Level Of Fatigue Damage Research Articles

Fatigue assessment of a composite railway bridge for high speed trains. Part II: Conditions for which a dynamic analysis is needed

This paper is a continuation of the paper titled “Fatigue assessment of a composite railway bridge for high speed trains-Part I: modeling and fatigue critical details” with the emphasis on conditions for which a dynamic analysis is needed for a fatigue assessment of the bridge. In part I, a fatigue assessment of the Sesia viaduct is performed by using both a static analysis and a dynamic analysis with a moving load model; fatigue damage levels of several concerned details are calculated and the fatigue critical details are identified. It is found that dynamic effects induced by the high speed trains reveal a big influence on the fatigue damage of the bridge. In this paper, conditions under which a dynamic analysis should be considered for the fatigue assessment of a bridge are investigated. Three different alternatives are considered: a static analysis taken into account a dynamic amplification factor, a dynamic analysis with a moving load model, or a dynamic analysis with multi-body models for the train vehicles include the train–bridge interaction effect. The effect of several parameters related to the train and the bridge is studied. The train speed and the mass ratio between the train and the bridge are identified as significant factors that determine the dynamic effect on the fatigue damage of the bridge.

Investigation of macro-turbulent flow structures interaction with a vertical hydrokinetic river turbine

Reported are a series of river velocity measurements taken one diameter upstream of a 25 kW vertical axis river kinetic turbine deployed in the Winnipeg River at Pointe du Bois, located 150 km northeast of Winnipeg. A load-bank located on the river shore provides a resistive load for the turbine generator. An acoustic Doppler velocimeter measures velocity fluctuations at a sample rate of 200 Hz. Analysis of acoustics Doppler velocimeter data measured upstream of the turbine quantifies the variation in the mean and fluctuating components of river flow velocity. Results show that in this particular test site the river contains eddies with the same order of magnitude as the turbine diameter or greater. Measurements upstream of the operating turbine indicate reduction in the mean velocity with eddies breaking down into smaller sizes. These smaller eddies increase the turbulence intensity of the incoming flow and increase the high frequencies energy level in the power spectra. ► Velocity measurements upstream of a 25-kW vertical turbine in a river are obtained. ► Solutions for ADV signal quality and signal despiking provided. ► Inflow is non-uniform and contains large eddies in the river. ► Operating turbine increases turbulence intensity and breaks large eddies in upstream. ► Higher turbulence intensity induces higher levels of fatigue damage.

Nondestructive Fatigue Damage Analysis of a Thin Asphalt Concrete Course Using the Wavelet Correlation Method

Abstract Stress wave analysis is employed herein as a nondestructive monitoring tool to assess the level of fatigue damage in a thin asphalt concrete (AC) overlay. A frequency-dependent cross-correlation procedure is developed to specify a stress wave at a desired frequency by using a wavelet kernel. This procedure is referred to as the wavelet correlation method (WCM). Once synthetic surface waves are constructed and subjected to simulated disturbances, such as structural damage or nearby frequencies, their phase velocities are computed using the WCM with over 96 % accuracy. The generated stress waves are periodically processed, while laboratory hot-mix asphalt pavements are trafficked by the third-scale model mobile loading simulator. The dispersion curves are then analyzed to validate that a wave of 16 kHz travels mainly within a 40∼60 mm thickness of a surface layer. Fatigue damage levels are quantified at intervals by the phase velocity that represents the AC elastic modulus. Microdamage healing of the AC during rest periods is then indexed and corrected by shifting the damage progress profile. Consequently, an early reduction in phase velocity, which is caused by microcracking, can be visually observed in the surface cracking once the phase velocity is reduced to about 50 % of the initial value regardless of pavement density and aggregate gradation. Thus, the WCM allows the optimal timing and scheduling of the preservation construction of a thin AC overlay by indicating the critical microdamage stage immediately prior to the visual evidence of surface cracking.

Using deformation modes to identify cracks in turbine engine compressor disks

AbstractRecent studies show that analytical predictions of crack growth in rotating components can be used in conjunction with displacement measurement techniques to identify critical levels of fatigue damage. However, investigations of this type traditionally have focused on the detection of damage at known flaw locations. This paper addresses the related problem of estimating damage associated with flaws at unknown locations, through the combined use of analytical models and measured vibration signatures. Because the measured data are insufficient to identify a unique solution for the location and severity of fatigue cracks, the function of the analytical model is to bound the extent of damage occurring at life-limiting locations. The prediction of remaining life based on estimates of worst-case fatigue damage and crack locations also is discussed.

