Showing papers on "Fatigue limit published in 1993"

The two thresholds of fatigue behaviour

Abstract: Two limiting thresholds to fatigue crack propagation are discussed. The first threshold is related to the microstructural texture and this threshold may therefore be deemed a material-based threshold. The second threshold is mechanically-based, and is related only to the stress state at the tip of a substantial defect. The material-based threshold is characterized in terms of Microstructural Fracture Mechanics (MFM) and the mechanically-based threshold is characterized in terms of Linear Elastic Fracture Mechanics (LEFM). The former condition is important when considering the fatigue limit of materials and components, while the latter is more applicable to the fatigue limit of structures. The different factors which affect the two threshold conditions are briefly presented. Finally, this paper discusses aspects of MFM relevant to the fatigue resistance of metals and components.

Tension-Tension Fatigue of Microcellular Polycarbonate: Initial Results:

Abstract: The fatigue life of microcellular polycarbonate specimens of five different relative densities ranging from 0.52 to 0.97 was measured in tension-tension tests on dumbbell specimens. Fatigue life va...

Influence of Loading Frequency on the Room-Temperature Fatigue of a Carbon-Fiber/SiC-Matrix Composite

Abstract: The influence of cyclic loading frequency on the tensile fatigue life of a woven-carbon-fiber/SiC-matrix composite was examined at room temperature. Tension-tension fatigue experiments were conducted under load control, at sinusoidal frequencies of 1, 10, and 50 Hz. Using a stress ratio ([sigma][sub min]/[sigma][sub max]) of 0.1, specimens were subjected to maximum fatigue stresses of 310 to 405 MPa. There were two key findings: (1) the fatigue life and extent of modulus decay were influenced by loading frequency and (2) the post-fatigue monotonic tensile strength increased after fatigue loading. For loading frequencies of 1 and 10 Hz, the fatigue limit (defined at 1 [times] 10[sup 6] cycles) was approximately 335 MPa, which is over 80% of the initial monotonic strength of the composite; at 50 Hz, the fatigue limit was below 310 MPa. During 1- and 10-Hz fatigue at a maximum stress of 335 MPa, the modulus exhibited an initially rapid decrease, followed by a partial recovery; at 50 Hz, and the same stress limits, the modulus exhibited an initially rapid decrease, followed by a partial recovery; at 50 Hz, and the same stress limits, the modulus continually decayed. The residual strength of the composite increased by approximately 20% after 1more » [times] 10[sup 6] fatigue cycles at 1 or 10 Hz under a peak stress of 335 MPa. The increase in strength is attributed in part to a decrease in the stress concentrations present near the crossover points of the 0[degree] and 90[degree] fiber bundles.« less

Effects of crystalline quality and electrode material on fatigue in Pb(Zr,Ti)O3 thin film capacitors

Abstract: Pb(Zr0.52Ti0.48)O3(PZT)/Y1Ba2Cu3Ox(YBCO) heterostructures have been grown by pulsed laser deposition, in which PZT films were epitaxial, highly oriented, or polycrystalline. These PZT films were obtained by varying the deposition temperature from 550 to 760 °C or by using various substrates such as SrTiO3(100), MgO(100), and r‐plane sapphire. PZT films with Pt top electrodes exhibited large fatigue with 35%–50% loss of the remanent polarization after 109 cycles, depending on the crystalline quality. Polycrystalline films showed better fatigue resistance than epitaxial or highly oriented films. However, PZT films with both top and bottom YBCO electrodes had significantly improved fatigue resistance for both epitaxial and polycrystalline films. Electrode material seems to be a more important parameter in fatigue than the crystalline quality of the PZT films.

Resistance of welded details under variable amplitude long-life fatigue loading

Abstract: This study was carried out with the objective of providing additional experimental data on the effects of infrequently exceeding the fatigue limit of welded steel highway bridge details. To accomplish this objective, eight full-size welded girders with Category E' web attachments and cover-plated flanges and Category C transverse stiffeners and diaphragm connection plates were subjected to long-life variable amplitude loading. It was found that fatigue cracking developed at the Category E' details when the constant amplitude fatigue limit was exceeded by more than 0.05% of the stress cycles. For the Category C details, fatigue cracks did not develop unless the peak stress range exceeded the constant amplitude fatigue limit by more than 33%. The results continued to support the extension of the exponential S-N relationship even when the effective stress range is below the constant amplitude fatigue limit.

