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At least 181 records · Page 10

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NASA NTRS1974

The partitioned strainrange fatigue behavior of coated and uncoated MAR-M-302 at 1000 C (1832 F) in ultrahigh vacuum

Elevated temperature (1000 C (1832 F)) vacuum fatigue tests have been conducted on uncoated and aluminide (PWA 45) coated cobalt-base MAR-M-302 alloy with the four different types of thermalmechanical reversed inelastic strain cycles (wave shapes) defined by the method of strainrange partitioning. Results of these tests indicated two major conclusions. First, there was no significant influence of the aluminide coating on the thermal-mechanical fatigue life of the MAR-M-302 alloy. Second, variations in the type of thermal-mechanical fatigue cycle applied caused significant variations of fatigue life for both coated and uncoated material. The longest lives were achieved with sigma sub pp type cycling, while the sigma sub cc cycle caused a reduction of fatigue life of about 1/2 order of magnitude with respect to the sigma sub pp life. The sigma sub cp type cycle caused a life reduction of between 1-1/2 and 2 orders of magnitude relative to the sigma sub pp life, while the sigma sub pc type cycle provided a fatigue life which appeared to be comparable to that generated by the sigma sub cc cycle.

Sheffler, K. D.↗

NASA NTRS1974

Fatigue life of metals under cyclic loading in vacuum

The effect of vacuum on the cyclic strength of metals is examined. Experimental data are presented of studies to clarify the mechanism of vacuum influence on fatigue failure. The dependence of copper and armco-iron fatigue life on vacuum level is studied. It is found that the presence of saturation on the curve of fatigue life versus vacuum level is common for the materials studied. The effect of vacuum on fatigue fracture of copper is studied over a wide range of deformations. It is shown that fatigue life increase under vacuum conditions and the observed characteristics of the fatigue fracture itself in vacuum are associated with homogenization of the plastic deformation under these conditions.

Verkin, B. I.↗

NASA NTRS1976

Rolling-element fatigue lives of AISI 52100 steel balls with several synthetic lubricants

Rolling-element fatigue tests were run with three synthetic lubricants with and without antiwear additives and with a paraffinic mineral oil at race temperatures of 336 to 353 K (146 146 to 175 F). The five-ball fatigue tester was used with steel balls to evaluate the relative fatigue lives with each of six lubricant-additive combinations. The tests were run at 5,520 MPa (800,000psi) maximum Hertz stress, 10,000 rpm shaft speed, and 30 deg contact angle. The lubricants tested have similar kinetic viscosities at 372 K (210 F) ranging from 0.034 to 0.089 sq cm/sec (3.4 to 8.9 cS). At these conditions, the mode of failure in the five-ball fatigue tester was classical subsurface rolling-element fatigue. The baseline for comparison of fatigue life was the paraffinic mineral oil without additives. The effects of the synthetic lubricants and their additives, which are useful for boundary lubrication, oxidation or foam inhibition, were evaluated.

Parker, R. J.↗

NASA NTRS1976

Low-cycle fatigue of two austenitic alloys in hydrogen gas and air at elevated temperatures

The low-cycle fatigue resistance of type 347 stainless steel and Hastelloy Alloy X was evaluated in constant-amplitude, strain-controlled fatigue tests conducted under continuous negative strain cycling at a constant strain rate of 0.001 per sec and at total axial strain ranges of 1.5, 3.0, and 5.0 percent in both hydrogen gas and laboratory air environments in the temperature range 538-871 C. Elevated-temperature, compressive-strain hold-time experiments were also conducted. In hydrogen, the cyclic stress-strain behavior of both materials at 538 C was characterized by appreciable cyclic hardening at all strain ranges. At 871 C neither material hardened significantly; in fact, at 5% strain range 347 steel showed continuous cyclic softening until failure. The fatigue resistance of 347 steel was slightly higher than that of Alloy X at all temperatures and strain ranges. Ten-minute compressive hold time experiments at 760 and 871 C resulted in increased fatigue lives for 347 steel and decreased fatigue lives for Alloy X. Both alloys showed slightly lower fatigue resistance in air than in hydrogen. Some fractographic and metallographic results are also given.

