Showing papers on "Fatigue limit published in 1971"

Application of fracture mechanics in the analysis of pavement fatigue

Abstract: THE PURPOSE OF THIS PAPER IS TO PREDICT THE FATIGUE LIFE OF PAVING MIXTURES IN TERMS OF MATERIAL CONSTANTS, GEOMETRY, BOUNDARY CONDITIONS, AND THE STATE OF STRESS. FATIGUE IS DEFINED IN TERMS OF CRACK INITIATION, INFLUENCES ON CRACK GROWTH, AND CRITICAL STRESS INTENSITY AT THE CRITICAL FAILURE POINT. CRACK GROWTH PARAMETERS WERE DETERMINED FROM BEAM LOADING EXPERIMENTS. FATIGUE LIFE ESTIMATES BASED ON THESE PARAMETERS AGREED CLOSELY WITH EXPERIMENTALLY DETERMINED VALUES.

Improved fatigue resistance of Al-Zn-Mg-Cu (7075) alloys through thermomechanical processing

Abstract: To decrease the accumulation of damage during long-life low-stress cyclic loading, microstructures must accommodate inelastic deformation by homogeneous or “dispersed” slip rather than by localized slip concentrations. In age-hardening aluminum alloys this requirement can be met by introducing a dense and uniform dislocation forest through suitable thermo-mechanical treatments. Such a treatment was developed for Al-Zn-Mg-Cu (7075) alloys, involving a process cycle of solution annealing, partial aging, mechanical working and final aging. The fatigue properties (S-N curves) of commercial and high-purity 7075TMT are compared with conventional 7075-T651 properties; with zero mean stress the alternating stress to cause failure in 107 cycles is more than 25 pct higher for commercial-purity 7075TMT and almost 50 pct higher for high-purity 7075TMT. The results emphasize the importance of microstructural control when high fatigue resistance is required.

The fatigue strength of fillet welded connections

Abstract: : The report describes a study of the fatigue behavior of fillet welded joints stressed perpendicular to the weld line. The study included an experimental phase in which the stress-life and cracking behavior of load and non-load carrying fillet weld joints was determined. This experimental study concentrated on the fatigue behavior in the transition region between high and low cycle fatigue. The second phase of the study was the determination of the fracture mechanics stress intensity factor for cracks at the weld toe or root of fillet welded joints. The finite element technique was used to determine the compliance of typical fillet welded joints.

Fatigue strength of high-yield reinforcing bars

Abstract: The stress range to which a reinforcing bar is subjected is the primary factor determining its fatigue life. For design purposes, there is a limiting stress range, the fatigue limit, above which a reinforcing bar will have a finite fatigue life and is certain to fracture. At stress ranges below the fatigue limit, a reinforcing bar will have a long fatigue life and may be able to sustain a virtually unlimited number of stress cycles. The magnitude of the fatigue limit depends on the minimum stress during each stress cycle and on the shape of the deformatins rolled onto the bar surface. It may also depend on the diameter and the grade of the bar. Increasing a tensile minimum stress was found to result in a decrease in fatigue strength. On the other hand the fatigue was found to incrase with an increasing compressive minimum stress. Bar diameter and grade of bar were found to influence the finite-life fatigue strength of reinforcing bars. The existence of a long-life fatigue effect due to these variables could not be established. Larger size bars have a lowered fatigue strength while higher grade bars have an increased fatigue strength. The effect of lug geometry on fatigue strength was found to be coupled with that of bar diameter. The larger the bar diameter, the greater was the effect of lug geometry.

Fatigue failure in high strength metals

Abstract: A recently developed theory of fatigue crack propagation in ductile metals is extended to high strength metals. The basic fatigue laws for such materials are derived and their behaviour compared with that of more ductile metals.