A comparison of the wear and fatigue properties of plasma-assisted physical vapour deposition TiN, CrN and duplex coatings on Ti-6Al-4V

Abstract: The study sets out to establish a comparison between duplex systems of plasma nitriding followed by plasma-assisted physical vapour deposition (PAPVD) of TiN deposited on Ti-6Al-4V, compared with PAPVD of TiN and CrN alone. The fatigue resistance has also been examined since conventional surface modifications can often impair fatigue resistance. A rubber-wheel-type abrasion tester and pin-on-disc sliding wear tester were used to examine the wear resistance of the coatings and the load-bearing capacity of the substrate respectively. A Wohler-type rotating tester was used to study the fatigue properties. Using smooth rotating-bending fatigue specimens tested in air at 5700 rev min-1 for 106 cycles or until failure it was found that TiN, CrN and the duplex coating did not impair fatigue resistance but actually improved the S-N curves and increased the endurance limit. From the wear results it was found that, although TiN and CrN do improve the wear resistance of Ti-6Al-4V significantly, it is the duplex coating that has much the greater load-bearing capacity on the titanium substrate and gives a significant improvement on PAPVD TiN or CrN coatings in sliding and abrasive wear conditions.

Fatigue of AZ91E-T6 Cast Magnesium Alloy

Abstract: The purpose of this research was to obtain room temperature fatigue behavior of AZ91E-T6 cast magnesium alloy and to determine if commonly used models that depict fatigues behavior are applicable to this cast alloy. Axial strain-controlled fatigue behavior using cylindrical specimens were employed to determine low cycle fatigue behavior. The conventional log-log total strain low cycle fatigue model properly represented the R = [minus]1 axial fatigue data. Significant means stress relaxation occurred for all R = 0 and [minus]2 axial fatigue tests. However, for the smaller strain amplitude tests with R = 0, sufficient mean stresses were retained such that fatigue life was reduced. The mean strains/stresses had little influence on the cyclic stress-strain curve which exhibited cyclic strain hardening. Mean stress effects were analyzed using the Morrow, SWT and Lorenzo-Laird models and similar, but oftentimes nonconservative, calculations resulting. Region 1 and 2 fatigue crack growth behavior was determined using C(T) specimens with load ratios R = P[sub min]/P[sub max] = 0.05 and 0.5. The commonly used low cycle fatigue and fatigue crack growth models appear to reasonably represent most of the results with this AZ91E-T6 cast magnesium alloy.

Cyclic deformation and fatigue of metals

Abstract: 1. General Features of the Fatigue Process (M. Klesnil et al.). Cyclic hardening/softening process. Nucleation of cracks. Fatigue crack propagation. 2. Low-Cycle Fatigue (M. Klesnil et al.). Cyclic plasticity. Hysteresis loop. Variable amplitudes. Stress and strain concentration in a notch. Effect of service factors. Fatigue life. References. 3. High-Cycle Fatigue (V.T. Troshchenko et al.). Basic information about inelasticity of metals. Mechanism of inelastic deformation of metals. Some laws of inelastic deformation and fatigue damage. Strain and energy criteria of fatigue damage of metals. Accelerated method of fatigue limit determination. References. 4. Resistance to Fatigue Crack Propagation (V.T. Troshchenko et al.). Strength of cyclically loaded components with cracks. Resistance of components to cracks, effect of service factors. References. 5. Operating Fatigue Life (M. Bily et al.). Operating loads. Modelling of operating processes. Calculation of operating fatigue endurance using material cyclic stress-strain properties. References. Conclusions. Subject index.

