Showing papers on "Fatigue limit published in 1979"

Fatigue Crack Propagation of Short Cracks

Abstract: Previous studies have shown that both threshold stress intensity factors and fatigue crack growth rates are dependent on crack size The average growth rates for very short cracks considerably exceed those given by conventional stress intensity-crack growth laws fitted to long crack data Elastic and elastic plastic fracture mechanics solutions are modified to predict this behavior of short cracks by introducing an effective crack length U into the solutions for intensity factors and the J integral method of analysis The threshold stress at a very short crack length approaches the fatigue limit of the ma­terial, and therefore the value of k can be obtained once the threshold stress intensity factor and the fatigue limit are known The accuracy of the term k in predicting crack growth rates for short cracks is found to be independent of the applied strain level It varies linearly with the grain size of material and can be considered at surface as a measure of the reduced flow resistance of surface grains due to their lack of con­straint

Compressive Fatigue Limits of Composite Restorative Materials

Abstract: Compressive fatigue limits at 5,000 stress cycles have been measured for seven commercial composite restorative materials. The average of the ratios between fatigue limit and compressive strengths is 0.64.

Fatigue Strength of Fillet Welded Cruciform Joints

Abstract: A method for determining the fatigue strength of fillet welded cruciform joints is presented. The inadequacy of the present fatigue specification is examined through comparison with experimental data. The significance of the lack of penetration occurring at the weld root upon experimental fatigue is presented. A fracture mechanics model of the fatigue behavior is developed and correlated with experimental results. Design equations based on the model and a design example are presented.

Elastic–plastic analysis of the fatigue limit for a material with small flaws

Abstract: Fatigue tests were carried out on mild steel with small cracks for which linear fracture mechanics is not effective. A fatigue limit criterion, based on the cyclic plastic zone size at a crack tip being a material constant at the fatigue limit, can effectively evaluate the effects of crack length and stress ratio. Regarding flaws as cracks, the theory gives fatigue limit values close to those obtained in experiments on specimens with natural defects, such as surface roughness, micro-shrinkage cavities, inclusions etc. The effect of water corrosion was also investigated.

The effect of hold time on the fatigue properties of a β-processed titanium alloy

Abstract: Several recent papers have demonstrated that dwell periods at peak stress can significantly reduce the number of cycles to failure in LCF tests on titanium alloys and can cause enhanced growth rates in fatigue crack propagation tests. In all cases cleavage or quasi-cleavage facet formation has been intimately linked with the dwell sensitive fatigue response. The present paper demonstrates that facets can also form during creep deformation at ambient temperatures and it proposes that the LCF dwell effect and facet formation under cyclic conditions is dependent on time dependent plastic strain accumulation. If hydrogen contributes to the failure process it is suggested this must be through an interaction with dislocations. The significance of the model for dwell sensitive fatigue crack propagation is discussed.

Fatigue of steel reinforcement bars in concrete: a review

Abstract: — During the last few years there has been an intensification of interest in the fatigue performance of steel reinforcement bars in concrete structures. Although fatigue has not proved to be a problem to date, loading cycles are becoming increasingly severe so that the margin of reserve strength is progressively being reduced. In this paper the main parameters associated with fatigue of reinforcement are reviewed with particular attention given to conditions related to highway bridges. Fatigue testing can be either axial in air or by bending of reinforced concrete beams. The latter is experimentally less convenient but simulates more closely the service environment. Endurances can be influenced by type of steel, geometry and size of the bars, nature of the loading cycle, welding and presence of corrosion. The relative behaviour of butt welded joints is considered and it is shown that the reduction in fatigue strength commonly attributed to flaws and to the stress concentrations associated with welds does not always occur for bending fatigue of reinforced concrete beams.

