Showing papers on "Fatigue limit published in 1970"

Surface Integrity of Machined Structural Components

Abstract: : A program has been run to evaluate the effects of different metal removal methods and variations of these methods on surface integrity. Three alloys were studied: beta rolled Ti-6Al-4V; AISI 4340, quenched and tempered, 50 Rc; and Inconel 718, solution treated and aged. Various grinding procedures caused the titanium alloy to exhibit a fatigue strength range of 13 to 62 ksi. The fatigue strength of 4340 due to grinding variables ranged from 62 to 102 ksi, while Inconel 718 showed a range of 24 to 60 ksi. Abusive grinding conditions always resulted in fatigue strengths at the minimum of these ranges. Milling variables exhibited a fatigue strength range of 32 to 72 ksi in the beta rolled titanium alloy. EDM and ECM on Inconel 718 yielded 22 and 39 ksi, respectively, compared to 60 ksi for gentle grinding. Guidelines for processing aerospace hardware considering surface integrity requirements are presented in the report.

Fatigue Behavior of a High-Density Graphite and General Design Correlation

Abstract: The fatigue behavior of a high-density graphite was investigated in reverse bending up to 5×108 cycles at room temperature. Fatigue life was analyzed statistically, using a Weibull distribution and a homologous stress. The homologous stress is the ratio of applied stress in fatigue to the expected first-cycle strength. The first-cycle strength was calculated from bending tests of similar materials or “mate” specimens, using the Weibull statistical strength theory to correct for the significant size effect. Homologous stress gives an essentially invariant fatigue correlation for many graphite grades and may be used to estimate fatigue life under a wide variety of operating conditions. The practical endurance limit for graphite corresponded to a homologous stress of about 47%.

Fatigue of a glass bead blasted nickel-base superalloy

Abstract: Room temperature fatigue crack initiation and propagation in the wrought nickel-base superalloy Udimet 700 were investigated with electropolished and glass bead blasted material. Cracks were found to initiate at the surface along coherent annealing twin boundaries oriented for maximum in-plane shear stress in both the electropolished and glass bead blasted conditions even though bead blasting more than doubled the fatigue strength. This increase was found to result from an enhanced crack initiation resistance, but even more importantly from a very pronounced retardation of early Stage I crack propagation by the residual compressive surface stress induced in glass bead blasting.

Fatigue behaviour of sintered metals and alloys

Abstract: Published information on the fatigue behaviour of sintered materials is reviewed.Porous sintered materials exhibit similar fatigue characteristics to cast and wrought materials, including fatigue limits in ferrous materials. Their endurance ratios are slightly lower than those of similar wrought materials and they may depend on porosity content. In some cases fatigue data for sintered materials show less scatter than those for similar wrought materials. The total porosity content, which is mainly determined by compacting conditions, is the most important factor influencing fatigue behaviour. Endurance limit decreases as the porosity content increases. In the copper- and iron-base materials investigated, fatigue behaviour is influenced only slightly by powder characteristics, sintering temperature, atmosphere, and time, and by post-sintering treatments. Environmental and surface conditions seem to influence the fatigue behaviour in the same manner as pore-free materials. However, notches have a les...

Notch root radii effects in the fatigue of polymers

Abstract: Wohler-type rotating bending fatigue tests have been performed on PVC cantilever specimens containing various sharp and blunt notches of known geometries. An attempt has been made to analyse the data obtained using linear elastic fracture mechanics concepts. Results from the sharp-notched specimens show a good correlation on a stress intensity factor basis and a fatigue limit is revealed. For blunt notches a stress intensity factor had to be calculated allowing for small flaws at the notch root, and some measure of correlation of the stress intensity factor at the fatigue limit for the various specimens is obtained.

Fatigue Tests on Prestressed Concrete I-Beams

Abstract: Six fatigue tests of prestressed concrete I-beams containing web reinforcement are reported. Each beam was initially subjected to a high overload, and then subjected to 2,000,000 cycles of a design equivalent repeated loading. The beams failed by either flexural or shear fatigue. The flexural fatigue life of the test beams was less than expected from available information on the fatigue strength of strand. The tests demonstrated that prestressed concrete beams have a remarkable shear fatigue resistance. Prestressed beams, with enough web reinforcement to develop their flexural capacity, can be subjected to overloads which cause extensive cracking without subsequent danger of a shear fatigue failure under design loads. Shear fatigue failures do not occur suddenly. There is considerable warning, as indicated by increasing deflection and increasing inclined crack width, before failure occurs. In these and other tests, shear fatigue failures did not occur when the range in inclined crack width was less than 0.006 in. under application of repeated loads.

