High Fatigue Loading Research Articles

The effect of carbon nanoparticles on the fatigue performance of carbon fibre reinforced epoxy

The aim of this study is to investigate the influence of different types of carbon nanoparticles like multi-wall carbon nanotubes (MWCNT) and few layered graphene (FLG) on the damage mechanisms of carbon fibre reinforced epoxy (CFRP) under fatigue loading up to the high cycle regime. Specimens were manufactured from in-house made prepregs to ensure good and reproducible nanoparticle distribution within the CFRP and penetration of the fibre rovings. The quasi-static mechanical properties of the crossply laminates were not affected by the nanoparticle modification. The fatigue life, however, increased significantly for both types of carbon nanoparticles, being most pronounced for FLG at high fatigue loads. Scanning electron microscopy-analysis revealed enormous plastic deformation of the matrix due to the nanoparticles. The fatigue life increasing effect was discussed by means of energy absorption. Nanoparticle-based damage mechanisms in CFRP were displayed and differences due to the varying nanoparticle structures identified.

Economic Aspects of Prognostics and Health Management Systems in the Wind Industry

Since Wind Turbines are one of the most dynamically stressed structures, all parts should be subjected to Prognostics and Health Management System. This is especially true for the supporting structure since it is exposed to high fatigue loads. The current technical trend in the O&M business is to improve the life-time of these supporting structures. In particular, when considering the supporting structure of a wind turbine from a civil engineering perspective; a long term approach is most beneficial in financial, ecological and social aspects. To meet the challenge of managing the life-time of wind turbine supporting structures efficiently, it is necessary to develop technical concepts assessing the consumed life-time of a wind turbine. Future PHM systems of wind turbines must include this function.The global O&M market in the wind energy industry grew in the period from 2005 to 2011 at a rate of around 18 per cent, annually. The main growth driver is the aging overall turbine park. Especially in the European onshore wind market there will be a profit migration of the O&M business at the expense of new construction until 2020. Until the year 2020, three quarters of the total profit in the wind energy industry will be occupied by O&M services (Oliver Wymann, 2011).This paper discusses the special economic aspects of Prognostics and Health Management Systems focusing on a remaining lifetime prediction as a basic maintenance system in application within the wind industry. Besides studies of the future O&M market development, concepts to lower the levelized cost of energy through PHM from a macroeconomic perspective will also be discussed.Keywords: Wind O&M market, Lifetime management, Levelized cost of energy, return on investment analysis.

Relevance of aerodynamic modelling for load reduction control strategies of two-bladed wind turbines

A new load reduction concept is being developed for the two-bladed prototype of the Skywind 3.5MW wind turbine. Due to transport and installation advantages both offshore and in complex terrain two-bladed turbine designs are potentially more cost-effective than comparable three-bladed configurations. A disadvantage of two-bladed wind turbines is the increased fatigue loading, which is a result of asymmetrically distributed rotor forces. The innovative load reduction concept of the Skywind prototype consists of a combination of cyclic pitch control and tumbling rotor kinematics to mitigate periodic structural loading. Aerodynamic design tools must be able to model correctly the advanced dynamics of the rotor. In this paper the impact of the aerodynamic modelling approach is investigated for critical operational modes of a two-bladed wind turbine. Using a lifting line free wake vortex code (FVM) the physical limitations of the classical blade element momentum theory (BEM) can be evaluated. During regular operation vertical shear and yawed inflow are the main contributors to periodic blade load asymmetry. It is shown that the near wake interaction of the blades under such conditions is not fully captured by the correction models of BEM approach. The differing prediction of local induction causes a high fatigue load uncertainty especially for two-bladed turbines. The implementation of both cyclic pitch control and a tumbling rotor can mitigate the fatigue loading by increasing the aerodynamic and structural damping. The influence of the time and space variant vorticity distribution in the near wake is evaluated in detail for different cyclic pitch control functions and tumble dynamics respectively. It is demonstrated that dynamic inflow as well as wake blade interaction have a significant impact on the calculated blade forces and need to be accounted for by the aerodynamic modelling approach.Aeroelastic simulations are carried out using the high fidelity multi body simulation software SIMPACK. The aerodynamic loads are calculated using ECN's AeroModule and NREL's BEM code Aerodynl3.