Fatigue Behavior and Life Prediction of Composite Laminates

Abstract: : This report presents a survey of some of the available data on the effects of fatigue in composite laminates. Both static and fatigue behaviors of unidirectional laminae are discussed in terms of the static failure and fatigue limit strains of the constituents as well as of the loading conditions. Subcritical failure modes in multidirectional laminates are delineated and correlated with the material responses such as temperature increase, modulus change, and fatigue notch insensitivity. Also included are the effects of test frequency and compression. Methods of predicting the fatigue strengths of off-axis laminae and of angle-ply laminates are summarized. Finally, two statistical life prediction models are discussed using the material 'age'. (Author)

High temperature low cycle biaxial fatigue of two steels

Abstract: — Biaxial low cycle fatigue tests at various temperatures and strain rates were performed on 1% Cr-Mo-V steel and AISI 316 stainless steel under combined torsional and axial loads. A correlation for fatigue strength has been derived, and it is also shown that if the Gough ellipse quadrant criterion is rephrased in terms of strain amplitudes, it may be used as a safe design rule for ductile metals in both the low and high cycle fatigue regimes.

Static and dynamic fatigue of polycrystalline alumina

Abstract: The fatigue failure of polycrystalline alumina was measured in a moist air environment at 30° C as a function of constant applied tensile stress and stressing rate. The good correlation found between the fatigue test data and fracture mechanics theory indicates that fatigue is controlled by the slow crack growth of pre-existing flaws and that static and dynamic fatigue test techniques adequately define the fatigue parameters needed for failure predictions. Comparisons of proof-test predictions with experiment indicate that the proof test can be effective in eliminating weak samples from the population and in assuring against the delayed failure of polycrystalline alumina in a moist environment.

Determination of fatigue crack initiation life

Abstract: — A reanalysis has been carried out on the results of Zachariah. An elastic-plastic analysis based on S-N data indicates that the mechanism of growth of very small cracks is similar to that expressed by linear elastic fracture mechanics, i.e. crack growth in mild steel is given by where Δγp is the plastic shear strain range and a is the crack length. Crack initiation occurs within a single grain and crack propagation is given by the above equation for cracks which can be smaller than a few microns. The size of an initiated crack is a function of stress level varying from 0·1–1·0 μm in the high strain fatigue regime to 2·3 μm close to the fatigue limit. The period of crack initiation is negligible in the HSF regime whilst the initiation period can dominate lifetime at low stress levels.

Corrosion fatigue of high-strength steel in low-pressure hydrogen gas

Abstract: Fatigue tests on 835M30 steel in hydrogen gas at atmospheric pressure have confirmed that two distinct forms of corrosion fatigue crack growth can occur. The dominant mechanism of corrosion fatigue is governed by whether the cyclic stress intensity level is above or below the threshold stress intensity value for hydrogen-induced crack growth under static load, and by the stress ratio and cyclic frequency employed during testing. It has been shown that existing mechanistic models are unable to account quantitatively for the observed corrosion fatigue behaviour at stress intensity levels below the static load threshold. However, a model designated the ‘Process Competition Model’ can be used for quantitative predictions of corrosion fatigue behaviour at stress intensity levels above the static load threshold. An extended version of the model is also described in which the effects of cyclic waveform are incorporated. The translation of crack propagation rates to cyclic lifetimes is discussed and the e...

Detection of plastic deformation during fatigue of mild steel by the measurement of Barkhausen noise

Abstract: In order to determine the potential of Barkhausen noise measurement as an ndt method, the effect of plastic deformation on the true RMS value of Barkhausen noise has been studied in a fine-grained mild steel. Due to cyclic straining, distinct changes in noise levels, the degree depending on the strain amplitude, occur even below the fatigue limit, indicating plastic deformation. It seems that by detecting the amount and variation during cycling, Barkhausen noise analysis could be used as an ndt method to predict the fatigue life of real steel structures without knowing their load levels.

A new criterion of fatigue strength of a round bar subjected to combined static and repeated bending and torsion

Abstract: Criteria of fatigue strength of a round bar subjected to combined static and repeated bending and torsion have already been published by Findley and Sines together with empirical formulas by Gough. However, it is only Sines’criterion, by which the fatigue limit under combined bending and torsion is calculated, when completely reversed and pulsating fatigue limits and mean stresses are known. The authors show that Sines’criterion does not conform strictly with experimental results and they correct Sines’criterion so that fatigue failure occurs when the octahedral shearing stress amplitude attains a material constant value which decreases not only in proportion to the octahedral normal mean stress, but also in proportion to the octahedral normal stress amplitude. Applying the authors’criterion to combined repeated bending and torsion, plus coexistent static bending and torsion, a design formula was derived. The formula was compared with experimental results of Gough.

