Showing papers on "Fatigue limit published in 1980"

Fatigue of metallic materials

Abstract: 1. Introduction. 2. Cyclic Stress-Strain Response. Mechanical properties. Microstructure. Theories of hardening and softening. 3. Fatigue-Crack Nucleation. Stress near surface. Sites of crack initiation. Near-surface dislocation structures. Surface relief and its relation to near-surface dislocation structures. Mechanisms of crack nucleation. Factors influencing crack nucleation. 4. Fatigue-Crack Propagation. Kinetics of crack growth. Fracture mechanics for fatigue cracks. Quantitative description of fatigue-crack propagation rate and threshold. Properties of plastic zone. Models of fatigue-cracks propagation and thresholds. Effects of miscellaneous factors on the crack-propagation rate. 5. Fatigue-Life Curves. Fatigue-life curve a versus f. Fatigue-life curve a versus f. Transformation of fatigue-life curves. Influence of cycle asymmetry. Hysteresis energy and fatigue life. Fatigue limit. Curves of constant damage. S/N curves of precracked bodies. Influence of temperature on fatigue life. 6. Notched Behaviour. Stress and strain concentration. Influence of notches on fatigue life. 7. Fatigue Life for Random Loading. Cyclic plasticity. Analysis of random variations of stress and strain. Prediction of fatigue life. References. Subject Index.

Estimation of the fatigue limit by progressively‐increasing load tests

Abstract: — On the basis of fatigue data derived from a large number of experiments, the load progression methods of Locati and Prot as well as a method operating on a thermometric basis have been comparatively evaluated in an experimental and analytical (statistical) study. The last-named thermo-method can provide useful estimates for a fatigue limit corresponding with ca. 0 to 5% probability of fracture with only 1 to 3 experiments. The Prot-method, set up and evaluated in a certain way, provides a useful estimate of the fatigue limit and, additionally, an approximate indication of the width of the transition range of the corresponding Wohler-type tests after experiments on not less than about ten specimens. The load progression, contrary to the original Prot-concept, must not necessarily be applied in a continuous mode, but can occur stepwise with increments Δσ smaller than ca. 15 Nm-2.

Flaw Characteristics in Dynamic Fatigue: The Influence of Residual Contact Stresses

Abstract: The effect of residual contact stresses on the dynamic fatigue response of surfaces containing indentation-induced flaws is studied. Indentation fracture mechanics is used to analyze the growth of well-defined “median/radial” cracks in combined residual (elastic/plastic) contact and applied (uniform) tensile fields, and thence to determine strength characteristics. In this way a general formulation is obtained for the fatigue strength at constant stress rate. Experimental confirmation of the essential predictions of the theory is obtained from strength tests on Vickers-indented soda-lime glass disks in water environment. It is thereby implied that residual stresses can have a significant deleterious influence on the fatigue behavior of any brittle solid whose controlling flaws have a contact history. Such effects need to be considered in the design of structural ceramics, most notably where fracture-mechanics calibrations of crack-velocity parameters are used for lifetime predictions.

Fatigue behavior of full-scale welded bridge attachments

Abstract: This report contains the findings of NCHRP Project 12-15(3). The objective of this study was to examine the fatigue strength of beams with web and flange lateral attachment plates. In addition to providing a more comprehensive data base for this type of detail, the program was intended to examine the influence of lateral bracing members on the out-of-plane distortion of the lateral plates. Further work also was undertaken during the experimental studies on the effectiveness of peening and gas tungsten arc remelting the fatigue-damaged connections and on the ability of drilled holes to arrest crack growth. A total of 18 beams, each with three welded gusset plate details, were tested in fatigue with stress ranges of 6 to 15 ksi. Several other details were welded to the girder web in order to simulate beam flanges framing into a web plate. The results of these tests were used to assess the adequacy of the applicable provisions of the AASHTO specifications. In addition, the influence of lateral bracing on the fatigue performance of the attachments was evaluated. Recommendations for modifications to current practice are included in the report.

Zero stress strength reduction and transitions in static fatigue of fused silica fiber lightguides

Abstract: This study shows that the transition in slope of static fatigue (log fracture stress vs. log time to failure) of fused silica fibers, is an intrinsic property of the glass which is modified by the presence of coating materials. The transition for fibers in water at 90°C occurs in the 300–400 Ksi range and is characterized by a change in slope and intercept. The slopes and intercepts preceding the transitions are dependent upon zero stress soak time in water at 90°C, however in the transition region they are unchanged by this treatment. At a stress level of about 160 Ksi the transition region appears to terminate in a more normal fatigue dependence. This may possibly be a fatigue limit. These observations suggest that changes will be required in the theories correlating static fatigue with crack growth parameters as determined from macro-crack experiments. Long-term extrapolations of fatigue data will accordingly have to incorporate modifications based on such changes.

