Showing papers on "Fatigue limit published in 1976"

A damage function and associated failure equations for predicting hold time and frequency effects in elevated temperature, low cycle fatigue

Abstract: A new approach is developed for predicting strain-controlled, low cycle fatigue life at elevated temperature using a proposed energy measure of fatigue damage. This measure of damage, defined as the net tensile hysteretic energy of the fatigue cycle, can be approximated by the damage function σTΔep, where σT is the maximum stress in the cycle and Δep is the inelastic strain range. The damage function is applied to predicting effects of hold time and frequency, when time-dependent damage occurs, through failure relations incorporating a variation of Coffin's frequency modified approach. Failure equations are developed for two postulated categories of time-dependent damage.

Static fatigue limit with particular reference to glass

Abstract: After a review of the literature on the static fatigue limit, it was concluded that the normal method for its determination, using time-to-rupture data, is inadequate. There are very few experimental demonstrations of the existence of a true fatigue limit, and it is difficult to measure its value with any statistical confidence. An alternative experimental approach is suggested which is based on measuring the change in strength distribution of a representative sample after a high-temperature anneal under a fixed tensile stress. This stress weakens weak specimens in the as-received distribution, whereas strong specimens gain in strength. Those specimens whose strength remains constant demonstrate the existence of a fatigue limit and can be used to calculate a numerical value. The results are analyzed on the basis of the Charles-Hillig stress corrosion theory, which has been slightly modified to facilitate the examination of the parameters associated with the fatigue limit. This analysis relates the static fatigue limit to the effective surface energy appropriate to the fatigue mechanism, i.e. corrosion caused by water vapor. Some implications of the modified model are discussed.

Fatigue of metallic glasses

Abstract: The fatigue behaviour of Ni49Fe29P14B6Si2, Ni48Fe29P14B6Al3 and Pd77.5Cu6Si16.5 metallic glasses is examined. In the finite lifetime regime the relationship between stress amplitude (σ a), fracture stress (σ f), mean stress (σ m) and cycles to failure (N f) isσ a=A(σ f−σm) (2N f) b , whereA andb are 16.9 and −0.40 respectively for reduced gauge section Ni49 strips (for σm ≲ 140 kg mm−2) and 27.0 and −0.44 for Pd base wires. These results are unusual in thatA ≫ 1. Consequently, a sharp discontinuity exists nearσ a(σ f−σ m) −1. In a simple tensile test failure occurs at σf(=σy) and 2Nf=1; for peak stresses only a percent or so less thanσ f the sample will withstand hundreds of cycles of stress. For uniform cross-section glassy metal filaments, a fatigue limit is observed at stress ratios (σ a/σ f) in the vicinity of 0.07 to 0.15. The fatigue limit for reduced section specimens is a factor of ∼ 2 higher. Fatigue failure of the Ni-Fe strips may occur under partially or fully plane stress or plane strain conditions, depending on sample thickness and stress. Final failure of the Pd77.5Cu6Si16.5 wires always occurs by general yielding of the remaining section.

Surface microcracking induced by weathering of polycarbonate sheet

Abstract: Polycarbonate sheet subjected to outdoor weathering for relatively short periods develops a network of surface microcracks on the side exposed to solar radiation. Artificial weathering and SEM microscopy were used to illustrate the process of microcrack formation. Microcracking occurs under the influence of light radiation in conjunction with cycling of either temperature and moisture or temperature alone. The use of radiation by itself, or even relatively severe cycling of temperature and humidity without radiation, does not induce microcracking. According to the proposed mechanism, the resin of the exposed surface undergoes, with weathering, a gradual reduction in strength owing to a lowering of its molecular weight as a result of photochemical degradation. Cyclic variation of temperature and humidity in natural and artificial weathering imposes on the surface material a type of stress fatigue. Such stress fatigue is caused by the non-uniform dimensional changes that result from thermal and moisture content gradients between the surface and the bulk of the resin and from inhomogeneities and defects. When the fatigue limit of the surface material at a microsite becomes lower than the physically induced stresses, the resin cracks.

Strength and Failure Predictions for Glass and Ceramics

Abstract: Fracture mechanics has provided the means for making failure predictions regarding the lifetime of a glass or ceramic component in service. The background necessary for making these failure calculations from crack velocity and fatigue strength data is presented. The analysis of these types of data gives conservative failure predictions for soda-lime glass in water. Also, how these fracture mechanics concepts (with the appropriate experimental data) can be used as an index to the fatigue resistance of a material is described.

