High Cycle Fatigue Failure Research Articles

Factors influencing the mechanism of superlong fatigue failure in steels

When the fatigue life N f of a specimen of 10 mm in thickness is longer than 10 8 cycles, the average fatigue crack growth rate is much less than the lattice spacing (∼0.1 A or 0.01 nm) that is 10 -11 to 10 -12 m/cycle. In the early stage of the fatigue process, the crack growth rate should be much less than the average growth rate, and accordingly we cannot assume that crack growth occurs cycle by cycle. In this paper, possible mechanisms for extremely high cycle fatigue are discussed. Of some possible mechanisms, a special focus was put on a newly found particular fatigue fracture morphology in the vicinity of the fracture origin (non-metallic inclusions) of a heat-treated alloy steel, SCM435, which was tested to N ≥ 10 8 . The particular morphology observed by SEM and AFM was presumed to be influenced by the hydrogen around inclusions. The predictions of the fatigue limit by the √area parameter model are ∼ 10% unconservative for a fatigue life of N f = ∼10 8 , though it successfully predicts the conventional fatigue limit defined for N = 10 7 . Thus, the fatigue failure for N ≥ 10 8 is presumed to be caused by a mechanism which induces breaking or releasing of the fatigue crack closure phenomenon in small cracks. In the vicinity of a non-metallic inclusion at the fracture origin, a dark area was always observed inside the fish-eye mark for those specimens with a long fatigue life. Specimens with a short fatigue life of N f = ∼10 5 do not have such a dark area in the fish-eye mark. SEM and AFM observations revealed that the dark area has a rough surface quite different from the usual fatigue fracture surface in a martensite lath structure. Considering the high sensitivity of high-strength steels to a hydrogen environment and the high hydrogen content around inclusions, it may be concluded that the extremely high cycle fatigue failure of high-strength steels from non-metallic inclusions is caused by environmental effects, e.g. hydrogen embrittlement coupled with fatigue.

Inclusion size distribution and endurance limit of a hard steel

High cycle fatigue failure of hard steels is dominated by a long crack initiation phase and a short crack propagation phase. The crack initiation sites are inclusions or surface defects. Due to this, the endurace probability of a hard steel part depends on its crack initiating inclusion size or surface defect size distribution. These consideration lead to the weakest-link concept which allows to calculate local and total endurance probabilities of the cycled parts. From these probabilities, the endurance limit and the probable crack initiation sites can be predicted. Base of the prediction is the knowledge of the endurance limits under three different load conditions. From these data and fracture-mechanic relations for the different inclusion types, the distributions of the crack initiating inclusion sizes can be predicted.

Status of PFBC and experience with materials performance during over 75 000 hours of operation in ABB'S PFBC plants

The key components of ABB's pressurized fluidized bed combined cycle (PFBC) plants are the specially designed gas turbine, which we refer to as the PFBC machine, and the pressurized fluidized bed boiler used to generate and superheat steam for expansion in a steam turbine. ABB Stal's 17 MWe GT35P and 70 MWe GT140P PFBC machines, respectively, are used in ABB's 100 MWe P200 and 425 MWe P800 modules. Particulate cleanup before expansion in the turbine sections is with cyclones. So far, over 75 000 hours of operation has been accumulated on P200 modules in the world's first PFBC plants, demonstrating that PFBC meets expectations. The GT35P machines have been found to perform as expected, although high cycle fatigue failures of the variable speed low pressure turbine blades have been experienced in the Tidd and Värtan plants, leading to minor modifications of the blades and also to a change of blade material. Following the observation of some wear of turbine blading in the GT35P machines, and as originally planned while these machines were being designed, erosion protective coatings have been applied at locations where secondary flows and the formation of vortices causes enhanced interaction between particulates and the turbine components. Wear on boiler tubing and membrane walls has been very limited, certainly no more severe than is typical for AFBC units, and found to be controllable through use of coatings, in the case of evaporator and other low temperature component surfaces, or by the application of sleeves at locations with enhanced tube—particle interaction. The flame sprayed Metcoloy-2 coating originally applied to all evaporator tubing has performed very well in the Tidd plant, whereas some very local flaking of this coating has been noted in the Escatron plant. By comparison, extensive flaking of this coating has occurred in the Värtan plant. This was found to involve oxidation of the bonding layer under the micro porous flame-sprayed coating, and embrittlement of the Metcoloy-2 layer. It is believed that differences in operating conditions, i.e., in surface temperature and corrosive environment, may explain these differences in performance. During the summer of 1996, the tube bundle in one of the two boilers at Värtan was replaced with a new tube bundle, in which the evaporator tubing has a sprayed and fused coating, which should be impervious to the combustion gases. Together, the experiences obtained in the different plants provide a very significant database which is applied for the continued improvement of existing plants and also in the design of new plants. The next P200 plant will be built in Germany for operation on brown coal. The first P800 plant will be built at the Karita site of Kyushu Electric Power Company, Japan. The GT140P machine for that plant has already been manufactured and is now being shop tested at the ABB Stal workshops in Finspong, before shipment to Japan. The new P800 boiler is being manufactured at IHI's Aioi factories in Japan. Future development of ABB's PFBC plants is foreseen to include raising the turbine inlet temperature through combustion of a topping fuel in order to reach thermal efficiencies which ultimately may be in the range of 50–53% (LHV). Otherwise, the main attention is focussed on improved availability, and further enhanced performance and fuel flexibility.

