Showing papers on "Fatigue limit published in 1991"

Fatigue Strength of Welded Structures

Abstract: Part 1 Welded joints under fatigue loading: Fatigue failure Significant features of welds in relation to fatigue Factors which affect fatigue of welded joints Fatigue of welded joints Conclusions. Part 2 Design rules: Fatigue design rules for welded steel joints Stresses used with the fatigue design rules Significance of weld imperfections Improving the fatigue strength of welded joints Typical service failures.

A general criterion for high cycle multiaxial fatigue failure

Abstract: A new simple general criterion of failure for high cycle multiaxial fatigue, τ a /t A,B +σ n,max /2σ T =1, is presented. The failure criterion is based on a critical plane approach where fatigue strength is a function of the shear stress amplitude and the maximum normal stress on the critical plane of maximum shear stress amplitude. The criterion takes account of whether case A cracks, growing along the surface, or case B cracks, growing into the surface, occur. It requires knowledge of the material properties, tensile strength, σ T , and reversed shear fatigue strength for case A, t A , or case B, t B , cracking whichever is relevant, t A is the reversed torsion fatigue strenght and t B is found from a case B cracking test case. The criterion is applicable in the region, 0.5t≤τ a ≤t, and 0≤σ n,max ≤σ T

Plasma surface engineering of low alloy steel

Abstract: The surface of low alloy steel (En40B) has been engineered in the plasma of a glow discharge via plasma nitriding and ion plating of titanium nitride (TiN) coatings on the nitrided substrates with the purpose of enhancing the surface properties and fatigue strength. The nitriding response of the steel has been accessed by the evaluation of phase composition, layer thickness, hardness profile, residual stresses and nitrogen and carbon distributions. The wear and fatigue characteristics of the plasma-nitrided steel have been investigated and simple models have been developed to describe the influence of such properties as depth and strength of the nitrided case on the fatigue limit and load-bearing capacity of the nitrided steel. In order to further improve the tribological properties and load-bearing capacity of the low alloy steel, a duplex plasma surface-engineering technique has been developed. This is achieved by plasma nitriding the steel first so as to produce a thick, strong subsurface and then depositing a thin, hard and wear-resistant TiN coating on the nitrided substrate by ion plating. Dry-sliding wear tests demonstrated that the duplex-treated steel, i.e. the TiN coating-nitrided steel composite, not only exhibited enhanced wear resistance over the as-nitrided steel (by a factor of 2–8) but also had much higher load-bearing capacity than the TiN coating on unnitrided steel. Optimization of the coating-substrate combination can be achieved by correct control of the plasma-nitriding, surface preparation and ion-plating processes.

Fatigue behavior of a 2xxx series aluminum alloy reinforced with 15 vol pct sicp

Abstract: The fatigue behavior of a naturally aged powder metallurgy 2xxx series aluminum alloy (Alcoa MB85) and a composite made of this alloy with 15 vol pct SiCp, has been investigated. Fatigue lives were determined using load-controlled axial testing of unnotched cylindrical samples. The influence of mean stress was determined at stress ratios of −1, 0.1, and 0.7. Mean stress had a significant influence on fatigue life, and this influence was consistent with that normally observed in metals. At each stress ratio, the incorporation of SiC reinforcement led to an increase in fatigue life at low and intermediate stresses. When considered on a strain-life basis, however, the composite materials had a somewhat inferior resistance to fatigue. Fatigue cracks initiated from several different microstructural features or defect types, but fatigue life did not vary significantly with the specific initiation site. As the fatigue crack advanced away from the fatigue crack initiation site, increasing numbers of SiC particles were fractured, in agreement with crack-tip process zone models.

