Fatigue Strength Reduction Factor Research Articles

A VARIABLE Kf‐NEUBERS RULE FOR PREDICTING FATIGUE CRACK INITIATION LIFE

AbstractAs is well known, the fatigue strength reduction factor due to a notch (Kf) in the modified Neuber's rule should not be considered a constant since it varies with different stress levels. In this paper, the definition and the variation of Kf are discussed on the basis of theoretical and experimental analyses, leading to the presentation of a variable Kf‐Neuber's rule. Comparing the results of experimental stress analyses and the fatigue life tests of specimens, the variable Kf‐Neuber's rule gives good predictions and is applied to the life prediction of an aircraft component.

Fatigue behavior of E-glass fiber reinforced phenolic composites: effect of temperature, mean stress and fiber surface treatment

Phenolic matrix is reinforced by unidirectional E-glass fibers with volume fractions of 0.30 and 0.45. Three different surface treatments are applied to the E-glass fibers. The composite specimens are tested at ambient condition and temperatures of 100°C 150° and 200°C with stress levels of R(σ min /σ max ) equal to 0 and 0.4 for load frequencies of 1.5, 10 and 25 Hz. Data are presented in terms of S/N curves and assessed by degradation of modulus based on compliance. For a particular fiber glass surface treatment and volume fraction, the composite specimen is notched and tested at room temperature and 200°C. A fatigue strength reduction factor K f is defined and obtained such that the results could be compared with those of the unnotched specimens. Notch effect is small if the hole diameter is equal to the specimen thickness; it would be important for larger hole sizes. Fractured surfaces are examined by the scanning electron microscope.

Evaluation of Fatigue Strength-Reduction Factor of Weldment under Elevated Temperature Considering Thickness Effect.

Analytical and experimental studies were conducted on the fatigue strength reduction factor (FSRF) of the weldments for breeder reactor structural design, focusing on the relationship between the strain constraint effect and the ratio of the welded metal breadth to the welded joint wall thickness. As the result, it was found that the FSRF for a 304SS-308 hot TIG welded joint is expected to be about 1. 1 for the wall thickness of the Japanese demonstration LMFBR vessel.

High cycle fatigue strength of butt welded joints of small diameter pipes

Fillet welds are usually used for joints of small pipelines with 2-inch nominal outer diameter or smaller. The fillet welded joints might not be strong enough to withstand the high cycle fatigue produced by steady-state vibrations, because they have inherent crack-like discontinuities in the root portion. The fatigue strength of butt welded joints is believed to be higher than that of fillet welded joints, so that it is desirable to use butt welded joints for small pipelines under steady-state vibrations. However, there are no available data on high cycle fatigue strength of small diameter piping butt welded joints. This paper presents new data on high cycle fatigue strength of butt welded joints in small diameter pipes and development of their fatigue strength reduction factors for design use.

Observations of the ripple-loading effect: Horstmann, M. and Gregory, J.K. Scr. Metall. Mater.25 11 (Nov 1991) 2503–2506

This paper is devoted to parametric study on creep-fatigue endurance of the steel type 316N(L) weldments at 550 °C identified as type 3 according to R5 Vol. 2/3 procedure classification. The study is implemented using a direct method known as the Linear Matching Method (LMM) and based upon the creep-fatigue evaluation procedure considering time fraction rule for creep-damage assessment. Seven configurations of the weldment, characterised by particular values of a geometrical parameter ρ, are proposed. Parameter ρ, which represents different grades of TIG dressing, is a ratio between the radius of the fillet of the remelted metal on a weld toe and the thickness of welded plates. For each configuration, the total number of cycles to failure N☆ in creep-fatigue conditions is assessed numerically for different loading cases defined by normalised bending moment M∼ and dwell period Δt. The obtained set of N☆ is extrapolated by the analytic function dependent on M∼,Δt and parameter ρ. Proposed function for N☆ shows good agreement with numerical results obtained by the LMM. Therefore, it is used for the identification of Fatigue Strength Reduction Factors (FSRFs) effected by creep, which are intended for design purposes, and dependent on Δt and ρ.