Evaluation of the Strength of Carburized Spur Gear Teeth Based on Fracture Mechanics

Abstract: A method is developed to simulate fatigue crack growth in a carburized gear tooth based on two-dimensional linear fracture mechanics. In the simulation, the stress intensity factor is calculated by means of a finite-element method (FEM) which takes into account the effect of residual stress. The fatigue crack growth rates in the case layer are estimated experimentally, and they are expressed by an experimental formula. A strength evaluation procedure for carburized gears is then proposed on the basis of the developed method of simulation. In the procedure, the critical length of the initial crack which is obtained from the fatigue strength is assumed to be equal to the crack length evaluated from the threshold stress intensity factor, and the basic load capacity is estimated. The critical lengths are evaluated for variously treated gears and illustrated as a function of surface condition.

Inhomogeneities in single-crystal components

Abstract: The temperature distribution during single-crystal solidification of the Ni-based superalloys SRR99 and CMSX-6 by the Bridgman process was measured at various withdrawal rates in dummy turbine blades. Asymmetric heat flux and cross-section transients influence the geometrical formation of the liquidus isotherm and consequently of dendritic growth. The probability of the formation of structural inhomogeneities such as microporosity, zebras, mosaic structures, etc. is largely dependent on the curvature of the solidification front, which in turn depends on the geometrical arrangement of the ceramic cluster and on the withdrawal rate. The proneness to recrystallization during γ′ solutioning at cross-section transients due to casting-induced deformation and misaligned dendrites was also examined. Recrystallization remains confined to highly deformed areas as long as microscopic inhomogeneities acting as obstacles are present during heat treatment. High cycle fatigue tests showed that defects such as porosity and local recrystallization reduce the fatigue strength to about half the fatigue strength of the defect-free materials.

An assessment of cold work effects on strain-controlled low-cycle fatigue behavior of type 304 stainless steel

Abstract: The influence of prior cold work (PCW) on low-cycle fatigue (LCF) behavior of type 304 stainless steel has been studied at 300, 823, 923, and 1023 K by conducting total axial strain-controlled tests in solution annealed (SA, 0 pct PCW) condition and on specimens having three levels of PCW, namely, 10, 20, and 30 pct. A triangular waveform with a constant frequency of 0.1 Hz was employed for all of the tests performed over strain amplitudes in the range of ±0.25 to ± 1.25 pct. These studies have revealed that fatigue life is strongly dependent on PCW, temperature, and strain amplitude employed in testing. The SA material generally displayed better endurance in terms of total and plastic strain amplitudes than the material in 10, 20, and 30 pct PCW conditions at all of the temperatures. However, at 300 K at very low strain amplitudes, PCW material exhibited better total strain fatigue resistance. At 823 K, LCF life decreased with increasing PCW, whereas at 923 K, 10 pct PCW displayed the lowest life. An improvement in life occurred for prior deformations exceeding 10 pct at all strain amplitudes at 923 K. Fatigue life showed a noticeable decrease with increasing temperature up to 1023 K in PCW state. On the other hand, SA material displayed a minimum in fatigue life at 923 K. The fatigue life results of SA as well as all of the PCW conditions obeyed the Basquin and Coffin-Manson relationships at 300, 823, and 923 K. The constants and exponents in these equations were found to depend on the test temperature and prior metallurgical state of the material. A study is made of cyclic stress-strain behavior in SA and PCW states and the relationship between the cyclic strain-hardening exponent and fatigue behavior at different temperatures has been explored. The influence of environment on fatigue crack initiation and propagation behavior has been examined.

Effects of laboratory asphalt concrete specimen preparation variables on fatigue and permanent deformation test results using strategic highway research program A-003A proposed testing equipment

Abstract: A study was carried out to determine the effects of laboratory specimen preparation variables on permanent deformation, fatigue, and flexural stiffness performance, as measured with test equipment and methods by a Strategic Highway Research Program contractor. The specimen preparation variables included in the project were binder type, aggregate type, fines content, air-void content, compaction method, mixing viscosity, and compaction viscosity. Asphalt rubber was included as one of the binders in the experiment. The test methods used were the constant-height repetitive shear test for permanent deformation and the controlled-stress beam apparatus for flexural fatigue and stiffness. The investigation indicates that the variables included in the study affect the test results. Of particular interest were the results showing that (a) compaction method (gyratory, rolling wheel, and kneading compaction were included in the study) is a significant factor in permanent deformation performance; (b) a reduction in fines content of 3% significantly affects both permanent deformation and fatigue performance; and (c) the temperatures at which a mix is mixed and compacted also significantly affect fatigue performance. In addition, the constant-height repetitive shear test results showed asphalt-rubber mixes to be superior to the conventional asphalt mixes at 60 deg C (140 deg F).