The Low-Cycle Fatigue Behaviour of Copper at Elevated Temperatures

Abstract: Tensile and high-strain fatigue tests have been carried out at elevated temperatures on specimens of oxygen-free, high-conductivity copper and vacuum-cast copper. Grain-boundary sliding and grain-boundary migration occur during testing to produce boundaries orientated preferentially at ∼ 45° to the stress axis. In oxygen-free, high-conductivity copper, cracking takes place at triple points as a result of stress concentrations induced by sliding. The cracks increase in length with further fatigue and eventually link together by ductile rupture to cause final failure. With vacuum-cast copper, triplepoint cracking is suppressed owing to the greater mobility of the grain boundaries and failure in tension and fatigue is due to rupture following plastic instability.

Fatigue Strength of Hot Rolled Deformed Reinforcing Bars

Abstract: FATIGUE TESTS ON #5, 8, AND 10 REINFORCING BARS (16, 25, AND 32 MM DIAMETER, RESPECTIVELY) WITH NOMINAL YIELD STRENGTHS OF 40, 60 AND 75 KSI (2800, 4200, AND 5300 KGF/CM SQUARED) ARE REPORTED. TESTS WERE CARRIED OUT ON REINFORCED CONCRETE BEAMS CONTAINING ONE SUCH BAR AND ON SPECIMENS MACHINED FROM THE BARS. ALTHOUGH THE FATIGUE STRENGTH OF THE MACHINED SPECIMENS VARIED DIRECTLY AS THE ULTIMATE TENSILE STRENGTH OF THE BARS, THE FATIGUE STRENGTHS OF THE BARS THEMSELVES WERE ESSENTIALLY CONSTANT AND EQUAL TO 30 KSI (2100 KGF/CM SQUARED) FOR A ZERO TO MAXIMUM TENSION CYCLE. IT IS SUGGESTED THAT THIS MAY BE CAUSED BY THE DECARBURIZED OUTER SURFACE OF THE BARS AND STRESS CONCENTRATIONS AT THE BASE OF THE DEFORMATIONS. /AUTHOR/

The role of creep in high temperature low cycle fatigue.

Abstract: The significance of the role that creep can play in governing high-temperature, low-cycle fatigue resistance is investigated by conducting strain cycling tests on two high-temperature stainless steel alloys and making concurrent measurements of stress, temperature, and strain at various frequencies The results are then analyzed in terms of damage imposed by creep and fatigue components It is shown that creep can play an important and sometimes dominant role in low cycle fatigue at high temperatures The results of the study include the findings that: (1) the simple life-fraction theory described is adequate for calculating creep damage when the cyclic creep rupture curve is used as a basis for analysis; (2) a method of universal slopes originally developed for room temperature use is sufficiently accurate at high temperature to be used to calculate pure fatigue damage; and (3) a linear creep-fatigue damage rule can explain the transitions observed from one failure mode to another

The Effect of Boundary Precipitates on the High-Temperature Fatigue Strength of Austenitic Stainless Steels

Abstract: Austenitic stainless steels of base composition Fe-18 Cr-12 Ni have been tested in low-stress-amplitude fatigue in the range 600–8000 C (873–1073K) (0.5–0.62 Tm ). Metallographic evidence, including transmission electron microscopy, illustrates the extent of boundary sliding and subsequent intergranular crack formation that occurs in the solid-solution 18/12 alloy. Pinning the grain boundaries, by M23C6 carbide precipitates, in an 18/12/0.05 C alloy improves the high-temperature fatigue properties by inhibiting both grain-boundary sliding and migration, so that the fracture-mode transition temperature is increased from ∼ 600 to 800°C. Nucleation of the triple-point cracks formed at the carbide/matrix interface at 800°C may be explained by the Smith-Barnby analysis of the stresses generated at particles along sliding boundaries. This analysis gives a value of 820 erg/cm2 (mJ/m2) for the carbide/matrix fracture surface energy. The experimental observations thus confirm the prediction that the fract...