Nonlinear response and sonic fatigue of National Aerospace Space Plane surface panels

T is becoming increasingly apparent that the development of supersonic/hypersonic vehicles such as the NASP cannot become a reality until the fatigue problem under severe aerodynamic, acoustic, and thermal environment is solved. Furthermore, with an increasing use of light-weight materials such as fiber-reinforced composites, metal matrix composites, and various intermetallic compounds, there is an urgent need for improved analytical methods for nonlinear response and sonic fatigue predictions. The proposed NASP concept involves operations at subsonic, supersonic, and hypersonic speeds. Thus, aerodynamic, acoustic, and thermal loadings should be considered for various phases of flight. In the early stages of the takeoff, the aft surface thermal protection systems will be in the near field of the engine exhaust noise. As the speed of the vehicle increases, the effect of engine exhaust noise (except in the near vicinity of exhaust nozzles) will decrease, and at Mach 1 and higher speeds the acoustic loads are expected to become negligible. However, at supersonic and hypersonic speeds, the fluctuating surface pressures due to convecting turbulent boundary layer will become significant. In addition, local impinging shocks on the structural surface could induce severe dynamic loads. Supersonic/hypersonic vehicles will be subjected to severe surface temperatures exceeding SCOOT.1'4 Structural surface components and control devices can be expected to behave in a highly nonlinear fashion with respect to both geometry and materials.4^8 No reliable analytical procedures have yet been developed which could handle these complexities and give meaningful results that are useful for the design of these vehicles. The thermal surface protection systems of aircraft structures are usually constructed from discretely stiffened panels and stiffened shells. A typical discretely stiffened panel is shown in Fig. 1. Due to anticipated high-surface temperatures of advanced supersonic/hypersonic vehicles, multi-wall and/ or multilayer constructions might need to be utilized for the design of thermal surfaces. High-cycle fatigue failures have occurred in discretely stiffened surface panels with the majority of cracks appearing in the near vicinity of the stiffening element.5'13 Thus, proper dynamic interaction between the panel and various stiffening elements should be taken into account when calculating the nonlinear response of the surface panels. A single bay panel with clamped or simple support boundary conditions might provide reasonable estimates on deformations,14'23 but the predictions of principal stresses of a discretely stiffened panel by a single panel model could be unrealistic. Since fatigue damage is controlled by local stress conditions, accurate stress predictions are essential for fatigue life estimates. A time domain Monte Carlo-type approach has been successfully applied to a variety of problems of a linear and nonlinear nature with complex random inputs.24'33 Utilizing the nonlinear time domain stress response solutions and fatigue information from constant amplitude coupon tests, preliminary fatigue damage models have been constructed. The adverse thermal conditions could result in degradation of strength, stiffness, and fatigue life. In addition, thermal inplane loads could induce buckling and snap through type vibrations of surface panels. The time domain procedures could account for these effects in the general formulation of the total response problem. General Features of Time-Domain Method For structural dynamic problems of nonlinear/probabilistic nature where close form or effective approximate solutions are not possible, the time domain Monte Carlo method could provide a feasible approach to construct practical solutions. The recent advent of high-speed digital computers has made this procedure a useful and effective method. To illustrate the basic features of the time domain approach, consider a generalized form of a second-order nonlinear equation

A general criterion for high-cycle multiaxial fatigue failure: McDiarmid, D.L. Fatigue Fract. Eng. Mater. Struct. 1991 14, (4), 429–453