A model relating low cycle fatigue properties and microstructure to fatigue crack propagation rates

Abstract: — The fatigue crack propagation rate is related to the cyclic flow stress and the cyclic ductility of materials by means of a ductility exhaustion mechanism. The da/dN vs. ΔK relationship is derived from the assumption that an elemental crack extension occurs when the sum of the fractional fatigue life of an element at various cycles equals unity. A comparison of predicted crack growth curves with those obtained experimentally for representative alloys shows good agreement. The threshold ΔK behavior has been explained in terms of the existence of a threshold plastic strain amplitude.

The mode III fatigue crack growth threshold for mild steel

Abstract: Fatigue cracks grow perpendicular to the maximum principal applied tensile stress, or put more precisely into fracture mechanics terms, in mode I. Like most generalizations this one has its exceptions, but it does mean [I] that fatigue crack growth data can conveniently be analyzed in terms of the range of mode I stress intensity factor, AK I. A threshold value of AKI, AKIt must be exceeded before a crack will propagate. Various techniques can be used to determine AKIt ; a simple approach is to define the threshold in terms of the fatigue limit of cracked specimens. Conventional specimens used to determine fatigue crack growth behaviour have the initial crack oriented perpendicular to the applied stress. In practice components fail by fatigue from crack-like flaws that are not necessarily at right angles to the maximum principal stress, and crack growth will be in general not be in the plane of the initial crack. Definition of threshold behaviour in terms of the fatigue limit of cracked specimens extends naturally to such combined mode situations; for example, AKIIIt may be defined as the critical value of AKIIIt , the range of the shear mode stress intensity factor necessary to cause growth with leads to failure, even though crack growth is not in the plane of the initial crack. It has recently been found [2] that the fatigue crack growth thresold behaviour of mild steel for pure mode II and combined mode I and II loadings depends on two separate factors: firstly, whether or not a mode I branch crack forms at or near the precrack tip under the loading being applied, and secondly, whether the loading is sufficient to cause continued crack growth from the branch. If such a branch forms easily, or is already present, threshold behaviour is controlled by AK I for the branch crack. If for some reason branch crack formation is suppressed, higher apparent threshold values are obtained. It follows that, for the understanding of mode Ill threshold behaviour, the stress intensity factor for a mode I branch at a mode III crack is required. Two dimensional cases involving small branch cracks have been extensively discussed, for example, [3]. The problem of a mode I branch at a mode III crack is more difficult because of its three dimensional nature. Approximate solutions to branched crack problems in the two dimensional case can be obtained [3] by solving the problem of an application of the prior traction on the branch, but merely ensuring that the "extended branch" (Fig. I) is traction free. Application of this method to a mode III branch at a mode III crack gives KIII (branch) = KIII [main), and is independent of the branch angle @. For an element of a mode I branch (Fig. 2), K I [branch) = KIII (main), but geometrical incompatabilities prevent assembly of these elements into a crack. Consideration of various possible configurations suggests [4] K I (branch) =

Subsurface crack initiation during fatigue as a result of residual stresses

Abstract: — The influence of shot peening on the bending fatigue strength of hardened specimens of a carbon steel is reported. Effects of residual compressive stresses after shot peening, as a function of distance from the surface, are discussed along with the evidence of scanning electron micrographs from fractured specimens. Subsurface crack initiation is reported at all stress amplitudes below a threshold value of 1100 N/mm2. Assuming that the fatigue strength is enhanced locally due to compressive residual stresses the experimental results can be explained with the aid of the Goodman relationship.

Fatigue-Limit Effect on Variable-Amplitude Fatigue of Stiffeners

Abstract: The effect of the constant-amplitude fatigue limit on the variable-amplitude fatigue life was determined experimentally. Tensile specimens with transverse stiffeners welded either automatically or manually were tested in such a way that, from one set of tests to another, an increasing number of cycles fell below the fatigue limit. Data were developed from 500,000 cycles — 60,000,000 cycles of loading. The root-mean-cube stress range was found to be a valuable transfer function between constant and variable-amplitude fatigue life data when its value exceeded the fatigue limit. When it was lower, the modification suggested by Yamada and the first writer—based on the assumption that those cycles in the histogram that fall below the fatigue limit do not drive the crack—led to good predictions of variable amplitude fatigue life.