Fatigue crack propagation growth rates under a wide variation of delta k for an astm a517 grade f(t-1) steel

Abstract: Fatigue crack propagation experiments on an ASTM A517 F(T-1) steel were carried out within the framework of linear-elastic fracture mechanics. At low stress intensity levels an examination of very slow fatigue crack propagation rates of the order of 0.000000001 in./cycle revealed the existence of a stress intensity threshold level below which fatigue crack propagation rates become diminishingly small (order of 0.000000001 in./cycle). Stress ratio was found to alter the threshold stress intensity with higher stress ratios resulting in somewhat depressed threshold values. The influence of stress ratio was also felt at stress intensity levels above the threshold with increased growth rate being associated with increased values of stress ratio. Low stress intensity level fatigue crack growth rate tests were conducted at room temperature in air, distilled water, and dry hydrogen gas. These environments did not affect the fatigue threshold. Identical values of the threshold stress intensity were recorded in each environment studied. Furthermore, for the range of slow growth rates, order of 0.000000001 to 0.0000001 in./cycle, and frequencies, 120 to 180 Hz, considered, these environments seemed to produce little effect on fatigue crack growth rate results. For higher cyclic growth rates (greater than 0.0000001 in./cycle) and stress intensity levels, the effect of material condition and heat treatment was evaluated. Plate material rated as ultrasonically good and poor was evaluated in two heat treated conditions. Examination of fatigue crack growth rates indicated that ultrasonic ratings do not necessarily give good indications of crack growth resistance. /Author/

Tensile behavior of subbase materials under repetitive loading

Abstract: RESULTS OF A STUDY OF THE FATIGUE CHARACTERISTICS OF ASPHALT-TREATED, CEMENT-TREATED, AND LIME-TREATED SUBBASE MATERIALS INDICATED THAT THE INDIRECT TENSILE TEST CAN BE USED SATISFACTORILY TO EVALUATE THE FATIGUE PROPERTIES OF TREATED MATERIALS UNDER REPEATED TENSILE STRESSES. IN ADDITION, THE GENERAL NATURE OF THE RELATIONSHIP BETWEEN APPLIED TENSILE STRESS AND FATIGUE LIFE WAS DETERMINED ALONG WITH THE INHERENT VARIATION ASSOCIATED WITH FATIGUE LIFE OF ASPHALT-TREATED MATERILAS. FOR THE ASPHALT-TREATED MATERIALS, THE RELATIONSHIPS BETWEEN TENSILE STRESS AND LOG FATIGUE LIFE WERE ESSENTIALLY LINEAR, WITH FAILURES OCCURRING AT TENSILE STRESSES RANGING FROM 8 TO 40 PSI, WHICH WERE APPROXIMATELY 6 TO 30 PERCENT OF THE STATIC INDIRECT TENSILE STRENGTH. SIGNIFICANT VARIATION IN FATIGUE LIFE OCCURRED, AND IT WAS FOUND THAT THE STANDARD DEVIATION VARIED LINEARLY WITH THE MEAN FATIGUE LIFE, WITH THE COEFFICIENT OF VARIATION RANGING FROM 30 PERCENT TO MORE THAN 75 PERCENT. RESULT CONCERNING THE FATIGUE LIFE, WITH THE COEFFICIENT OF VARIATION RANGING FROM 30 PERCENT TO MORE THAN 75 PERCENT. RESULTS CONCERNING THE FATIGUE LIFE RELATIONSHIP FOR THE CEMENT-TREATED AND LIME-TREATED MATERIALS SUGGEST THAT THERE IS A CRITICAL STRESS LEVEL ABOVE WHICH THE FATIGUE LIFE IS VERY SHORT AND BELOW WHICH THE FATIGUE LIFE IS VERY LONG. THE TENSILE FATIGUE CHARACTERISTICS OF ASPHALT-TREATED MATERIALS WERE FOUND TO BE AFFECTED BY TYPE OF ASPHALT CEMENT, ASPHALT CONTENT, COMPACTION TEMPERATURE, AND MIXING TEMPERATURE. WITHIN THE RANGE TESTED, IT WAS FOUND THAT FATIGUE LIFE WAS INCREASED BY USING A MORE VISCOUS ASPHALT CEMENT, HIGHER COMPACTION TEMPERATURE, THE HIGHER MIXING TEMPERATURE. IT WAS ALSO CONCLUDED THAT THERE IS AN OPTIMUM ASPHALT CONTENT FOR MAXIMUM FATIGUE LIFE. IN ADDITION, A SIMPLE PREDICTIVE EQUATION WAS DEVELOPED WHICH ADEQUATELY DESCRIBED THE FATIGUE LIFE OF THE SPECIMENS TESTED. THE FATIGUE LIFE OF THE ASPHALT-TREATED MATERIALS WAS FOUND TO CORRELATE WITH INITIAL STIFFNESS, INITIAL TENSILE STRAIN, AND THE RATIO BETWEEN TENSILE STRESS AND STRENGTH, BUT THESE CORRELATIONS WERE ASSOCIATED WITH A LARGE AMOUNT OF VARIATION. NO CORRELATION WAS FOUND BETWEEN FATIGUE LIFE AND PERCENT OF AIR VOIDS. /AUTHOR/