Evaluation of Weldment Creep and Fatigue Strength-Reduction Factors for Elevated-Temperature Design

New explicit weldment strength criteria in the form of creep and fatigue strength-reduction factors were recently introduced into the American Society of Mechanical Engineers Code Case N-47, which governs the design of elevated-temperature nuclear plant components in the United States. This paper provides some of the background and logic for these factors and their use, and it describes the results of a series of confirmatory creep-rupture and fatigue tests of simple welded structures. The structures (welded plates and tubes) were made of 316 stainless steel base metal and 16-8-2 weld filler metal. Overall, the results provide further substantiation of the validity of the strength-reduction factor approach for ensuring adequate life in elevated-temperature nuclear component weldments.

Fatigue Strength of Deteriorated Steel Highway Bridges

The effect of long-term environmental exposure and an extended history of highway loading was quantitatively determined for steel bridge beams that had been in service more than 50 years. Fatigue tests were conducted on specimens removed from the tension flange of truss stringers salvaged from four bridge replacement projects. Based on the fatigue strength reduction factor \IK\df\N determined on a basis of mean fatigue life for all specimens in one bridge, the fatigue sensitivities encountered were less than that associated with an AASHTO Category C welded connection detail. For individual specimens, the maximum \IK\df\N value determined was 3.0. At 36-ksi (248-MPa) stress range, average specimen lives exceeded 300,000 cycles for all bridges, for both center-span (maximum prior stress history) and near-support (insignificant prior stress history) specimens. The corrosion deterioration experienced by the plain (nonwelded) simple-span steel bridge beams in this study caused the average fatigue strength to be reduced by approximately one AASHTO fatigue category.

Fretting fatigue of a [formula omitted]NiCrMoV low pressure turbine shaft steel — the effect of different contact pad materials and of variable slip amplitude

The fretting fatigue strength of a 2 1 2 % NiCrMoV low pressure turbine shaft steel has been measured when in contact with fretting pads machined from En3, 3%CrMoV, 8%Mn8%Ni4%Cr and 2 1 2 % NiCrMoV steels of different hardnesses. The fatigue strength reduction factors at a mean stress of zero were all in the range 2.6 to 3.2. Fractographic and metallographic studies of both fatigue specimens and contact pads revealed major differences in the type and extent of surface damage between the various material combinations. Fretting fatigue failures occurred over a wide range of relative slip amplitude with a minimum in fatigue life at a slip of approximately 25 μm. The experimental observations indicate that the initiation and growth of fretting fatigue cracks are controlled by the surface cyclic tangential stress which is a combination of the cyclic frictional stress induced by fretting and the bulk alternating stress.

ＳＵＳ３０４溶接継手の５５０℃におけるクリープ疲労強度とそのひずみ集中による検討

Strain controlled fatigue and creep-fatigue tests at 550°C were carried out on two kinds of SUS304 welded joints. The results obtained are summarized as follows;(1) Fatigue strength reduction factor of welded joints, in the case without strain holding, shows a maximum value at the strain range of 0.3%, and decreases rapidly with a decrease in strain range.(2) Creep-fatigue strength of welded joints, in the case of strain range of more than 0.5%, can be reasonably evaluated by strain concentration in a weld metal region, which is strongly affected by thermal softening and/or dynamic strain aging behaviors of weld metal and base metal.(3) Creep-fatigue strength of welded joints in a low strain range region is considerably affected by creep damage, which increases due to elastic follow-up during strain holding period.

Critical examination of valve design according to sub-section B 3500 of the french design and construction rules for mechanical components of PWR nuclear Islands (RCCM January 1985 edition)

The aim of this paper is to make easier the understanding of sub-section B 3500 of the RCCM Code concerning the design rules of class 1 valve bodies. The RCCM Code is applicable to mechanical components of PWR nuclear islands and is published by the French Society for Design and Construction rules for Nuclear Island Components (AFCEN). The RCCM design rules for class 1 valves (sub-section B 3500) are reviewed and the agreement between the principle of sub-section B 3200 and the rules of sub-section B 3600 concerning piping design is examined. The equations used in the code are examined with respect to their origin, their justification and their conservatism. The applicability of the body shape rules and their relationship with fatigue strength reduction factors are also discussed. A parametric analysis is presented to show the influence of transition radii under different loads, including thermal shock. The study specifically examines the stress concentration factors obtained from a 3D finite element analysis of a tee shaped valve under mechanical loading (pressure, bending, etc.). The objective of this study is to understand and improve the existing rules so that ultimately a program can be written which will allow the stress calculations to be confirmed quickly using a microcomputer.