Transient Liquid Phase Bonding (TLPB) makes use of the isothermal solidification of a liquid interlayer through the formation of intermetallic compounds (IMC). This work investigates the Ag3Sn IMC formation in a thin, electroplated Ag–Sn interlayer system appropriate for TLPB – while covering as-coated conditions, growth initiation up to a few minutes of holding time – with experimental and numerical methods. Both solid-state and solid–liquid reactions at 200, 250 and 300 °C, respectively, were covered under identical and near-isothermal conditions with a novel experimental setup. Morphological aspects like the lateral IMC grain size or the scallop roughness were proven to strongly depend on temperature. Growth kinetics were captured by power laws, growth exponents n were determined as 0.24, 0.36 and 0.29, parabolic growth constants k as 1.32 ⋅ 10−11, 4.04 ⋅ 10−10 and 7.24 ⋅ 10−10 cm2/s at 200, 250 and 300 °C, respectively, and the activation energy Q as 30.6 kJ/mol. Predominant diffusion mechanisms, i.e. volume or GB diffusion, as well as coarsening effects were identified as temperature and time dependent at early stages. A mathematical model was developed based on the Arrhenius relationship including a growth correction function zIMC,corr to calculate and predict IMC growth as a function of temperature and time. The study was completed by multiphase-field simulations which targeted to mimic kinetic and morphological features of Ag3Sn IMC under various conditions. Good agreement between experiments and simulations was found for a limited set of parameters which allowed drawing conclusions about the dominant transport mechanisms for the reacting elements.

Fatigue behavior of thermosetting‐polyester‐matrix sheet‐molding compounds

AbstractTwo sheet molding compounds, one containing 65 weight percent E‐glass fiber and no filler (SMC‐65) and the other containing 50 weight percent glass and 15 weight percent CaCO3 filler were subjected to sinusoidal, load‐control, tension‐tension fatigue. The macroscopic behavior showed some notch sensitivity for both materials, but particularly for SMC‐65. The extent of failure damage over the specimen length decreased with decreasing load amplitude. Different failure mechanisms appear to operate under high‐ and low‐cycle fatigue conditions. Microscopic surface damage studies at stresses near the endurance limit showed several stages in the fatigue failure. For SMC‐65, these stages are, sequentially, matrix crack initiation and growth, matrix pulverization, fiber fracture, and fiber pullout. A similar, but less distinct pattern was observed for SMC‐50. Acoustic emission was also followed during fatigue cycling near the endurance limit. Distinct amplitude distribution peaks for each failure process were seen. It is suggested that matrix fracture, permanent deformation and Euler buckling are all important in high cycle fatigue failure.

Costly low pressure bearing support cracking problem resolved through additive damping

The component is the low pressure bearing support (LPBS) of the TF‐41 jet engine. The LPBS was experiencing high cycle fatigue failures in the four vanes used to supply and scavage oil for the front bearing. High cycle fatigue failure in this vane case assembly is a general problem in currently operating turbine engines. A program was conducted to design and engine test a constrained layer damping wrap to control the problem. This project was conducted as a cost savings alternative to the current high maintenance effort to repair the high cycle fatigue cracks, and the redesign of the component and implementation of the new component in the fleet. The requirement for the damping treatment was to significantly reduce the dynamic response (and thus reduce the dynamic stresses) of the oil carrying vanes in their operating temperature range; and, the thickness of the application was to be not greater than 0.016 in., so as not to affect engine performance. The laboratory test showed a factor of ten decrease in dynamic response due to the application of the damper wrap, while the engine test cell run showed a dynamic stress reduction of eighty to ninety percent. This paper will describe the utilization of modal analysis data for damping treatment planning, the process for damping treatment design optimization, and proof testing on an operational engine. [This work was supported by the United States Air Force Materials Laboratory.]

High cycle fatigue of tantalum carbide reinforced nickel base eutectics at room temperature

The high cycle fatigue response of two advanced tantalum carbide strengthened eutectic superalloys has been determined at room temperature. Since these alloys will be coated in service, the effects of variables associated with coating processes were given special attention. Both alloys showed a well defined fatigue limit. It was concluded that the maximum stress obtained in the cycle at the fatigue limit coincided with the tensile stress at which the matrix yielded. Above this stress level cyclic deformation of the matrix resulted in fiber failure, a necessary precursor of specimen failure. Detailed observation of the sequence of events leading to fiber failure and subsequent early crack growth in the matrix over a broad range of alturnating stresses confirmed that fiber fracture was the critical crack nucleation step in high cycle fatigue failure of the two alloys. It was shown that several changes in the surface condition did not affect the fatigue life of the alloys. However, when the samples were prestrained to crack the carbide fibers, they failed when cycled, at alternating stresses below the fatigue limit for virgin material. A similar loss of fatigue life was observed after heat treatment of one of the alloys. Fractographic examination indicated that this degradation resulted from enhanced crack nucleation at sigma platelets which were present as a result of the heat treatment.