Fatigue strength of rolled and welded steel beams

Abstract: Fatigue tests were conducted on rolled and welded steel beams to provide information that can be used as a basis for the development of design specifications. The principal variables were grade of steel, type of beam and detail, and stress variables. Three grades of steel were included in the study: ASTM A36, A441 and A514. Two of the four basic beam types investigated are examined hereiin, namely plain-rolled and plain-welded beams. It was found that: (1) Stress range was the dominant stress variable for all rolled and welded beams; (2) fatigue life was not significantly influenced by grade of steel; (3) rolled beams provided the longest life and the most scatter of the data; and (4) plain-welded beams without attachments constitute an upper bound for welded beams. The treatment of plain-welded beams the same as base metal or rolled beams grossly overestimates their fatigue life.

Correlation between the Fatigue Limit of a Material with Defects and Its Non-Propagating Crack : Some Considerations Based on the Bending or Torsional Fatigue of a Specimen with a Diametrical Hole

Abstract: From our experiments carried out about the bending and torsional fatigue of S35C specimens with a diametrical hole (each specimen has a round hole or its connected holes), the following results were obtained. 1) In the case of a specimen with a diametrical hole, the root radius of the notch at the branch point is constant (ρ0c0.5 mm in our material), as in the case of the bending fatigue test of the circumferential notched specimen. This means that the fatigue limit of the material with defects is mainly determined by the limit stress for propagation of its non-propagating crack. 2) when the fatigue limit of a specimen with a diametrical hole is determined by the crack strength (whenρ<ρ0), the fatigue limit is mainly dominated by the diametrical hole in the perpendicular direction of the maximum tensile stress. This means that the fatigue limit of a material with defects is more influenced by the size of defect than by the sharpness of defect. 3) It is case of a plain specimen, τw/σwc0.6; but in the case of the specimen with a diametrical hole, τw/σw=0.750.9. This shows that the effect of defect in the bending fatigue is greater than in the torsional fatigue.

The Effect of Load Interaction and Sequence on the Fatigue Behavior of Notched Coupons

Abstract: CONSTANT AMPLITUDE FATIGUE TESTS WITH VARIOUS CONDITIONS OF OVERLOADS WERE CONDUCTED TO UNDERSTAND THE EFFECTS OF LOAD INTERACTION AND SEQUENCE IN NOTCHED COUPONS. THE RESULTS WERE ANALYZED WITH PARTICULAR EMPHASIS ON THE LOCAL RESIDUAL STRESSES AT NOTCHES. FATIGUE LIFE RESULTS FROM THE TESTS HAVING A VARIATION IN THE NUMBER OF CYCLES BETWEEN THE PERIODIC OVERLOADS WHICH REINFORCE THE CONCEPT OF A FADING RESIDUAL STRESS. OTHER FATIGUE LIFE TRENDS COULD BE PREDICTED BY ASSUMING A STABLE RESIDUAL STRESS. CONDITIONS WHICH COULD CAUSE CYCLE-DEPENDENT OVERLOAD RESIDUAL STRESSES ARE DISCUSSED. AN EXPONENTIAL ANALYSIS FORM IS SUGGESTED FOR CALCULATING THE CYCLE-DEPENDENT LOCAL STRESSES FOR APPLICATION TO CUMULATIVE DAMAGE PREDICTIONS. /ASTM/

Effect of External Geometry of Reinforcement on the Fatigue Strength of a Welded Joint