Fatigue of Yttria‐Stabilized Zirconia: I, Fatigue Damage, Fracture Origins, and Lifetime Prediction

Abstract: ~Uniaxial tension-compression fatigue behavior of 3-mol%yttria-stabilized tetragonal zirconia polycrystals was investigated. Hysteresis in the stress-plastic strain curve featured cumulative plastic strain and weakened elastic stiffness. Fracture statistics in terms of cycle-to-failure depends strongly on the maximum stress and less on the stress amplitude. Preexisting processing flaws were identified as the fracture origins in all cases. We suggest that microcracking is the dominant mechanism of fatigue damage, that nucleation of fatigue crack is usually not necessary, and that fatigue lifetime is primarily controlled by crack propagation, which is most sensitive to the maximum stress. [Key words: fatigue, fracture, zirconia, cracks, stress.]

Tensile and Fatigue Behavior of Silicon Carbide Fiber‐Reinforced Aluminosilicate Glass

Abstract: The onset of damage accumulation in ceramic-matrix composites occurs as matrix microcracking and fiber/matrix debonding. Tension tests were used to determine the stress and strain levels to first initiate microcracking in both unidirectional and cross-ply laminates of silicon carbide fiber-reinforced aluminosilicate glass. Tension–tension fatigue tests were then conducted at stress levels below and above the matrix cracking stress level. At stress levels below matrix microcracking, no loss in stiffness occurred. At stresses above matrix cracking, the elastic modulus of the unidirectional specimens exhibited a gradual decrease during the first 10 000 cycles, and then stabilized. However, the cross-ply material sustained most of the damage on the first loading cycle. It is shown that fatigue life can be related to nonlinear stress–strain behavior of the 0° plies, and that the cyclic strain limit was approximately 0.3%.

The fatigue performance of machined surfaces

Abstract: It is well known that surface condition has a strong effect on fatigue life, and that most surfaces produced by conventional manufacturing operations such as machining and forging have poorer fatigue behaviour than polished surfaces commonly used for laboratory specimens. As yet, there are no reliable quantitative models to predict the behaviour of such surfaces; the problem is a multi-parameter one, involving surface roughness, surface microstructure and residual stress. High-cycle fatigue data was obtained for En19 steel, using four types of machined surface, produced by: polishing, grinding, milling and shaping. Residual stress was eliminated by heat treatment. Fatigue limit data were plotted as a function of roughness parameters using Kitagawa-type diagrams, and compared to simple notch-based and crack-based models. It was found that, whilst both theories tended to be overly conservative, fracture mechanics approaches are useful for relatively low roughness, when the surfaces can be modelled as a series of short cracks. For higher roughness a notch-based approach is appropriate.

Flexural fatigue performance of steel fiber reinforced concrete - influence of fiber content, aspect ratio, and type

Abstract: The performance of steel fiber reinforced concrete under flexural fatigue loading is examined in terms of fiber content (0.5 to 1.5 percent by volume), fiber aspect ratio (47 to 100), and fiber type (4 types). Data from 194 fatigue tests and 135 complementary static loading tests are presented both as S-N relationships, with the maximum stress expressed as a percentage of the strength under static loading, and as relationships between actual stress and number of loading cycles. The relationships between actual stress and number of cycles depend primarily on fiber content. The best performance, a 100,000 cycle endurance limit of 6.9 MPa, is obtained with 1.5 % by volume of 75 aspect ratio cold-drawn wire fibers in a concrete with a w/(c plus f) of 0.49. For 0.5 % of the same fibers, the 100,000 cycle limit is only 5.2 MPa despite a lower w/(c plus f) of 0.39

An Experimental Study of Fatigue Strength for Adhesively Bonded Tubular Single Lap Joints

Abstract: In this paper, manufacturing technology of the tubular single lap adhesive joint was studied to obtain reliable and optimal joint quality. In addition, a surface preparation method and a bonding process for the joint were devised. The effect of the adhesive thickness and the adherend roughness on the fatigue strength of the joint was experimentally investigated. From experiments, it has been found that the fatigue strength of the joint increased as the adhesive thickness decreased and the optimal arithmetic surface roughness of the adherends was about 2 μm.

