the extent of deformation induced martensitic transformation.

Fatigue behaviour of coke drum materials under thermal-mechanical cyclic loading

Coke drums are vertical pressure vessels used in the delayed coking process in petroleum refineries. Significant temperature variation during the delayed coking process causes damage in coke drums in the form of bulging and cracking. There were some studies on the fatigue life estimation for the coke drums, but most of them were based on strain-fatigue life curves at constant temperatures, which do not consider simultaneous cyclic temperature and mechanical loading conditions. In this study, a fatigue testing system is successfully developed to allow performing thermal-mechanical fatigue (TMF) test similar to the coke drum loading condition. Two commonly used base and one clad materials of coke drums are then experimentally investigated. In addition, a comparative study between isothermal and TMF lives of these materials is conducted. The experimental findings lead to better understanding of the damage mechanisms occurring in coke drums and more accurate prediction of fatigue life of coke drum materials.

Probabilistic integrity assessment of turbine disks in presence of potential defects

Rotor disks for gas turbines are heavy components and are usually designed following a safe-life approach, where the low-cycle fatigue analysis is carried out referring to design life curves with suitable probabilistic margins. However, in the case of such a heavy component there is the possibility of rare occurrence of undetected defects or, better, the need to identify the defect acceptability for the different rotor regions (considering stress, temperature, mission profile). In order to carry out such calculations for a turbine rotor disk a software named {AStrID} (Assessment of the Structural Integrity of Disks) has been developed in close cooperation between Politecnico di Milano and Ansaldo Energia (AEN). This paper summarizes its background and some of the relevant features.

A Simple Format for Failure Probability under LCF and its Application to a Complex Component

Turbine components are usually designed onto safe-life approach, where the low-cycle fatigue analysis is based onto design life curves with suitable probabilistic life margins. However, in order to design for a given reliability, the definition of the design curve should not only include the life variability but also the scatter of applied load. Unfortunately, in the literature there are few indications which only refer to safety factors under HCF, without any specific discussion of the case of LCF.In this paper, we firstly propose a simple format for calculating reliability for an assessment point ▪. The format is then used for estimating failure probability adopting the Socie's criterion for a notched component whose elasto-plastic response was simulated by FEA. Results show the need of implementing a correct dependence between ɛ− N curve and material volume.

Life Line Characteristics of Solid Insulation under Electrical Stress

Insulating materials subjected to electrical and thermal-electrical stresses exhibit life line characteristics which show a tendency to reach an electrical threshold. Their fit is investigated with reference to several sets of data derived from accelerated life tests performed on different insulating materials. Life line models provide the estimates of failure time percentiles at selected probabilities and stresses so that life curves at fixed failure probabilities can be drawn. These characteristics should help in the choice of compatible thermal and electrical stresses for the design of insulation systems, apparatus and components. The tests to be conducted to plot the characteristic curves involve Class B insulation (up to 8 specimens) under three active stress levels. Subsequently, using the data acquired, the Weibull probability distribution curves are plotted using STATISTICA software.

Modelling and experimental study of fatigue of powder metal steel (FC-0205)

The microstructure sensitive multistage fatigue model captured the fatigue life of a powder metal FC-0205 steel alloy. Uniaxial strain controlled fatigue data and microstructure information from sets of high and low porosity specimens calibrated the model. Strain–life behaviour depicted that above the plastic strain limit of 0·002 mm mm−1 in the low cycle fatigue regime, where ubiquitous plasticity occurred, the different porosity levels gave distinct, visibly different results. However, specimens tested below the plastic limit in the high cycle fatigue regime, where failure was dominated by local cyclic microplasticity, showed unclear fatigue lives at different porosity levels. Fractography using scanning electron microscopy showed no clear presence of striations; however, asserted striations in powder metal specimens were similar to geometrical features observed on fracture surfaces of monotonically loaded specimens. The experimental and microstructure data calibrated a fatigue model that allowed for satisfactory prediction of the varying porosity specimen strain–life curves.

Effect of Internal Ribs on Fatigue Performance of Sandwich Panels with GFRP Skins and Polyurethane Foam Core

This paper investigates the cyclic fatigue behavior of sandwich panels composed of glass fiber reinforced polymer (GFRP) skins connected by longitudinal Z-shape ribs and low-density polyurethane foam core. Eighteen 1,143×635×78-mm panels with ribs of two different flange widths, including three control static tests, were tested in fatigue under fully reversed loading (R=−1) and fully unloaded (R=0) conditions, to maximum loads (Pmax) of 20–70% of their ultimate static strength (Pult). Fatigue life curves were established and compared with those of panels without any ribs. It was shown that internal ribs increased static strength and stiffness by 80 and 66%, respectively, but were only beneficial within the low cycle fatigue range. Under Pmax of 50% Pult, number of cycles to failure (Nf) at R=−1 was only 6% of Nf at R=0 in the ribbed panels. Fatigue failure consistently initiated by flange debonding of the rib from skin. In panels loaded to higher than Pult of their nonribbed counterparts, final shear failure of foam core occurred simultaneously with rib debonding, whereas in panels loaded below this level, additional fatigue life was attained after rib debonding, until core shear failure, but at significantly lower stiffness. To achieve an Nf of 2M cycles, Pmax should not exceed 35 and 21% of Pult at R=0 and −1, respectively. A three-dimensional (3D) failure surface (Haigh diagram) was established for the ribbed panels. It predicts fatigue life for a given mean and amplitude loads.