A very high cycle fatigue thermal dissipation investigation for titanium alloy TC4

Titanium alloy TC4 is widely used in aeronautics applications where it is subjected to high frequency fatigue loads. Tests are performed to investigate the alloy fatigue behavior sustaining ultrasonic fatigue load in Very High Cycle Fatigue (VHCF) regime. Thermal dissipation for the alloy in 20kHz frequency is studied and a model is proposed to describe the temperature increment in the framework of thermodynamics by estimation of the anelastic and inelastic thermal dissipation at microscopic active sites in the reference element volume. The failure probability prediction method is used to evaluate the VHCF dispersion based on the two scale model and fatigue thermal dissipation analysis.

Structural Integrity of a Shaft Subjected to Multiaxial Fatigue Loads in Presence of Short Crack

The behavior of short cracks, focusing on the study of small manufacturing flaws in a critical structure working under a high frequency load spectrum, is a complex and very current topic and its investigated in this study. The modeling and predicting of the effect of these flaws is a matter of interest also for industrial manufacturers with regards to the future design of high stressed components. Aeronautical components in general, but particularly helicopter components, are subjected to a high number of fatigue cycles that potentially cause the propagation of very small flaws and thus require careful analysis. The application of multi-axial loading conditions such as axial and torsional load complicate the assessment by provoking a multimode propagation. A high cycle multi-axial fatigue loading condition (torsional and axial) on a tube that is representative of a helicopter main transmission shaft has thus been investigated in this work. The presence of a small flaw has been artificially created on the surface of the tube and the behavior of the defect in terms of the threshold of the propagation of a crack emanating from it under high cycles fatigue due to axial and torsional stresses has been investigated by means of analytical and FE models. Experimental test have also been carried out to validate the results of the simulations.

Tower Vibration Control of Active Stall Wind Turbines

Negative aerodynamic damping amplifies tower vibrations on turbines with stall rotors. Under certain conditions, these vibrations may rise to a level that causes a shutdown. Active stall turbines have a pitch system, and the objective of the work was to develop pitch control algorithms, which would minimize the probability of tower vibrations rising to the shutdown level. The results of a root cause analysis show that vibrations are largest when coherence of the wind across the rotor is high, and turbulence intensity is low. High magnitude tower vibrations are found to occur if all blades have similar angles of attack and these are in the narrow region in which aerodynamic damping is negative. The first control algorithm uses individual pitch control to ensure that all three blades do not have the same angle of attack. The second algorithm uses de-rating to eliminate the forcing function that causes the instability. The algorithms are described and simulation results are presented. Measured data from thousands of V82-1.65 MW turbines with the software in operation are presented. The control algorithms are found to reduce substantially the number of vibration shutdown events. The tower vibration control software is now in operation on more than 5 GW of installed wind turbines. The fact that high fatigue loads occur under conditions of unusually low turbulence and high coherence is the opposite from the normal wind turbine design situation. This provides a different perspective on wind turbine loads and control.

Experimental Investigation of Effect of Gear Milling Parameters of Al-B<sub>4</sub>C Composite Gears on Surface Roughness

The gears which are being used in offshore applications are to be manufactured by using materials which are having heat resistance, corrosion resistance , high strength and with standing for higher fatigue loading. This research work deals with the experimental study and analysis of various parameters of gear milling process which would effects the microstructure and surface finish of gears. Analysis have been done on various cutting parameters during the gear cutting process of Aluminium boron carbide composites. The A6061 is reinforced with Boron carbide by using the stir casting process with 10 wt. % and 15 wt. % of B4C compositions .The experimental evaluation of effect of milling parameters on surface finish and microstructure are carried out for gear milling process by varying the feed,depth of cut and the corresponding cutting force, vibration and roughness during the machining of tooth have been measured.

Potential of Metal Fibre Felts as Passive Absorbers in Absorption Silencers

The growing noise exposure of residents, due to a rising number of flights, causes significant impacts on physical health. Therefore it is necessary to reduce the noise emission of aircrafts. During take-off, the noise generated by the jet engines is dominating. One way to lower the noise emission of jet engines is to build an absorption silencer by using porous liners. Because of the high thermic and corrosive attacks as well as high fatigue loads, conventional absorbers cannot be used. A promising material is sintered metal fibre felts. This study investigates the suitability of metal fibre felts for the use as absorption material in silencers. The influences of pore morphology, absorption coefficient, determined with perpendicular sound incidence, as well as geometric parameters of the silencer to the damping are identified. To characterise the material, the parameters fibre diameter, porosity and thickness are determined using three-dimensional computer tomography images. The damping potential of absorption silencers is measured using an impedance tube, which was modified for transmission measurements. The essential parameter to describe the acoustic characteristics of porous materials is the flow resistivity. It depends on the size, shape and number of open pores in the material. Finally a connection between pore morphology, flow resistivity of the metal fibre felts and damping potential of the absorption silencer is given.