Fatigue fracture of bone plates

Abstract: Summary Of all the possible causes of bone plate failure, by far the most common is failure by fatigue. The effect of the environment and loading regime on the fatigue life is discussed. The use of the corrosion potential for the evaluation of fatigue experiments and the detection of fatigue cracks in explanted plates is described. The authors demonstrate that the surface finish and the possibility of a loss in fatigue strength after plate remodelling are factors that should not be ignored when testing bone plates.

Environmental effects on fatigue fracture mode transitions observed in aluminium alloys

Abstract: — Fatigue crack propagation tests were carried out in different environments on 7075–T6 and 2024–T3 centre-cracked sheet specimens. Observations were made on the macroscopic transition from tensile mode to shear mode. The transition is suppressed by an aggressive environment, whereas it is promoted by an inert environment. As a consequence there is no unique correlation between the state of stress and the mode of cracking. Both the state of stress and the environment have a significant effect on the mode of cracking. A simple model for the effect of environment on fatigue crack growth is presented. The implications for crack growth under corrosion fatigue conditions are discussed.

The Impact of High-Speed Reclosure of Single and Multi-Phase Systems Faults on Turbine-Generator Shaft Torsional Fatigue

Abstract: The successive electrical transients caused by system faults and subsequent switching operations associated with the widely practiced technique of automatic high-speed reclosing give rise to mechanical stresses in nearby turbine-generators. Depending upon many factors, these stresses may exceed the endurance limit of certain machine parts and thus cause slight or severe fatigue damage which is cumulative over the life of machines.

Welded structrue having improved mechanical strength and process for making same

Abstract: A welded structure and a method for making same wherein a plurality of weld metals, such as martensitic steel and austenitic steel, differing in coefficient of thermal expansion from each other are deposited in a plurality of layers thicknesswise of the welded structure in a gap defined by structural members to be welded in such a manner that a layer of the weld metal of higher coefficient of thermal expansion is covered by a layer of the weld metal of lower coefficient of thermal expansion in a weld formed, to improve brittle fracture strength, fatigue strength and stress corrosion cracking resistance of the weld. By subjecting the welded structure to stress relief annealing treatment, it is possible to produce compressive stress on the surface of the layer of the weld metal of lower coefficient of thermal expansion.

Variable amplitude fatigue in relation to highway bridges

Abstract: With the introduction of welded construction in highway bridges it has become necessary to assess designs for fatigue. Current methods of calculating endurances involve constant amplitude data and assumption that the Palmgren-Miner Law can be used to sum the damage caused by different stresses. In order to investigate the efficacy of this method laboratory tests have been conducted on representative welded connections using variable amplitude loading at endurances of up to 415 million cycles. Using a Rayleigh spectrum of stresses it is shown that for endurances relevant to bridge service, the role of stresses below the constant amplitude fatigue limit is critically important. If they are ignored, calculated endurances can be many times too optimistic. An accurate allowance for the effects of low stresses can be made by representing the constant amplitude data by a curve having a higher stress exponent for stresses below the fatigue limit. At the longest endurance tested, only about 0.25 per cent of the stresses exceeded the fatigue limit and these caused 9 per cent of the calculated damage. Under an axle load spectrum, endurances were six times longer than calculated. This was found to be due to an interactive effect caused by small numbers of high stresses which effectively retard the rate of crack propagation. In practice, shapes of stress spectra differ from axle load spectra and situations in which measured stresses involve small numbers of high values are rare. In all cases, conservative endurances were estimated using the method given in the British standard for bridge design. (Author/TRRL)

Improvement of Ti alloy fatigue properties by Pt ion plating

Abstract: The effect of Pt ion plating on the high cycle axial fatigue life of Ti-6Al-2Sn-4Zr-2Mo specimens was studied at room temperature and 455 °C. Unlike other coatings, the plated material tested in this work showed an increase in fatigue strength when compared to uncoated material. The fatigue strength improvement was greater at 455 °C than at room temperature. Coated specimens cycled close to the fatigue limit at 455 °C demonstrated the highest improvement, which was associated with subsurface fatigue crack initiation. The uncoated specimens, tested under similar conditions, failed by cracking at the surface. Sectioning analysis showed no defects in the subsurface initiation sites. The initiation was related to groups of similarly aligned coarse α platelets. It is suggested that the Pt ion plating caused some surface hardening as well as oxidation resistance at elevated temperatures leading to the suppression of surface crack initiation.

Fatigue Strength of Trip Steels.

Abstract: : Comparison of the S-N curves of warm-extruded TRIP steels, heat treated to give two stabilities with respect to deformation-induced martensitic transformation during testing, reveals a beneficial effect of the transformation on fatigue behavior under stress control conditions. The proportionality of fatigue strength to ultimate tensile strength is maintained to higher strength levels relative to other steels. For a stress ratio of R = 0.1, a fatigue strength at 10 to the seventh power cycles of 180 ksi (1,240 MPa) is obtained. (Author)

Steel-polypropylene-steel laminate: a new weight reduction material

Abstract: The dent resistance, flexural stiffness, fatigue strength, and formability are characterized for a steel-polypropylene-steel (S-P-S) laminate for automotive applications. Many of these physical properties are comparable to those of steel. The S-P-S laminate takes advantage of the I-beam principle in bending. The highest stress is in the surface, with the lowest stress at the neutral axis. The material has a high stiffness-to-weight ratio, with the potential to reduce component weight by 50% with no loss in flexural stiffness. Applications where bending stiffness is the major design criterion include: seat backs, load floors, covers, narrow body panels, truck trailer sides, and interior trim. One disadvantage of S-P-S is loss of bolt torque due to creep by the polypropylene core. This loss is a function of the applied stress level, core thickness and temperature.