Effect of the welding profile generated by the modified indirect electric arc technique on the fatigue behavior of 6061-T6 aluminum alloy

This paper reports the effect of the welding profile generated by the modified indirect electric arc (MIEA) technique on the fatigue behavior of 6061-T6 welded joints. The calculations are based on the effect of the stress concentration factor produced by the characteristic geometry of the welding profile formed during the welding process. It was found that the fatigue life of welded samples using this welding technique was larger in comparison with data reported in the literature. Also, measurements of microhardness and tension testing were performed to account for the effect of different levels of fatigue damage on the mechanical properties of these welds. Experimental findings reveal that, with reference to undamaged samples, fatigue damage increases the microhardness in the weld metal and heat affected zone (HAZ) whereas it only produces a moderate increase in yield strength of approximately 14% for a 75% of fatigue damage. Tensile and fatigue tests indicate that the thermal affection undergone by the plates during fusion welding dictates failure in the HAZ under tension stress but not under fatigue. The failure mechanism under fatigue is discussed in terms of theoretical analyses and fractography.

Simplified Nonlinear Temperature Curling Analysis for Jointed Concrete Pavements

The assumption of a linear temperature change through the slab depth has been overwhelmingly used in pavement analysis since Westergaard proposed a curling solution for rigid pavements. However, the actual temperature profiles through the slab thickness are primarily nonlinear. These nonlinear temperature profiles produce stresses that can be divided into three components: a uniform temperature stress, an equivalent linear curling stress, and a nonlinear self-equilibrating stress. It is the self-equilibrating stress component that often goes unaccounted for in concrete pavement stress prediction and can significantly affect the tensile stress magnitude and critical location. This paper presents a solution for a piecewise method and proposes a simplified method termed NOLA, or nonlinear area, that easily captures the effect of temperature nonlinearity on rigid pavement responses. The proposed NOLA method enables the use of a three-dimensional temperature frequency distribution that allows simple postprocessing of rigid pavement curling stress solutions derived from a linear temperature assumption. The impact of accounting for self-equilibrating stresses in terms of projected fatigue damage levels and critical cracking locations is also explored using a mechanistic-based rigid pavement analysis program called RadiCAL.

Effect of fatigue damage on the dynamic tensile behavior of 6061-T6 aluminum alloy and AISI 4140T steel

Usually material properties are determined from damage free materials, but it is not well known how these properties vary with respect to previous fatigue damage. In the present work the dynamic response of fatigue damaged 6061-T6 aluminum alloy and AISI 4140T steel specimens subjected to impact loading was investigated. Samples subjected to previous damage under high cycle fatigue and low cycle fatigue were tested. Different fatigue damage levels were considered. In addition, the effect of previous fatigue damage on the quasi-static behavior, ductility and fracture mechanism was also evaluated for both materials. A tensile Hopkinson bar apparatus was used in this work to investigate the dynamic response of the pre-fatigued specimens. Projectile speeds ranged from 18 m/s to 30 m/s giving strains rates from 550 to 2850 s −1. The quasi-static mechanical properties of aluminum are not affected by the way the fatigue damage is induced. The dynamic properties, however, are sensitive to the previous fatigue damage, but are not affected by the strain rate. In the steel case, and when damage is induced by strain control, the mechanical properties are influenced by the previous fatigue damage. The dynamic properties are sensitive to the previous fatigue damage and depend on the strain rates. The analysis results show an increase in the ductility of the aluminum alloy when increasing the fatigue damage level; the steel exhibits an opposite behavior, a decrease in the ductility when increasing the damage level. The results show how the previous fatigue damage can modify the quasi-static and dynamic mechanical properties of the tested materials.

周辺固定された二層積層ハニカムコアサンドイッチパネルの局部圧縮疲労

The fatigue property of a honeycomb core sandwich panel (hereafter, HSP for brevity) has been studied in consideration of usage as a floor panel. In previous papers, fundamental properties were reported; 1) A fatigue life as the floor panel was defined from the growth curve of dent depth. 2) It was possible to design by calculating the stress of crack initiation site on dent. 3) A fatigue damage level was predicted from the dent depth. In this study, it was attempted to study the local compression fatigue property of peripherally clamped double layer honeycomb core sandwich panel for the sake of wide application. Obtained results were summarized as the follows; 1) The higher a rigidity of HSP was, the shorter a fatigue life was. 2) The taller a core height near an indenter was, the deeper a depth of dent was when HSP had been fractured except a single layer HSP. 3) There were two steady regions in double layer HSPs. In consideration of the fatigue life and rigidity except self-weights, the double layer HSP that the core height near indenter is small is superior to the single layer HSP.