Fatigue strengthening of damaged metallic beams using prestressed unbonded and bonded CFRP plates

Bonded fiber reinforced polymers (FRPs) reinforcement systems have traditionally been found to be an efficient method for improving the lifespan of fatigued metallic structures and have attracted much research attention. Nevertheless, the performance of a bonded FRP reinforcement system under fatigue loading is basically dependent on the FRP-to-metal bond behavior. In this paper, a prestressed unbonded reinforcement (PUR) system was developed. The proposed PUR system can be used as an alternative to bonded FRP reinforcement, particularly when there is concern about the effects of high ambient temperatures, moisture, water and fatigue loading on the FRP-to-metal bond behavior. The performance of cracked beams strengthened by the PUR system was compared with that of cracked beams strengthened by the prestressed bonded reinforcement (PBR) system. A theoretical method was developed to estimate the level of prestressing sufficient to arrest fatigue crack growth (FCG). Furthermore, the method was used to examine different passive, semi-active and active crack modes with a loaded, strengthened beam. The mechanism by which a prestressed FRP plate forms a compressive stress field at the vicinity of the crack tip was also examined. Finite element (FE) modeling was conducted and the results were compared with experimental results.

Crack initiation of natural rubber under high temperature fatigue loading

AbstractVulcanized natural rubber (NR) under quiescent thermal oxidation aging and high temperature fatigue loading with small strain amplitude was investigated by the infrared spectroscopy, scanning electron microscopy, and positron annihilation lifetime spectroscopy measurements. IR results demonstrated the thermal oxidation degradation process of vulcanized NR at 85°C. During high temperature fatigue loading, nanoscale cracks and voids that are generated by the combined impact of thermal oxidation and cyclic loading were detected. Further investigation suggests that the nucleation effect of dissolved vapor and gas in the low molecular weight domains of the NR under fatigue loading accounts for the appearance of nanocracks. This work provides some new insight into the crack initiation mechanism of NR during high temperature fatigue loading, which has not been clearly understood. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Experimental research on the behaviour of high frequency fatigue in concrete

Calculating the fatigue strength of concrete under the cyclic load of vehicles when designing bridges is an issue which is receiving more and more attention from many engineers and researchers. Based on this fact, fatigue tests of plain concrete under constant-amplitude and stepping-amplitude cyclic loads were conducted. The mechanism which damages plain concrete specimens under high frequency fatigue loads was analysed and a non-linear accumulative fatigue formula that causes the damage was proposed. A fatigue equation P– S– N that considers the failure probability p′ was given. The results of this research are a good preparation for further studies into high frequency fatigue tests of concrete cylinders reinforced with carbon fibre.

Comparison of various 9–12%Cr steels under fatigue and creep-fatigue loadings at high temperature

Abstract The present article compares the cyclic behaviour of various 9–12%Cr steels, both commercial grades and optimized materials (in terms of creep strength). These materials were subjected to high temperature fatigue and creep-fatigue loadings. TEM examinations of the microstructure after cyclic loadings were also carried out. It appears that all the tempered ferritic–martensitic steels suffer from a cyclic softening effect linked to the coarsening of the subgrains and laths and to the decrease of the dislocation density. These changes of the microstructure lead to a drastic loss in creep strength for all the materials under study. However, due to a better precipitation state, several materials optimized for their creep strength still present a good creep resistance after cyclic softening. These results are discussed and compared to the literature in terms of the physical mechanisms responsible for cyclic and creep deformation at the microstructural scale.

Predicting Effect of Pressure, Shaft Velocity and Surface Finish on Depth of Wear of Lining Thickness of Engine Bushing by Experimentation