An improved a.c. potential drop method for detecting surface microcracks during fatigue tests of unnotched specimens

Abstract: —The detection and measurement of surface microcracks in unnotched specimens are becoming more important, both from the point of view of description of crack growth by linear elastic fracture mechanics and from the classical endurance limit approach. Theoretical analyses of the current distribution in a cylindrical test specimen show promise that the a.c. potential drop method will become more sensitive for surface microcracks when higher current frequencies are applied. This effect was experimentally affirmed during fatigue tests on unnotched cylindrical specimens. With a 40 kHz current frequency semicircular surface microcracks with an area of 0.0066 mm2 (0.05% of the specimen cross-section) were detected. For accurate and reproducible crack growth measurements, a 5 kHz current frequency is preferable.

Fatigue Strength of Anchor Bolts

Abstract: The results of an experimental study of the fatigue strength of anchor bolts in tension and bending are presented. The influence of bolt diameter, steel strength, thread size, and method of forming the threads is examined. An analysis of the results indicated that these variables did not significantly influence fatigue performance. The fatigue design strength of single nut anchor bolts was found to be adequately represented by the current bridge and building fatigue specification Category E. Tightened double nutted anchor bolts in tension and bending produced fatigue lines comparable to Category C of the fatigue specifications.

Applicability of Crack Propagation Data to Failure Prediction in Porcelain

Abstract: Failure probability of porcelain was measured as a function of time under constant stresses in various environments. The experimental results were compared with the theoretical predictions derived from the measured distribution of fracture stress and subcritical crack growth data. The experimental and theoretical results nearly agreed for the short-term lifetime (less than ∼1000 s), after which failure probability was less than predicted. The applicability of crack propagation data to failure prediction was studied and the very slow crack-growth data near the fatigue limit were determined to be necessary for predicting the long-term lifetime of porcelain structures.

Effects of thermomechanical treatments on the mechanical behavior of eutectoid steel

Abstract: High strength, good ductility, and superior fatigue behavior were produced in fully pearlitic steel by thermomechanical treatments (TMT) in which heavy amounts of cold rolling were followed by rapid annealing above theA1 temperature. Alignment of the cementite fibers in a soft ferrite matrix by TMT gives a number of beneficial effects on mechanical behavior without the brittleness inherent to cold rolling alone. A linear regression equation was developed to quantitatively relate prior austenite grain size and interlamellar spacing with the yield strength. TMT involving lower annealing temperatures for short times resulted in optimal fatigue behavior relative to both random coarse pearlite and 75 pct cold rolled pearlite. This superiority was manifested by higher fatigue ratios (stress at 107 cycles/UTS) and higherS/N curves normalized with respect to either UTS or hardness. Transverse samples showed better fatigue strength than longitudinal ones because of anisotropy in both mechanical fibering of the microstructure and in the crystallographic texture of the ferrite matrix.

Fatigue of Hot Isostatic Pressed WC-12%Co Alloys

Abstract: Fatigue fracture properties of hot isostatic pressed WC-12%Cc alloys having coarse and fine WC particles were investigated and macro-and microfractografic analyses were carried out. The results obtained are as follows: (1) Fatigue strength increases with increase of WC particle size. (2) Fatigue strength of hot isostatic pressed alloy becomes larger than those of conventionally sintered alloy. (3) Fractographic analysis reveals the existence of characteristic fracture surface of fatigue. A some region of fatigue fracture surface looks weakly bright. In this region, saw tooth type crack can be observed. These pattern consist from the Co (h.c.p.) phase.

Fatigue Strength Of Tubular T- And X-Joints

Abstract: The results of fatigue tests on 40 tubular T- and X-joints are presented. The chord diameters of the joints are 168, 457 and 914 mm. The most important conclusions are that the lifetime decreases with increasing joint size and that in the high cycle range (N 107) some test results of the large joints fall below the AWS-X curve.

A Hydrostatic Stress-Sensitive Relationship for Fatigue Under Biaxial Stress Conditions

Abstract: Failure criteria for elastic- and plastic-strain controlled fatigue under biaxial loading are proposed. The criterion for elastic strain is defined in terms of the equivalent strain from the distortion energy theory and that for the plastic part as the corresponding equivalent plastic strain, but modified to include a function of the hydrostatic stress component. These criteria are incorporated into the derivation of a relationship for fatigue life in terms of the principal strain amplitude, analogous to a conventional form for uniaxial loading. An analysis of some limited available data from the literature shows the potential predictive capability of the relationship.