Showing papers on "Fatigue limit published in 1994"

Low-Cycle Fatigue Behavior of Reinforcing Steel

Abstract: ASTM A615 grade 40 ordinary deformed‐steel reinforcing bars with a specified minimum yield strength of 276 MPa (40 ksi) and ASTM A722 high‐strength prestressing thread bars with a specified ultimate strength of 1,083 MPa (157 ksi) were experimentally evaluated for their low‐cycle fatigue behavior under axial‐strain‐controlled reversed cyclic tests with strain amplitudes ranging from yield to 6%. All tests were performed on virgin (unmachined) specimens to closely simulate seismic behavior in structural concrete members. A methodology is suggested to identify incipient (first‐cracking) failure of test specimens. The experimental data were evaluated with existing fatigue models, which related stress‐strain quantities to the failure life. Additional energy‐based fatigue models are proposed that relate various stress and/or strain quantities to the dissipated energy. The study demonstrates that the modulus of toughness and low‐cycle fatigue life for both the low‐ and high‐strength materials are similar. Based...

Inclusion rating by statistics of extreme values and its application to fatigue strength prediction and quality control of materials

Abstract: The inclusion rating method by statistics^ extreme values (IRMSE) using y/area of inclusions as the size parameter enables one to discriminate between current super-clean steels. Moreover, IRMSE enables one to predict the size {y/area-m,^ of maximum inclusions contained in domains larger than the inspection domain. The statistical di.'itribution of s/area^, can be used for the quality control of materials and for the prediction of a scatter band of fatigue strength. Practical procedures of inclusion rating and prediction of a scatter band of fatigue strength arc shown.

The cyclic properties of engineering materials

Abstract: The basic fatigue properties of materials (endurance limit, fatigue threshold and Paris law constants) are surveyed, inter-related and compared with static properties such as yield strength and modulus. The properties are presented in the form of Material Property Charts. The charts identify fundamental relationships between properties and, when combined with performance indices (which capture the performance-limiting grouping of material properties) provide a systematic basis for the optimal selection of materials in fatigue-limited design.

Instructions for a New Method of Inclusion Rating and Correlations with the Fatigue Limit

Abstract: Many inclusion rating methods already exist [1–6], some of which have been adopted as the standards for particular countries or industries. However, with the existing methods, it is difficult to evaluate the relationship between the fatigue limit and the type, size, or distribution of the inclusions. Recent works have shown that inclusions behave as small defects and that the quantitative effect on the fatigue strength can be assessed from an evaluation of the square root of the projected area of the largest inclusion, on a plane perpendicular to the maximum principal stress direction. This parameter, designated √areamax, contained in a definite volume, can be evaluated using the statistics of extremes of the inclusion distribution.

Finite element analysis estimates of cement microfracture under complete veneer crowns

Abstract: Long-term clinical failures of complete veneer crowns are commonly attributed to microleakage of the cement. Excessive stress or fatigue cycling may create cement microfractures and promote microleakage. Two-dimensional (2D) finite element analysis (FEA) was selected to determine stress levels and distributions on dental cements resulting from 10 MPa occlusal loads on single-unit complete artificial veneer crowns during various clinical conditions. Sixteen 2D-FEA computer models were generated for a mandibular first premolar to study the effects of (1) marginal configuration (shoulder for all-ceramic crown versus chamfer for type III gold alloy crown), (2) four types of cement (zinc phosphate, polycarboxylate, glass ionomer, and composite resin), and (3) two thicknesses of cement (25 and 100 μm) for single-cycle loads and fatigue loading. There was almost no difference between a chamfer and shoulder marginal configuration except at the edge of the margin where the chamfer finish lines reached 2 to 8 times greater stresses. There were minimal effects for thickness of cement and marginal configurations. Stresses were slightly less for thicker cement. Fatigue analysis was based on estimated stress versus number of cycle curves for cements and resulted in stresses below the estimated endurance limit. If the average occlusal loading levels were 10 MPa, there did not appear to be a risk of microfracture in dental cement because of mechanical loading.

High toughness low yield ratio, high fatigue strength steel plate and process of producing same

Abstract: A steel plate having a high toughness, low yield ratio and high fatigue strength is provided by preserving the fine metallographical microstructure of martensite or bainite while austenitizing extremely fine portions of the microstructure, and during cooling, dispersing the portions as martensite, retained austenite, cementite or mixture thereof in a tempered martensite or tempered bainite phase.