Jaske, C. E.↗

NASA NTRS1977

Fatigue damage in notched composite laminates

This communication describes the results of an experimental investigation of the fatigue characteristics of a boron-epoxy laminate containing an unloaded circular hole. The study focuses upon characterization of the fatigue damage incurred and its influence upon axial tensile residual strength. Two apparent anomalies in the fatigue behavior for this laminate were observed. First, the fatigue damage was observed to propagate tangentially to the edge of the notch (parallel to the direction of the load), in contrast to propagation in contemporary metallic materials which is perpendicular to the load direction. Second, the residual strength characteristics for the laminate have revealed a significant increase in residual strength subsequent to a fatigue loading of 500,000 cycles at R = 0.1 and S = 0.8 and 0.67 (S = maximum stress/static ultimate). Failure analyses of the fatigue damage are conducted utilizing ultrasonic 'C' scan techniques.

Pipes, R. B.↗

NASA NTRS1979

Experimental and analytical study of fatigue damage in notched graphite/epoxy laminates

Both tension and compression fatigue behaviors were investigated in four notched graphite/epoxy laminates. After fatigue loading, specimens were examined for damage type and location using visual inspection, light microscopy, scanning electron microscopy, ultrasonic C-scans, and X-radiography. Delamination and ply cracking were found to be the dominant types of fatigue damage. In general, ply cracks did not propagate into adjacent plies of differing fiber orientation. To help understand the varied fatigue observations, the interlaminar stress distribution was calculated with finite element analysis for the regions around the hole and along the straight free edge. Comparison of observed delamination locations with the calculated stresses indicated that both interlaminar shear and peel stresses must be considered when predicting delamination. The effects of the fatigue cycling on residual strength and stiffness were measured for some specimens of each laminate type. Fatigue loading generally caused only small stiffness losses. In all cases, residual strengths were greater than or equal to the virgin strengths.

Whitcomb, J. D.↗

NASA NTRS1978

Statistical fatigue of graphite/epoxy angle-ply laminates in shear

A three-parameter fatigue and residual strength degradation model has been proposed to predict statistically the fatigue behavior of composite laminae under axial shear loadings. The fatigue behavior includes the fatigue life and the fatigue damage expressed in terms of the residual strength degradation. An experimental test program using graphite/epoxy /+ or - 45 deg/2s laminates has been conducted to generate statistically meaningful data in order to examine the validity of the theoretical model. It is shown that the correlation between the theoretical predictions and the test results on the statistical distributions of the fatigue life and the residual strength is excellent. Test results on the shear modulus degradation are also presented and discussed in detail in order to provide insight for the establishment of a shear modulus degradation model.

Yang, J. N.↗

NASA NTRS1978

An engineering approach to the prediction of fatigue behavior of unnotched/notched fiber reinforced composite laminates

An engineering approach is proposed for predicting unnotched/notched laminate fatigue behavior from basic lamina fatigue data. The fatigue analysis procedure was used to determine the laminate property (strength/stiffness) degradation as a function of fatigue cycles in uniaxial tension and in plane shear. These properties were then introduced into the failure model for a notched laminate to obtain damage growth, residual strength, and failure mode. The approach is thus essentially a combination of the cumulative damage accumulation (akin to the Miner-Palmgren hypothesis and its derivatives) and the damage growth rate (similar to the fracture mechanics approach) philosophies. An analysis/experiment correlation appears to confirm the basic postulates of material wearout and the predictability of laminate fatigue properties from lamina fatigue data.

Kulkarni, S. V.↗

NASA NTRS1979

Thermographic measurement of fatigue damage

The paper presents a thermographic technique developed to measure the extent of fatigue damage in composite materials during fatigue loading. It is noted that heat generated by cyclic loading of fatigue damaged material raises the surface temperature. These temperatures were measured with an infrared camera and were used as boundary conditions in a finite element heat transfer program, which has been developed especially to calculate the extent of the heat generation zone, and thereby to define the fatigue damage zone. It is reported that the finite element program was verified by comparing calculated heat generation with the actual heat generation for a simple heat transfer problem that had a closed form solution. Damage zones are calculated for several boron/epoxy fatigue specimens from thermograms of specimens. The calculated damage zones were compared with damage detected by C-scan, X-ray, and scanning electron microscope (SEM) examinations. It is concluded that the analysis was effective in locating the boundaries of the fatigue damage zones.