Hydrodynamic Cu-Pb-Sn material journal bearings are widely used in automobile and industrial application because of its simplicity, efficiency and low cost. The bearing is often subjected to many stops and starts with unknown load cycles. During this transient period, friction is high and bushes become progressively worn-out, thus inducting certain disabilities. The bushes are provided with a lining of Cu-Pb-Sn material which is found in the range of 450 to 600 micron. The bearing designers are not provided the attention toward this dimension as in practice the failure of bushes observed by seizer, scoring, pitting, cavitations, loss of Babbitt due to high fatigue loads etc. The total depth of wear of healthy journal bearing is observed 150 to 180 micron up to 40000 kms run. The aim of present experimental work is to determine the effect of variable load, sliding velocity of shaft and deterministic surface roughness (R a ) of lining material on sliding wear behaviour and depth of wear of lining thickness(d w ) of Cu-Pb-Sn material bush, which is widely used as bush material in automobile engine. The highest temperature zone was determined and the bush samples are marked circumferentially as a, b, c, d, e, f, g in front side and a', b', c', d', e', f', g' rear side in that region. The relationship between depth of wear of lining thickness (d w ) versus load, shaft speed, surface roughness is established by using the experimental results and regression model. The numerical result indicates that the surface roughness is most important bearing characteristics and the combined effect of load, shaft speed and surface roughness on depth of wear of lining material particularly in high temperature zone.

Predicting effect of pressure, shaft velocity and surface finish on depth of wear of lining thickness of engine bushing by experimentation

Hydrodynamic Cu-Pb-Sn material journal bearings are widely used in automobile and industrial application because of its simplicity, efficiency and low cost. The bearing is often subjected to many stops and starts with unknown load cycles. During this transient period, friction is high and bushes become progressively worn-out, thus inducting certain disabilities. The bushes are provided with a lining of Cu-Pb-Sn material which is found in the range of 450 to 600 micron. The bearing designers are not provided the attention toward this dimension as in practice the failure of bushes observed by seizer, scoring, pitting, cavitations, loss of Babbitt due to high fatigue loads etc. The total depth of wear of healthy journal bearing is observed 150 to 180 micron up to 40000 kms run. The aim of present experimental work is to determine the effect of variable load, sliding velocity of shaft and deterministic surface roughness (Ra) of lining material on sliding wear behaviour and depth of wear of lining thickness(dw) of Cu-Pb-Sn material bush, which is widely used as bush material in automobile engine. The highest temperature zone was determined and the bush samples are marked circumferentially as a, b, c, d, e, f, g in front side and a’, b’, c’, d’, e’, f’, g’ rear side in that region. The relationship between depth of wear of lining thickness (dw) versus load, shaft speed, surface roughness is established by using the experimental results and regression model. The numerical result indicates that the surface roughness is most important bearing characteristics and the combined effect of load, shaft speed and surface roughness on depth of wear of lining material particularly in high temperature zone. Keywords: Crank shaft bush, test rig, depth of wear, lining thickness, surface roughness.

Assurance of reliable time limits in fatigue depending on choice of failure simulation: Energy density versus stress intensity

The principle of least variance is applied to evaluate the reliability of the design conditions of the Runyang cable-stayed bridge. Monitored fatigue load in service data are analyzed in conjunction with the specimen fatigue crack growth data for bridge steel. Aside from size differences, the interactive effects of material behavior with load amplitude and frequency would vary with the depicted physical model for the reliability of life prediction. Based on the same crack growth history in time or cycle, the two choice selected for comparison are stress intensity factor (SIF) range, and the strain energy density (SED) range. Reliability is found to depend on the trade off between load amplitude and frequency. Considered are high-amplitude; low-frequency and low-amplitude; high-frequency. In each case, the chances are the reliable time span of fatigue crack growth will not coincide with the useful portion of bridge life, simply because the load frequency must be anticipated as an educated estimate. It is subject to change. Conversion of the crack length fatigue cycle history to the corresponding time history requires the specification of load frequency that can set the time span of the useful life. This is demonstrated for the Runyang bridge, where approximately 30 MPa and 8 MPa would correspond to the high and low fatigue load, respectively. Significant variances were found for the SIF and SED models. The difference can be attributed to the inclusion of the mean stress in the SED that is more forgiving since it accounts for both the stress and strain effects, in contrast to the SIF model that leaves out the strain and the mean stress. Since the principle of least variance refers to the average of the R-integrals, the results based on the linear sum (LS) and root mean square (RMS) will differ quantitatively, but not qualitatively. The obvious mismatch of the fatigue load used to determine the material property and that for the bridge design can be adjusted and absorbed into the appropriate choice for the load frequency, a compensating factor not realized up to now. To this end, the weighted functions in the R-integrals further emphasize long run effects of the least variance reliability analysis. Attention is called to Changeability in addition to determinability and probability for predicting the time to failure. That is to better anticipate the change in the fatigue load frequency, to which the assistance of health monitoring should provide.