Effect of microstructure on the fatigue strength of an austempered ductile iron

Abstract: Rotating bending fatigue tests were carried out on austempered ductile iron containing 1.5 wt% nickel and 0.3 wt% molybdenum. The ductile iron was austenitized at 900 or 1050 °C and then austempered at 280 or 400 °C for different lengths of time to obtain different microstructures. The fatigue strength was correlated with the amount of retained austenite and its carbon content, which were both determined by X-ray diffraction technique. While the tensile strength decreased with increasing retained austenite content, the fatigue strength was found to increase. Carbide precipitation was found to be detrimental to fatigue strength. Lower austenitizing temperature resulted in better fatigue strength.

Fatigue predictions and scatter

Abstract: — Different aspects of fatigue design problems are indicated and uncertainties are listed. Scatter as observed in many laboratory studies is analyzed. It is argued that scatter of crack initiation and crack growth are different issues. Various sources of scatter are discussed and illustrative examples are presented. Comments are given on statistical distribution functions, scatter under Variable-Amplitude loading, and scatter in service. The discussion touches upon the experience and meaning of scatter of laboratory test series related to practical problems.

A review of modelling small-crack behavior and fatigue-life predictions for aluminum alloys

Abstract: The small-crack effect, where small fatigue cracks grow faster and at lower stress-intensity factors than large cracks, has been found to be significant for many materials and loading conditions. In this paper, plasticity effects and crack-closure modelling of small fatigue cracks are reviewed. A crack-closure model with a cyclic-plastic zone-corrected effective stress-intensity factor range (related to the cyclic J-integral) and microstructural data on crack-initiation sites were used to calculate small-crack growth rates and fatigue lives for unnotched and notched specimens made of two aluminum alloys. The crack-closure transient from the plastic wake was shown to be the dominant cause of the small-crack effect and plasticity effects on the cyclic-plastic zone-corrected stress-intensity factor range were negligible except at extremely high stress levels. Small-crack growth rates and fatigue lives under both constant-amplitude and spectrum loading from tests and analyses agreed well.

A Theory of Fatigue Crack Initiation

Abstract: A theory of fatigue crack initiation based on the Gibbs free energy consideration is proposed. The theory is extended to the cases when the free surface effect and interface effect are considered.

Mechanical and Environmental Factors in the Cyclic and Static Fatigue of Silicon Nitride

Abstract: The effects of environment on cyclic and static fatigue behavior were investigated with hot-pressed silicon nitride materials. Tests were conducted at ambient temperature on standard compact tension specimens, and a dc electric potential technique was used to monitor crack lengths in situ. The results indicate that the environmental sensitivity of our materials under both cyclic and static loading mirrors that of durable glasses in static fatigue. The materials were most sensitive to water in the environment, while changes in pH had no significant effect in the range tested. In addition, NH3 was much less reactive with our materials than with vitreous SiO2. In some cases, the intergranular glass appears to be the site of environmental interaction. Evidence was also found that cyclic fatigue is not simply a manifestation of static fatigue. Cyclic fatigue was seen to occur in the absence of measurable static fatigue, and the data indicate that the mechanism of cyclic fatigue involves damage to the crack wake shielding zone.

Mechanical properties of vacuum-mixed acrylic bone cement.

Abstract: The thrust of the present work was the experimental determination of the uniaxial static compressive and fully reversed tension-compression fatigue properties of CMWTM 3 acrylic bone cement whose consituents were mixed in a properietary chamber while simultaneously subjected to a vacuum. Selected indices of performance in this material are: mean static compressive strength, 81.4 MPa; mean compressive modulus of elasticity, 1.95 GPa; endurance limit, 8.1 MPa; and characteristic fatigue life (using a three-parameter Weibull fit to the fatigue test data obtained at a stress of ±10 MPa), 238 712 cycles. The difficulties in comparing results obtained using different cement formulations, preparation conditions, and test conditions are detailed. With this in mind, it is suggested that the present results are within the range of values reported by previous workers for other formulations mixed using a variety of methods. The clinical significance of the present results is discussed. © 1994 John Wiley & Sons, Inc.