Thermal-Mechanical, Low-Cycle Fatigue of AISI 1010 Steel

Abstract: This study was undertaken to develop an understanding of the fatigue resistance of AISI 1010 steel under conditions of combined thermal and mechanical strain cycling in air. Comparative evaluations were made with existingthermal-mechanical fatigue data on carbon steels and with results of a comprehensive, companion study of the fatigue behavior of this same steel under isothermal conditions. Thermal-mechanical fatigue behavior was investigated for constant-amplitude, fully reversed, strain cycling of uniaxially loaded specimens at three ranges of temperature: (a) 93 to 316°C (200 to 600°F), (b) 93 to 427°C (200 to 800°F), and (c) 93 to 538°C (200 to 1000°F). Experiments were conducted both with maximum strain in phase with maximum temperature and out of phase with maximum temperature. Considering differences in experimental techniques and the difficulties associated with conducting these types of experiments, the present data agreed with similar data from the literature in limited instances where comparisons could be made. Thus, these results were considered to be representative of this type of steel, and they provided a significant extension of existing knowledge on the thermal-mechanical fatigue resistance of low-carbon steel. Dynamic strain aging was observed to cause more cyclic hardening in these experiments than in isothermal fatigue experiments. In terms of total or plastic strainrange, out-of-phase cycling was more deleterious than in-phase cycling, and thermal-mechanical fatigue life was much less than isothermal fatigue life. However, on the basis of stable stress amplitude, there was little difference in fatigue life between in-phase and out-of-phase cycling, and the fatigue life was reasonably well correlated with isothermal fatigue results.

Initiation and propagation of fatigue crack in pre-strained material

Abstract: Experimental investigations of the effect of pre-straining on the crack initiation and propagation in fatigue have been carried out on mild steel and stainless steel. X-ray analysis of the material has clearly revealed that pre-straining of the material introduces surface compressive residual stress. With increasing degree of pre-strain the crack nucleation period is delayed and the rate of crack propagation is decreased. The maximum stress intensity factor K max is related to the rate of crack propagation by a relation da/dN=CK max q , where the value of the exponent q is approximately 2.5. The constant C decreases with increasing pre-strain. Thus it has been observed that the increase in fatigue life is contributed by the delay in crack nucleation and a decrease in the rate of crack propagation. The increase in the endurance limit appears to bear a linear relation with the degree of pre-strain.

Effect of grain size, cold work, loading frequency and temperature on fatigue limit of mild steel

Abstract: Fatigue tests are performed on mild steel (0.07 pct ‡C) having different grain sizes or 5 and 10 pct cold work at frequencies of 83.3 Hz (LF) and 23 kHz (HF) at room and elevated temperatures. The fatigue limit was found to be a function ofd −1/2, whered is grain size, for HF loading at different temperature conditions. The ratio between fatigue limits determined at various frequencies and different temperatures during testing was found not to be strongly dependent on grain size. Prior cold work increased the HF fatigue limit atRT. HF tests at room temperature displayed higher fatigue limits than for LF loading. Increases in temperature during HF tests significantly decreased the fatigue limit for different grain sizes and various amounts of cold work. These results are explained by physical-metallurgical models.

Joint aircraft loading/structure response statistics of time to service crack initiation

Abstract: A reliability analysis for predicting the statistical distribution of time to fatigue crack initiation for aircraft structures in service is presented. The present analysis utilizes the statistical data of the specimen fatigue tests, the full-scale structure tests, and the statistical dispersion of aircraft service loads. The statistical distribution of the time to fatigue crack initiation of the full-scale structure under laboratory loading spectrum is assumed to be Weibull. The service loads for gust turbulences are modeled as Poisson processes for transport-type aircraft, while the maneuver loads are modeled as compound Poisson processes for fighter and training aircraft. It is found that the statistical distribution of time to fatigue crack initiation for aircraft structures in service is not Weibull and that the prediction on the basis of the Weibull distribution is unconservative, in particular in the early service time.

Environmental Effects on Graphite-Epoxy Fatigue Properties

Abstract: Effects of torsional and flexural fatigue on the long-time Integrity of advanced graphite-epoxy structural composites have been investigated. Torsional fatigue tests were run at stress ratios of R = 0 (zero to maximum, repeated) and R = -1 (zero mean stress) on unidirectional, angleply, and woven graphite fiber materials in air and water at room temperature and at 74 C. Flexural fatigue tests (four-point bending) with R = -1 were run in air and water at room temperature, and with R = 0 in air. Results show that, in torsional cycling, both water environment and higher test temperature contribute to significant degradation of torsional stiffness. The degradation of stiffness from torsional stress cycling was observed to be much greater with R = -1 than with simple R = 0 cycling. The effect of environment also is greater in the fully reversed cycling. Flexural fatigue results on +/- 30 deg material show a large fatigue effect, with fatigue limits of less than 50% and 30% of the static failure strength for specimens tested under stress ratios of R = 0 and R = -1, respectively. Compliance measurements indicate that the final failures are preceded by damage initiation and accumulation, which begins at about 1% of the specimen life.

Effect of use of galvanized steel on the durability of reinforced concrete

Abstract: Results of atmospheric exposure with cyclic application of NaCl solution on prestressed black and galvanized steel reinforcing bars are reported. Fatigue tests were conducted after 8 months. High strength reinforcing steel bars require limited crack widths for proper durability, so galvanized bars have an added safety factor in aggressive exposures. After vibration testing to failure, black bars were rusted at cracks while galvanized bars showed little attack. Concrete adhered tightly to galvanized bars near cracks. Tests showed galvanized bars lost less fatigue strength than black bars, and that they could tolerate greater crack widths.