Low-temperature fatigue of 316L and 316LN austenitic stainless steels

Abstract: This article is concerned with the cyclic properties of 316L-type austenitic stainless at 300 and 77 K. The role of nitrogen alloying and of the temperature decrease is examined during low-cycle fatigue (LCF) and fatigue crack propagation. Fatigue resistance is enhanced by the addition of nitrogen in steel at both test temperatures. The results are discussed on the basis of micro-structural observations. Planar slip of dislocations is found in the nitrogen-containing steel and is favored by a decrease in test temperature. To some extent, the influence of interstitial nitrogen on the fatigue properties is related to its role in stabilizing austenite observed during cooling as well as during straining.

Influence of Stress Ratio on the Elevated‐Temperature Fatigue of a Silicon Carbide Fiber‐Reinforced Silicon Nitride Composite

Abstract: The influence of stress ratio on the tensile fatigue behavior of a unidirectional SiC-fiber/Si,N.,-matrix composite was investigated at 1200°C. Tensile stress ratios of 0.1, 0.3, and 0.5 were examined. Fatigue testing was conducted in air, at a sinusoidal loading frequency of 10 Hz. For peak fatigue stresses below the proportional limit of the composite (approximately 195 MPa at 1200°C) specimens survived 5 X lo6 cycles, independent of stress ratio. At peak stresses above the proportional limit, fatigue failures were observed; fatigue life decreased significantly as the stress ratio was lowered from 0.5 to 0.1. Creep appears to be the predominant damage mechanism which occurs during fatigue below the pro

Fatigue limit and crack arrest in alkali-containing silicate glasses

Abstract: Crack growth, including fatigue limit and crack arrest, have been investigated for glasses of the systemsxNa2O-11Al2O3-(89-x)SiO2,xNa2O-(100-x)SiO2 andxNa2O-7CaO-(93−x)SiO2 in water as well as in acid and alkaline solutions. From studies of the dependence of the crack arrest on the alkali content of the glass, the kind of alkali (K+, Na+, Li+), the pH of the corrosive medium, the ageing time and ageing loading in conjunction with measuring the alkali leaching behaviour, the basic mechanism of crack arrest and fatigue limit can be concluded. Owing to load- and medium-dependent diffusion processes, a crack-growth retarding leached layer at the crack is generated with modified strength and crack growth properties compared to the bulk properties. In high alkali-containing glasses the process is additionally stimulated by stresses produced in the leached layer at the crack tip and at the crack surfaces.

Effect of Machining Processes on the Fatigue Strength of Hardened AISI 4340 Steel

Abstract: The effect of two finishing processes, namely, cutting and grinding, on the fatigue strength of hardened AISI 4340 steel was investigated. Three sets of flat tensile specimens were prepared by first machining into the general shape of the fatigue specimen standard, then they were hardened to HRC 54. The final grinding was carefully performed on one set of specimens. Two sets of specimens were fly cut to obtain a surface finish comparable to the ground surface. The residual stress distribution, surface structure, and surface profiles were determined. Fatigue testing was accomplished on these specimens in tension under load control. All the residual stress patterns were compressive, but the residual stress created by fly cutting reached a much deeper layer than that created by grinding. Fly cutting also produced a surface with a higher fatigue strength than the grinding did.