Abstract: Low alloy high tensile steels are of limited use in welded structures under alternating load, because welded joints in those steels have not greater fatigue strength than similar joints in mild steels. It has been believed that a metallurgical factor is responsible for the low fatigue strength of the welded joint.Nevertheless it has been made clear recently that all metallurgical microstructures in the heat-affected zone (HAZ) of welds are not weak to fatigue and not very notch-sensitive. Only in the welded joint of larger heat-input than the proper welded condition for the steel, the softening zone of HAZ may decrease the fatigue strength by a few kg/mm2.In order to examine the effect of the external geometry of welded joints, investigations were carried out using simulated weld reinforcement specimens. In these previous papers, the external geometry of reinforcement was not carefully observed, so that the effect of a very small toe radius upon the fatigue strength has been ignored. A reduction in the fatigue strength as large as 70% remained unclarified.The present research was devoted to studying the effect of the external geometry of the reinforcement on fatigue strength. For this purpose, a careful observation of the external geometry of the toe of the reinforcement was performed. It was found that the acute flank angle θ ranged from about 20 to 60 deg and the radius of curvature R at the toe of the reinforcement ranged from about 10 to 0.02 mm. Fatigue tests under pulsating tension were conducted on simulated butt-welded specimens of weldable high tensile steel. The profile of the weld reinforcement was controlled by varying θ and R. How the geometry of the reinforcement toe influences the fatigue strength of the welded joint has been revealed. The stress concentration factor at the toe of the reinforcement with 60 deg flank angle and 0.02 mm toe radius, affecting the fatigue crack initiation, was deduced from photo-eastic experiments and determined to be 3.5 by the finite element method. The fatigue notch factor of the mild steel was 1.9 and that of 80 kg/mm2 class high tensile steel was 3.2 for the stress concentration factor of 3.5. From these values, the butt-welded joint in these steels can be explained well.The main reason for the very poor fatigue strength of the welded joint is the stress concentration associated with the severe discontinuity of the external geometry at the toe of the weld reinforcement.

On the Probabilistic Determination of Scatter Factors using Miner's Rule in Fatigue Life Studies

Abstract: A PARTICULAR TWO-PARAMETER FAMILY OF LIFE-LENGTH DISTRIBUTIONS FOR FATIGUE LIFE IS ASSUMED. THIS FAMILY, FIRST FORMULATED BY FREUDENTHAL AND SHINOZUKA IN 1961, WAS SYSTEMATICALLY EXAMINED BY BIRNBAUM AND SAUNDERS IN 1968 WHERE IT WAS DERIVED, USING CONSIDERATIONS FROM RENEWAL THEORY, FOR THE NUMBER OF CYCLES NEEDED TO FORCE A FATIGUE-CRACK EXTENSION TO EXCEED A CRITICAL VALUE. BY EMPLOYING THIS NEW FAMILY, TOLERANCE BOUNDS ARE OBTAINED FOR THE POPULATION OF LIFE TIMES UNTIL FATIGUE FAILURE UNDER A PROGRAMMED LOAD. THIS IS ACCOMPLISHED BY UTILIZING A GENERALIZATION OF MINER'S RULE WHICH COMPUTES THE MEAN LIFE UNDER THE PROGRAMMED LOAD IN TERMS OF THE MEAN LIVES UNDER SIMPLER PROGRAMMED LOADS AT STRESS LEVELS FOR WHICH DATA ARE AVAILABLE. SUCH BOUNDS HAVE NEVER BEEN OBTAINED PREVIOUSLY FOR ANY OTHER LIFE-LENGTH DISTRIBUTION AND THE CONFIDENCE LEVEL EXACTLY DETERMINED. THIS PAPER CONCLUDES WITH AN APPLICATION OF THESE RESULTS TO A SET OF REAL FATIGUE DATA. /ASTM/

Fatigue of Bolted High Strength Structural Steel

Abstract: Results of fatigue tests on bolted connections in high strength structural steel conducted at the University of Illinois are presented. The principal parameter varied was the bolt type; one additional variation was included also. Constant amplitude fatigue tests were conducted at various stress ratios, with particular emphasis on zero-to-tension testing. The results indicate improved fatigue strength for high strength structural steel (yield strength = 50.0 ksi) compared to A36 steel. Connections with A325 bolts exhibit approximately the same fatigue strength as those in which A490 bolts are used. A comparison of results of present study with those from other sources indicates that stress based on gross section area gives a better correlation with the number of load cycles to failure of steel in bolted connections than net section stress.