Fatigue of advanced materials

Abstract: The development of toughened ceramics over the past 10 to 15 years is arguably one of the most important materials breakthroughs of this century. Monolithic and composite ceramic materials having fracture toughnesses up to an order of magnitude higher than those available 20 years ago have been produced using technologies based on scientific understanding and micromechanical models for in situ phase transformation, fiber bridging, ductile-particle toughening, and other toughening mechanisms. The irony of this, however, is that although ceramics can now be seriously considered for many structural applications, they can also, contrary to popular belief, be susceptible to degradation under cyclic fatigue loading. This is true even when the loading is fully compressive. As a result, a great deal of attention is now being paid to ceramic fatigue, largely because of the importance of cyclic loading in many of the potential applications for ceramics, such as gas-turbine and reciprocating engines. However, because the field is in its infancy, only limited fatigue property data have been documented, understanding of salient fatigue mechanisms has not been achieved, and the design of ceramic microstructures for optimum fatigue resistance has yet to be attempted.

Microstructure and plasticity of two molybdenum-base alloys (TZM)

Abstract: In two different commercial Mo-base alloys (TZM), produced by vacuum melting and by powder metallurgy respectively, microstructural differences (particle size and chemistry, grain and subgrain sizes, dislocation density) have been found which affect the observed mechanical properties of the material. The compression-creep properties at 1423 K show a negligibly small creep rate at a stress of approximately 200 MPa. Trapping of dislocations by particles is proposed to be the controlling deformation mechanism during creep. The microstructure and fatigue properties of TZM welds were also investigated. Friction welds showed the best mechanical properties. Fatigue measurements in load control at room temperature and 1123 K show that the endurance limit of the vacuum-melted alloy is higher than that of the powder-metallurgically processed alloy.

Composite-to-metal tubular lap joints: strength and fatigue resistance

Abstract: The axial strength and fatigue resistance of thick-walled, adhesively bonded E-glass composite-to-aluminum tubular lap joints have been measured for tensile and compressive loadings. The joint specimen bonds a 63 mm OD aluminium tube within each end of a 300 mm long, 6 mm thick E-glass/epoxy tube. Untapered, 12.5 mm thick aluminium adherends were used in all but four of the joint specimens. The aluminum adherends in the remaining four specimens were tapered to a thickness of 1 mm at the inner bond end (the bond end where the aluminum adherend terminates). For all loadings, joint failure initiates at the inner bond end as a crack grows in the adhesive adjacent to the interface. Test results for a tension-tension fatigue loading indicate that fatigue can severely degrade joint performance. Interestingly, measured tensile strength and fatigue resistance for joints with untapered adherends is substantially greater than compressive strength and fatigue resistance. The joint specimen has been analyzed in two different ways: one approach models the adhesive as an uncracked, elastic-perfectly plastic material, while the other approach uses a linear elastic fracture mechanics methodology. Results for the uncracked, elastic-plastic adhesive model indicate that observed bond failure occurs in the region of highest calculated stresses, extensive bond yielding occurs at load levels well below that required to fail the joint, and a tensile peel stress is generated by a compressive joint loading when the aluminum adherends are untapered. This latter result is consistent with the observed joint tensile-compressive strength differential. Results of the linear elastic fracture mechanics analysis of a joint with untapered aluminum adherends are also consistent with the observed differential strength effect since a mode I crack loading is predicted for a compressive joint loading. Calculations and a limited number of tests suggest that it may be possible to selectively control the differential strength effect by tapering the aluminum adherends. The effect of adherend material and thickness on fracture mechanics parameters is also investigated. The paper concludes by examining the applicability of linear elastic fracture mechanics to the joints tested.