Tensile and fatigue strength of hydrogen-treated Ti-6Al-4V alloy

Abstract: Tensile, fatigue and fractographic data on Ti-6AI-4V microstructures attained through a series of post-L3-annealing treatments which used hydrogen as a temporary alloying element are presented. Hydrogen-alloying treatments break up the continuous grain boundary 0c and colony structure, and produce a homogeneous microstructure consisting of refined s-grains in a matrix of discontinuous 13. These changes in microstructural morphology result in significant increases of the yield strength (974 to 1119 MPa), ultimate strength (1025 to 1152 MPa) and high cycle fatigue strength (643 to 669 MPa) compared to respective values for lamellar microstructures (902, 994, 497 MPa). The strengths are also significantly greater than the strengths of equiaxed microstructures (914, 1000, 590 MPa). The strengths of hydrogen-alloy treated samples are therefore superior to strengths attainable via other thermal cycling techniques. The fatigue fracture surfaces of the hydrogen-alloy treated samples were topographically similar to equiaxed samples. Fatigue crack initiation was characterized by faceted regions. As crack length and AK increased, the crack surface changed to a rounded, ductile topology, with microcracks and locally striated regions. Fracture primarily followed the ~-[3 interfaces. This is rationalized by the fact that hydrogen-alloyed microstructures are very fine Widmanstatten microstructures having reduced aspect ratios, and these microstructures fail along =-~ interfaces.

Energy stored in a specimen under fatigue limit loading conditions

Abstract: This paper presents results of measuring the energy stored in a specimen made from Ck 35 carbon steel loaded with an alternating one-directional stress whose value is equal to the fatigue limit. The energy stored in the specimen was determined in an indirect way as the difference between the mechanical energy expended in the specimen and the energy released into the surroundings as heat. The proposed technique is a modified version of the dynamic hysteresis loop method. The energy released as heat was measured with an electric modelling method. It was found that the energy stored during one cycle in a specimen loaded with a stress equal to the fatigue limit was larger than zero (i.e., with Nf→ ∝, the stored energy value theoretically also tends to infinity). To make the theory of the experiment compatible with the fundamental assumption stating that the amount of energy needed to cause fracture is constant irrespective of loading conditions, it is necessary to introduce a rheological function accounting for all reversible processes occurring in a material under cyclic loading.

Fracture mechanics analysis of fatigue resistance of spot welded coach‐peel joints

Abstract: Resistance spot welding is the most widely used joining method in automobile manufacture. The number, location, and quality of welds are some of the factors that influence the performance of welded subassemblies, and body panel structures. Therefore, design optimization requires knowledge of not only sheet metal behavior, but also weld behavior under service loadings. A linear elastic fracture mechanics approach was employed in this study to estimate the fatigue lives of spot welds subjected to tearing loads in a coach-peel specimen. Using a finite element method (FEM), the initial J-integral values for five coach-peel joints, each with different geometries, were calculated. Fatigue tests conducted on the same weld geometries provided life data. The experimental data were used to derive a relationship between the initial elastic J-integral values (ΔJe) and the fatigue life. It was found that the total fatigue life (Nf) of a weld at one applied stress range is related to its range of J-integral value such that a ΔJe vs Nf log-log plot gives a straight line relationship. This relationship can be used to evaluate the effects of geometrical variables on the fatigue life of coach-peel joints. The results show that, within the dimension range studied here, the effects of geometrical variables on the fatigue resistance can be ranked in the following decreasing order: weld eccentricity, sheet thickness and weld nugget diameter.

Method for improving machinability of titanium and titanium alloys and free-cutting titanium alloys

Abstract: The machinability of titanium or a titanium alloy is improved without adversely affecting the hot workability and fatigue strength or corrosion resistance by addition of P: 0.01-1.0% along with one or both of S: 0.01-1.0% and Ni: 0.01-2.0%, or along with S: 0.01-1.0%, Ni: 0.01-2.0%, and REM: 0.01-5.0%, on a weight basis.

Fatigue behaviour of steel fiber reinforced concrete

Abstract: The results of an experimental investigation on the fatigue characteristics and residual strength of steel fiber reinforced concrete (SFRC) are reported. The testing program included flexural specimens as well as split-cylinders and cubes reinforced with two fiber types at a low volume content. One of the fibers was of the deformed slit-sheet type available at aspect ratios of 45 and 60. It is shown that SFRC has a better fatigue response than plain concrete and that the deformed slit-sheet fiber has an effect almost identical to hooked-end fiber of similar dimensions. There is no increase in residual strength measured by split-tension when specimens are subjected to fatigue stress above the endurance limit. Fatigue characteristics of SFRC from this testing program as well as previous works can be interpreted as a function of the fiber factor (i.e. a parameter accounting for volume fraction, aspect ratio and fiber type) to provide design charts. More experimental work is needed to provide an acceptable database for fatigue design of SFRC.