Fatigue Strength in Shear of Prestressed Concrete I-Beams

Abstract: THE RESULTS OF STATIC AND REPEATED LOAD SHEAR TESTS ON 17 POST-TENSIONED I-BEAMS, WITH WEB REINFORCEMENT, ARE PRESENTED. THE EFFECT OF FATIGUE ON THE DIAGONAL CRACKING STRENGTH AND THE EFFECT OF A LOAD HISTORY ON THE SUBSEQUENT STATIC DIAGONAL CRACKING STRENGTH IS REPORTED. THE CRITERION OF FATIGUE FAILURE, IN ALL CASES, WAS FOUND TO BE STIRRUP FRACTURE. THE NUMBER OF CYCLES TO FIRST STIRRUP FRACTURE AND THE NUMBER OF CYCLES TO COMPLETE BEAM FAILURE ARE GIVEN FOR THREE MAXIMUM LOAD VALUES. THE MINIMUM LOAD WAS CONSTANT IN ALL TESTS. THE RESULTS ARE SUMMARIZED IN THE FORM OF STRESS CYCLES-PROBABILITY CURVES. /AUTHOR/

The room temperature fatigue behavior of nickel-base superalloy crystals at ultrasonic frequency

Abstract: Previous investigations have invariably observed strain rate related deformation effects as the fatigue frequency is raised to the ultrasonic range. Through room temperature tests on strain rate insensitive nickel-base superalloy single crystals of Mar-M200, we have shown that another effect of increasing the fatigue frequency to the ultrasonic range is in the suppression of the deleterious influence of environment. It was found that above a stress amplitude of 30,400 psi the fatigue lives of crystals ultrasonically fatiguedin air increase with decreasing stress in a manner which is functionally similar to, that of crystals conventionally fatiguedin vacuum. Similarly, the fracture surfaces of ultrasonically fatigued crystals have a dimpled appearance over most of their areas which is characteristic of locally ductile failure and identical to, the appearance of crystals failed at conventionally frequency in vacuum. These results, along with a kinetic analysis of gaseous adsorption, indicate that the major effect of increasing the fatigue frequency to the ultrasonic, range is in the suppression of the influence of oxygen in enhancing the rate of crack propagation. In addition, the short test times involved in running large numbers of cycles have allowed for the determination of the fatigue limit in a nickel-base superalloy. This is the first indication of no-fail behavior in this type of alloy.

Fatigue of iron and of Fe-Si 5% down to liquid helium temperature and the relationship between fatigue and brittleness

Abstract: Fatigue of Armco iron and of Fe-Si 5% is studied at liquid nitrogen and liquid helium temperatures in symmetric torsion, and by estimating the plastic strain amplitudes from internal friction measurements and from comparison with the static stress-strain curves. It is shown that, contrary to previous experiments of MacCammon and Rosenberg, iron in pure shear still shows a definite fatigue curve also at 4°K. The corresponding plastic strain amplitude at the fatigue limit is of the order of 10−4. For Fe-Si 5%, on the other hand, fatigue at 4°K almost disappears, and this disappearance seems associated with a critical value of the strain to fracture of the order of 10−5. In the representation of Manson and Coffin of the plastic strain amplitude as a function of the number of cycles it is shown that the slope of the curves decreases with increasing brittleness and for the silicon alloy becomes almost zero at liquid helium temperature. The results obtained on the relationship between fatigue and britt...

Effects of Surface Condition on the Mechanical Properties of Titanium and its Alloys

Abstract: : The report reviews the available literature on the effects of commonly encountered surface conditions on the properties of titanium and its alloys. The material properties of major concern include fatigue strength, tensile strength and ductility, and bend ductility. (Author)