Mechanical Properties Characterization of an SMC-R50 Composite

Abstract: Materials scientists and design engineers considering the use of composites molded from sheet molding compound (SMC) for structural applications need reliable mechanical property data for these composites. In response to this need, the mechanical properties of a well documented, carefully prepared SMC-R50 composite over a temperature range of - 40 degree C to 121 degree C are reported. The properties determined include Young's modulus, shear modulus, and Poisson's ratio, thermal coefficient of expansion, tensile fatigue, tensile creep, and ultimate strengths in tension, compression, shear and flexure. All tests are based on one composite material, thus, conclusions about the relationship between different mechanical properties can be made. For example, the tensile and compressive moduli of SMC-R50 composite are virturally the same. Also, Young's modulus, shear modulus, and Poisson's ratio obey an established equation relating these constants. In addition, this study establishes the existence of a fatigue limit for SMC-R50 composite.

Fatigue crack initiation in hot isostatically pressed Ti-6Al-4V castings

Abstract: Precision castings of the Ti-6Al-4V alloy containing pore defects were hot isostatically pressed (HIP) in an attempt to improve the high cycle fatigue strength. Although all defects were healed, the fatigue strength was still low when compared to β-processed wrought material with a similar microstructure. Fatigue-crack initiation analysis, which included precision sectioning, indicated that early fatigue-crack initiation sites were related to relatively large α-platelet colonies and massive grain-boundary α(GBα) phase. Shear across the large colonies or along the GBα interfaces provided large initial cracks which resulted in the fatigue-life degradation. Large colonies and massive GBα developed in the HIP healed zones of the casting pores. In spite of the total HIP pore closure, the fatigue-strength improvement was small when compared to wrought material due to the coarse microstructure that developed in some locations during the HIP cycle. Large planar-shear initiation facets across several colonies were also observed. The multicolony faceted shear is the result of the Burgers relation between the colony orientations and increases the chance of early fatigue-crack inititation.

Thermal-mechanical low-cycle fatigue under creep-fatigue interaction on type 304 stainless steels

Abstract: — In this study, in-phase and out-of-phase thermal fatigue tests at the temperature ranges of 473–823 and 573–873 K were carried out on three kinds of 304 stainless steel as well as isothermal low-cycle fatigue tests at 823 and 873 K, in order to investigate the properties of thermal fatigue strength under creep-fatigue interaction. Based on the relation between the fatigue life and the failure mode, the time-dependent effect on the fatigue life was discussed. Also, an attempt was made to apply the strain range partitioning method to the thermal fatigue life prediction. It is difficult to evaluate the thermal fatigue life at high temperatures simply from the isothermal fatigue life under the same strain condition. It was also found that an unbalanced creep strain during tensile loading, which increased the number of intergranular cracks, gave the largest damage to the material. By the strain range partitioning method, it was possible to predict the isothermal fatigue life and the thermal fatigue life at the low temperature range within a factor of 1·5. On the other hand, the thermal fatigue life at the high temperature range could only be predicted within a factor of 3. However, further detailed investigations are required on the technique of partitioning the inelastic strain range and predicting the effects of dynamic strain ageing and recovery during strain holds.

Flexural fatigue behaviour of plain concrete

Abstract: The paper describes simple flexural fatigue tests carried out on small concrete beams, simply supported and loaded at the middle third points. Three different types of concrete were used. Preliminary tests were carried out to determine the effect of rate of loading on fatigue performance. The programme covered ages at time of loading up to 5 years with comparable information on static loading up to 10 years. It was concluded that the results of the tests, which covered a wide range of moisture conditions and age, suggest that fatigue behaviour reflects closely the behaviour under conventional "static" loading. For example, increase of strength and stiffness with age result in a proportionate increase in fatigue performance. On the other hand, a decrease in strength resulting from surface drying results in a decrease in fatigue performance. Equally, concrete mixes having an inherently higher flexural strength also have a proportionately higher fatigue strength. The effects of a wide range of mix variables and test conditions can be approximated by a single mean endurance curve in which the fatigue loading is expressed as a proportion of the static flexural strength under the same conditions. This approach can form the basis of a useful practical method of predicting the fatigue performance of concrete under particular combinations of conditions, from a knowledge of its static behaviour under the same conditions. (TRRL)