Whitcomb, J. D.↗

NASA NTRS1980

Mechanisms of fatigue damage in boron/aluminum composites

Tensile fatigue tests were conducted on several laminates of boron/aluminum (6061-0). In laminates with 0 deg fibers on the outside, an analysis that identifies "shakedown" conditions predicted the stress amplitude below which no fatigue damage accumulated. A fatigue damage accumulation model which relates matrix fatigue cracking and the overall laminate properties is described. A model for the saturation damage stage development is presented, that identical laminates, tested in directions 90 deg apart (such that one layup has 90 deg outer plies and the other 0 deg), have different fatigue behaviors due to the stacking sequence. The 90 deg plies on the surface develop cracks earlier than predicted by shakedown. An attempt was made to explain this stacking sequence effect. Variable load history effects on the fatigue damage response were investigated. Tests reveal that for a given stress ratio the specimen seeks the saturation damage state for the largest stress range to which it is subjected. It was also found that little damage is generated by shifting a given stress range down, whereas significant damage may be created by shifting it upward. The laminate stresses were always tensile.

Johnson, W. S.↗

NASA NTRS1980

Statistical fatigue of unnotched composite laminates

A general analytical, phenomenological model is developed for characterizing fatigue damage accumulation in unnotched composite laminates, and experimental results are used to evaluate the model. The fatigue model, which is developed for various types of cyclic loadings, is based on the assumption that the residual strength reduction is a monotonically decreasing function of the fatigue life. Parameters are determined from baseline test data consisting of one set of residual strength data, one set of static strength data, and one set of constant amplitude fatigue life data. Once the parameters are determined, the model is able to predict: (1) the statistical distribution of the fatigue life, and the residual strength under constant and variable amplitude loads, and (2) the effect of periodic proof tests, without any additional experimental data. Experimental test results show that predictions made from the present model agree quite well with the experimental results. The model is also applicable to fatigue of bonded or bolted composite joints.

Yang, J. N.↗

NASA NTRS1982

The thermal fatigue resistance of H-13 Die Steel for aluminum die casting dies

The effects of welding, five selected surface coatings, and stress relieving on the thermal fatigue resistance of H-13 Die Steel for aluminum die casting dies were studied using eleven thermal fatigue specimens. Stress relieving was conducted after each 5,000 cycle interval at 1050 F for three hours. Four thermal fatigue specimens were welded with H-13 or maraging steel welding rods at ambient and elevated temperatures and subsequently, subjected to different post-weld heat treatments. Crack patterns were examined at 5,000, 10,000, and 15,000 cycles. The thermal fatigue resistance is expressed by two crack parameters which are the average maximum crack and the average cracked area. The results indicate that a significant improvement in thermal fatigue resistance over the control was obtained from the stress-relieving treatment. Small improvements were obtained from the H-13 welded specimens and from a salt bath nitrogen and carbon-surface treatment. The other surface treatments and welded specimens either did not affect or had a detrimental influence on the thermal fatigue properties of the H-13 die steel.

Source record↗

NASA NTRS1983

Fatigue crack initiation and propagation in several nickel-base superalloys at 650 C

The modes of crack initiation and propagation of several nickel-base superalloys have been examined after fatigue and creep-fatigue testing at 650 C. In fatigue, crack initiation was transgranular and frequently associated with porosity or inclusions in the higher strength alloys. These defects were usually located at the surface, except for tests at low strain ranges where larger, internal defects often initiated failure. Although fatigue crack initiation was transgranular, in those alloys with grain sizes of less than 15 microns, fatigue crack growth quickly became intergranular. This transition was environmentally assisted and did not occur for subsurface cracks until the crack broke through to the atmosphere. In the creep-fatigue cycle, which included a 900 s tensile dwell, crack initiation and propagation were both intergranular in all alloys.

Gayda, J.↗

NASA NTRS1983

A review of modern approaches to fatigue reliability analysis and design

Metal fatigue is a principal mode of failure in components of mechanical systems. But fatigue design factors (e.g., stress, fatigue strength) are subject to considerable uncertainty. Therefore, relative to fatigue, reliability methods are appropriate for purposes of safety checking of designs, risk assessment, failure analysis, and development of code statements. Described herein are four methods which can be effectively employed for fatigue reliability analysis, (1) Monte Carlo methods, (2) the lognormal format, (3) the Weibull format, (4) the Rackwitz-Fiessler algorithm. Examples of the application of each are presented. In summary, no general reliability method can be recommended for all situations involving fatigue. The approach has to be tailored to the problem.