SNotch Effects Under Ultrasonically Vibrating Stress

In this study, stress concentration factors and fatigue strength reduction factors are obtained for the case of a round bar of mild steel with a V-groove under ultrasonically vibration stress. The stress concentration factors are determined by FEM analysis on the eigenmodes. The fatigue strength reduction factors are obtained by combining the above analysis and the results of the ultrasonic fatigue tests. These factors are determined under ultrasonically vibrating stress. These results could make the ultrasonic fatigue test more popular than the ordinary fatigue test.

Notch effects under ultrasonically vibrating stress.

In this study, stress concentration factors and fatigue strength reduction factors are obtained in the case of a round bar of mild steel with a V-groove under ultrasonically vibrating stress. The stress concentration factors are determined by analysis on eigenmodes using FEM. The fatigue strength reduction factors are also obtained by combining the analysis and the results of the ultrasonic fatigue tests. The factors under ultrasonically vibrating stress are firstly determined. Based on these results the ultrasonic fatigue test may become more popular as an ordinary fatigue test.

Fatigue behaviour of porous polysulphone surface coatings for orthopaedic applications

Porous polysulphone (PPSF) has recently been selected as a candidate material for use as a prosthetic surface coating in orthopaedic applications to acommodate ‘bone ingrowth’ for fixation to the skeletal system. The tension-loaded fatigue behaviour of PPSF was characterized based on the interpretation of stress/life ( S/ N) data. Results of this investigation indicate that the fatigue life of PPSF is governed by crack propagation across the sintered necks of the porous structure. In addition, experimental data revealed that the fatigue strength-reduction factor, K f , remains equivalent to the tensile notch strength-reduction factor for a wide range of fatigue lives. However, analytical results indicate that the material has a low notch sensitivity and that a single application of a theoretical elastic stress-concentration factor in a fatigue analysis would be inappropriate.

Techniques for dealing with child sexual abuse : by Arlene Baxter Charles C. Thomas (2600 South First Street, Springfield, Illinois 62717), 1986, 145 pp., hardcover—$20.75

The 3rd Edition of API 579-1/ASME FFS-1 2016 Fitness-For-Service includes a new Part 14 dedicated to fatigue assessment. An important section in this part covers the fatigue assessment of welded joints. In this paper, an overview of the fatigue methods for welded joints is provided and extensions are recommended. First, an overview is given of the classical fatigue method used in the ASME B&PV Code based on smooth bar fatigue curves in conjunction with a fatigue strength reduction factor. In addition, the mesh insensitive structural stress method is outlined using an equivalent stress parameter based on fracture mechanics considerations in conjunction with a master S-N curve based on the analysis of over 2000 high and low cycle S-N test data. The resulting master S-N curve approach is applicable to high cycle fatigue and low cycle fatigue if a Neuber correction is introduced. In this paper, a new structural strain method is presented to extend the early structural stress based master S-N curve method to the low cycle fatigue regime in which plastic deformations can be significant while an elastic core is present. With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure can be eliminated, such as its use of Neuber’s rule in approximating structural strain beyond yield. The earlier mesh-insensitive structural stress based master S-N curve method can now be viewed as an application of the structural strain method in the high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke’s law. Thus, both low cycle and high cycle fatigue behavior can now be treated in a unified manner. In the low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy a linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. A PVRC Joint Industry Project is currently sponsoring work on the structural strain method that will lead to its incorporation in the next edition of API 579-1/ASME FFS-1.

はめあい軸の疲労強度に及ぼす腐食環境の影響

In order to investigate the effect of corrosive environment on fatigue strength, rotating bending fatigue tests were carried out on shrink-fitted specimens. The materials of axles tested were SUS 304 stainless steel, SCM430 alloy steel and S45C carbon steel, and those of bosses were SUS304 and SF55 for SUS304 axle, SUS420J1 for SCM430 axle, S45C for S45C axle. The tests were carried out in air, city water and 3% salt water. The fatigue strength reduction factor of shrink-fitted specimens in 3% salt water was approximately 2 for SF55+S30C, 3 for S45C+S45C, and 4 for SCM 430+SUS420.It was considered that the corrosion fatigue strength (CF) of shrink-fitted specimens depended on CF of these axles because CF of shrink-fitted specimens was affected by the sensitivity to corrosion of axle. CF of SUS304+SUS304 specimen increased more than the fatigue strength in air. In SCM430+SUS420 specimen fatigue fracture occurred at the plane part of specimen in city water and CF in salt water reduced to about 1/4 of that in air. It was concluded that the reduction of CF was due to galvanic corrosion, stress concentration and fretting near the shrink-fitted end.