Fatigue Life of Riveted Railway Bridges

Abstract: This dissertation, which focuses on the fatigue life of riveted steel railway bridges, is based on: - A series of full-scale fatigue tests of nine riveted stringers taken from a railway bridge built in 1896 A review of and comparison with the different full-scale fatigue tests on riveted railway bridge members, which have been conducted in other laboratories over the years, is also given - A number of field tests on riveted railway bridges Some 15 bridges have been investigated by the author since 1989 - A theoretical study of the load-carrying capacity of riveted connections Special emphasis is given to the influence of the clamping force The phenomenon of fatigue cracking in riveted railway bridges is also accounted for The structural members and locations where fatigue cracking is likely to occur are especially in foucs The main finding is that there seems to be a substantial remaining fatigue life of riveted railway bridges still in use today The full-scale fatigue tests have shown that the common standard fatigue design curve for riveted members and details (virgin specimens) is underestimating the fatigue life The fatigue damage accumulation of the old bridge stringers, at the start of the laboratory tests, was found to be negligible, ie the stresses due to the in-service loading history have in general been low In situ strain measurements on several riveted railway bridges also show that the stresses seldom, if ever, exceed the fatigue limit for riveted details Ultrasonic testing, which has shown an absence of fatigue cracks, confirms these conclusions The fracture toughness is in general low for the steel in old bridges, but there are a number of circumstances that indicates that the ductility is adequate despite this fact For example low in-service stresses, low strain rates, the absence of fatigue cracks or other major defects, small plate thicknesses and an inherent structural redundancy of built-up riveted bridge members all contribute to a safe behaviour with respect to brittle fracture Especially the inherent structural redundancy of such members has, during the full-scale fatigue tests, led to the arrest of a propagating fatigue crack when passing from one member part to another

Fatigue crack growth in fiber-reinforced metal-matrix composites

Abstract: Fatigue crack growth in fiber-reinforced metal-matrix composites is modeled based on a crack tip shielding analysis. The fiber/matrix interface is assumed to be weak, allowing interfacial debonding and sliding to occur readily during matrix cracking. The presence of intact fibers in the wake of the matrix crack shields the crack tip from the applied stresses and reduces the stress intensity factors and the matrix crack growth rate. Two regimes of fatigue cracking have been simulated. The first is the case where the applied load is low, so that all the fibers between the original notch tip and the current crack tip remain intact. The crack growth rate decreases markedly with crack extension, and approaches a “steady-state”. The second regime occurs if the fibers fail when the stress on them reaches a unique fiber strength. The fiber breakage reduces the shielding contribution, resulting in a significant acceleration in the crack growth rate. It is suggested that a criterion based on the onset of fiber failure may be used for a conservative lifetime prediction. The results of the calculations have been summarized in calibrated functions which represent the crack tip stress intensity factor and the applied load for fiber failure.

Effects of dc electric field on mechanical properties of piezoelectric ceramics

Abstract: The effects of dc electric field on mechanical properties such as hardness, fracture toughness, flexural strength and flexural fatigue strength of piezoelectric ceramics were investigated. Hardness and fracture toughness were not influenced by the dc electric field, where as the flexural strength and the fatigue strength degraded with the absolute value of the electric field. The degradation of the flexural strength and the acceleration of fatigue were greater under an electric field whose direction was opposite to the poling direction than an electric field whose direction was the same as the poling direction. The main cause of strength degradation and fatigue acceleration in piezoelectric ceramics is estimated to be the microscopic internal stress which is generated at the grain boundary by the piezoelectric and domain switching deformations.

Creep behavior of hand-mixed Simplex P bone cement under cyclic tensile loading.

Abstract: Acrylic cement, used for the fixation of total hip replacements and other orthopedic implants, is a subject of renewed scientific interest as a result of recent hypotheses about dynamic, longterm mechanical failure mechanisms suspected to play a role in prosthetic loosening. Little is known, however, about the long-term mechanical behavior of cement. In this study, the dynamic creep deformation of hand mixed acrylic cement was examined in laboratory tests. Strain patterns found represented the familiar creep process consisting of a primary, a secondary, and a tertiary creep phase. Specimens dynamically loaded with a maximum stress of 3 MPa from 0 were subject to creep of about 50% of the elastic strain after 250 000 loading cycles. A linear relationship between the logarithmic values of the creep-strain and the number of loading cycles was found. Specimens exposed to higher loads showed significantly higher creep-strains. No relationship could be established between the strain levels and the porosity of the specimens. Specimens dynamically loaded with a maximal stress of 7 or 11 MPa from 0 failed during the tests. The number of loading cycles to failure was similar to fatigue strength data reported in earlier literature. © 1994 John Wiley & Sons, Inc.

Effect of glass fiber-matrix polymer interaction on fatigue characteristics of short glass fiber-reinforced poly(butylene terephthalate) based on dynamic viscoelastic measurement during the fatigue process

Abstract: Fatigue behaviors of glass fiber-reinforced poly(butylene terephthalate) (PBT) were studied based on dynamic viscoelastic measurements during the fatigue process. The fatigue strength of glass fiber-reinforced PBT was greatly improved by strengthening the interfacial adhesion between glass fiber and matrix PBT. The heat generation rate under cyclic fatigue for PBT reinforced with surface-unmodified short glass fiber was always larger than that reinforced with surface-modified short glass fiber because of the large net imposed strain amplitude of PBT matrix which occurred due to the interfacial debonding under cyclic fatigue. A fatigue fracture criterion based on the magnitude of hysteresis energy loss being consumed for a structural change was established for the PBT/short glass fiber composites in consideration of glass fiber-matrix polymer interfacial interaction. © 1994 John Wiley & Sons, Inc.