Mechanical properties of a composite inlay material following post-curing

Abstract: Compressive strength, fatigue limit, hardness and wear factors were determined for two shades of a composite inlay material both with and without post-cure annealing! The effect of the annealing procedure was to cause a small general improvement in properties. This was more noticeable in the Dentin shade of the material than in the Enamel shade. It is doubtful whether the small improvement in properties would have any significant effect on clinical durability.

Hygrothermal degradation and fracture process of advanced fibre-reinforced plastics

Abstract: An investigation has been carried out on the effects of water absorption on the mechanical properties and fatigue strength of unidirectionally reinforced carbon-epoxy (CFRP) and aramid-epoxy (AFRP). T-1/347 normal-type CFRP absorbed more water than MM-1/982X heat-resistant-type CFRP. After a long immersion of about 7 months, however, MM-1/982X composites absorbed water rapidly and the fatigue strength was considerably decreased. In T-1/347 wet specimens preconditioned in water for 2 months a high amount of water absorption degraded the matrix and decreased the strength of the fibre-matrix interface, thereby worsening the mechanical properties. For MM-1/982X composites a small amount of water absorption moderately decreased the interfacial strength and increased the ductility of the matrix, thereby improving the mechanical properties. However, the fatigue lives of wet specimens of both CFRPs were decreased in water. Water absorption lowered the yield strength of the epoxy resin, thereby changing the shape of the stress-strain curves of AFRP. This had no effect on the tensile strength of the composites. Water absorption increased the fatigue strength of both Dupont's Kevlar-49 and Teijin's Technora AFRP. The water absorption increased the ductility of the matrix and enabled local realignment of the fibres. The Technora composite had a stronger interface than the Kevlar-49 composite. Water absorption lowered the interfacial strength of both AFRPs and changed the failure mechanisms. In these tests the synthesized evaluation of acoustic emission (AE) signals using several AE parameters was carried out and the fracture mechanisms were discussed in terms of the observations of internal damage by a scanning acoustic microscope and of fracture surfaces by a scanning electron microscope.

Toward a sound understanding of dislocation plasticity

Abstract: A novel theory is presented for work hardening in stage II shown by single crystals of ductile face-centered cubic (fcc) metals. The theory is based on the assumption that slipbands are highly elongated ellipsoidal zones with weak interiors, oriented at small, alternating angles with the crystallographic slip planes. Consideration of the stacking of such ellipsoids, together with a condition for the nucleation of a new slipband at the center of an obstacle nearby, yields both the average angle of inclination and the shear offset in the bands, both of which remain constant throughout stage II. The theory makes several predictions concerning the internal stress distribution and the plastic behavior. Before expounding the theory, the concepts upon which it is based are discussed in the context of two much simpler problems: the work hardening of dispersionhardened metals and the cyclic work hardening which forms the basis of understanding fatigue properties. The picture confirms in a natural way that the value of σ III , the stress which marks the end of stage II hardening, may be taken to be equal to the fatigue endurance limit.

Design approaches for edge delamination resistance in laminated composites

Abstract: This paper reviews the material and structural approaches to increase resistance of delamination initiation and growth. In the material approach, it was found that currently available toughened thermosets and thermoplastic composites can significantly improve delamination resistance for static, but not fatigue, loading. In the structural approach, it was concluded that stitching and interleafing are effective ways to resist delamination as a result of impact. However, terminating and discretizing the critical ply, as well as selective interleafing, are the most effective ways to increase delamination resistance for both static and fatigue in-plane loadings. 25 refs.