Effect of Specimen Size and Preparation on the Fatigue Strength of a Plain Carbon Steel Tested in Rotating Bending and in Torsion

Abstract: A survey of the literature indicates that, in the presence of a stress gradient, the fatigue strength of large components is generally less than the fatigue limit exhibited by small specimens of similar material. In some cases the ‘size effect’ reported is of alarming proportions. However, a closer examination of the literature and the results of some exploratory tests in rotating bending suggest that the effects observed may not be attributable simply to size but may also be dependent on the final stages of specimen preparation.An extensive test programme carried out on notch-free 0·32 C steel specimens, ranging in minimum diameter from 0·05 to 05 in in torsion and from 0·05 to 1·6 in in bending, provides results showing four main features. As other workers have found, very small specimens have generally high fatigue limits and as-polished specimens have higher fatigue limits than as-heat-treated specimens. For specimens that were normalized, carefully machined and polished, then stress-relieved and fina...

Fatigue Performance Of Friction Stir Butt Welds In A6000 Series Aluminum Alloy

Abstract: Friction stir welding is a novel solid state joining process for making low cost, energy efficient butt welds in aluminum alloy extrusions. The plate edges are clamped against a backing plate and the material is plastically deformed and stirred by a rotating tool moving along the joint line. The resulting weld bead is flush with the surface and exhibits little distortion. The material in the weld and heat affected zone (HAZ) has a fine-grained microstructure and a high tensile strength compared with welds produced by conventional arc welding methods. The present investigation was undertaken to determine the fatigue properties of friction stir welds in 5 mm thick plates in an AA6082 alloy. Extruded plates in the T4 condition were used in the test program. S-N tests in pulsating tension at R = 0.5 were performed on specimens with the weld transverse to the stress direction. Reference tests were made on the base material. Crack growth data were obtained for material in the weld metal, in the HAZ and base material. S-N tests were also made on conventional MIG butt welds from the same batch material to enable a comparison of the two welding methods. The results indicate that the fatigue strength of transverse friction stir welds is approximately 50 percent higher than the fatigue strength of MIG butt welds. The crack growth rates obtained for the weld material were lower than in the base material, probably due to a more fine grained microstructure in the weld region. INTRODUCTION The friction stir welding process has recently been developed as a cost effective alternative to conventional metal inert gas (MIG) and tungsten inert gas (TIG) Transactions on Engineering Sciences vol 8, © 1995 WIT Press, www.witpress.com, ISSN 1743-3533 226 Surface Treatment Effects II welding in aluminum alloys [1]. A major advantage of friction stir welding is that it is a solid state process involving a much lower heat input than that required in conventional arc welding methods. The weld itself and its adjacent narrow heat affected zone both have a very fine-grained microstructure with high mechanical strength. The high tensile strength of the weld material and the favorable geometry would also indicate that friction stir welds could have high levels of fatigue strength. A testing program was implemented to determine the fatigue properties of transverse butt welds of two alloys in the AA6000 series. The data presented in this paper are results from introductory tests on specimens fabricated from extruded plates in AA6082 material in the T4 temper condition. THE FRICTION STIR WELDING PROCESS In friction stir welding the plates to be joined are clamped on a backing plate to prevent movement A cylindrical shouldered tool with a specially profiled pin is rotated at a high speed, see Fig. la. The pin is slowly brought into contact with the joint line, and the material is heated by friction and plasticised in an annular volume around the pin. As the pin is lowered into the plates, soft material is extruded at the surface. Upon further lowering of the pin and movement along the joint line the shoulder face contacts the plate surface and the plasticised material is compressed against the face of the shoulder. The soft material is mashed by the leading face of the pin profile and transported to the trailing face of the pin where it consolidates and cools to form a solid-phase weld. The generation of a friction stir weld has many similarities with extrusion seam welds that form when material is joined in the weld chamber of an extrusion die [2]. The material flow, however, is somewhat different due to the more extensive mechanical mixing of the material from the two plates in the friction stir process. The properties of the weld are closely related to the tool technology. The tool bit shape and material determines the heating, plastic flow and forging pattern. Development of the friction stir welding process has up to now been concentrated mainly on butt and lap joints, however, introductory tests have shown that friction stir welding is suitable for a wide range of joint configurations [4], as shown in Fig. 2. EXPERIMENTAL PROGRAM The specimens were fabricated from AA6082 alloy plate material, in the T4 (as-extruded) condition. The plate thickness was 5 mm. The mechanical properties are listed in Table 1. Transactions on Engineering Sciences vol 8, © 1995 WIT Press, www.witpress.com, ISSN 1743-3533 Surface Treatment Effects II 227 Table 1. Mechanical properties of the AA6082 alloy in T4 temper.