Crack Tip Plasticity Associated with Corrosion Assisted Fatigue.

Abstract: : Preliminary data is presented on the effect of a water vapor environment on the deformation within the plastic zone of fatigue cracks in 7075-T6. Results in the water vapor environment is compared to those in a vacuum environment. High spatial resolution observations have been made using a special cyclic stage for the SEM and strains have been determined using the stereoimaging technique. Crack tip opening is shown to be a power function of the distance behind the crack tip, in agreement with a theoretical derivation of this correlation. The crack tip strain correlates with the crack opening at 1 micrometer behind the crack tip. Crack tip strains are shown to vary considerably for a fixed cyclic stress intensity. By ignoring some data, a preliminary analysis is made which indicates that the water vapor environment lowers crack tip strains. Strain distribution within the plastic zone is shown to fit a logarithmic function, as opposed to a power function, although there is still some uncertainty in this result. Work on 7075-T6 and the powder metallurgy alloy MA-87 is continuing, with definitive results expected next year. (Author)

Influence of inelastic strains on the fatigue limit of metals during bending

Abstract: 1. The calculated values of the relative difference between bending and tension-compression fatigue limits (on a base of 107 cycles) due to inelastic cyclic strains obtained for most of the materials investigated do not exceed 5% which is considerably less than the experimental values. The only exceptions are certain high-ductility austenitic and low-alloy steels. 2. At stresses corresponding to endurances of 104–105 cycles a substantial (up to 40%) difference between real and nominal stresses due to inelastic strains was observed in the case of the carbon, austenitic, and low-alloy steels strained in cyclic bending; this must be taken into account in designing for strength and in constructing models of fatigue fracture of metals.

Environmental Effects on Initiation and Propagation of Fatigue Cracks in High Strength Steel

Abstract: Environmentally enhanced fatigue crack initiation and propagation characteristics of a high strength steel have been discussed in regard to the influences of stress cycle frequency and of tempering temperature. A small amount of moisture or an aqueous environment considerably decreases the fatigue strength of the steel tempered at a temperature lower than 400°C due to the increased sensitivity to hydrogen embrittlement (HE). The co-operative action of fatigue and HE due to the occluded hydrogen results in an enhanced crack growth characteristics can be well explained by the occulusion process of hydrogen. The facet area fraction by HE increases with an increase by hydrogen content, though striation-like secondary cracking is observable on almost whole surfaces in argon. A fruitful cathodic protection can not be expected for the corrosion fatigue of a high strength steel having a high hydrogen content.

Phase composition and fatigue strength of alloy VT22

Abstract: 1. The transformation of β phase in alloy VT22 may occur with formation of martensitic α' phase of acicular form or nonmartensitic rounded α phase, which determines the fatigue strength of the alloy. 2. Raising the quenching temperature from 740 to 780°, which increases the amount of β phase from 25 to 70%, does not change the character of the transformation, which is inherent. The amount of residual β phase does not depend on the transformation mechanism.

Elevated Temperature Fatigue of Aluminum Alloy 5454 [Addendum to “Low Cycle Fatigue of Aluminum Alloys”]

Abstract: Fatigue data for aluminum alloy 5454 in the annealed condition and in the half-hard and stabilized condition, at a test temperature of 300°F (149°C), are given. These data are developed from load-controlled tests and cover the cyclic life region from about 1 × 105 to 2 × 106. The data supplement previously presented data from strain-controlled tests covering the cyclic life region below 1 × 104 cycles.

Rail whose tumbling fatigue life is prolonged

Abstract: PURPOSE: S content in steel constituents is suppressed in low level and MnS is decreased by the combined addition of Cu, Ti, Zr, Ca, REM Thereby, wear and fatigue strength resistances of steel are improved COPYRIGHT: (C)1977,JPO&Japio

Comparison of fatigue deformation and fracture in a dispersion-strengthened and a conventional nickel-base superalloy

Abstract: INCONEL alloy MA 753 is a dispersion strengthened nickel-base superalloy made by mechanical alloying which combines γ’ precipitation hardening and yttria dispersion strengthening with good oxidation and sulfidation resistance. At temperatures up to 1227 K (1750°F), the fatigue strength of MA 753 is greater than that of a conventional wrought superalloy which has a composition close to that of the MA 753 matrix. Fatigue strength at elevated temperatures is strongly dependent on testing frequency. This behavior is correlated with the strain rate dependence of tensile strength. Fatigue crack initiation sites and propagation modes in MA 753 are discussed as a function of temperature and microstructure and compared to those in the conventional superalloy. The transition from transgranular to intergranular fracture mode in MA 753 occurs at a higher temperature than found in conventional nickel-base superalloys. While the γ’ precipitate controls the fatigue strength at low and intermediate temperatures, the oxide dispersoid and carbides also affect deformation in this temperature range. At elevated temperatures, fatigue deformation is controlled by the dispersoid and carbides.