Fatigue damage characterization using surface acoustic wave nonlinearity in aluminum alloy AA7175-T7351

Nonlinear ultrasonic (NLU) harmonic generation system was used to characterize the fatigue damage in a flat hour-glass, high strength Al–Cu–Zn–Mg alloy, AA7175-T7351 specimens. Experiments were carried out to introduce controlled levels of fatigue damage under constant amplitude loading to determine the NLU response using surface acoustic wave (or Rayleigh mode) at regular intervals of fatigue life. The NLU parameter (A2/A12) plotted as a function of percentage of fatigue life shows two peaks for all the samples tested, independent of the amplitude of fatigue loading. The first peak appeared between 40%–50% of fatigue life and the second peak between 80%–90% of fatigue life. Among the two flat surfaces of the specimen, a higher nonlinearity response was observed on the surface which had the first crack initiation. The appearance of two peaks in the nonlinear response during fatigue damage progression is explained based on the dislocation dynamics and dislocation-crack interaction present in the specimens during the fatigue process.

Stress fracture healing: Fatigue loading of the rat ulna induces upregulation in expression of osteogenic and angiogenic genes that mimic the intramembranous portion of fracture repair

Woven bone is formed in response to fatigue-induced stress fractures and is associated with increased local angiogenesis. The molecular mechanisms that regulate this woven bone formation are unknown. Our objective was to measure the temporal and spatial expression of osteo- and angiogenic genes in woven bone formation in response to increasing levels of fatigue-induced damage. We used the rat forelimb compression model to produce four discrete levels of fatigue damage in the right ulna of 115 male Fischer rats. Rats were killed at 0 (1 h), 1, 3 and 7 days after loading. Using qRT-PCR, we quantified gene expression associated with osteogenesis ( BMP2, Msx2, Runx2, Osx, BSP, Osc), cell proliferation ( Hist4), and angiogenesis ( VEGF, PECAM-1) from the central half of the ulna. The spatial distribution of BMP2, BSP and PCNA was assessed by immunohistochemistry or in situ hybridization in transverse histological sections 1, 4, and 7 mm distal to the ulnar mid-diaphysis. One hour after loading, BMP2 was significantly upregulated in neurovascular structures in the medial ulnar periosteum. Expression of angiogenic markers ( VEGF, PECAM-1) increased significantly between Day 0 and 1 and, as with BMP2 expression, remained upregulated through Day 7. While Osx and BSP were upregulated on Day 1, the other osteogenic genes ( Msx2, Runx2, Osx, BSP and Osc) were induced on Day 3 in association with the initiation of periosteal woven bone formation and continued through Day 7. The magnitude of osteogenic gene expression, particularly matrix genes ( BSP, Osc) was significantly proportional the level of fatigue damage. The woven bone response to fatigue injury is remarkably similar to the “intramembranous” portion of fracture repair — rapid formation of periosteal woven bone characterized by early BMP2 expression, cell proliferation, and upregulation of osteogenic genes. We speculate that woven bone repair of fatigue damage may be an abbreviated fracture response without the requirement for endochondral repair. We conclude that bone fatigue repair is a process similar to intramembranous fracture repair characterized by increases in the expression of genes associated with angiogenesis, cell proliferation and osteoblastogenesis, and that the response from the local vasculature precedes the osteogenic response to fatigue loading.

Subrupture tendon fatigue damage

The mechanical and microstructural bases of tendon fatigue, by which damage accumulates and contributes to degradation, are poorly understood. To investigate the tendon fatigue process, rat flexor digitorum longus tendons were cyclically loaded (1-16 N) until reaching one of three levels of fatigue damage, defined as peak clamp-to-clamp strain magnitudes representing key intervals in the fatigue life: i) Low (6.0%-7.0%); ii) Moderate (8.5%-9.5%); and iii) High (11.0%-12.0%). Stiffness, hysteresis, and clamp-to-clamp strain were assessed diagnostically (by cyclic loading at 1-8 N) before and after fatigue loading and following an unloaded recovery period to identify mechanical parameters as measures of damage. Results showed that tendon clamp-to-clamp strain increased from pre- to post-fatigue loading significantly and progressively with the fatigue damage level (p <or= 0.010). In contrast, changes in both stiffness and hysteresis were significant only at the High fatigue level (p <or= 0.043). Correlative microstructural analyses showed that Low level of fatigue was characterized by isolated, transverse patterns of kinked fiber deformations. At higher fatigue levels, tendons exhibited fiber dissociation and localized ruptures of the fibers. Histomorphometric analysis showed that damage area fraction increased significantly with fatigue level (p <or= 0.048). The current findings characterized the sequential, microstructural events that underlie the tendon fatigue process and indicate that tendon deformation can be used to accurately assess the progression of damage accumulation in tendons.