Experimental and Analytical Studies on the High Cycle Fatigue of Thin Film Metal on PET Substrate for Flexible Electronics Applications

This paper addresses the behavior of thin-film metal coated flexible substrates under high cyclic bending fatigue loading. Polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are widely used substrates in the fabrication of microelectronic devices. Factors affecting the fatigue life of thin-film coated on a flexible PET substrates were studied, including thin-film thickness, film material, bending radius, temperature, and humidity. A series of experiments for sputter-deposited copper and aluminum coated on a PET substrate were conducted. Electrical resistance and crack growth rate were monitored during the experiments at specified time intervals. In addition, a finite element model was built to simulate the bending of thin-films on flexible substrate structure. Layered shell elements were used in the model. Stress intensity and stress distribution across the film were obtained and compared with the experiments. Initial results of copper-coated PET showed a great agreement between the model and the experimental results.

Experimental and Numerical Investigation on the High Velocity Impact Response of GLARE with Different Thickness Ratio

GLARE is a fiber metal laminate that is a novel hybrid composite, consisting of thin aluminum and glass/epoxy layers. The main advantage of GLARE is its high fatigue and impact loading resistance. Also its areal density is lower than metals or composites when they are used alone. In fact, it has benefits of metals and composites simultaneously. In this paper some 2/1 GLARE laminates are manufactured and impacted by 8.7mm diameter blunt cylinder projectiles at energies up to that required to achieve complete perforation of the target using a helium gas gun. Aluminum and composite layers in these laminates have different thicknesses so the effect of changing thickness of aluminum or composite layers on the ballistic performance of GLARE, ie. Ballistic limit velocity and specific perforation energy, can be investigated. The efficient thickness ratio to maximize the specific perforation energy is obtained, too. The same tests are analyzed numerically by LS-DYNA and the results show good agreement with experimental data. The results are discussed and commented upon.

A Comparison of the Prediction of Fatigue Damage and Crack Growth in Adhesively Bonded Joints Using Fracture Mechanics and Damage Mechanics Progressive Damage Methods

Lifetime prediction of adhesively bonded joints under constant amplitude fatigue has been carried out using two finite element-based, progressive damage modelling methods. The first method used a fracture mechanics (FM)-based crack growth law in which the relationship between strain energy release rate and crack growth rate under fatigue was determined from experiments using compound double cantilever beam specimens. It was found that the FM approach predicted the fatigue life well at higher fatigue loads but under-predicted the fatigue life at lower fatigue loads. This was attributed to the increasing importance of crack initiation at lower fatigue loads. This problem was solved in the second predictive method, which was based on a continuum damage mechanics approach. A power law relationship to plastic strain was used to define the damage rate. The damage law was able to simulate damage evolution prior to crack growth and excellent predictions of fatigue life were found at all fatigue loads.

Evaluation of the fatigue defect population in an elastomer using X‐ray computed micro‐tomography

AbstractAs elastomeric materials are heterogeneous by nature, their fatigue behavior is strongly driven by the initiation and the growth of cavities. In this study, X‐ray micro‐tomography is used to describe the fatigue mechanisms at a microscale. This non destructive method has already been widely applied to elastomeric materials to control the fillers size and dispersion or to analyze the cavitation induced under high hydrostatic pressure fatigue loading, for example. Here, this technique is used with a good resolution to analyze the evolution of the defects population during a fatigue campaign on hourglass shaped axisymmetric specimens. The initiation and propagation mechanisms are clearly shown on 3D observations, and the influences of the maximum principal strain and of the number of cycles on several parameters (size repartition, porosity, and defect volumic density) are investigated. A scenario for the fatigue damage evolution is proposed and some fatigue initiation criteria are finally discussed, using the results obtained at the microscopic scale. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers.

Fatigue of Laserbeam Welded Joints of High-Carbon Steel Strips

The high-carbon steel strips are one of the most widely used base materials of bandsaw blade manufacturing. These materials have sufficient strength and ductility to cope with the high fatigue load of the bandsaw blades. These endless strips are produced by welding, and therefore the weld and the heat affected zone have different mechanical properties, like tensile strength and fatigue resistance, than the base material. These properties of the weld can be influenced by preheat and post weld heat treatment. Regarding to the latest industrial requirements, the application of laserbeam welding was examined to produce higher standard bandsaw blade. The laserbeam welded joints has lower heat input and narrower heat affected zone compared to metal inert gas (MIG) welding, which is currently used in bandsaw blade manufacturing. To assure the proper mechanical properties and sufficient resistance to fatigue, an examination was carried out to determine the effect of preheat temperature and post weld heat treatment time on the mechanical properties and fatigue behaviour of the laserbeam welded joint.