Wirsching, P. H.↗

NASA NTRS1984

Dynamic response and acoustic fatigue of stiffened composite structure

The results of acoustic fatigue and dynamic response tests performed on L-1011 graphite-epoxy (GrE) aileron and panel components are reported. The aileron featured glass microballoons between the GrE skins. Tests yielded random fatigue data from double and single cantilever coupons and modal data from impedance hammer and loudspeaker impulses. Numerical and sample test data were obtained on combined acoustic and shear loads, acoustic and thermal loads, random fatigue and damping of the integrally stiffened and secondary bonded panels. The fatigue data indicate a fatigue life beyond 10 million cycles. The acoustic data suggested that noise transmission could be enhanced in the integrally stiffened panels, which were more acoustic-fatigue resistant than were the secondary bonded panels.

Soovere, J.↗

NASA NTRS1985

Fatigue of graphite/epoxy buffer strip panels with center cracks

The effects of fatigue loading on the behavior of graphite/epoxy panels with either S-Glass or Kevlar-49 buffer strips is studied. Buffer strip panels are fatigued and tested in tension to measure their residual strength with crack-like damage. Panels are made with 45/0/-45/90 sub 2s layup with either S-Glass or Kevlar-49 buffer strip material. The buffer strips are parallel to the loading direction and made by replacing narrow strips of the 0-degree graphite plies with strips of either 0-degree S-Glass/epoxy or Kevlar-49/epoxy on a one-for-one basis. The panels are subjected to a fatigue loading spectrum MINITWIST, the shortened version of the standardized load program for the wing lower surface of a transport aircraft. Two levels of maximum strain are used in the spectrum with three durations of the fatigue spectrum. One group of panels is preloaded prior to the application of the fatigue cycling. The preload consists of statistically loading the spectrum in tension until the crack-tip damage zone reaches the ajacent buffer strips. After fatigue loading, all specimens are statistically loaded in tension to failure to determine their residual strengths.

Bigelow, C. A.↗

NASA NTRS1985

Surface fatigue life and failure characteristics of EX-53, CBS 1000M, and AISI 9310 gear materials

Spur gear endurance tests and rolling-element surface fatigue tests are conducted to investigate EX-53 and CBS 1000M steels for use as advanced application gear materials, to determine their endurance characteristics, and to compare the results with the standard AISI 9310 gear material. The gear pitch diameter is 8.89 cm (3.50 in). Gear test conditions are an oil inlet temperature of 320 K (116 F), an oil outlet temperature of 350 K (170 F), a maximum Hertz stress of 1.71 GPa (248 ksi), and a speed of 10,000 rpm. Bench-type rolling-element fatigue tests are conducted at ambient temperature with a bar specimen speed of 12,500 rpm and a maximum Hertz stress of 4.83 GPa (700 ksi). The EX-53 test gears have a surface fatigue life of twice that of the AISI 9310 spur gears. The CBS 1000M test gears have a surface fatigue life of more than twice that of the AISI 9310 spur gears. However, the CBS 1000M gears experience a 30-percent tooth fracture failure which limits its use as a gear material. The rolling-contact fatigue lines of RC bar specimens of EX-53 and ASISI 9310 are approximately equal. However, the CBS 1000M RC specimens have a surface fatigue life of about 50 percent that of the AISI 9310.

Townsend, D. P.↗

NASA NTRS1986

Grain boundary oxidation and oxidation accelerated fatigue crack nucleation and propagation

Fatigue life at elevated temperatures is often shortened by oxidation. Grain boundary oxidation penetrates deeper than the surface oxidation. Therefore, grain boundary oxide penetration could be the primary cause of accelerated fatigue crack nucleation and propagation, and the shortened fatigue life at elevated temperatures. Grain boundary oxidation kinetics was studied and its statistical scatter was analyzed by the Weibull's distribution function. The effects of grain boundary oxidation on shortened fatigue life was analyzed and discussed. A model of intermittent microruptures of the grain boundary oxide was proposed for the fatigue crack growth in the low frequency region. The proposed model is consistent with the observations that fatigue crack growth rate in the low frequency region with hold time at K sub max is inversely proportional to cyclic frequency and that crack growth is intergranular.

Liu, H. W.↗