Studies in fretting fatigue with special reference to the effect of stress distributions on the fatigue strength of assembled components

Abstract: When an assembly (such as a shrink-fitted shaft/hub configuration) is subjected to oscillating loads, two components of the assembly may experience oscillating slip relative to each other, giving rise to fretting on the interface between the two components. If, subsequently, a fatigue failure originates in that fretting region, the nominal failure stress is often considerably lower than the fatigue strength of the appropriate material. The object of the research reported in this thesis was to investigate a hypothesis which explains the (relatively) low magnitude of fretting fatigue strengths as a consequence of macroscopic stress concentrations in the fretted region. Such stress concentrations are a function of the overall geometry of the assembly and it was suggested that, as a result, stress distributions in assemblies should be similar to stress distributions in geometrically similar one-piece (solid) components. In an assembly, high stress concentrations near an interface are partially relieved by slip (which gives rise to fretting) and it was thought that an assembly should be stronger, therefore, than the equivalent solid. Even so, it was expected that a detailed analysis of the stress distribution in an assembly would predict stress concentrations of sufficiently high magnitude to explain failure under the low nominal stresses found in practice, thus demonstrating that fretting, itself, does not greatly affect fatigue strengths. Fatigue tests were carried out using solid and assembled specimens of various shapes and a method was developed, using finite element techniques, to analyse the stresses in the assemblies, taking full account of slipping. The theoretical stress analysis showed that, for certain geometries and systems of loading, the stress distributions in an assembly may differ considerably from those in a geometrically similar solid. Furthermore, when the fatigue results were examined in the light of the stress analysis, it was apparent that fretting can reduce the fatigue strength of a material by more than 60%. To explain this result, a new hypothesis is proposed. It is suggested that under conditions of oscillating slip, the discontinuous contact between two surfaces of an assembly results in large stress concentrations in the regions of actual contact. It is shown that such stress concentrations can be of sufficient magnitude and affect a sufficiently large volume of material to cause failure at a nominal stress considerably lower than the fatigue strength of the material concerned. Such a hypothesis is supported by the results of this investigation and also by the results of several other workers.

The relationship between discontinuous yielding and cyclic behavior in polycrystalline molybdenum

Abstract: The fatigue properties of polycrystalline molybdenum at room temperature have been determined by axial tension-compression tests in both stress and strain control modes to give lives of up to 106 cycles. Under the appropriate conditions, the endurance limit can be considerably reduced,e.g., the stress to cause failure in 106 cycles decreases from ∼42,000 psi at 5 cps to ∼28,000 psi at 0.5 cps. The fatigue resistance as assessed by either strain control or by stress control is discussed in relation to the tensile and creep behavior, and it is concluded that the reduction in the long life fatigue resistance with decrease in the cyclic frequency is due to the time dependent generation of a mobile dislocation population. The previously accepted value of 0.6 for the ratioFL/UTS for bcc metals can be attributed to this frequency effect. A lower value for this ratio, 0.4, can be derived from the high frequency data by the use of an extrapolation technique. This lower value is confirmed by long life, low frequency tests, in agreement with the value for fcc metals. The importance of obtaining fatigue data under comparable states of mobile dislocation density is emphasized, particularly for materials exhibiting pronounced tensile yield points, in order that meaningful comparisons can be made between different materials.

Fatigue behaviour of alpha-titanium and alpha-titanium–hydrogen alloys

Abstract: The fatigue behaviour of coarse grained α-titanium designated as I.M.I, titanium-115 has been examined at room temperature. The fatigue specimens were loaded in tension—compression about a zero mean load at a frequency of approximately 100 c/sec. The overall fatigue failure stress level was significantly increased by the additions of 40 ppm and 140 ppm hydrogen to the α-titanium. Metallographic examination revealed extensive permanent fatigue damage associated with twins formed during the cyclic loading. This damage in the form of cracks at the twin-matrix interface was primarily associated with {11 1} type twins in both surface and internal grains. Twin formation and the associated fatigue damage is considered to play a significant role in the fatigue failure process. When hydrogen is present in this form of titanium hydride precipitates fatigue strengthening of α-titanium by hydrogen is considered to be a consequence of the restriction of twin formation by the precipitates.