Linear and nonlinear acoustic control of accumulated fatigue damage in steel

We present results of linear and nonlinear acoustic testing of steel samples with different levels of fatigue damage. The steel specimens were tested under programed cyclic loading on a fatigue testing machine and accumulated different levels of fatigue damage. No visible surface-crack formations during fatigue cycling were observed. In other words, the emphasis was placed on the characterization of continued but physically invisible in service life conditions damage in different materials and structures. Both linear and nonlinear acoustic techniques were used to assess damage accumulation. (1) Impulse resonant acoustic spectroscopy (IRAS) is based on analysis of the free-sample vibration after impact excitation. It demonstrated the increasing of the resonance frequencies and Q factor with damage accumulation. (2) Nonlinear resonant acoustic spectroscopy (NRAS) is based on measurement of the resonance response for different levels of acoustic excitation. The amplitude-dependent frequency shift for damaged steel was observed to increased with damage accumulation. (3) Nonlinear wave modulation spectroscopy (NWMS) implies the modulation of ultrasound wave by lower frequency vibration. High level of the side-band components for damaged samples were observed. The comparison of different methods is given.

Load alleviation in tilt rotor aircraft through active control; modelling and control concepts

This paper presents the first results from research into active control of structural load alleviation (SLA) for tiltrotor aircraft carried out in the European ‘critical technology’ RHILP project. The importance of and the need for SLA in tiltrotors are discussed, drawing on previous US experience reported in the open literature. The paper addresses the modelling aspects in some detail; hence forming the foundation for both the FLIGHTLAB simulated XV-15 and EUROTILT configurations. The primary focus of attention is the suppression of in-plane rotor yoke loads for pitch manoeuvres in airplane mode; without suppression these loads would result in a very high level of fatigue damage. Multi-variable control law design methods are used to develop controller schemes and load suppression of 80-90% is demonstrated using rotor cyclic control, albeit at a 20-30% performance penalty. However, rotor flapping transients tend to increase by the action of the SLA system. A dual-objective control design approach demonstrates the effectiveness of suppressing both loads and flapping simultaneously.

VIBRATION-BASED DAMAGE IDENTIFICATION USING RECONSTRUCTED FRFS IN COMPOSITE STRUCTURES

This work aims to establish a vibration-based damage identification method for fiber-reinforced laminated composites and their sandwich construction. This new on-line structural damage identification technique uses the structural dynamic system reconstruction method exploiting the frequency response functions (FRFs) of a damaged structure. To verify the effectiveness of this damage identification method, the frequency responses obtained by vibration testing of fatigue-damaged laminated composites and honeycomb sandwich beams with debonding are examined according to the extent of the damage via the fatigue-damage load cycle for laminated composites, and via the debonding extent for honeycomb sandwich beams. The changes of the peaks and valley of the FRFs according to the debonding extent and the fatigue load cycles are examined, and the area changes in the FRFs are also discussed as the damage index. The residual FRFs or the difference between intact and damaged FRFs are newly defined for application of the on-line damage identification method. Finally, the delamination extent for the sandwich beams and the fatigue damage level for the laminated composites can be easily identified in terms of the changes in natural frequencies and damping ratios of the reconstructed FRFs for these damaged composite structures.

Characterization of Microdamage and Healing of Asphalt Concrete Mixtures

The pseudo-strain concept based on Schapery’s extended nonlinear elastic-viscoelastic correspondence principle is demonstrated by a considerable database to be able to characterize both microdamage and microdamage healing during the damage process. A change in pseudo stiffness can be used to quantify microdamage and healing during the fatigue test. Pseudo stiffness decreases consistently with increasing number of loading cycles, indicating that microdamage occurs during the fatigue test. The significant recovery of pseudo stiffness after rest periods demonstrates a healing effect due to rest periods. Dissipated pseudo-strain energy is a strong and consistent quantifier of damage and healing. High levels of cumulative dissipated pseudo-strain energy are consistent with high levels of fatigue damage, whereas low levels of cumulative pseudo-strain energy are associated with fatigue damage resistance. Pseudo stiffness is the chord slope of the stress-pseudo strain hysteresis loop. Since linear viscoelastic-based time effects are eliminated in this approach, pseudo stiffness and/or pseudo-strain energy are superior indicators of damage than either stiffness or total dissipated strain energy. The effects of rest periods on fatigue life extension due to healing of microcracks are significant. It has been demonstrated that longer rest periods result in more healing, and in turn in greater fatigue life.