Experimental investigation of the fatigue strength of plain concrete under high compressive loading

Abstract: Fatigue tests are conducted on plain concrete cylinders subjected to axial cyclic compression. The upper level of the cyclic stress ranges between 60% and 90% of the static compressive strength. A method based on volume strain measurements is used to predict the individual static strength of each tested sample. The results of fatigue tests are presented in a load versus cycles to failure diagram. The recorded values of the longitudinal strains are, plotted in terms of number of cycles. The test figures are discussed assuming that the failure is a result of both time and cycle dependent damage. For loading levels ranging up to 80% of the static strength, the number of cycles to failure shows hardly any variation with the effects of time. Above this level, the fatigue strength proves to be more sensitive to time dependent effects, as the level of loading increases.

Phosphorus and carbon segregation: Effects on fatigue and fracture of gas-carburized modified 4320 steel

Abstract: Phosphorus and carbon segregation to austenite grain boundaries and its effects on fatigue and fracture were studied in carburized modified 4320 steel with systematic variations, 0.005, 0.017, and 0.031 wt pct, in alloy phosphorus concentration. Specimens subjected to bending fatigue were characterized by light metallography, X-ray analyses for retained austenite and residual stress measurements, and scanning electron microscopy (SEM) of fracture surfaces. Scanning Auger electron spectroscopy (AES) was used to determine intergranular concentrations of phosphorus and carbon. The degree of phosphorus segregation is directly dependent on alloy phosphorus and carbon content. The degree of carbon segregation, in the form of cementite, at austenite grain boundaries was found to be a function of alloy phosphorus concentration. The endurance limit and fracture toughness decreased slightly when alloy phosphorus concentration was increased from 0.005 to 0.017 wt pct. Between 0.017 and 0.031 wt pct phosphorus, the endurance limit and fracture toughness decreased substantially. Other effects related to increasing alloy phosphorus concentration include increased case carbon concentration, decreased case retained austenite, increased case compressive residual stresses, and increased case hardness. All of these results are consistent with the phosphorus-enhanced formation of intergranular cementite and a decrease in carbon solubility in intragranular austenite with increasing phosphorus concentration. Differences in fatigue and fracture correlate with the degree of cementite coverage on the austenite grain boundaries and the buildup of phosphorus at cementite/matrix interfaces because of the insolubility of phosphorus in cementite.

Effect of elevated temperature exposure on cast gamma titanium aluminide (Ti48Al2Cr2Nb)

Abstract: In order to effectively replace the nickel-base alloys currently in use, cast gamma alloys must possess a wide range of mechanical property capabilities, including tensile strength, ductility, fracture toughness, fatigue strength and creep resistance. These properties must be retained after exposure to the high temperature environment of gas turbines. One phenomenon that has surfaced recently is the effect of moisture and hydrogen on tensile ductility. Prior to the work reported here, it had not been determined if these effects occur in alloys such as Ti-48Al-2Cr-2Nb. The function of the Cr and Nb is to improve strength, ductility, creep resistance and oxidation resistance. The results show that both ductility-reducing phenomena do indeed occur in Ti-48Al-2Cr-2Nb, and that they can interact to sharply lower room temperature ductility after surprisingly modest exposures.

Cumulative damage of fiber-reinforced elastomer composites under fatigue loading

Abstract: Fracture mechanisms under fatigue loading were assessed in the case of nylon fiber-reinforced elastomer matrix composite which represents the actual carcass of bias aircraft tires. Under uniaxial tension, the angle-plied carcass composite specimens were subjected to a considerably large interply shear strain before failure. The composite specimens exhibited semi-infinite fatigue life when stress amplitude was below a threshold level, i.e., fatigue endurance limit. Under cyclic stresses exceeding the endurance limit, localized damage in the form of fiber-matrix debonding and matrix cracking was formed and developed into delamination eventually leading to gross failure of the composite. The process of damage accumulation was accompanied by heat generation and a continuous increase of cyclic strain (dynamic creep). Fatigue lifetime and the resistance to damage accumulation of aircraft tire carcass composite were strongly influenced by cyclic frequency. The use of higher cyclic frequency resulted in shorter f...