Dual strength blade of 17-4ph stainless steel

Abstract: A PROCESS IS DESCRIBED FOR HEAT TREATING A PRECIPITATION HARDENING STAINLESS STEEL CONTAINING, NOMINALLY, 17% CHROMIUM AND 4% NICKEL FOR OBTAINING VARIOUS MECHANICAL PROPERTIES. THE STEPS INCLUDE A SOLUTION HEAT TREATMENT, A CONDITIONING TREATMENT. A FIRST STAGE AGING TREATMENT AND A FINAL AGING TREATMENT. VARIOUS MECHANICAL PROPERTIES ARE DESCRIBED INCLUDING FATIGUE STRENGTH, AND DAMPING CAPACITY. UTILITY IS DEMONSTRATED IN THE TURBINE BLADE FIELD.

Quenching injection molded polycarbonate parts for fatigue resistance

Abstract: A process for the production of void free enhanced impact fatigue strength polycarbonate parts wherein parts are molded in a heated die and immediately after the molding cycle quenched in a liquid bath such as ice water.

On strain ageing and the fatigue limit

Abstract: Metallographic evidence, provided by a low-carbon steel that was strain-cycled under varying conditions of strain ageing, has shown that active fatigue slip-bands are kept narrow when strain ageing is rapid but, in general, dislocations within the active bands are not captured by solute-locking. It is concluded that, near room temperature, strain ageing must promote fatigue strength by strengthening the fatigue-hardened material that surrounds regions of strain concentration, so that the spread of fatigue damage is inhibited.

Crack propagation during fatigue experiments

Abstract: In the present paper a very simple phenomenological theory of crack propagation during fatigue experiments is presented. It is shown that an important role is played by work-hardening to explain both the existence of a fatigue limit and the well known deviations from Miner's Rule in the case of tests conducted at varying maximum applied stresses. Although approximate, the theory can account for the main phenomena involved in fatigue experiments.

Effects of carbon content on the strength of carburized steel

Abstract: 1. Other conditions being equal, the strength of carburized steel depends on the yield strength of the core and the residual compressive stresses in the case. 2. With increasing carbon concentrations in the steel the yield strength of the core increases, while the compressive stresses in the case decrease. There is an optimal carbon concentration ensuring the highest strength of the carburized steel. 3. The optimal carbon concentration depends on the al'oying of the steel and the size of the piece. In carburized steels Kh2N4 and KhGT with small sections (8–12 mm in diameter) the bending strength and fatigue limit are highest at carbon concentrations of 0.19–0.24%.

Dynamic fracture toughness tests on a302-b steel

Abstract: Using dynamic loading, fracture toughness tests were made on annealed, and normalized and tempered A302-B steel at temperatures from minus 320 to 125 Fahrenheit. Fatigue precracked compact tension specimens were used. The specimens were separated into two groups with one group fatigue precracked under a higher load level than the other. Higher fracture toughness values were obtained for the specimens precracked at the higher load. Dynamic K sub Ic values determined in this investigation were lower than static K sub Ic values for the same materials as reported earlier by another investigator.

Cumulative Damage in Push-Pull Fatigue of Fillet-Welded Mild Steel Plate Subjected to Narrow Band Random Loading

Abstract: Push-pull fatigue tests have been made in constant amplitude loading and in narrow band random loading on fillet-welded cruciforms made from 3/8 in thick mild steel plate to B.S. 1501-151 Grade 28. The test frequency was 250 Hz, mean tensile stresses of 0, 5 and 10 tonf/in2were employed and tests were extended for endurances of up to 108cycles.Curves have been fitted to the S-N results using the relationship N(S — So)α= C and on the assumption that this may be extrapolated beyond 108cycles, curves showing the effect of mean stress are given for both constant amplitude loading and narrow band random loading for endurances up to 1012cycles. Suitable factors of safety should be applied to these stresses before use in design. For endurances beyond 107cycles, an increase in tensile mean stress from 0 to 10 tonf/in2reduced the fatigue strength by about 50 per cent in both constant amplitude loading and random loading.Reasonable agreement was found at all mean stresses between the experimental random loading S-N...