Fatigue Failure in Polysilicon Not Due to Simple Stress Corrosion Cracking

In the absence of a corrosive environment, brittle materials such as silicon should be immune to cyclic fatigue. However, fatigue effects are well known in micrometer-sized polycrystalline silicon (polysilicon) samples tested in air. To investigate the origins of this phenomenon in polysilicon, we developed a fixed-grip fracture mechanics microspecimen but could find no evidence of static stress corrosion cracking. The environmental sensitivity of the fatigue resistance was also investigated under cyclic loading. For low-cycle fatigue, the behavior is independent of the ambient conditions, whether air or vacuum, but is strongly influenced by the ratio of compressive to tensile stresses experienced during each cycle. The fatigue damage most likely originates from contact stresses at processing-related surface asperities; subcritical crack growth then ensues during further cyclic loading. The lower far-field stresses involved in high-cycle fatigue induce reduced levels of fatigue damage. Under these conditions, a corrosive ambient such as laboratory air exacerbates the fatigue process. Without cyclic loading, polysilicon does not undergo stress corrosion cracking.

Effects of accelerated testing on porcine bioprosthetic heart valve fiber architecture

We undertook the following study to quantitatively assess the changes in porcine bioprosthetic heart valve (PBHV) fiber architecture to increasing levels of fatigue damage using an in vitro accelerated test model. PBHVs were subjected to 0–500 million test cycles at 16 Hz, and small-angle light scattering (SALS) was used to quantify the gross fiber structure of the cusps. The degree of gross fiber alignment remained essentially constant from 0 to 500 million cycles over the entire cusp. Increasing fiber orientation randomness, indicative of local damage, was observed only in the vicinity of the nodulus of Arantii after 50 million cycles. The SALS data from the damaged regions suggested shearing between fiber layers, which may be part of the failure process and accelerates valve failure. Histological analysis revealed a relatively intact gross fiber structure with the collagen fiber crimp remaining, although delamination and de-registration of the crimp was also observed. Accelerated tested PBHVs also demonstrated a pronounced ‘sagging’, which began at the earliest cycle number tested (1.4 million cycles) and whose rate decreased logarithmically with cycle number. Results of this study suggest that PBHV cusps can alter their shape without any visually apparent material yielding or fiber failure under continual cyclic loading. Further, while most of the 4 mmHg pressure fixed PBHV’s gross fiber architecture remains unchanged after 500 million cycles of accelerated testing, localized accumulated fiber damage can occur on a sub-visual structural level as early as 50 million cycles.

A model for fatigue crack propagation and remodelling in compact bone.

The process of fatigue in bone is of interest for a number of reasons. Fatigue damage in vivo can eventually lead to stress fracture, and may also act as a stimulus for bone remodelling and adaptation. The aim of this paper is to develop a theoretical model which describes the growth of fatigue cracks, especially of microcracks. The growth behaviour of microcracks is complicated by their interactions with the surrounding microstructure. This problem has been identified by researchers working on fatigue in engineering materials. Their work can be adapted to develop an equation in which the growth rate of cracks is related to applied stress conditions and also to a microstructural parameter, d, which is defined as the spacing of barriers to crack growth. The model can be used to generate stress/life data for comparison with in vitro fatigue experiments. It can also be used to investigate two hypotheses: that microcracking stimulates repair and that the level of fatigue damage can act as a signal to initiate adaptation processes of deposition or resorption.

Evaluation of fatigue damage on the mechanical properties of fiber reinforced cement pastes

Type III Portland cement samples reinforced with 0.01 volume fraction of chopped steel fibers were subjected to cyclic loading in tension with zero minimum stress and maximum stress amplitudes varying from 0.72 to 1.38 of the first crack strength of the composite. The number of cycles-to-fracture at different stress amplitudes (S-N curve) indicated good fatigue resistance in stress amplitudes representing up to 80% of the range between first crack and ultimate composite strengths. A number of specimens cycled at a stress amplitude 38% above the first crack strength were monotonically fractured in tension following different levels of prior fatigue damage. Pre-cycled specimens exhibited a rapid drop in Young's modulus, substantial increase in ultrasonic attenuation, but insignificant decay in composite strength and post-cracking ductility. It was concluded that fatigue damages, short of propagating interface cracks to the ends of the fibers, would not adversely affect the composite strength and post-cracking fiber pull-out behavior.