Effects of Stress Concentration and Reinforcement on the Fatigue Strength of Rectangular Openings

Large rectangular openings on the strength deck are a major concern of the designers of commercial and military ships. Currently used methods are inaccurate for the prediction of stress concentration factor (SCF). There is no commonly accepted criterion for the maximum stress. The current design method cannot be used to explain the behaviors of the structure when the stress at a small area is calculated as greater than the ultimate strength of the metal. Nor can it explain the failures that occur at relatively low nominal stress. All these problems can be better explained and analyzed by the concept of fatigue strength and fatigue life. This paper proposes some new methods for the prediction of SCF, approximate methods for the prediction of fatigue strength reduction factor due to stress concentration, and fatigue life of openings. Validation of the new method is also included.

Cavitation erosion of cast iron in 3% salt water

Summary Vibratory cavitation erosion tests were performed on cast irons andsteels in 3% salt water, and the results were compared with previous findings in ion-exchanged water. The ratio of the mass loss rate in 3% salt water to that in ion-exchanged water is greater for pearlitic than ferritic cast irons. It is also greater for spheroidal graphite than for gray cast iron. These results are explained by the increased number and/or average length of erosion cracks in 3% salt water. The erosion resistances of cast irons and steels in 3% salt water do not improve with increasing values of Hv2/Eβg (where βe is the fatigue strength reduction factor for cast iron) as expected from those in ion-exchanged water. In 3% salt water, cast irons show only one-half the erosion resistance of steels with the same value of Hv2/Eβg. The relation that the mass loss rate is proportional to the nth power of the horn amplitude was observed. However, the exponent n is smallest for the pearlitic materials in corrosive liquid.

A study of cavitation erosion of cast iron

Cavitation erosion tests were carried out in ion-exchanged water using four cast irons (gray or spheroidal graphite) together with carbon steels with the same matrix as the cast irons. The erosion mechanism of the cast irons is explained through the growth of eroded cracks which depends on both the notch action of graphite and the notch sensitivity of the matrix. The erosion resistances of the cast irons and of the carbon steels were compared. A linear relation was obtained from the logarithmic curve between the reciprocal of the maximum mass loss rate and H v 2/ Eβ g (where H v is the Vickers microhardness, E is the elastic modulus and β g is the fatigue strength reduction factor for cast iron). It is thus concluded that the cavitation erosion resistance of cast iron is less by a factor of (1/ β g ) 1.7 than that of carbon steel of the same hardness as the cast iron matrix.

Structure-property relationships in graphitic cast irons

Some techniques used to study fatigue of cast irons are discussed and used to study the structure-property relationships in graphitic cast irons. A wide range of cast irons have been assessed to give a better understanding of the effects of metallurgical structure and graphite morphology on the fatigue behaviour of these materials. It is concluded that the ‘true’ fatigue strength reduction factor is one of the most significant parameters by which to consider improvements in fatigue properties. Increasing the amount of spheroidal graphite is suggested as a method of improving the fatigue properties of CG cast irons and yielding alloys with great potential importance.

Torsional Fatigue Strength of a Shrink Fitted Shaft

Torsional fatigue strength of several types of shrink fitted shaft couplings is estimated based on the results of stress analysis, in which the changes of frictional coefficient and contact pressure during cyclic loading are taken into account. Then these estimated strengths are confirmed by torsional fatigue tests. The results are summarized as follows. The firictional coefficient in the slip region increases steadily with an increase of cycles and saturates for more than 104 cycles. The saturated value of it is about 0.7. Torsional fatigue strength reduction factor of shrink fitted shaft is found to be Kf=1.28, which is far smaller than the rotational bending fatigue strength reduction factor (Kf=2∼3). Anti-fretting measures such as groove, taper are found to be not so effective for torsionalfatigue as for rotational bending fatigue.